ALBERT

Description: We estimated frequency-dependent attenuation of coda waves ([IMG]/medium/109eq1.gif" ALT="Formula "〉) and body waves ([IMG]/medium/109eq2.gif" ALT="Formula "〉 and [IMG]/medium/109eq3.gif" ALT="Formula "〉) in 1.5-24 Hz by applying the single isotropic scattering theory and the extended coda-normalization method, respectively, in the crust beneath the Andaman Sea. We used 43 aftershocks of the 13 September 2002 earthquake (Mw 6.5) in the Andaman Sea recorded by three stations installed in the Andaman Islands. The coda Q factors calculated from the amplitude decay rate of the S-wave coda show a dependence on frequency and lapse time. We found that with the increase in lapse time window from 10 to 40 s, Q0 (QC at 1 Hz) increases from 55 to 153, while the frequency-dependent coefficient n decreases from 1.1 to 0.94. The average frequency-dependent relations of [IMG]/medium/109eq4.gif" ALT="Formula "〉 vary from 0.02f-1.1 to 0.01f-0.94 with an increase in lapse time window from 10 s to 40 s, respectively. The values of [IMG]/medium/109eq5.gif" ALT="Formula "〉 and [IMG]/medium/109eq6.gif" ALT="Formula "〉 corresponding to spectral amplitude decays show strong frequency dependence and are expressed as 0.02f-1.01 and 0.01f-1.0, respectively. Our results are consistent with those of other seismically active regions. The ratio [IMG]/medium/109eq7.gif" ALT="Formula "〉 is found to be larger than unity for the whole frequency range. We separated intrinsic absorption ([IMG]/medium/109eq8.gif" ALT="Formula "〉) and scattering attenuation ([IMG]/medium/109eq9.gif" ALT="Formula "〉) using the independent estimates of [IMG]/medium/109eq10.gif" ALT="Formula "〉 and [IMG]/medium/109eq11.gif" ALT="Formula "〉. The results show that [IMG]/medium/109eq12.gif" ALT="Formula "〉 is close to [IMG]/medium/109eq13.gif" ALT="Formula "〉 and both of them are larger than [IMG]/medium/109eq14.gif" ALT="Formula "〉, suggesting that coda decay is predominantly caused by intrinsic attenuation.

Description: The earthquake ground-motion prediction equations developed for the Next Generation Attenuation of Ground Motions (NGA-West) project in 2008 have established a new baseline for the estimation of ground-motion parameters, such as peak ground acceleration, peak ground velocity, and spectral acceleration, for shallow crustal earthquakes in active tectonic regions. We perform statistical goodness-of-fit analyses to quantitatively compare the predictive capabilities of the NGA models and their predecessors, using several testing subsets of the master database used in model development. In addition, we perform a blind comparison test using 1060 ground-motion records from seven recent earthquakes recorded in California: the 2003 M 6.5 San Simeon event, 2004 M 6.0 Parkfield event, 2005 M 5.2 Anza event, 2007 M 5.4 Alum Rock event, 2008 M 5.4 Chino Hills event, 2010 M 7.2 Baja event, and 2010 M 5.7 Ocotillo event. We assess how modeling decisions regarding the regression dataset, functional forms, input parameters, and model complexity influence the models' predictive capabilities. By comparing the performance of each model, we discuss various ground-motion modeling strategies and offer recommendations for model development. We find that increased model complexity does not necessarily lead to increased prediction accuracy, that the inclusion of aftershocks in regression datasets may result in decreased predictive capabilities for mainshocks, and that the use of measured site characteristics leads to greatly improved ground-motion predictions. A model validation framework is introduced to assess the prediction accuracy of ground-motion prediction equations and to aid in their future development.

Description: Changes to building codes in the last decade, lowering the probability at which design ground motions for geotechnical applications are defined, have led to an urgent need for a probabilistic approach/tool for liquefaction potential assessment. We propose a consistent approach for probabilistic liquefaction hazard analysis (PLHA) that is based on probabilistic seismic hazard analysis and incorporates a reliability-based liquefaction potential evaluation method based on shear-wave velocity data. The method directly takes the joint probability distribution of peak ground acceleration and moment magnitude into account. We demonstrate the method for four Canadian cities, employing our interim updated seismic hazard models for eastern and western Canada. Using the developed method and representative site profiles, PLHA is implemented for four major cities across Canada with the aim of investigating the impact of regional seismic characteristics on liquefaction hazard assessment. Sensitivity analysis indicates that different magnitude ranges of dominant contributing seismic events have significant impact on the extent of liquefaction hazard. More specifically, for a given seismic excitation level, the relatively high hazard contributions from small-to-moderate earthquakes in eastern Canada leads to less significant liquefaction potential, in comparison with similar sites in western Canada.

Description: There are many engineering applications that require an understanding of the nature of strong ground motions adjacent to and spanning across faults. Unfortunately, such near-field observations at distances less than 100 m of fault rupture are few and incomplete. In this study a 3D finite-difference method is used to simulate strong ground motions for a hypothetical Mw 6.5 earthquake at sites within a few tens of meters of the fault to document the nature of strong ground motion at pairs of sites across the fault as a first step toward providing ground-motion input for engineering design applications. We employ several distributed slip kinematic models to examine ground-motion variability. We also examine the ground motions for fault scenarios ranging from vertical strike-slip to low-angle thrust faulting. The results show that the motions have two primary components: (1) far-field waves that undergo focusing and amplification due to finite-source rupture directivity and (2) near-field waves that are sensitive to the tectonic rebound, or fling, of the closest section of the fault to the recording stations. Both the far-field and near-field controlled motions result in nonstationary pulse-like velocity waveforms that have many implications for the design of engineered structures located close to or spanning faults.

Description: In 2006 we published an earthquake hazard model called LASSCI (LAyered Seismogenic Source model in central Italy). In October 2008 we began to update the model for use in 5- and 10-year forecasts. The LASSCI-2006 model is supported by good fault-based definitions of seismogenic sources and simple physically motivated models of earthquake occurrence; the LASSCI-2009 model has been improved by revision of the error propagation assumptions and increased accuracy of the earthquake probabilities. The 6 April 2009 earthquake that struck L'Aquila occurred on the model fault having the highest probability of occurrence in the 2009 revised LASSCI forecast: it is therefore consistent with our model assumptions. Furthermore, peak ground accelerations were in reasonable agreement with the values having 90% probability of not being exceeded in 50 yr. In the revised 2009 model, the aggregate probability of having a maximum-sized event in the next 5 yr on at least one of the neighboring sources (less than 25 km distance away) decreases in L'Aquila from 10% to 7% after the earthquake occurrence, but still remains a maximum there along the central Apennines. The LASSCI models 2006 and 2009, featuring characteristic fault sources and time dependence, seem to be suitable for guidance in reconstruction and seismic retrofit in the central Appenines.

Description: In many seismic applications, a constant quality factor is used to describe the constitutive laws of viscoelastic materials, characterized by frequency-independent attenuation characteristics. In such cases, the frequency dependence of the medium's properties is not taken into account. To overcome this drawback, we proposed an elegant finite difference time domain implementation, with an auxiliary differential equation technique to explicitly solve any stress-strain relation. This scheme is inherited from the formalism of electromagnetism and is based on the separation of the propagation equations from the constitutive law defined in the stress-strain equation. The conventional assumption of a constant quality factor assumption can then be easily avoided in the modeling of seismic-wave propagation in viscoelastic media. We developed such a method and simulated synthetic traces over a simple, 2D viscoelastic homogeneous medium using a Zener model. Wave propagation phase velocities were estimated by means of dispersion analysis and appeared to match theoretical values over a reasonable frequency range. We also measured the material's attenuation behavior by studying the quality factor, thereby validating our approach.

Description: We analyzed temporal changes in the quality factor (Q) of P waves transmitting through a fracturing Westerly granite sample under triaxial compressive conditions, using the rise time of the initial wave. Broadband recording was performed throughout, even after the differential stress reached the peak strength. We calibrated a transducer assembly and corrected the recorded waveforms. Using theoretical waveforms, we derived a new equation relating QP, the rise time for the velocity waveform, and the travel time. QP remained at [~]23 before the start of dilatancy and decreased to [~]17 after the peak stress was reached. A large nonlinear decrease in QP was not clearly observed, while the first wave amplitude clearly decreased rapidly after the occurrence of the peak stress.

Description: We use the 2D finite element method to determine how geometrical parameters determine whether rupture will propagate across a linked stepover in a strike-slip fault. The end segments of the fault system are aligned in the direction of maximum shear, and the length and angle of the linking segment are allowed to vary. We observe that ruptures propagate through extensional stepovers with steeper angles and longer linking segments than otherwise equivalent compressional stepovers. These different rupture behaviors form distinct regions in angle-stepover-length parameter space; the boundary between these regions takes the shape of an asymptotic curve in both the extensional and compressional cases. Models in which the size of the entire fault system was made larger or smaller revealed that the location of the boundaries between regions of different rupture behavior do not scale linearly with the system size; it was easier to rupture steeper and relatively longer stepovers in fault systems that were larger overall. A separate set of models in which the stress field is rotated so that the parallel end segments were optimally aligned for rupture significantly altered the rupture behavior curves; in this stress field, it was easier to rupture compressional stepovers with steeper angles and longer linking segments than it was to rupture equivalent extensional stepovers. In both the case in which the end segments are aligned with the direction of maximum shear and the case in which the end segments are optimally oriented for rupture, the angles at which rupture could no longer propagate through the entire fault corresponded with peaks in the fault's S value.

Description: The 6 April 2009 Mw 6.3 L'Aquila earthquake, central Italy, has been recorded by the Irpinia Seismic Network (ISNet) about 250 km southeast of the epicenter. Up to 19 three-component accelerometer stations could be used to infer the main source parameters with different seismological methods. We obtained an approximate location of the event from arrival times and array-based back-azimuth measurements and estimated the local magnitude (6.1) from an attenuation relation for southern Italy. Assuming an omega-square spectral model, we inverted S-wave displacement spectra for moment magnitude (6.3), corner frequency (0.33 Hz), stress drop (2.5 MPa), and apparent stress (1.6 MPa). Waveform modeling using a point source and an extended-source model provided consistent moment tensors with a centroid depth around 6 km and a prevalently normal fault plane solution with a dominant directivity toward the southeast. The relatively high corner frequency and an overestimated moment magnitude of 6.4 from moment tensor inversions are attributed to the rupture directivity effect. To image the rupture geometry, we implemented a beamforming technique that back-projects the recorded direct P-wave amplitudes into the earthquake source region. A northwest-southeast striking rupture of 17 km length is imaged, propagating with an average velocity up to 3 km/s. This value is significantly higher than our estimate of 2.2 km/s from S-wave spectra. Our case study demonstrates that the use of array techniques and a dense accelerometer network can provide quick and robust estimates of source parameters of moderate-sized earthquakes located outside the network.

Description: On 8 June 2008 a strong (Mw 6.4) earthquake occurred in the area of northwest Peloponnese, western Greece. The event originated in the lower crust and was caused by the rupture of a previously unknown fault probably inherited from past tectonic phases. In this study we perform a stress inversion of all available focal mechanisms in this area in order to obtain an estimate of the regional stress field. It is shown that the maximum principal stress axis has an azimuth of N273{degrees}E and forms an angle of 63{degrees} with the fault's strike, which is well-constrained by seismological observations. This implies that the fault was severely misoriented with respect to the prevailing stress field assuming friction coefficients in the range 0.65-0.85. Calculation of pore-fluid factors for a variety of input parameters seems to confirm the presence of elevated fluid pressure near the hypocenter because they reach superhydrostatic to lithostatic values (0.80-1.0). The source of these fluids is probably of deep origin and may have to do with upper mantle degassing. This suggestion is supported by the presence of mantle helium in spring waters close to the epicenter and by low Pn velocities consistent with partially molten mantle beneath northwest Peloponnese.

Description: We use seismographic data from 12 local, shallow earthquakes of magnitude MN 1.0-3.1 that occurred near Sudbury, Ontario, recorded at three stations within 30 km of the source, to examine ground-motion attenuation at close distances. This is an important issue for development of ground-motion prediction equations in eastern North America and for the inference of earthquake source parameters from regional earthquake observations. We show that high-frequency (3-10 Hz) horizontal-component ground motions decay at a geometric spreading rate of approximately R-1.3 in the first 25-30 km from the earthquake source (where R is the hypocentral distance). This is in agreement with the geometric spreading rate determined by Atkinson (2004) and used in the ground-motion prediction equations of Atkinson and Boore (2006). Vertical-component ground-motions in this distance range appear to decay at a slightly slower rate of R-1.1.

Description: We investigated the source scaling of earthquakes (Mw 4.6-8.9), mostly from the Taiwan orogenic belt, and made a global compilation of source parameters to examine the scaling self-similarity. Finite-fault slip models (12 dip-slip and 7 strike-slip) using mainly dense strong-motion data and teleseismic data from Taiwan were utilized. Seven additional earthquakes (M〉7) were included for further examination of scaling of large events. We determined the effective length and width for the scaling study was M0[~]L2 and M0[~]L3 for the events less than and larger than the seismic moment of [IMG]/medium/464eq1.gif" ALT="Formula "〉, respectively, regardless of the fault types, suggesting a nonself-similar scaling for small to moderate events and a self-similar scaling for large events. Although the events showed variation in stress drops, with the exception of three events with high stress drops, most of the events had stress drops of 10-100 bars. The observed bilinear relation is well explained by the derived magnitude-area equation of Shaw (2009) when we considered only events with stress drops of 10-100 bars and a seismogenic thickness of 35 km. The bilinear feature of the regressed magnitude-area scaling holds for ruptured areas up to about 1000 km2 for our seismogenic thickness of 35 km. For the events having rupture areas larger than that, the average slip becomes proportional to the rupture length. The distinct high stress drop events from blind faults in the western foothill of Taiwan yield local high peak ground accelerations (PGAs) when compared to the Next Generation Attenuation model. Regardless of the relative small magnitudes of these events, the high PGAs give the region higher seismic hazard potential and thus require special attention for seismic hazard mitigation.

Description: Identifying changes in the rate of background earthquakes is a common method to detect transients in seismic-activity rates, and these transients can often be linked to changes in physical properties of the crust or to the recording homogeneity of seismic networks. In this article, we consider the effect that various sources of uncertainty have on seismicity-rate estimation, and we describe a Monte Carlo method to compute the statistical significance of seismicity-rate changes. We consider real earthquake catalogs and synthetic catalogs that represent Poissonian background seismicity. We find that the choice of declustering algorithm and the variation in the related parameter values have the greatest impact on seismicity-rate-change calculations. This sensitivity is a direct consequence of the fact that there is no unique or universally applied approach to separate aftershocks and foreshocks from background events. Hypocenter location uncertainty also influences the statistical significance of seismicity-rate changes; its relevance depends on the ratio of the location uncertainty to the extension of the seismicity-rate anomaly. We find that magnitude uncertainty and edge effects associated with seismicity declustering have less impact than the other factors, but their influence is not negligible. In an example application, we suggest that, when uncertainties are considered, the reported seismicity-rate changes prior to the 1992 Landers and Big Bear mainshocks decrease significantly.

Description: Theoretical considerations and empirical regressions show that, in the magnitude range between 3 and 5, local magnitude, ML, and moment magnitude, Mw, scale 1:1. Previous studies suggest that for smaller magnitudes this 1:1 scaling breaks down. However, the scatter between ML and Mw at small magnitudes is usually large and the resulting scaling relations are therefore uncertain. In an attempt to reduce these uncertainties, we first analyze the ML versus Mw relation based on 195 events, induced by the stimulation of a geothermal reservoir below the city of Basel, Switzerland. Values of ML range from 0.7 to 3.4. From these data we derive a scaling of ML[~]1.5Mw over the given magnitude range. We then compare peak Wood-Anderson amplitudes to the low-frequency plateau of the displacement spectra for six sequences of similar earthquakes in Switzerland in the range of 0.5[≤]ML[≤]4.1. Because effects due to the radiation pattern and to the propagation path between source and receiver are nearly identical at a particular station for all events in a given sequence, the scatter in the data is substantially reduced. Again we obtain a scaling equivalent to ML[~]1.5Mw. Based on simulations using synthetic source time functions for different magnitudes and Q values estimated from spectral ratios between downhole and surface recordings, we conclude that the observed scaling can be explained by attenuation and scattering along the path. Other effects that could explain the observed magnitude scaling, such as a possible systematic increase of stress drop or rupture velocity with moment magnitude, are masked by attenuation along the path.

Description: We numerically simulate seismic wave propagation from the 1999 Mw 7.4 earthquake in Izmit, Turkey, using a 3D finite difference method based on published finite-source models obtained by waveform inversions. This earthquake has been reported, based on observations at the near-fault station SKR, as an example of supershear rupture propagation toward the east. Although the modeled ground motion does show a characteristic Mach wave from the fault plane, it is difficult to identify any particular effects in terms of peak ground velocity (PGV), an important parameter in earthquake engineering. This is because the fault's spatial heterogeneity is strong enough to mask the properties of supershear rupture, which has been reported through several numerical simulations mostly based on homogeneous fault conditions. This article demonstrates the importance of studying ground motions for known earthquakes through numerical simulations based on finite-fault source models.

Description: Forward directivity effects are known to cause pulselike ground motions at near-fault sites. We propose a comprehensive framework to incorporate the effects of near-fault pulselike ground motions in probabilistic seismic hazard analysis (PSHA) computations. Also proposed is a new method to classify ground motions as pulselike or non-pulselike by rotating the ground motion and identifying pulses in all orientations. We have used this method to identify 179 recordings in the Next Generation Attenuation (NGA) database (Chiou et al., 2008), where a pulselike ground motion is observed in at least one orientation. Information from these 179 recordings is used to fit several data-constrained models for predicting the probability of a pulselike ground motion occurring at a site, the orientations in which they are expected relative to the strike of the fault, the period of the pulselike feature, and the response spectrum amplification due to the presence of a pulselike feature in the ground motion. An algorithm describing how to use these new models in a modified PSHA computation is provided. The proposed framework is modular, which will allow for modification of one or more models as more knowledge is obtained in the future without changing other models or the overall framework. Finally, the new framework is compared with existing methods to account for similar effects in PSHA computation. Example applications are included to illustrate the use of the proposed framework, and implications for selection of ground motions for analysis of structures at near-fault sites are discussed.

Description: We have synthesized the characteristics of the seismogenic zone in the east Los Angeles basin by analyzing earthquake data recorded during the past 30 years (1981-2010). The seismicity is distributed along the Whittier fault, with the majority of earthquakes located adjacent to the south side, in the depth range from 0 to 9 km, with b value of 1.1{+/-}0.05 and mostly normal and strike-slip faulting. Within the depth range of 9-12 km, the seismicity is scattered uniformly across the region, the b value is 1.0{+/-}0.05, and all three faulting styles are present. At the deepest depths (12-18 km), seismicity is sparse and primarily limited to a few clusters striking north; these deeper earthquakes primarily have reverse fault motion, and the b value is 0.78{+/-}0.04. Inversion of high-quality focal-mechanism data for the orientation of the regional stress field showed that the direction of maximum compressional stress rotates from N12{degrees}W at shallow depth to due north at the bottom of the seismogenic zone. Similarly, a depth dependence is observed in stress drops calculated from P-wave source spectra, which indicate stress drop generally increases from [~]7 MPa at shallow depth (3 km) to [~]53 MPa at the base of the seismogenic zone (17 km). Overall, our results provide new evidence for the vertical partitioning of styles of deformation and state of stress within this complex fault system in the east Los Angeles basin.

Description: Broadband waveform inversion of ground velocities in the 0.02-0.10 Hz frequency band is successfully applied to 181 earthquakes with ML[≥]3 of the April 2009 L'Aquila, Italy, earthquake sequence. This was made possible by the development of a new regional crustal velocity model constrained by deep crustal profiles, surface-wave dispersion and teleseismic P-wave receiver functions, and tested through waveform fit. Although all earthquakes exhibit normal faulting, with the fault plane dipping southwest at about 55{degrees} for the majority of events, a subset of events had much shallower dips. The issue of confidence in the derived parameters was investigated by applying the same inversion procedure by two groups who subjectively selected different traces for inversion. The unexpected difficulty in modeling the regional broadband waveforms of the mainshock as a point source was investigated through an extensive finite-fault modeling of broadband velocity and accelerometer data, which placed the location of major moment release up-dip and about 4-7 s after the initial first-arrival hypocentral parameters.

Description: Since 2003, 39 small earthquakes have been detected off the coast of central Oregon in the nominally locked part of the Cascadia subduction zone, where very little seismic activity has been recorded in spite of a paleoseismic record of great subduction events. Although the regional earthquake bulletin reports depths of 29 and 28 km for the two largest events (Mw 4.9 and Mw 4.7, which occurred in 2004), analysis by Trehu et al. (2008) indicates that they were low-angle thrust events that occurred on the plate boundary at depths of 9-11 and 16 km, respectively. Because of sparse onshore station coverage, most of the smaller events have large location uncertainties. Double-difference relative location of 30 of these earthquakes reveals two tight clusters approximately 30 km apart; each cluster is associated with one of the two larger events. Within each cluster, relocation reduces the hypocenter depth spread from 〉15 km to

Description: Evansville, Indiana, is one of the closest large urban areas to both the New Madrid Seismic Zone, where large earthquakes occurred in 1811-1812, and the Wabash Valley Seismic Zone, where there is evidence of several large prehistoric earthquakes in the last 14,000 yr. For this reason, Evansville has been targeted as a priority region for urban seismic-hazard assessment. The probabilistic seismic-hazard methodology used for the Evansville region incorporates new information from recent surficial geologic mapping efforts, as well as information on the depth and properties of near-surface soils and their associated uncertainties. The probabilistic seismic-hazard calculation applied here follows the method used for the 2008 United States Geological Survey (USGS) national seismic-hazard maps, with modifications to incorporate estimates of local site conditions and their uncertainties, in a completely probabilistic manner. The resulting analysis shows strong local variations of acceleration with 2% probability of exceedance in 50 yr, which are clearly correlated with variations in the thickness of unconsolidated soils above bedrock. Spectral accelerations at 0.2-s period range from 0.6 to 1.5g, values that are much greater than those of the USGS national seismic-hazard map, which assume B/C site conditions with an average shear-wave velocity of 760 m/s in the top 30 m. The presence of an ancient bedrock valley underlying the current Ohio River flood plain strongly affects the spatial pattern of accelerations. For 1.0-s spectral acceleration, ground motions are significantly amplified due to deeper soils within this structure, to a level comparable to that predicted by the national seismic-hazard maps with D site conditions assumed. For PGA and 0.2-s spectral acceleration, ground motions are significantly amplified outside this structure, above the levels predicted by the national seismic-hazard maps with uniform D site conditions assumed.

Description: We compare our recent ground-motion prediction equations (GMPEs) for western North America (WNA; Boore and Atkinson, 2008 [BA08]) and eastern North America (ENA; Atkinson and Boore, 2006 [AB06]; Atkinson, 2008 [A08]) to newly available ground-motion data. Based on these comparisons, we suggest revisions to our GMPEs for both WNA and ENA. The revisions for WNA affect only those events with M[≤]5.75, while those for ENA affect all magnitudes. These are simple modifications to the existing GMPEs that bring them into significantly better agreement with data. The wealth of new data clearly demonstrates that these modifications are warranted; we therefore recommend the use of the updated equations for seismic hazard analyses and other applications. More detailed studies are under way by many investigators (including ourselves) to develop a new generation of ground-motion models in both WNA and ENA from scratch, through a comprehensive reevaluation of source, path, site, and modeling issues. In time, those more complete models will replace those proposed in this study. However, as the new models will be several years in development, we recommend using the modified models proposed herein, labeled BA08' (for WNA), AB06' (for ENA), and A08' (for ENA, to replace A08), as interim updates to our existing models. The proposed models are in demonstrable agreement with a rich database of ground motions for moderate-magnitude earthquakes in both WNA and ENA and are constrained at larger magnitudes by the BA08 magnitude and distance scaling.

Description: The 2009 Mw 6.3 L'Aquila earthquake produced an impressive number of rotational effects on vertically organized objects such as chimneys, pillars, capitals, and gravestones. We present a dataset of such effects that consists of 105 observations at 37 different sites and represents a compendium of earthquake-induced instances of rotational effects that is unprecedented in recent times. We find that the absolute majority of the reported effects were observed in the epicentral zone and that most of the observations are located where the Mercalli-Cancani-Sieberg intensity is between 7 and 8-9. The evident asymmetry in the distribution of the rotational effects resembles the southeastward directivity of the macroseismic effects and highlights a significant convergence between rotations and damage. Finally, we perform some qualitative analyses to recognize and evaluate which geological and seismological parameters can be significant contributors to local rotations. We find that surface geology and amplification of the seismic motion at each reported location strongly influence the occurrence and the nature of the earthquake-induced rotational effects. Conversely, the contribution of the pattern of slip distribution on the fault plane plays only a secondary role in enhancing the rotational motion at each site.

Description: Current ground-motion prediction equations invariably assume that site conditions at strong-motion stations, often characterized by the average shear-wave velocity to a depth of 30 m (VS30), are known to a uniform accuracy. This is, however, rarely the case. In this article, we present a regression procedure based on generalized least-squares and maximum-likelihood approaches that take into account the varying uncertainties on VS30. Assuming that VS30s for various groups of stations are known to different accuracies, application of this procedure to a large set of records from the Japanese KiK-net shows that the regression coefficients are largely insensitive to the assumption of nonuniform uncertainties. However, this procedure allows the computation of a site-specific standard deviation ({sigma}) that should be used for sites where VS30 is known to different accuracies (e.g., a site only specified by class or a site with a measured VS profile). For sites with a measured VS profile, this leads to lower site-specific{sigma} than for a site that is poorly characterized because this technique explicitly models the separation between the epistemic uncertainty in VS30 and the aleatory variability in predicted ground motion.

Description: In this paper, we adopt three ground-motion simulation techniques (the stochastic finite-fault simulation code from Motazedian and Atkinson, 2005; the hybrid deterministic-stochastic approach with approximated Green's functions from Pacor et al., 2005; and the broadband hybrid integral-composite technique with full-wavefield Green's functions from Gallovi[c] and Broke[s]ova, 2007), with the aim of investigating the different performances in near-fault strong-motion modeling and prediction from past and future events. The test case is the 1980 M 6.9 Irpinia earthquake, the strongest event recorded in Italy in the last 30 years. First, we simulate the recorded strong-motion data and validate the model parameters by computing spectral acceleration and peak amplitude residual distributions. The validated model is then used to investigate the influence of site effects and to compute synthetic ground motions around the fault. Afterward, we simulate the expected ground motions from scenario events on the Irpinia fault, varying the hypocenters, the rupture velocities, and the slip distributions. We compare the median ground motions and related standard deviations from all scenario events with empirical ground-motion prediction equations (GMPEs). The synthetic median values are included in the median {+/-} 1 standard deviation of the considered GMPEs. Synthetic peak ground accelerations show median values smaller and with a faster decay with distance than the empirical ones. The synthetics total standard deviation is of the same order or smaller than the empirical one, and it shows considerable differences from one simulation technique to another. We decomposed the total standard deviation into its between-scenario and within-scenario components. The larger contribution to the total sigma comes from the latter, while the former is found to be smaller and in good agreement with empirical interevent variability.

Description: The 1999 Chi-Chi, Taiwan, earthquake (Mw 7.6) was the largest earthquake to strike Taiwan in the twentieth century. Its surface rupture extends more than 100 km in a north-south trend. However, the northern end of this surface deformation abruptly terminates at an area between the Tachia river and the Taan river, where a broad pop-up structure with east to northeast strike (quite different from the north-south-striking main thrust) is found. This pop-up structure is composed of several regions of parallel thrusts and back thrusts and total shortening reaches about 9 m. In order to reveal the displacement field in the pop-up structure and slip vectors of branch faults, we use a digital cadastral system with about 17,000 control points to calculate coseismic displacement around the Shihkang area. The digital cadastral system was originally conceived to survey land and building boundaries. The cadastral system affords high-density control points that reach about 2800 points/km2, and the accuracy is to within {+/-}11 cm. Outside of the pop-up structure, the horizontal displacements vectors are 8-9 m in about 330{degrees}-340{degrees}, changing as they pass through the surface rupture. The azimuth of horizontal displacement shows large variation with the azimuth from 360{degrees} to 315{degrees} and displacement amounts from about 4-8 m inside the pop-up structure. In addition, according to these cadastral data, we found surface ruptures that had not been observed in previous studies that could be of importance to future disaster assessment.

Description: Since the beginning of the Global Geodynamics Project (GGP) in 1997, the number of superconducting gravimeters (SGs) has increased to reach 30 operating sites today. Data from this network allow a comparison of the noise levels of the different contributing stations. The knowledge of the noise levels at each site is important in any combination of data to determine global Earth parameters, for example, the stacking of the data in the search for elusive signals, like the gravity variations associated with the translational mode of the inner core. We use a standardized procedure based on the computation of the residual power spectral densities (PSDs) over a quiet time period in order to evaluate the combined instrument plus site noise in the long-period seismic band (0.3 mHz-1 mHz). The experience at Strasbourg (France) has shown some improvements from the TT70-T005 full-size instrument to the C026 compact model in terms of noise reduction, while the most recent Observatory SG types, OSG044 at Bad Homburg (Germany) and OSG052 at Sutherland (South Africa), do not show any further improvement with respect to the compact models, respectively CD30 and CD037, operating at the same stations. At Black Forest Observatory (BFO) in Germany, the experience of the dual-sphere OSG with a lower sphere heavier than usual has shown that the instrumental and site conditions make this station the least noisy one at frequencies larger than 0.1 mHz. The noise analysis using the longest time-series available has shown that the noise level at these sites is mostly stable (within 1{sigma}) over the years. The comparison with some seismological noise models shows that the best SG sites are less noisy than long-period seismometers below 1 mHz. However, the noise level of the best SGs is still at the limit of detection of the subseismic translational mode of the inner core.

Description: North Korea detonated its first known nuclear device, a moderate-sized event in the northeastern corner of the Korean Peninsula, on 9 October 2006. A second one, several times more powerful, was set off nearby on 25 May 2009. Both were recorded at high signal-to-noise ratio (SNR) by a modern broadband (0.03-30 Hz) network of seismographs deployed since 2004 along the Sino-Korean border, and by station MDJ due north of ground zero. Spanning a wide range of station azimuth (259{degrees}-11{degrees}), the near-regional (1.3{degrees}-3.3{degrees}) paths are all purely continental and away from continental margins, making the resulting data uniquely suited for assessing the capabilities of an out-country network to verify the compliance by North Korea with a future comprehensive nuclear test ban treaty. The mb(Lg) magnitudes for the 2006 and 2009 nuclear explosions were determined anew, giving 4.32{+/-}0.13 and 4.86{+/-}0.13, respectively. We show that the MS-mb(Lg)method correctly differentiates these nuclear explosions from natural earthquakes, although the differentiation was not achieved in studies based on published mb(Lg) values. An analysis of the Pg:Lg ratio, derived from recordings of the two nuclear tests and of four regional earthquakes selected for their comparable magnitudes (4.2[≤]mb[≤]4.8), reveals that the ratios associated with the explosion and earthquake populations showed surprisingly little overlap for a broad frequency range of 3 to 11 Hz. In principle, MS-mb can also be used to correctly identify the nuclear tests as explosions, but the mb measurement for such moderate-sized events is not as robust as the mb(Lg) measurement, rendering this traditionally favored method less practical.

Description: Assessing the completeness magnitude Mc of earthquake catalogs is an essential prerequisite for any seismicity analysis. We employ a simple model to compute Mc in space based on the proximity to seismic stations in a network. We show that a relationship of the form [IMG]/medium/1371eq1.gif" ALT="Formula "〉, with d the distance to the kth nearest seismic station, fits the observations well, k depending on the minimum number of stations being required to trigger an event declaration in a catalog. We then propose a new Mc mapping approach, the Bayesian magnitude of completeness (BMC) method, based on a two-step procedure: (1) a spatial resolution optimization to minimize spatial heterogeneities and uncertainties in Mc estimates and (2) a Bayesian approach that merges prior information about Mc based on the proximity to seismic stations with locally observed values weighted by their respective uncertainties. Contrary to the current Mc mapping procedures, the radius that defines which earthquakes to include in the local magnitude distribution is chosen according to an objective criterion, and there are no gaps in the spatial estimation of Mc. The method solely requires the coordinates of seismic stations. Here, we investigate the Taiwan Central Weather Bureau (CWB) seismic network and earthquake catalog over the period 1994-2010.

Description: Korea Polar Research Institute has been operating a broadband seismic station (KSJ1) at the King George Island (KGI), Antarctica, since 2001. Examining ambient seismic noise levels using power spectral analysis for the period of 2006-2008 at the KSJ1, we observed a seasonal pattern at a 4-10 s period. The amplitude of double-frequency (DF) microseism reaches a peak in May. Correlation of the DF energy and its predominant period with significant ocean-wave height and peak wave period models from the WAVEWATCH III and polarization analysis consistently indicate that ocean swell in the Drake Passage is a possible source to excite the DF microseism at the KGI. We also found that the temporal variation of DF amplitude is coincident with the seasonal change of ocean-acoustic ambient noise level around the KGI, which implies that incorporating long-term seismic and hydroacoustic noise information might give us an opportunity to figure out the characteristics of local climate variation near the Antarctic Peninsula.

Description: We have obtained a Poisson's ratio image of the uppermost mantle beneath China by performing tomographic inversion of travel-time differences between Sn and Pn. The arrival pairs were selected from the Annual Bulletin of Chinese Earthquakes from 1985 to 2007 and from the International Seismological Center (ISC) data set between 1985 and 2005. The data include 58,663 arrival pairs from 10,486 earthquakes recorded at 204 stations. The average Poisson's ratio is 0.266. The preliminary tomographic results show that (1) the pseudowave velocity is high and the velocity ratio (VP/VS) and Poisson's ratio are low in the stable cratons around the Tibetan Plateau such as the Tarim and Junggar basins, the Ordos craton, and the southern region of the Sichuan basin; (2) a low pseudowave velocity and high velocity ratio and Poisson's ratio exist in the central and northern Tibetan Plateau, the North-South Seismic Zone, and north China; and (3) the high velocity ratio and Poisson's ratio region in the Tibetan Plateau extends to the surrounding cratons, suggesting that the uplift of the Tibetan Plateau results from a high Poisson's ratio or partially melted rocks beneath the plateau and that the softer rocks have intruded into the upper mantle of surrounding cratons.

Description: In August 1989, an earthquake sequence including ten events with 6.3[≥]M[≥]5.5 in the first two days produced widespread ground deformation in the Dobi graben of central Afar. Numerous surface breaks with complex geometry, including fresh scarplets with vertical throws up to 30 cm high and open fissures up to 30 cm wide, were observed. Coseismic slip incremented the deformation (normal faulting, block tilting, and counterclockwise rotation of basaltic slices) accumulated in the last 2 m.y. in the transfer zone between the Dobi and Hanle grabens. By combining maps of surface ruptures, relative event relocations with the local Djibouti network, published focal mechanisms, and source sizes, we tentatively relate most of the mainshocks of the sequence to slip on individual faults. The largest shocks at 11h16 on 20 August 1989 (MS 6.2) and at 1h09 on 21 August 1989 (MS 6.3) ruptured southern segments of the southwestern bounding fault of the graben. A dozen other faults also slipped along the edges of, and inside, the graben. On average, triggered seismic faulting propagated about 35 km northwestward along the graben in about 20 hr. Slip on the main faults was coupled with slip on secondary antithetic faults branching from them at depth. Although the Dobi earthquakes ruptured part of the fault array between the Asal rift (1978 sequence) and the Serdo region (1969 sequence), an approximately 50-km-long gap subsists along the Der'ela half-graben. We infer the patterns of surface faulting in the Dobi sequence, which coinvolved bookshelf-faulting about both horizontal and vertical axes, to typify the complexity of coseismic stress release in central Afar and in other active zones of distributed extension (e.g., Iceland, Abruzzi, Basin and Range).

Description: The scale of previously proposed methods for mapping site-response ranges from global coverage down to individual urban regions. Typically, spatial coverage and accuracy are inversely related. We use the densely spaced strong-motion stations in Parkfield, California, to estimate the accuracy of different site-response mapping methods and demonstrate a method for integrating multiple site-response estimates from the site to the global scale. This method is simply a weighted mean of a suite of different estimates, where the weights are the inverse of the variance of the individual estimates. Thus, the dominant site-response model varies in space as a function of the accuracy of the different models. For mapping applications, site-response models should be judged in terms of both spatial coverage and the degree of correlation with observed amplifications. Performance varies with period, but in general the Parkfield data show that: (1) where a velocity profile is available, the square-root-of-impedance (SRI) method outperforms the measured VS30 (30 m divided by the S-wave travel time to 30 m depth) and (2) where velocity profiles are unavailable, the topographic slope method outperforms surficial geology for short periods, but geology outperforms slope at longer periods. We develop new equations to estimate site response from topographic slope, derived from the Next Generation Attenuation (NGA) database.

Description: A cumulative frequency-magnitude relation, the Gutenberg-Richter law, dominates the statistics of the occurrence of earthquakes. Although it is an empirical law, some authors have tried to give some physical meaning to its a and b parameters. Here, we recall some theoretical expressions for the probability of occurrence of an earthquake with magnitude M in terms of a and b values. A direct consequence of the maximum likelihood estimation (MLE) and the maximum entropy principle (MEP) is that a and b values can be expressed as a function of the mean magnitude of a seismic sequence over a certain area. We then introduce the definition of the Shannon entropy of earthquakes and show how it is related to the b value. In this way, we also give a physical interpretation to the b value: the negative logarithm of b is the entropy of the magnitude frequency of earthquake occurrence. An application of these concepts to two case studies, in particular to the recent seismic sequence in Abruzzi (central Italy; mainshock Mw 6.3, 6 April 2009 in L'Aquila) and to an older 1997 sequence (Umbria-Marche, central Italy; mainshock Mw 6.0, 26 September 1997 in Colfiorito), confirms their potential to help in understanding the physics of earthquakes. In particular, from the comparison of the two cases, a simple scheme of different regimes in succession is proposed in order to describe the dynamics of both sequences.

Description: Our study used space-based interferometric synthetic aperture radar (InSAR) and feature tracking on sub-meter-resolution optical imagery pairs to characterize surface deformation resulting from the 4 September 2010 Mw 7.1 Darfield, 22 February 2011 Mw 6.3 Christchurch, and 13 June 2011 Christchurch earthquakes (dates in local time), each of which occurred in the Canterbury region of the South Island of New Zealand. A rapid, coordinated international emergency response is often required when strong-motion earthquakes hit urban areas. Unfortunately in these cases relief workers often have little information about the location or the extent of damage. Remote sensing can rapidly provide maps of certain key variables (i.e., building damage, potential loading of nearby faults, etc.) to relief workers on the ground. These maps can cover broad areas on time scales that are only limited by the revisit time of the satellite or aircraft. Critically, imagery types such as satellite-based synthetic aperture radar (SAR) have long repeat times of up to 46 days at present, although the existence of overlapping tracks and multiple satellite platforms effectively reduces the repeat time somewhat. Here we demonstrate the impact of commercial optical imagery that can be acquired within hours to days after an earthquake, with the goal of supporting relief efforts in future earthquakes on a more rapid timescale than can be achieved with SAR imagery alone. We demonstrate that these sub-meter-resolution scenes are feasible tools for deriving near-fault surface displacements for use in fault slip inversions, even in areas of heavy agricultural activity. The Darfield and Christchurch earthquakes present an opportunity to observe postseismic deformation related to multiple moderate (

Description: On 4 September 2010, a magnitude M = 7.1 earthquake struck the Canterbury region on the South Island of New Zealand. The epicenter of the earthquake was located near Darfield, about 40 km west of the central business district (CBD) of the city of Christchurch and at a depth of about 10 km. Extensive damage was inflicted on lifelines and residential houses due to widespread liquefaction and lateral spreading in areas close to major streams, rivers, and wetlands throughout the city of Christchurch and the town of Kaiapoi. In the months following the Darfield M 7.1 earthquake, numerous aftershocks were felt across the city. Almost six months after the Darfield mainshock, on 22 February 2011, the Canterbury region was hit by a magnitude M = 6.3 earthquake. The epicenter was located near Lyttelton, only 6 km to the southeast of the Christchurch CBD and at a depth of 5 km. In spite of its smaller magnitude, this earthquake resulted in more damage to pipeline networks, transport facilities, residential houses/properties, and multistory buildings in the CBD than the September 2010 event, mainly because of the short distance to the city and the shallower depth. Although there were no casualties after the 2010 Darfield earthquake, which is sort of a miracle considering the magnitude of the earthquake, the 2011 Christchurch earthquake resulted in a significant number of casualties due to the collapse of multistory buildings and unreinforced masonry structures in the Christchurch city center. As of 1 June 2011, 181 casualties were reported (New Zealand Police; http://www.police.govt.nz/list-deceased). While it is extremely regrettable that the 2011 Christchurch earthquake resulted in significant casualties, engineers and seismologists now have a hard-to-find opportunity to learn the response of ground and structures to two large-scale earthquakes that occurred less than six months apart. From a geotechnical...

Description: Following the Christchurch 2011 earthquake, the Basilicata University (Potenza, Italy) organized a field trip to New Zealand mainly to examine structural engineering issues but also to investigate the similarity between this event and the L'Aquila 2009 quake that struck central Italy. In both cases an event with magnitude slightly above 6 occurred on a blind fault underlying an area inhabited by a population of the order of hundreds of thousands, killing a few hundred people and severely damaging the city center, and in both cases a site amplification study was available before the event. At the same time there were striking differences between the two earthquakes in maximum recorded acceleration, the nonlinear behavior of soils, and the occurrence of liquefaction. It was also an opportunity to look at some issues related to the use of microtremor measurements, in particular...

Description: Following both the 4 September 2010 Mw 7.1 Darfield and 22 February 2011 Mw 6.2 Christchurch, New Zealand, earthquakes, Geotechnical Extreme Events Reconnaissance (GEER) team members from the United States and New Zealand visited the affected areas to assess geotechnical related damage (e.g., Allen et al. 2010a, b). As shown in Figure 1, liquefaction was pervasive in large portions of the region after both earthquakes. The widespread liquefaction caused extensive damage to residential properties, water and wastewater networks, high-rise buildings, and bridges. For example, nearly 15,000 residential houses and properties were severely damaged from liquefaction and lateral spreading. More than 50% of these houses were damaged beyond economic repair. Also, portions of the central business district (CBD) were severely damaged by liquefaction during the Christchurch earthquake. It is estimated that approximately 30% of the buildings in the CBD were damaged beyond repair, although not all of the damage resulted from liquefaction. Among the field tests performed by the GEER team were the dynamic cone penetrometer (DCP) test (Sowers and Hedges 1966) and spectral analysis of surface waves (SASW) test (Stokoe et al. 1994). Both of these tests can provide information about the liquefaction susceptibility of soil and are relatively portable, making them suitable for rapid post-earthquake reconnaissance field studies. The objective of this paper is to provide an overview of DCP and SASW tests performed across the Christchurch region and to summarize the comparison of the observed versus predicted liquefaction occurrence during both the Darfield and Christchurch earthquakes...

Description: The 22 February (local time) MW ~6.2 Christchurch earthquake occurred within the aftershock region of the 4 September 2010 MW 7.1 Darfield (Canterbury) earthquake (Gledhill et al. 2011). Both the Darfield and Christchurch earthquakes occurred on previously unknown faults in a region of historically low seismicity, but within the zone of plate boundary deformation between the Pacific and Australian plates. The Darfield earthquake caused surface rupture up to 5 m (Quigley et al. 2010, forthcoming), but none has been observed associated with the Christchurch earthquake. Geodetic data indicate that strain has been slowly accumulating within the region (Wallace et al. 2007; Beavan et al. 2002), and the presence of active subsurface faults was known or suspected (e.g., Pettinga et al. 2001). Earthquakes of magnitude up to 7.2 in this region had been allowed for in the national seismic hazard model (Stirling et al. 2002), but the observed high apparent stresses (Fry and Gerstenberger 2011, page 833 of this issue) and high ground accelerations (Fry et al. 2011, page 846 of this issue) had not been anticipated, particularly those experienced in the Christchurch event. These and other factors (Fry and Gerstenberger 2011, page 833 of this issue; Fry et al. 2011, page 846 of this issue; Holden 2011, page 783 of this issue), plus the close proximity of the February earthquake to Christchurch city center, were responsible for the major damage caused by the earthquake (e.g., Kaiser et al. 2011). A large amount of geodetic ground-displacement data is available to constrain the source of the earthquake, in part because we reoccupied nearly 200 GPS sites that had been observed following the Darfield earthquake, and in part because a number of space agencies collected synthetic aperture radar (SAR) data over...

Description: Large earthquakes within seismogenic crust are generally thought to require the pre-existence of large fault structures. Such fault structures appear to evolve by the progressive growth and amalgamation of smaller faults and fractures (Cowie and Scholz 1992). In the course of their evolution some components of an evolving fault system may be inherited from previous tectonic episodes while others may be newly formed in the prevailing tectonic stress field. With increasing displacement and amalgamation of sub-structures, fault structures tend to become “smoother,” less complex, and perhaps weaker (Wesnousky 1988). The 2010–2011 Canterbury earthquake sequence occurred within the upper crust of the South Island of New Zealand around 100 km southeast from the fast-moving (20–30 mm/yr) Alpine and Hope fault strike-slip components of the Pacific-Australia transform fault system linking into the southern Hikurangi Margin subduction zone (Figure 1). As of 15 July 2011, the sequence has included three major shocks: the Mw 7.1 Darfield earthquake (3 September 2010 UTC) followed by an Mw 6.2 event on 21 February 2011 UTC and an Mw 6.0 event on 13 June 2011 UTC, along with a rich aftershock sequence that includes 27 shocks with Mw 〉 5.0. Rupturing occurred on previously unrecognized faults that appear to be components of a highly segmented E-W structure concealed beneath alluvial cover and/or Neogene volcanics. Some subsurface information is, however, available from seismic reflection lines and gravity surveys (e.g., Field et al. 1989). Here we seek to demonstrate how this complex sequence has likely arisen through reactivation under the contemporary tectonic stress field of a mixture of comparatively newly formed and older inherited fault structures...

Description: On 22 February 2011 at 12:51 p.m. local time, a moment magnitude Mw 6.3 earthquake occurred beneath the city of Christchurch, New Zealand, causing an level of damage and human casualties unparalleled in the country's history. Compared to the preceding 4 September 2010 Mw 7.1 Darfield earthquake, which occurred approximately 30 km to the west of Christchurch, the close proximity of the 22 February event led to ground motions of significantly higher amplitude in the densely populated regions of Christchurch. As a result of these significantly larger ground motions, structures in general, and commercial structures in the central business district in particular, were subjected to severe seismic demands and, combined with the event timing, structural collapses accounted for the majority of the 181 casualties (New Zealand Police 2011). This manuscript provides a preliminary assessment of the near-source ground motions recorded in the Christchurch region. Particular attention is given to the observed spatial distribution of ground motions, which is interpreted based on source, path, and site effects. Comparison is also made of the observed ground motion response spectra with those of the 4 September 2010 Darfield earthquake and those used in seismic design in order to emphasize the amplitude of the ground shaking and also elucidate the importance of local geotechnical and deep geologic structure on surface ground motions...

Description: Almost six months after the MW 7.1 Darfield (Canterbury) earthquake, on 22 February 2011 at 12:51 p.m. (local time), an MW 6.2 earthquake struck the city and suburbs of Christchurch—the largest city on the South Island of New Zealand, with about 400,000 inhabitants. The Christchurch earthquake can be considered one of the greatest natural disasters recorded in New Zealand. The death toll was more than 180, with around 2,000 people injured, and structures already weakened by the Darfield event and its aftershocks were badly affected (Cubrinovski and Green 2010; Tonkin and Taylor Ltd. 2010; Kam et al. 2011). The earthquake was generated by an oblique thrust fault located between the Australian and Pacific plates, within about 6 km of the city center. It is worth recalling that prior to the Darfield event there was no surface evidence of the fault that generated the Christchurch earthquake on February 2011, nor of the Greendale fault, recognized as responsible for the September 2010 earthquake (Quigley et al. 2010). During the last decade a set of seismic surveys across the Canterbury Plains had been carried out (Green et al. 2010), but they did not reveal any convincing evidence of the Greendale fault and there was no clear indication that a major earthquake was imminent in this particular region. Beyond the effects and the consequences of the seismic event, the attention of the scientific community was drawn to two aspects that had a primary role in the Christchurch earthquake: 1) the extremely severe, strong ground shaking observed, especially on the vertical component; and 2) the widespread liquefaction phenomena across the city (Cubrinovski and Green 2010; Green et al. 2011, page 927 of this issue). Between September 2010 and June 2011 the Canterbury area experienced three major earthquakes with...

Description: During the period between September 2010 and June 2011, the city of Christchurch was strongly shaken by a series of earthquakes that included the 4 September 2010 (Mw = 7.1), 26 December 2010 (Mw = 4.8), 22 February 2011 (Mw = 6.2), and 13 June 2011 (Mw = 5.3 and Mw = 6.0) earthquakes. The moment magnitude (Mw) values adopted in this paper are taken from GNS Science, New Zealand (http://www.geonet.org.nz); they are 0.1 units higher than the corresponding Mw values reported by the U.S. Geological Survey (http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usb0001igm/). These earthquakes produced strong ground motions within the central business district (CBD) of Christchurch, which is the central heart of the city just east of Hagley Park and encompasses approximately 200 ha. Some of the recorded ground motions had 5% damped spectral accelerations that surpassed the 475-year return-period design motions by a factor of two. Ground shaking caused substantial damage to a large number of buildings and significant ground failure in areas with liquefiable soils. The 22 February earthquake was the most devastating. It caused 181 fatalities and widespread liquefaction and lateral spreading in the suburbs to the east of the CBD and in areas within the CBD, particularly along the stretch of the Avon River that runs through the city. There were pockets of heavy damage in the CBD, including the collapse of two multistory reinforced concrete buildings, as well as the collapse and partial collapse of many unreinforced masonry structures including the historic Christchurch Cathedral in the center of the CBD. Soil liquefaction in a substantial part of the CBD adversely affected the performance of many multistory buildings, resulting in global and differential settlements, lateral movement of foundations, tilt of buildings, and bearing failures. The Mw = 6.2, 22 February 2011 earthquake...

Description: The Canterbury earthquake sequence began in September 2010 with the Mw 7.1 (source: GeoNet catalog, http://geonet.org.nz/canterbury-quakes/) Darfield earthquake that ruptured the previously unknown 40-km-long Greendale fault 30 km west of Christchurch (Gledhill et al. 2011). Extreme ground accelerations as high as 1.8 g near the epicenter were recorded. The event caused intense liquefaction in the eastern suburbs of Christchurch as well as closer to downtown, near the course of the Avon River. The Darfield earthquake was followed by a major aftershock on 22 February local time (21 February UTC) of magnitude Mw 6.2 (source: GeoNet), but Me 6.7 (source: USGS, http://earthquake.usgs.gov/earthquakes/eqinthenews/2011/usb0001igm/neic_b0001igm_e.php). This earthquake was centred only a few kilometers south of the Christchurch city center. Extremely high accelerations (as high as 2.2 g) were also recorded near the epicenter (Kaiser, Benites et al. 2011). In addition to the extreme liquefaction seen after the Darfield earthquake, this event also caused landslides, large rockfalls, widespread damage to earthquake-risk buildings in Christchurch, and, most tragically, about 180 casualties. Another large aftershock of Mw 6.0 (source: GeoNet), but with Me 6.7 (source: USGS), subsequently occurred on 13 June local time (12 June UTC) just a few kilometers south of the February event, causing further damage, landslides, rockfalls, and liquefaction. Following the Darfield earthquake, the GeoNet network (New Zealand National Hazard Monitoring Network) and its regional component the CanNet network (Berrill et al. 2011) was supplemented by the deployment of 13 additional strong motion instruments regionally (and another nine following the February earthquake). We used this dense network of strong motion instruments to constrain the source kinematics of the February event. We present the inversion scheme and discuss its limitations. These results are preliminary, since more thorough data processing is needed; however, they already provide a key model that will help in understanding the sequence...

Description: The region in and around Christchurch, encompassing Christchurch city and the Selwyn and Waimakariri districts, contains more than 800 road, rail, and pedestrian bridges. Most of these bridges are reinforced concrete, symmetric, and have small to moderate spans (15–25 m). The 22 February 2011 moment magnitude (Mw) 6.2 Christchurch earthquake induced high levels of localized ground shaking (Bradley and Cubrinovski 2011, page 853 of this issue; Guidotti et al. 2011, page 767 of this issue; Smyrou et al. 2011, page 882 of this issue), with damage to bridges mainly confined to the central and eastern parts of Christchurch. Liquefaction was evident over much of this part of the city, with lateral spreading affecting bridges spanning both the Avon and Heathcote rivers. The majority of bridge damage was a result of liquefaction-induced lateral spreading, with only four bridges suffering significant damage on non-liquefiable sites. Abutments, approaches, and piers suffered varying levels of damage, with very little damage observed in the bridge superstructure. However, bridges suffered only a moderate amount of damage compared to other structural systems. Because some bridges critical to the city infrastructure network sustained substantial damage, extensive traffic disruption occurred immediately following the event. This paper presents a summary of field observations and subsequent analyses on the damage to some of the bridges in the Canterbury region as a result of the Christchurch earthquake. Reference is also made to the performance of bridges following the 4 September 2010 Mw 7.1 Darfield earthquake (Gledhill et al. 2011), and details of damage progression are presented where applicable. The ground motion characteristics for both events and the regional soil conditions are first described. We provide descriptions of the damage at each selected bridge site and compare observations of liquefaction with predicted response using in situ test data...

Description: The February 2011 Christchurch, New Zealand, earthquake was highly destructive, causing a number of buildings to collapse and killing many people. We examined the properties of strong ground motions in this earthquake using the records released by GeoNet (http://www.geonet.org.nz/). We also investigated the damage around the seismic stations to determine the relationship between structural damage and strong ground motions. SEISMIC GROUND MOTION INTENSITIES AND ELASTIC RESPONSE SPECTRUM The locations of the seismic stations in our study are shown in Figure 1. Accelerograms and the elastic acceleration response spectra, with a damping factor of 0.05 in the maximum horizontal direction, are shown in Figures 2 and 3, respectively. Peak ground accelerations (PGA) and peak ground velocities (PGV) are shown in Table 1. Ij and I1–2 are also shown in Table 1. Ij is JMA (Japan Meteorological Agency) seismic intensity (Tables 2, 3 and 4). It is publicly used to describe the damaging power of seismic shaking in Japan. I1–2 is also an index like Ij. It was defined by Sakai, Kanno, and Koketsu (2002, 2004) based on elastic responses between 1 and 2 seconds period that were closely related with heavy structural damage (the subscript 1–2 means between 1 and 2 seconds) and represents the damaging power of an earthquake much better than...

Description: Probabilistic seismic-hazard assessments of the central and eastern United States (CEUS) require estimates of the size of the largest possible earthquake (Mmax). In most of the CEUS, sparse historical seismicity does not provide a record of moderate and large earthquakes that is sufficient to constrain Mmax. One remedy for the insufficient catalog is to combine the catalog of moderate to large CEUS earthquakes with catalogs from other regions worldwide that are tectonically analogous to the CEUS (stable continental regions, or SCRs). After the North America SCR, the largest contribution of earthquakes to this global SCR catalog comes from a Southeast Asian SCR that extends from Indochina to southeasternmost Russia. Integration and interpretation of recently published geological and geophysical results show that most of these Southeast Asian earthquakes occurred in areas exposing abundant alkaline igneous rocks and extensional faults, both of Neogene age (last 23 million years). The implied Neogene extension precludes classification of the areas as SCR crust. The extension also reduces the number of moderate and large Southeast Asian historical earthquakes that are available to constrain CEUS Mmax by 86 percent, from 43 to six.

Description: An earthquake of Mw 6.1–6.31 (Beavan et al. 2011, page 789 of this issue) that struck Christchurch, New Zealand, on 22 February (21 February, UTC) produced recorded ground motion acceleration over 2 g. The event caused widespread damage with dense recordings of non-linear site behavior. Globally, dense near-field recordings of shallow intraplate earthquakes are rare. It is possible that extreme ground motions are common with this type of earthquake and that their rarity is merely a function of inadequate seismic sampling in the near field of such low-probability, high-potency events. To better define the nature of these events, we calculate apparent stress (ta) of the three largest earthquakes in the Canterbury sequence and compare them to global and regional data. We then place recorded PGA and spectral accelerations into the context of regional and global ground motion prediction equations and discuss the implications of high-stress events for future seismic hazard estimates for the region. For the February event, we also briefly explore the implications of directivity on measured ground motions in central Christchurch. The earthquakes that occurred in the Canterbury region of the South Island, New Zealand, from September 2010 to the present have disproportionately large energy magnitudes (Me) to their moment magnitudes (Mw). They have produced the largest ground motions ever measured in New Zealand. The sequence began with the Mw 7.1 earthquake that occurred about 40 km west of the city of Christchurch on 4 September 2010. The maximum recorded ground acceleration recorded during the event was over 1.25 g, which was experienced near the intersection of the triggering thrust on which the rupture began and the strike-slip Greendale fault that carried most of the moment in the earthquake (Gledhill et al. 2010). Peak ground accelerations (PGA) in the central business district of Christchurch averaged...

Description: On 22 February 2011 New Zealand time (21 February UTC), the MW 6.2 Christchurch earthquake occurred just 7 km southeast of the center of Christchurch city, New Zealand (Fry et al. 2011, Holden 2011, page 783 of this issue). There were 181 confirmed fatalities, and the damage to Christchurch city is estimated to be NZ$15 billion–$NZ20 billion (US$12 billion–US$16 billion). The event was well-recorded by the broadband and strong-motion national-scale GeoNet network (Petersen et al. 2011) as well as by the Canterbury regional strong-motion network (Avery et al. 2004). Since the 22 February earthquake, more than 2,700 further aftershocks have been recorded up to 1 May 2011, including 21 events with local magnitude (ML) greater than 5. Here we describe the initial relocation analysis for these aftershocks. The Mw 6.2 Christchurch earthquake is part of the larger aftershock sequence of the Mw 7.1 Darfield earthquake, which occurred at 16:35 3 September UTC, 2010. Seismological, GPS, and InSAR data all suggest that the earthquake rupture process for the Mw 7.1 Darfield earthquake involved failure of multiple fault segments (Beavan et al. 2010; Gledhill et al. 2011). A surface rupture for that earthquake, now termed the Greendale fault, extending ~29.5 km and located ~4 km south of the epicenter, is consistent with strike-slip faulting with an average horizontal surface displacement of ~2.5 m (Quigley et al. 2010). The vast majority of the 7,400+ aftershocks following the Darfield earthquake are shallow, at less than 15 km depth. Figure 1 shows that, although many of the aftershocks occurred near the surface trace of the Greendale fault, intense clusters of aftershock activity have also occurred at the western and eastern ends of the Darfield fault trace, as well as north-northwest from the Darfield epicenter. The distribution of...

Description: The Canterbury earthquakes of 2010 and 2011 have produced some of the strongest ground motions ever measured in New Zealand. Many of the highest acceleration recordings arose from seismic stations within the city of Christchurch (population ~377,000). A dense array of strong-motion seismometers was in place prior to the mainshock of 4 September 2010. Subsequent to the mainshock, numerous rapid response accelerometers were installed in the Canterbury Plains, Banks Peninsula, and in the city itself (Gledhill et al. 2011; Cochran et al. 2011). Many of the strongest aftershocks were recorded by this dense amalgamation of permanent and temporary arrays and provide a detailed record of variable ground motion throughout the region during the aftershock sequence. The most extreme ground motions were recorded during the Mw 6.2 earthquake of 22 February 2011 that struck a few kilometers to the south of Christchurch (Beavan et al. 2011, this issue; Holden et al. 2011, this issue; Bannister et al. 2011, this issue), generating severe damage throughout the city. In fact, damage to the built environment and ground liquefaction was much more widespread in the February event than in the September Mw 7.1 mainshock (Kaiser et al. 2011). This event is one of the best-recorded shallow thrust earthquakes in the near field. Recorded peak ground acceleration (PGA) in February exceeded 2 g near the epicenter and was greater than 0.6 g over much of the central and eastern suburbs (Figure 1). At these near-source stations, vertical accelerations were generally markedly higher than horizontal accelerations. The large accelerations can be reasonably well explained by the combination of the proximity of the February event to Christchurch...

Description: The M 7.1 Darfield earthquake occurred 40 km west of Christchurch (New Zealand) on 4 September 2010. Six months after, the city was struck again with an M 6.2 event on 22 February local time (21 February UTC). These events resulted in significant damage to infrastructure in the city and its suburbs. The purpose of this study is to evaluate the performance of global predictive models (GMPEs) using the strong motion data obtained from these two events to improve future seismic hazard assessment and building code provisions for the Canterbury region. The Canterbury region is located on the boundary between the Pacific and Australian plates; its surface expression is the active right lateral Alpine fault (Berryman et al. 1993). Beneath the North Island and the north South Island, the Pacific plate subducts obliquely under the Australian plate, while at the southwestern part of the South Island, a reverse process takes place. Although New Zealand has experienced several major earthquakes in the past as a result of its complex seismotectonic environment (e.g., M 7.1 1888 North Canterbury, M 7.0 1929 Arthur's Pass, and M 6.2 1995 Cass), there was no evidence of prior seismic activity in Christchurch and its surroundings before the September event. The Darfield and Christchurch earthquakes occurred along the previously unmapped Greendale fault in the Canterbury basin, which is covered by Quaternary alluvial deposits (Forsyth et al. 2008). In Figure 1, site conditions of the Canterbury epicentral area are depicted on a VS30 map. This map was determined on the basis of topographic slope calculated from a 1-km grid using the method of Allen and Wald (2007). Also shown are the locations of strong motion stations. The Darfield event was generated as a result of a complex rupture mechanism; the recordings and geodetic data...

Description: Strong aftershocks following major earthquakes present significant challenges for infrastructure recovery as well as for emergency rescue efforts. A tragic instance of this is the 22 February 2011 Mw 6.3 Christchurch aftershock in New Zealand, which caused more than 100 deaths while the 2010 Mw 7.1 Canterbury mainshock did not cause a single fatality (Figure 1). Therefore, substantial efforts have been directed toward understanding the generation mechanisms of aftershocks as well as mitigating hazards due to aftershocks. Among these efforts are the prediction of strong aftershocks, earthquake early warning, and aftershock probability assessment. Zhang et al. (1999) reported a successful case of strong aftershock prediction with precursory data such as changes in seismicity pattern, variation of b-value, and geomagnetic anomalies. However, official reports of such successful predictions in geophysical journals are extremely rare, implying that deterministic prediction of potentially damaging aftershocks is not necessarily more scientifically feasible than prediction of mainshocks. A potentially more effective approach for aftershock hazard mitigation is described by Bakun et al. (1994) for the case of the Loma Prieta earthquake. This approach relies on the rapid detection of an aftershock using a dense observation network in the rupture area of the mainshock and subsequent broadcast of an alert to more distant sites. Recent progress in rapid determination of epicenter and magnitude involving a small number of stations and short time window of P waveforms (Allen and Kanamori 2003; Wan et al. 2009; Wang et al. 2009) make the approach of earthquake early warning more effective for regions not very close to the rupture area of the mainshock. Such an approach might have been useful for aftershocks of the 2008 Wenchuan earthquake, where megacities such as Chengdu are about 90 km away and 20 seconds were available for rapid mitigation response...

Description: The 22 February 2011 magnitude 6.2 Christchurch earthquake, centered southeast of Christchurch, was part of the aftershock sequence that has been occurring since the September 2010 magnitude 7.1 quake near Darfield, 40 km west of the city. The Christchurch earthquake killed more than 180 people, damaged or destroyed more than 100,000 buildings, and is New Zealand's most deadly disaster since the earthquake that struck the Napier and Hastings area on 3 February 1931. This special focused issue of Seismological Research Letters, which I had the fortune to edit, contains a selected set of 19 original technical papers. These papers cover different aspects of the 2011 Christchurch earthquake from seismological, geodetic, geological, and engineering perspectives. The first eight papers focus on earthquake source modeling, fault stress variation, and aftershock sequence. The paper by Guidotti et al. presents three-dimensional numerical simulations of the Christchurch earthquake by comparing different fault and interface models. Using data from a dense network of strong motion instruments, Holden et al. presents the inversion scheme for constraining the source kinematics of the Christchurch event. The constrained geodetic...

Description: The Mw = 6.3 earthquake of February 22 was the strongest seismic event in a series of damaging aftershocks in and around Christchurch after the Darfield earthquake on 4 September 2010. The source of the Darfield earthquake was in a sparsely populated area and thus it caused no loss of life. Serious damage was mainly due to extensive liquefaction. By contrast, the Christchurch earthquake was generated on a fault in close proximity to the city, resulting in a death toll of 181 people. The Canterbury Plains are covered with river gravels that hide any evidence of past fault activity in this region. The newly revealed Greendale fault was therefore completely unknown. Only a portion of it was revealed on the ground surface during the Darfield earthquake. The second fault (the one that ruptured in February 2011) appears to be a continuation of the first, although no fault structure directly connecting the faults has been recognized. There is a debate among seismologists at this point whether this is a different fault from Greendale one or not (NHRP 2011a; NHRP 2011b; Geonet 2011). Due to its magnitude, shallow depth and close proximity to the city, the February earthquake proved particularly destructive for the central business district (CBD) of Christchurch, where buildings suffered extensive damage. Thanks to a dense network of strong ground motion stations, a large number of records have been obtained, which provide valuable information on the event and offer the possibility of relating the extent of damage to actual measurements of ground shaking. Apart from the southern part of the city on the hills and the Lyttelton port area, Christchurch is built on deep estuarine soil, which has been shaped in the last thousands of years by the ever-changing riverbed. Fine sands—the dominant soil type...

Description: The objective of this paper is to summarize the performance of the levees (or stopbanks) along the Waimakariri and Kaiapoi rivers during the 4 September 2010 Mw 7.1 Darfield and 22 February 2011 Mw 6.2 Christchurch, New Zealand, earthquakes. Shortly after their arrival in the Canterbury area in the mid-nineteenth century European settlers started constructing drainage systems and levees along rivers (Larned et al. 2008). In particular, flooding of the Waimakariri River and its tributaries posed a constant threat to the Christchurch and Kaiapoi areas. The current levee system is a culmination of several coordinated efforts that started in earnest in the 1930s and is composed of both primary and secondary levee systems. The primary levee system is designed for a 450-year flood. Damage estimates for scenarios where the flood protection system is breached have been assessed at approximately NZ$5 billion (van Kalken et al. 2007). As a result, the performance of the levee system during seismic events is of critical importance to the flood hazard in Christchurch and surrounding areas. During the 2010 Darfield and 2011 Christchurch earthquakes, stretches of levees were subjected to motions with peak horizontal ground accelerations (PGAs) of approximately 0.32 g and 0.20 g, respectively. Consequently, in areas where the levees were founded on loose, saturated fluvial sandy deposits, liquefaction-related damage occurred (i.e., lateral spreading, slumping, and settlement). The performance summary presented herein is the result of field observations and analysis of aerial images (New Zealand Aerial Mapping 2010, 2011), with particular focus on the performance of the levees along the eastern reach of the Waimakariri River and along the Kaiapoi River. In the sections that follow, we first present background information about the levee system. This is followed by an overview of the performance of the levees during the Darfield...

Description: To be unbiased and uniform across a wide geographical area, seismic hazard assessments based primarily on earthquake recurrence rates require that the same magnitude scale be used for all earthquakes evaluated. Increasingly, moment magnitude, MW, is seen as the magnitude of preference. Moment magnitude, however, was not routinely calculated in the past for earthquakes in Canada, necessitating the conversion from other magnitude types in common use. This step is complicated by the fact that several magnitude scales are routinely reported for Canadian earthquakes with the choice being influenced primarily by geography and to a lesser extent by the size of the earthquake. This paper focuses on eastern Canada, where mN is the most commonly used magnitude scale. Conversions to MW are established and evaluated. The simple conversion of applying a constant is sufficient. However, the conversion is time dependent with the constant changing from 0.41 to 0.53 in the mid-1990s.

Description: INTRODUCTION Ground-motion prediction equations (GMPEs) are one of the key input parameters for seismic hazard assessment (e.g., Petersen et al. 2008) and for the site-specific ground-motion response spectrum (GMRS) for nuclear power plants (U.S. Nuclear Regulatory Commission 2007). Examples of GMPEs developed for the central and eastern United States include Somerville et al. (2001), Silva et al. (2002), Campbell (2003), and Atkinson and Boore (2006). Led by Pacific Earthquake Engineering Research, a significant effort is under way to systematically develop GMPEs for the central and eastern United States, next-generation attenuation for the eastern North America, or so-called NGA-East. The development of GMPEs in the central and eastern United States is hindered by the lack of ground-motion observations, particularly from large earthquakes (M 7 or larger). It is desirable to compare any GMPE developed for the central and eastern United States with the ground-motion observations from similar earthquakes in other stable continental regions (SCR) of the world. For example, Cramer and Kumar (2003) compared ground motions recorded by structural response recorders (i.e., engineering seismoscopes) within 300 km fault-distance from the 2001 Bhuj, India, earthquake (M 7.6) to several published GMPEs for the central and eastern United States. On 12 May 2008, a large intra-plate earthquake (M 7.9)...

Description: A new technique to determine uncertainty estimates for wave parameters obtained by seismic-wave gradiometry is established using the multiwavelet transform. Wave gradiometry uses spatial gradients as measured by a small-scale seismic array to estimate the wave parameters of propagation azimuth, slowness, geometrical spreading, and radiation pattern. The advantage of wave gradiometry is that the wave parameters are estimated on a point-by-point basis for the entire seismogram. Previous work on wave gradiometery employed the continuous wavelet transform to decompose the wave field into narrow-band realizations prior to gradiometric analysis so that the wave parameters are resolved as functions of both time and frequency. To build on this approach, I incorporated the multiwavelet transform. The multiwavelet transform decomposes the wave field into a series of mutually orthogonal wavelet coefficients, which can then be analyzed by wave gradiometry. Each result can then be treated as a sample from a statistical population. The method is tested on synthetic data, with and without noise. A relatively low degree of uncorrelated noise can have a significant impact on the quality of the results. Finally, multiwavelet gradiometry is applied to a single earthquake recorded by a small subset of USArray stations and shows that the method accurately and robustly determines the wave parameters.

Description: The formation and uplift of Holocene marine terraces along a 160-km-long stretch of the Wairarapa coast in the southern Hikurangi subduction margin, North Island, New Zealand, are interpreted in terms of causative faults and associated large earthquakes during the past circa 7000 cal yr B.P. Distinctive stepped terrace morphology, the clustering of radiocarbon ages on each uplifted terrace, and the historic occurrence of coseismic uplift in other parts of eastern and southern North Island, support the contention that coastal uplift occurred suddenly and repeatedly during large-magnitude earthquakes. The rupture of five west-dipping reverse or reverse-oblique faults within the Australian plate, whose surface traces lie a short distance seaward of the present shoreline, are inferred to be responsible for coastal uplift. Individual structures range in length from 25 to 60 km. These lengths, together with uplift events in the range of 1-4 m, suggest earthquakes in the range of Mw 7-7.5. Approximately 20% of the radiocarbon ages are anomalously young for their elevations, but many have ages indistinguishable from accepted ages for terrace uplift. We infer that many of the anomalous ages are from samples deposited by tsunami that occurred in association with offshore fault rupture on adjacent sections of the coast.

Description: The Colima-Jalisco (CJ) region in northwestern Mexico has generated large-magnitude earthquakes at least since 1800. For example, during the last century, three large, destructive, shallow-thrust subduction earthquakes occurred on 3 and 18 June 1932 with MS of 8.2 and 8, respectively, and on 9 October 1995 (Mw 8, MS 7.4). This historical seismicity and the lack of seismic recordings in the CJ region pose important constraints for the computation of reliable seismic-hazard studies for sites in this region of Mexico. Towards this aim, we have used a hybrid method to generate broadband (BB) synthetics for the Mw 8 CJ 1995 earthquake for the recording sites of the near (MZ) intermediate (CG), and far (COL) fields. The low-frequency (LF, [≤]0.5 Hz) synthetics were simulated by applying a 3D finite-difference method, and the high frequencies (HF, 〉0.5 Hz) were generated by the empirical Green's function technique. Finally, matched filters were applied to the LF and HF synthetics to obtain the BB time series. The LF synthetics were computed from a finite-fault description of the source with four asperities in a 2.5D model constrained by gravity and seismological data. Our preferred model includes an approximation of a thin accretionary prism. For the HF modeling, we also used the four-asperity source model as well as the recordings of the foreshock and aftershock of the Mw 8 1995 mainshock. Based on the comparisons of the BB synthetics with the observed strong ground motions for the 1995 CJ earthquake at the three stations, we believe that our hybrid method is a first step toward the generation of more reliable estimates of the seismic hazards in CJ region. Further improvement in the hazard estimates depends on the urgent deployment of seismological and strong ground motion infrastructure in the CJ region.

Description: The Acambay earthquake of 1912 (M[~]7.0), which occurred in the central Trans-Mexican Volcanic Belt (CTMVB) about 100 km west-northwest of Mexico City, has been thought to be a critical scenario event in the estimation of seismic hazard of the city. We use seismograms of two small earthquakes located near Actopan (15 December 2003; 18 May 2010) and recorded at station CUIG, a hill-zone site in Mexico City, as empirical Green's functions (EGFs) for the Acambay region. Because Actopan, like Acambay, is situated in the CTMVB and both are located at about the same distance from CUIG, the use of the recordings of the Actopan earthquakes as EGFs is partly justified. We first analyze data of the two small earthquakes at a local station, DHIG, to estimate their seismic moment and stress drop. As there is considerable uncertainty in the estimated stress drops of the two events, we choose a reasonable range of values for them and apply a technique of random summation of EGFs to simulate ground motion at CUIG from a postulated Mw 7 earthquake. The estimated geometric mean horizontal peak ground acceleration (PGA) and peak ground velocity (PGV) at CUIG range from 2.7 to 9.7 cm/s2 and from 1.1 to 3.0 cm/s, respectively. Ground-motion maps for the entire city are presented using a simulated trace at CUIG and the known transfer functions of many sites within the city. The results are consistent with reported seismic intensities in Mexico City, and PGA and peak ground displacement (PGD) at a 2-s period at the seismic station of Tacubaya during the Acambay earthquake. Estimated ground motions suggest that a repeat of the event does not present significant hazard to Mexico City.

Description: The coda of earthquake motions and microtremors are sometimes referred to as diffuse-wave fields. They are generated by the multiple scattering due to the complexity of the Earth. It is accepted that the average cross correlation between the diffuse-field motions at pairs of receivers, in the frequency domain, is proportional to the imaginary part of the Green's function between these locations. The average autocorrelation of a single receiver is also proportional to the imaginary part of the Green's function when both the source and receiver are the same. In this study we explored the application of diffuse-field concepts to analyze earthquake records at a site when its site effect can be described using a 1D model. We derived a corollary of Claerbout's result for a 1D layered medium. We found that the imaginary part of the Green's function at the free surface is proportional to the square of the absolute value of the corresponding transfer function for a plane, vertically incident wave. We considered a set of incoming plane waves (of P, SV, and SH types) with varying azimuths and incidence angles. After summing up a few hundred synthetics with inclined incidences we obtained horizontal-to-vertical (H/V) spectral ratios that match the ratios estimated from the simple theory of diffuse field. By using observed records in Japan, we found that the earthquake H/V ratios are quite stable and converge rapidly regardless of what part of the waveform is used, except the P-wave part. We also found that their spectral characteristics can be reproduced well by the velocity structures estimated in previous studies. However, theory and observation were not in perfect agreement, which in turn means that the inversion of a 1D structure could be accomplished by adopting the proposed theory for earthquake H/V spectral ratios.

Description: Refraction microtremor (ReMi) and multichannel analysis of surface waves (MASW) are effective approaches to estimate shallow shear-wave velocity (VS) structure often needed to estimate ground motions using recent ground motion prediction relations. Interferometric MASW (IMASW) uses slowness-frequency slant-stack analyses combined with interferometric time-domain dispersion analyses to improve resolution of lower-frequency Rayleigh-wave dispersion to better constrain VS. Cross-correlation interferometry is used to obtain deterministic correlation Green's function (CGF) IMASW seismograms from ambient-noise and/or active-source wave fields contained in ReMi and/or MASW data. The CGFs are processed using the multiple-filter technique to estimate phase and group dispersion. In the IMASW approach, active seismic sources ensure that the stationary-phase contributions to cross correlations dominate CGF responses. In a single IMASW profile, each geophone represents a virtual source, and the IMASW approach stacks CGF common-offset data from all virtual sources to obtain a single averaged forward- and reverse-record section. CGF time-domain and slowness-frequency phase-slowness estimates are combined with CGF time-domain group slowness estimates for a consistency check on dispersion picks. A multistate Monte Carlo approach is used to estimate mean slowness depth and slowness uncertainties. IMASW is evaluated with passive ReMi data from two sites and active-source IMASW at six sites with independent downhole velocity-depth logs. Comparison of six P-S suspension log-IMASW profile pairs across the Van Norman Complex in northern San Fernando Valley shows that, on average, 30-m-depth shear-wave velocity estimates between the two methods differed by 200-m-deep borehole site.

Description: Recent moderate-sized earthquakes (ML[≤]5.2) that occurred in northern Italy demonstrate that the currently available ground-motion prediction equations (GMPEs) largely underestimate shaking in the Po Plain region at hypocentral distances greater than 70 km. In order to investigate this phenomenon, we collected a set of peak ground acceleration (PGA) observations for weak and moderate earthquakes in the area. Nonparametric regression analysis of PGA observations as a function of magnitude and hypocentral distance shows that PGA is systematically enhanced for distances between 70 and 200 km. An indirect estimation through the analysis of the attenuation of macroseismic intensities suggests that the effect also applies to strong earthquakes (ML〉5.5). We performed numerical modeling experiments to investigate the cause of the phenomenon. The characteristics of the computed synthetic seismograms indicate that the enhancement of ground motion is mainly an effect of the reflection of S waves at the Moho (SmS phase). The analysis of both real and synthetic data shows that the Moho reflection effect is maximized at hypocentral distances between 90 and 150 km, where the PGA increases by a factor larger than 2.4.

Description: We applied the coda-derived source spectrum method of Mayeda et al. (2003) to earthquakes in and around the Korean peninsula. After empirical calibrations, we derived source spectra of the earthquakes. From the coda-derived spectra, we estimated valuable source parameters such as the seismic moment, corner frequency, and radiated energy for small events with Mw

Description: We have determined the locations of more than 20,000 aftershocks (as small as moment magnitude Mw -4.4 or even smaller) following an M 2 event in a South African gold mine, using manually picked arrival times. Spatial clustering into five groups was clearly discerned. A majority of the aftershocks formed a planar cluster ([~]4 m in apparent thickness, [~]100x80 m in areal extent). This cluster is thought to delineate the rupture area of the mainshock because its orientation and spatial extent were consistent with the nodal plane of the centroid moment tensor (CMT) solution and with the corner frequency of the mainshock, respectively. The cluster's attitude suggests that the mainshock was a Mohr-Coulomb failure (or formation of a shear rupture surface in intact rock at an angle that obeys the Coulomb failure criterion) that took place in a vertical compression stress field that is indicated by borehole breakout patterns. The aftershock distribution also shows that the mainshock rupture was largely confined to the interior of a 25-m-thick vertical dike, although there are indications of interactions taking place between the rupture and the dike's material boundary with the host rock.

Description: We propose that a controlling parameter of static stress drop during an earthquake is related to the scaling properties of the fault-surface topography. Using high resolution laser distance meters, we have accurately measured the roughness scaling properties of two fault surfaces in different geological settings (the French Alps and Nevada). The data show that fault-surface topography is scale dependent and may be accurately described by a self-affine geometry with a slight anisotropy characterized by two extreme roughness exponents (HR), H||=0.6 in the direction of slip and H{perp}=0.8 perpendicular to slip. Disregarding plastic processes like rock fragmentation and focusing on elastic deformation of the topography, which is the dominant mode at large scales, the stress drop is proportional to the deformation, which is a spatial derivative of the slip. The evolution of stress-drop fluctuations on the fault plane can be derived directly from the self-affine property of the fault surface, with the length scale ({lambda}) as std{Delta}{sigma}({lambda}){propto}{lambda}HR-1. Assuming no characteristic length scale in fault roughness and a rupture cascade model, we show that as the rupture grows, the average stress drop, and its variability should decrease with increasing source dimension. That is for the average stress drop{Delta}{sigma} (r){propto}rHR-1, where r is the radius of a circular rupture. This result is a direct consequence of the elastic squeeze of fault asperities that induces the largest spatial fluctuations of the shear strength before and after the earthquake at local (small) scales with peculiar spatial correlations.

Description: Earthquakes of M[~]5 have repeatedly occurred at the same location on the plate boundary off Kamaishi, northeast Japan, with a mean recurrence interval of about 5.5 years. The latest two events (M 4.8 on 13 November 2001, and M 4.7 on 11 January 2008) were successfully observed by the broadband seismic network of Tohoku University covering the Tohoku District of northeast Japan. We estimated the source processes of the 2001 and 2008 events by carefully picking the onsets of P and S waves and by inverting seismic waveforms recorded by the network. The results show that both events were caused by the rupture of the same asperity patch (diameter, [~]1 km). As the previous 1995 event was also reported to have ruptured the source area of the 2001 event, at least the last three events (1995, 2001, and 2008) in this earthquake sequence are thought to have been caused by repeated ruptures of the same asperity. A closer examination, however, reveals a small discrepancy in the slip distribution between the last two events, which explains the difference in the high-frequency components of the seismograms. The regions in which slip was smaller during the 2001 event than during the 2008 event nearly coincide with the source areas of the smaller repeating earthquakes that occurred just before the 2001 event. This finding suggests that the activity of smaller events immediately before the mainshock can influence the slip distribution of the mainshock.

Description: The deep earthquake (h=650 km) that occurred on 11 April 2010 south of Granada, Spain, has been studied using the inversion of body waves at teleseismic and regional distances. We have obtained a solution of dip-slip motion on either a vertical plane or a nearly horizontal plane with the pressure axis dipping 45{degrees} to the east. The horizontal plane is chosen as the rupture plane, with rupture propagating from east to west on the basis of directivity effects at teleseismic distances and differences of the waveforms at regional distances. The comparison of these results with the focal mechanisms of four other deep earthquakes that occurred in the same area shows similar rupture processes. The origin of this deep seismic activity remains an open question. Tomographic studies have shown the existence of an anomalous body in this region that extends from 200- to 700-km depth. The olivine-spinel phase transitions or shear melting along horizontal planes inside of this body may be an explanation for the occurrence of these earthquakes.

Description: Four independent ground-motion simulation codes are used to model the strong ground motion for three earthquakes: 1994 Mw 6.7 Northridge, 1989 Mw 6.9 Loma Prieta, and 1999 Mw 7.5 Izmit. These 12 sets of synthetics are used to make estimates of the variability in ground-motion predictions. In addition, ground-motion predictions over a grid of sites are used to estimate parametric uncertainty for changes in rupture velocity. We find that the combined model uncertainty and random variability of the simulations is in the same range as the variability of regional empirical ground-motion data sets. The majority of the standard deviations lie between 0.5 and 0.7 natural-log units for response spectra and 0.5 and 0.8 for Fourier spectra. The estimate of model epistemic uncertainty, based on the different model predictions, lies between 0.2 and 0.4, which is about one-half of the estimates for the standard deviation of the combined model uncertainty and random variability. Parametric uncertainty, based on variation of just the average rupture velocity, is shown to be consistent in amplitude with previous estimates, showing percentage changes in ground motion from 50% to 300% when rupture velocity changes from 2.5 to 2.9 km/s. In addition, there is some evidence that mean biases can be reduced by averaging ground-motion estimates from different methods.

Description: Dynamic rupture calculations using heterogeneous stress drop that is random and self-similar with a power-law spatial spectrum have great promise of producing realistic ground-motion predictions. We present procedures to specify initial stress for random events with a target rupture length and target magnitude. The stress function is modified in the depth dimension to account for the brittle-ductile transition at the base of the seismogenic zone. Self-similar fluctuations in stress drop are tied in this work to the long-wavelength stress variation that determines rupture length. Heterogeneous stress is related to friction levels in order to relate the model to physical concepts. In a variant of the model, there are high-stress asperities with low background stress. This procedure has a number of advantages: (1) rupture stops naturally, not at artificial barriers; (2) the amplitude of short-wavelength fluctuations of stress drop is not arbitrary: the spectrum is fixed to the long-wavelength fluctuation that determines rupture length; and (3) large stress drop can be confined to asperities occupying a small fraction of the total rupture area, producing slip distributions with enhanced peaks.

Description: The SH-wave scattering induced by a lower semielliptic convex topography is studied here. A rigorous series solution is obtained via the region-matching technique. The method of separation of variables in elliptic coordinates is adopted to express the pertinent wave fields in terms of an infinite series containing products of radial and angular Mathieu functions with unknown coefficients. Steady-state responses for some parameters are calculated and discussed. Because the present surficial configuration may collect wave energy in much the same way as a concave mirror does with converging light waves, the potential focusing effects are further probed in the frequency domain. Numerical results demonstrate that the geometry under consideration is indeed capable of causing a localized high concentration of wave energy underground, which may have a great influence on the structures below the convex topography such as mountain tunnels.

Description: We study dynamic rupture propagation on flat faults using 2D plane strain models featuring strongly rate-weakening fault friction (in a rate-and-state framework) and off-fault Drucker-Prager viscoplasticity. Plastic deformation bounds stresses near the rupture front and limits slip velocities to [~]10 m/s, a bound expected to be independent of earthquake magnitude. As originally shown for ruptures in an elastic medium (Zheng and Rice, 1998), a consequence of strongly rate-weakening friction is the existence of a critical background stress level at which self-sustaining rupture propagation, in the form of self-healing slip pulses, is just barely possible. At higher background stress levels, ruptures are cracklike. This phenomenology remains unchanged when allowing for off-fault plasticity, but the critical stress level is increased. The increase depends on the extent and magnitude of plastic deformation, which is influenced by the orientation of the initial stress field and the proximity of the initial stress state to the yield surface.

Description: Branching point process models such as the epidemic-type aftershock sequence (ETAS) models introduced by Ogata (1988, 1998) are often used in the description, characterization, simulation, and declustering of modern earthquake catalogs. The present work investigates how the parameters in these models vary across different tectonic zones. After considering divisions of the surface of the Earth into several zones based on the plate-boundary model of Bird (2003), ETAS models are fit to the occurrence times and locations of shallow earthquakes within each zone. Computationally, the expectation-maximization (EM) type algorithm of Veen and Schoenberg (2008) is employed for the purpose of model fitting. The fits and variations in parameter estimates for distinct zones are compared, and seismological implications are discussed. In particular, we find that estimated background seismicity rates range by a factor of nearly 500 for interplate and trench events, respectively; the estimated productivity parameter, governing the relationship between the magnitude of an earthquake and its expected number of direct aftershocks, ranges by a factor of more than five from events in the slow-moving zone to events in active continental areas, suggesting a much higher rate of swarming in the ridges than in the trenches and active continental zones. Despite the pronounced differences between the seismicity patterns and parameter estimates in the different zones, the ETAS model with few parameters and with the same functional form seems to fit reasonably well to the seismicity in each zone.

Description: The [~]1300-km-long rupture zone of the 2004 Andaman-Sumatra megathrust earthquake continues to generate a mix of thrust, normal, and strike-slip faulting events. The 12 June 2010 Mw 7.5 event on the subducting plate is the most recent large earthquake on the Nicobar segment. The left-lateral faulting mechanism of this event is unusual for the outer-rise region, considering the stress transfer processes that follow great underthrusting earthquakes. Another earthquake (Mw 7.2) with a similar mechanism occurred very close to this event on 24 July 2005. These earthquakes and most of their aftershocks on the subducting plate were generated by left-lateral strike-slip faulting on north-northeast-south-southwest oriented near-vertical faults, in response to north-northwest-south-southeast directed compression. Pre-2004 earthquake faulting mechanisms on the subducting oceanic plate are consistent with this pattern. Post-2004, left-lateral faulting on the subducting oceanic plate clusters between 5{degrees} N and 9{degrees} N, where the 90{degrees} E ridge impinges the trench axis. Our study observes that the subducting plate off the Sumatra and Nicobar segments behaves similarly to a chip of the India-Australia plate, deforming in response to a generally northwest-southeast oriented compression, an aspect that must be factored into the plate deformation models.

Description: The wealth of accelerometric recordings collected by the K-NET and KiK-net networks in Japan since 1996 provides a unique opportunity to improve our understanding of many important seismological research questions. Subsets of these data have been used for many case studies, most of them, however, not focusing specifically on the best practices for data selection and giving relatively little attention to the properties and peculiarities directly observable from the data. Yet for many applications, these steps are an important prerequisite for successful and reliable analysis. For this reason, we devote this article to the extraction of a large data set of surface and borehole recordings from the K-NET and KiK-net databases with strong emphasis on data quality and reliability. The final data set available for subsequent work consists of 78,840 records from 2201 earthquakes covering the Japan Meteorological Agency (JMA) magnitude range 2.7-8, observed at 1681 sites throughout Japan. We explain how this data set has been compiled, including automatic phase picking and relocation of events. We also present an overview of the general features of the data set, providing important information for subsequent analysis. Strong amplification effects at high frequencies are immediately visible on the surface recordings. Furthermore, there is a clear presence of downgoing waves in the borehole records, as deconvolution of borehole/surface recording pairs indicates.

Description: A reanalysis of the varve chronology from hydraulic piston sediment cores was carried out to establish better uncertainty estimates on ages of prehistoric debris-flow deposits (DFDs) in the last 4000 yr. Saanich Inlet is an anoxic fiord located in southeast Vancouver Island near the city of Victoria, British Columbia. It contains annually laminated (varved) marine mud deposited in anoxic conditions. Interlayered with these Holocene varves are massive layers of coarser sediments deposited by submarine debris flows. It has been previously interpreted that these flows were induced by earthquake shaking. Two of the DFDs correspond to known earthquakes: A.D. 1946 Vancouver Island (M 7.3) and the A.D. 1700 Cascadia plate-boundary subduction earthquake (M 9). Based on varve counts, 18 DFDs (310, 410-435, 493-582, 767-887, 874-950, 1001-1133, 1163-1292, 1238-1348, 1546-1741, 1694-1811, 1859-2104, 2197-2509, 2296-2483, 2525-2844, 2987-3298, 3164-3392, 3654-4569, 3989-4284 yr ago from A.D. 2010 datum) were correlated among two or more cores during this time period, suggesting an average return period of strong shaking from earthquakes of about 220 yr. Nine of the DFDs overlap with the age ranges for great plate-boundary earthquakes that have been determined by other paleoseismic studies: coastal subsidence and offshore turbidity deposits. The remaining nine events give an average return period of about 470 yr for strong shaking from local earthquakes. The peak ground acceleration calculated from a recurrence relation based on statistics from local earthquakes for a 470-yr period is 0.30g, which corresponds to the upper range of Modified Mercalli Intensity (MMI) VII (seven). Historical data from Vancouver Island and other areas show that this level of shaking (MMI VII) is sufficient to trigger submarine landslides.

Description: We studied the correlation between the final event magnitude and four parameters obtained from the early portion of P and S phases for a set of high quality subduction events. These relationships are used in the framework of earthquake early-warning systems for real-time magnitude estimation. The investigated parameters are the low-pass-filtered peak displacement (PD), the integral of the velocity squared (IV2), and the predominant and characteristic periods ({tau}p and{tau} c). We created a dataset from the continuous records of the first two weeks following the 14 November 2007 Mw 7.8 Tocopilla (Chile) earthquake. The dataset includes 69 events with magnitudes greater than 4, among them the main event (Mw 7.8), the main aftershocks of Mw 6.7 occurred on November 15, and 4 events with magnitude greater than 6. The low-pass-filtered PD read on short P-phase and S-phase windows is well correlated with the final magnitude, confirming previous results. Indeed when examining 2-s time windows of P waves, we did not observe any saturation effect for magnitudes greater than 6.5; however, there is a slope change in the regression curve. A similar result is obtained from the integral of squared velocity computed over short windows around P and S waves. The characteristic and predominant periods are correlated with magnitudes up to Mw 6; but they clearly do not scale with the magnitude for the stronger events. Our observations offer insight into the feasibility of an early-warning system in Chile.

Description: Lateral variation in crustal attenuation of California is calculated by inverting 25,330 synthetic Wood-Anderson amplitudes from the California Integrated Seismic Network (CISN) for site, source, and path effects. Two-dimensional attenuation (q or 1/Q) is derived from the path term, which is calculated via an iterative least-squares inversion that also solves for perturbations to the site and source terms. Source terms agree well with initial CISN MLs, and site terms agree well with a prior regression analysis; q ranges from low attenuation at 0.001 (Q=1000) to high attenuation at 0.015 (Q=66), with an average of 0.07 (Q=143). The average q is consistent with an amplitude decay function (logA0) for California when q is combined with a simple geometrical spreading rate. Attenuation in California is consistent with the tectonic structure of California, with low attenuation in the Sierra batholith and high attenuation at The Geysers, at Long Valley, and in the Salton trough possibly due to geothermal effects. Also, path terms are an order of magnitude smaller than site and source terms, suggesting that they are not as important in correcting for ML.

Description: The 1300-km rupture of the 2004 interplate earthquake terminated at around 15{degrees} N, in the northernmost segment of the Andaman-Nicobar subduction zone. This part of the plate boundary is noted for its generally lower level seismicity, compared with the southern segments. Based on the Global Centroid Moment Tensor (CMT) and National Earthquake Information Center (NEIC) data, most of the earthquakes of Mw[≥]4.5 prior to 2004 were associated with the Andaman Spreading Ridge (ASR), and a few events were located within the forearc basin. The 2004 event was followed by an upward migration of hypocenters along the subducting plate, and the Andaman segment experienced a surge of aftershock activity. The continuing extensional faulting events, including the most recent earthquake (10 August 2009; Mw 7.5) in the northern end of the 2004 rupture, suggest the reduction of compressional strain associated with the interplate event. The style of faulting of the intraplate events before and after a great plate boundary earthquake reflects the relative influences of the plate-driving forces. Here we discuss the pattern of earthquakes in the Andaman segment before and after the 2004 event to appraise the spatial and temporal relation between large interplate thrust events and intraplate deformation. This study suggests that faulting mechanisms in the outer-ridge and outer-rise regions could be indicative of the maturity of interplate seismic cycles.

Description: The self-noise of a seismic instrument is a fundamental characteristic used to evaluate the quality of the instrument. It is important to be able to measure this self-noise robustly, to understand how differences among test configurations affect the tests, and to understand how different processing techniques and isolation methods (from nonseismic sources) can contribute to differences in results. We compare two popular coherence methods used for calculating incoherent noise, which is widely used as an estimate of instrument self-noise (incoherent noise and self-noise are not strictly identical but in observatory practice are approximately equivalent; Holcomb, 1989; Sleeman et al., 2006). Beyond directly comparing these two coherence methods on similar models of seismometers, we compare how small changes in test conditions can contribute to incoherent-noise estimates. These conditions include timing errors, signal-to-noise ratio changes (ratios between background noise and instrument incoherent noise), relative sensor locations, misalignment errors, processing techniques, and different configurations of sensor types.

Description: The investigated area, located around the Rio-Antirrio Strait, Central Greece, has been the target of a seismic microzonation campaign. Seventy seismic stations have been deployed for a period of 4 months, recording in continuous mode. Despite the high level of urban noise, we compiled a data set of 95 earthquakes recorded at most of the 70 sites. By employing the attributes of self-organizing maps (SOMs), a quality-control and signal-improving method is proposed. A SOM (Kohonen, 1997) is a type of unsupervised neural network. The main property of SOMs utilized is that while the competitive learning algorithm on whom this method is based maps the input data on an n-dimensional grid of neurons, the topological relations (proximity of patterns in input data) are preserved in the output space. SOM is applied to the horizontal-to-vertical spectral ratios (HVSR) of every weak event analyzed for each station separately and allows a better evaluation of the stability of the HVSR.

Description: We present analyses of the noise wave field in the vicinity of Virgo, the Italian-French gravitational wave observatory located close to Pisa, Italy, with special reference to the vibrations induced by a nearby wind farm. The spectral contribution of the wind turbines is investigated using (1) onsite measurements, (2) correlation of spectral amplitudes with wind speed, (3) directional properties determined via multichannel measurements, and (4) attenuation of signal amplitude with distance. Among the different spectral peaks thus discriminated, the one at frequency 1.7 Hz is associated with the greatest power, and under particular conditions it can be observed at distances as large as 11 km from the wind farm. The spatial decay of amplitudes exhibits a complicated pattern, which we interpret in terms of the combination of direct surface waves and body waves refracted at a deep ({approx}800 m) interface between the Plio-Pleistocenic marine, fluvial, and lacustrine sediments and the Miocene carbonate basement. We develop a model for wave attenuation that allows determining the amplitude of the radiation from individual turbines, which is estimated on the order of [IMG]/medium/568eq1.gif" ALT="Formula "〉 for wind speeds over the 8-14 m/s range. On the basis of this model, we then develop a predictive relationship for assessing the possible impact of future wind farm projects.

Description: The spatial autocorrelation (SPAC) method is a useful tool for interpreting ambient noise, and several theoretical tools based on this method are available for data collection, processing, and analysis. The method uses the three components of seismic microtremors recorded simultaneously with arrays of different configurations and yields dispersion curves of Rayleigh and Love waves. Existing theories of the SPAC method are based on an idealized circular array with an infinite number of stations (the continuous model). Errors due to the use of a finite number of stations have been derived theoretically for vertically polarized ambient noise. This paper presents an extension of that error theory to horizontally polarized ambient noise so that it is valid for a circular array with M uniformly spaced stations. The quantities that are obtained as the output of the SPAC method, however, depend on the ratio of the power spectral densities (PSDs) of Rayleigh and Love waves, which are difficult to identify beforehand. Instead we introduce a quantity,{kappa} , that does not depend on the PSD ratio and therefore is more convenient for separating the properties of Rayleigh and Love waves. We derive an expression of{kappa} for a finite, M-station circular array ({kappa}M) and compare it with what it should be in the continuous model ({kappa}T). We illustrate{kappa} M and{kappa} T using field data from a site in Tehran and discuss their discrepancies. Finally, we demonstrate how we have estimated Love-wave phase velocities using the calculated {kappa}M.

Description: We collected the strong-motion accelerograms from the 2010 Jiasian earthquake ([IMG]/medium/701eq1.gif" ALT="Formula "〉) that struck southern Taiwan recorded by the Taiwan Strong-Motion Instrumentation Program (TSMIP) stations to perform an off-line test for our proposed short-distance earthquake early-warning (EEW) approach. The 2010 Jiasian earthquake demonstrates the vital need for the short-distance EEW. The tested short-distance EEW method is a threshold-based approach relying on the amplitude of the filtered vertical displacement. Our result shows that the EEW lead times by the tested method are significantly improved compared to those of the current EEW systems (EEWS) in Taiwan for short-distance warning. For sites located at the epicentral distances of about 25 km the EEW lead times are of 2 to 4 s, ahead of the arrival of the strong ground acceleration. We propose that this threshold-based EEW method might be used for target areas and objects of those in the vicinity of seismogenic zones around the world where warnings are most needed.

Description: This paper is motivated from the need to extract the characteristic time and length scales of strong pulselike ground motions with a mathematically formal, objective, and easily reproducible procedure. The investigation uses wavelet analysis to identify and extract energetic acceleration pulses (not velocity pulses) together with their associated frequency and amplitude. The processing of acceleration records with wavelet analysis is capable of extracting pulses that are not detected visually in the acceleration records, yet they become coherent and distinguishable in the velocity records. Most importantly, the proposed analysis is capable of extracting shorter duration distinguishable pulses (not necessarily of random character) that override the longer near-source pulses that are of significant engineering interest. The study elaborates on the role of the weighting function in the definition of the wavelet transform and concludes that longer pulses are captured when less suppressive weighting functions are implemented in the wavelet transform. We examine the capability of several popular symmetric and antisymmetric wavelets to locally match the energetic acceleration pulse. We conclude that the exercise to identify the best-matching wavelet shall incorporate, in addition to the standard translation and dilation-contraction of the wavelet transform, a phase modulation together with a manipulation of the oscillatory character (addition of cycles) of the wavelet. This need leads to the extension of the wavelet transform to a more general wavelet transform during which the mother wavelet is subjected to the four above-mentioned modulations. The mathematical definition and effectiveness of this extended wavelet transform is presented in this paper. The objective identification of the pulse period, amplitude, phase, and oscillatory character of pulselike ground motions with the extended wavelet transform introduced in this paper makes possible the immediate use of closed-form expressions published by other investigators to estimate the peak response of elastic and inelastic systems.

Description: We describe a prototype detection framework that automatically clusters events in real time from a rapidly unfolding aftershock sequence. We use the fact that many aftershocks are repetitive, producing similar waveforms. By clustering events based on correlation measures of waveform similarity, the number of independent event instances that must be examined in detail by analysts may be reduced. Our system processes array data and acquires waveform templates with a short-term average (STA)/long-term average (LTA) detector operating on a beam directed at the P phases of the aftershock sequence. The templates are used to create correlation-type (subspace) detectors that sweep the subsequent data stream for occurrences of the same waveform pattern. Events are clustered by association with a particular detector. Hundreds of subspace detectors can run in this framework a hundred times faster than in real time. Nonetheless, to check the growth in the number of detectors, the framework pauses periodically and reclusters detections to reduce the number of event groups. These groups define new subspace detectors that replace the older generation of detectors. Because low-magnitude occurrences of a particular signal template may be missed by the STA/LTA detector, we advocate restarting the framework from the beginning of the sequence periodically to reprocess the entire data stream with the existing detectors. We tested the framework on 10 days of data from the Nevada Seismic Array (NVAR) covering the 2003 San Simeon earthquake. One hundred eighty-four automatically generated detectors produced 676 detections resulting in a potential reduction in analyst workload of up to 73%.

Description: The western escarpment of the Sea of Marmara has recently been recognized as the site of intensive gas emissions escaping from the seafloor. Visual observations with the Nautile submersible indicate that gas escapes from elongated tensile cracks oriented to the northwest in the direction of the maximum principal stress. Here, we report results from a 25-day test in 2007 with four ocean-bottom seismometers (OBSs) showing that this area is also characterized by microseismic activity. A cluster of 13 small-magnitude earthquakes aligned northwest occurred in less than 30 hr at shallow crustal depth below the western slope of the Tekirdag basin. The only two focal mechanisms resolvable using land and sea-bottom data reveal normal faulting with strike-slip components, consistent with the stress field expected in this area. It is suggested that tectonic strain below the western slope of the Tekirdag basin contributes to maintaining a high permeability in fault zones and that the fault network provides conduits for deep-seated fluids to rise up to the seafloor.

Description: The southernmost [~]100 km of the San Andreas fault has not ruptured historically. It is imperative to determine its rupture history to better predict its future behavior. This paleoseismic investigation in Coachella, California, establishes a chronology of at least five and up to seven major earthquakes during the past [~]1100 yr. This chronology yields a range of average recurrence intervals between 116 and 221 yr, depending on assumptions, with a best-estimate average recurrence interval of 180 yr. The most recent earthquake occurred c.1690, more than 300 yr ago, suggesting that this stretch of the fault has accumulated a large amount of tectonic stress and is likely to rupture in the near future, assuming the fault follows a stress renewal model. This study also establishes the timing of the past 5-6 highstands of ancient Lake Cahuilla since A.D. 800. We found that earthquakes do not tend to occur at any particular stage in the lake cycle.

Description: Previous approaches used to determine the rates of different earthquakes on a fault have made assumptions regarding segmentation, have been difficult to document and reproduce, and have lacked the ability to satisfy all available data constraints. We present a relatively objective and reproducible inverse methodology for determining the rate of different ruptures on a fault or fault system. The data used in the inversion include slip rate, event rate, and other constraints such as an optional a priori magnitude-frequency distribution. We demonstrate our methodology by solving for the long-term rate of ruptures on the southern San Andreas fault. Our results imply that a Gutenberg-Richter distribution is consistent with the data available for this fault; however, more work is needed to test the robustness of this assertion. More importantly, the methodology is extensible to an entire fault system (thereby including multifault ruptures) and can be used to quantify the relative benefits of collecting additional paleoseismic data at different sites.

Description: Waveform inversion is applied to P and S waveforms selected from microearthquakes of duration magnitude (Md) between 1.8 and 2.4 recorded at station PARM in Stahl Farm, Missouri, operated by the Cooperative New Madrid Seismic Network in the upper Mississippi Embayment to resolve the one-dimensional (1D) velocity structure of uppermost near-surface soils. Constrained by vertical seismic profiling (VSP) and standard cone penetration test (SCPT) data at nearby sites, we begin by defining a gradient velocity structure in the soils. A reference velocity model is constructed by grid search in which the observed P and PS wave shapes, [IMG]/medium/93eq1.gif" ALT="Formula "〉 travel time, and P/PS amplitude ratio are the controlling factors in the systematic selection from among 136,000 models. A waveform inverse scheme is then implemented to obtain the least spectral misfit and best waveform correlation between synthetic and observed data. Four simultaneous inversions of joint waveforms from P and S wave types are performed. Resolved four 1D final models with associated uncertainties are listed in this article. The uppermost 7 m of near-surface soil exhibit very low velocities, through which VP and VS are in the ranges of 140 m/s to 470 m/s and 95 m/s to 215 m/s, respectively. This noninvasive technique demonstrates that the observed high-frequency reverberations (1-17 Hz) of P, S, and PS waveforms from local microearthquakes can be utilized to provide a view of the near-surface soil structure. Although there are trade-offs between layer thickness and slowness, this method explains much of the high-frequency site response due to microearthquake wave propagation through the very low-velocity, near-surface soils.

Description: Calculation of earthquake scarp ages from scarp morphology usually assumes that scarp materials reach their angle of repose immediately after a rupture. However, observations of the 1959 Hebgen Lake, Montana, earthquake scarp and similar features worldwide confirm that scarps require a finite period of mass failure to reach the initial conditions for hillslope diffusion, so the age of features less than 1000 yr old cannot be accurately estimated with methods based only on the linear diffusion equation. We apply a numerical model of this interval of mass failure degradation to vertical initial-angle scarps from the 1959 rupture at Hebgen Lake, Montana. The mass failure rate coefficient, RM, ranges from 1.0x10-2 to 1.2x10-1 m{middle dot}yr-1 for young scarps at Hebgen Lake and nine other locations, and has little or no dependence on climate conditions such as annual temperature range or average rainfall. Including an interval of mass failure gives more accurate age estimates where a scarp age is of the same order as the characteristic mass failure relaxation time of 10-1000 yr.

Description: Standard single-component spectral ratios used to estimate localized site effects may be generalized by using a 3x3 matrix transfer function between the three-component motion observed at a reference (rock) site and that at a nearby soil site. This complex matrix (G matrix) represents the complete three-dimensional site-response within the linear regime. No prior knowledge of the geology at the soil site is required, and G may be computed directly from the spectral responses of earthquakes recorded at both sites. Earthquakes from different hypocentral locations may be used but must be selected so that the body-wave incidence at both stations is nearly vertical and must be time-windowed to avoid surface waves. G is calculated using a stochastic inverse of the observed response spectra and corresponding noise covariance for each response. This method of analysis depends on the availability of a good quality reference (rock) site, as do standard spectral ratio techniques that compare rock and soil site records. We have applied this method to determine site effects to data recorded by strong ground motion seismographs near Wellington, North Island, New Zealand. Magnitudes of the diagonal elements of G are similar to standard single-component spectral ratios, and large values of the off-diagonal matrix elements are seen at some locations. At these locations the maximum response is better estimated from the principal values of the transfer function matrix. The orientation at which the maximum amplification (up to 14 in one case) occurs varies widely according to location.

Description: Pn-wave energy refracts through the uppermost mantle, with the first seismic wave arrival at distances of [~]200 to [~]1500 km from crustal sources. The Pn phase provides important constraints on source type, location, and magnitude, but its propagation is complicated by frequency-dependent sensitivity to the Earth's sphericity and lithospheric velocity structure. Converging on an acceptable Pn geometric spreading correction and specifying its uncertainties, a requirement for accurately determining frequency-dependent attenuation models for Pn, depends on improved understanding of the behavior of Pn geometric spreading for various heterogeneous models. We investigate the effects of radial mantle lid velocity gradients, mantle lid random volumetric velocity heterogeneities, and Moho topography on Pn geometric spreading using reflectivity and two-dimensional (2D) finite-difference 1-Hz wave propagation calculations for elastic Earth models. Mantle lid velocity gradients systematically modify the frequency-dependent geometric spreading from that found for models with constant velocity but retain the same overall functional form. Pn amplitudes are also sensitive to the presence of modest 2D random lateral velocity heterogeneities within the uppermost mantle, with geometric spreading approaching a power-law behavior as the root mean square strength of heterogeneity increases. 2D Moho topography introduces scatter into the amplitude of Pn, but the overall behavior remains compatible with that for a laterally homogeneous model. Given the lack of knowledge of specific small-scale structure for any particular Pn path, the preferred geometric spreading parameterization is the frequency-dependent model for a constant mantle lid velocity structure unless Pn travel-time branch curvature can constrain the radial gradient in the mantle lid.

Description: The definition of a reference bedrock condition representative of a region of interest is of great significance in seismic-hazard assessment. It is highly beneficial when ground-motion prediction equations are referenced to a specific site condition, particularly in the case of site-specific seismic-hazard analyses. When known, the effect of any given site with respect to the reference can then be applied to the predicted ground motion. However, the choice of a reference velocity profile is not straightforward, mainly due to the high variability of the velocity structure in the shallower layers. A new method to define the regional reference rock profile is proposed. The method relates quarter-wavelength average velocity at a site to frequency-dependent amplification. A reference bedrock velocity profile can then be directly defined in relation to expected amplification characteristics over a number of sites. We compare 27 quarter-wavelength velocity profiles from seismic station locations in Switzerland with empirical amplification functions derived from spectral modeling. From this comparison, a set of frequency-dependent calibration relationships is established. Assuming that the reference profile is defined by a lack of any relative amplification, the quarter-wavelength velocity profile that corresponds to unitary spectral amplification can be extracted. The reference velocity profile can then be obtained through an inversion procedure and defines the reference for the ground-motion prediction equation (GMPE). The proposed reference velocity profile is compared with previous reference velocity profiles. A good agreement is found between the different methods. Additionally, an estimation of the transfer function for the Swiss reference rock condition is provided. This can be used to correct recorded or estimated spectral amplitudes for the local response of the reference site. Finally, it is shown that the coefficients from the aforementioned correlations can be used to estimate a generic amplification function at any site with a known quarter-wavelength velocity profile.

Description: Ground-motion prediction equations are commonly developed using regression techniques that treat the input variables or parameters as exact, neglecting the uncertainty associated with the measurement of shear-wave velocity, moment magnitude, and site-to-source distance. This parameter uncertainty propagates through the regression procedure and results in a model uncertainty that overestimates the inherent variability of the ground motion. We present methods of quantifying the uncertainty of the input parameters and incorporating the parameter uncertainty into the regression of ground-motion data using a Bayesian framework. The quantified parameter uncertainty of VS30 is explicitly accounted for in this study, and a reduction in the sigma (standard deviation of ground-motion prediction equation in natural log units) of on average 5% and upwards of 10% in the longer period spectral ordinates is realized.

Description: Seismicity rates and ground-motion prediction equations (GMPEs) are the key uncertainties in probabilistic seismic hazard analysis (PSHA). We explore the impact of new findings and knowledge from seismological and ground-motion studies on seismic hazard assessment for eastern and western Canada. Updated information includes the reevaluation of seismicity rates and their interpretation in terms of seismic source zones and the use of new GMPEs. We refer to our model as an interim updated seismic hazard model, as it does not treat all uncertainties comprehensively; rather, we address the impact of key uncertainties. Based on our updated interim seismic hazard model, uniform hazard spectra (UHS) at four major cities across Canada are obtained and compared with UHS in the current seismic hazard maps of Canada (2005/2010), which are based on a 1995 seismic hazard model developed by the Geological Survey of Canada. Sensitivity analysis highlights the significant impact of seismicity smoothing in low-to-moderate seismic regions (eastern Canada), while GMPEs are important for all regions. Moreover, our interim updated seismic hazard model can readily produce seismic hazard curves as well as seismic hazard deaggregation results for various site conditions and for multiple probability levels; this capability is essential for carrying out advanced earthquake engineering analyses.

Description: The horizontal-to-vertical component (H/V) spectral ratio of the small and moderate earthquake ground motions for the shear-wave window was used as an estimation of the site response in the New Madrid seismic zone (NMSZ). The database used in this study consisted of 500 broadband seismograms from 63 events of magnitude Mw 2.5 to 5.2, recorded on 11 stations operated by the University of Memphis Center for Earthquake Research and Information (CERI) at the University of Memphis. All the broadband stations were located within the Mississippi embayment. Soil deposits overlying the rock basement of the embayment strongly affected the amplitudes of the ground motions. The horizontal-to-vertical component ratios were evaluated for the frequency range of 0.2 to 20 Hz. The observed average H/V ratios suggested site amplification between 2 and 4 in the low-frequency range (f[≤]5 Hz) for stations located on the lower shear-wave velocity deposits (lowlands). The higher shear-wave velocity deposits (uplands) indicated low-frequency amplification between 1.5 and 3 Hz. The observed average H/V ratios were also compared with the soil amplifications in the upper Mississippi embayment developed by Romero and Rix (2005) from the 1D (equivalent linear) method for generic regional profiles. The H/V ratios were also compared with the theoretical quarter-wavelength approximation. These comparisons suggested that the H/V ratios could be a first estimate of the site amplifications. Finally, the variability of the H/V ratios with distance was examined and no discernible trends were found; therefore, the path effect model developed by Zandieh and Pezeshk (2010) for the vertical ground motions in NMSZ using the database of this study was also applicable for the horizontal ground motions.

Description: The staggered-grid finite-difference (SFD) method is widely used in numerical modeling of wave equations. Conventional SFD stencils for spatial derivatives are usually designed in the space domain. However, when they are used to solve wave equations, it becomes difficult to satisfy the dispersion relations exactly. Liu and Sen (2009c) proposed a new SFD scheme for one-dimensional (1D) scalar wave equation based on the time-space domain dispersion relation and plane wave theory, which is made to satisfy the exact dispersion relation. This new SFD scheme has greater accuracy and better stability than a conventional scheme under the same discretizations. In this paper, we develop this new SFD scheme further for numerical solution of 2D and 3D scalar wave equations. We demonstrate that the modeling accuracy is second order when the conventional 2M-th-order space-domain SFD and the second order time-domain finite-difference stencils are directly used to solve the scalar wave equation. However, under the same discretization, our 1D scheme can reach 2M-th-order accuracy and is always stable; 2D and 3D schemes can reach 2M-th-order accuracy along 8 and 48 directions, respectively, and have better stability. The advantages of the new schemes are also demonstrated with dispersion analysis, stability analysis, and numerical modeling.

Description: The Bengkulu Mw 8.4 earthquake on 12 September 2007, close in time and space to the 2004 Mw 9.2 Sumatra-Andaman and 2005 Mw 8.7 Nias megathrust events, suggests that it could be a triggered earthquake. It was located in the southern portion of the historic 1833 Mw 8.9 rupture. However, it appears perplexing that the portion of the Sunda subduction zone between the Nias and Bengkulu rupture patches, which last ruptured in a 1797 Mw 8.7 event, did not recur first. Coulomb failure stress (CFF) modeling of the 2004 and 2005 megathrust earthquakes and subsequent postseismic relaxation processes fails to explain why the 2007 patch ruptured before the northern 1797 segment. Surprisingly, the much smaller 2000 Mw 8.0 Enggano earthquake produced a much larger positive CFF change at the 2007 hypocenter and may help to explain the southern location of the 2007 earthquake. Investigation of changes in seismicity rates in the region following the 2004-2005 events shows that the megathrust earthquakes may have dynamically triggered slip near the northern end of the 2007 rupture zone. A large increase in seismicity levels following the 2000 earthquake may also have influenced the eventual initiation point of the 2007 earthquake. Regrettably, the section of the Sunda megathrust near Siberut that last ruptured in 1797 still poses a great seismic hazard to the region as the only segment not to have ruptured in the sequence of twenty-first century megathrust earthquakes.

Description: Careful analysis of strong-motion recordings of 13 medium magnitude earthquakes (3.7[≤]M[≤]6.5) in the Parkfield, California, area shows that very modest levels of shaking (approximately 3.5% of the acceleration of gravity) can produce observable changes in site response. Specifically, I observe a drop and subsequent recovery of the resonant frequency at sites that are part of the USGS Parkfield dense seismograph array (UPSAR) and Turkey Flat array. While further work is necessary to fully eliminate other models, given that these frequency shifts correlate with the strength of shaking at the Turkey Flat array and only appear for the strongest shaking levels at UPSAR, the most plausible explanation for them is that they are a result of nonlinear site response. Assuming this to be true, the observation of nonlinear site response in small (M