ALBERT

Description: With the implementation of the USGS National Earthquake Information Center Prompt Assessment of Global Earthquakes for Response system (PAGER), rapid determination of earthquake moment magnitude is essential, especially for earthquakes that are felt within the contiguous United States. We report an implementation of moment tensor processing for application to broad, seismically active areas of North America. This effort focuses on the selection of regional crustal velocity models, codification of data quality tests, and the development of procedures for rapid computation of the seismic moment tensor. We systematically apply these techniques to earthquakes with reported magnitude greater than 3.5 in continental North America that are not associated with a tectonic plate boundary.Using the 0.02–0.10 Hz passband, we can usually determine, with few exceptions, moment tensor solutions for earthquakes with Mw as small as 3.7. The threshold is significantly influenced by the density of stations, the location of the earthquake relative to the seismic stations and, of course, the signal-to-noise ratio. With the existing permanent broadband stations in North America operated for rapid earthquake response, the seismic moment tensor of most earthquakes that are Mw 4 or larger can be routinely computed. As expected the nonuniform spatial pattern of these solutions reflects the seismicity pattern. However, the orientation of the direction of maximum compressive stress and the predominant style of faulting is spatially coherent across large regions of the continent.

Description: The U.S. Geological Survey National Earthquake Information Center (NEIC) uses a variety of classical network-averaged magnitudes (e.g., m b and M s ) and waveform modeling procedures to determine the moment magnitude ( M w ) of an earthquake from teleseismic observations. Initial magnitude estimates are often inaccurate because of poor azimuthal control (sampling of the focal sphere) and/or intrinsic limitation of each method to a specific range of event size. To provide faster and more accurate estimates of the moment magnitude, source duration, and source complexity, NEIC is exploring the use of a variation of the empirical Green’s function (EGF) deconvolution procedure. This approach uses a predicted focal mechanism derived from the Global Centroid Moment Tensor Catalog to compute teleseismic P -wave synthetic seismograms, which are then deconvolved from observed P and SH waveforms to determine station-specific M w , source time function, and a network-averaged M w . Our EGF approach is validated using broadband waveforms from 246 earthquakes in the magnitude range M w  6.0–9.1. Within approximately 13 min of earthquake origin time, our procedure using teleseismic P waves only computes an M w that lies within ±0.25 of the final W -phase M w in the magnitude range 6–8. Using later arriving teleseismic SH phases results in an M w that lies within ±0.12 of the W -phase M w . For magnitude 8 or larger earthquakes, we underestimated the moment magnitude by up to 0.8 magnitude units, primarily due to the initial P phase not containing the total seismic moment release. Long-period phases such as the W -phase and surface waves that better characterize total moment release can also be incorporated in the processing.

Description: We provide a complete description of the characteristics of excitation and attenuation of the ground motion in the Lake Van region (eastern Turkey) using a data set that includes three-component seismograms from the 23 October 2011 M w  7.1 Van earthquake, as well as its aftershocks. Regional attenuation and source scaling are parameterized to describe the observed ground motions as a function of distance, frequency, and magnitude. Peak ground velocities are measured in selected narrow frequency bands from 0.25 to 12.5 Hz; observed peaks are regressed to define a piecewise linear regional attenuation function, a set of excitation terms, and a set of site response terms. Results are modeled through random vibration theory (see Cartwright and Longuet-Higgins, 1956 ). In the log–log space, the regional crustal attenuation is modeled with a bilinear geometrical spreading characterized by a crossover distance at 40 km: fits our results at short distances ( r 〈40 km), whereas is better at larger distances (40〈 r 〈200 km). A frequency-dependent quality factor, Q ( f )=100( f / f ref ) 0.43 (in which f ref =1.0 Hz), is coupled to the geometrical spreading. Because of the inherent trade-off of the excitation/attenuation parameters ( and ), their specific values strongly depend on the choice made for the stress drop of the smaller earthquakes. After choosing a Brune stress drop Brune =4 MPa at M w =3.5, we were able to define (1) an effective high frequency, distance- and magnitude-independent roll-off spectral parameter, eff =0.03 s and (2) a size-dependent stress-drop parameter, which increases with moment magnitude, from Brune =4 MPa at M w  3.5 to Brune =20 MPa at M w  7.1. The set of parameters mentioned here may be used in order to predict the earthquake-induced ground motions expected from future earthquakes in the region surrounding Lake Van.

Description: The M w  5.8 earthquake of 23 August 2011 (17:51:04 UTC) (moment, M 0 5.7 x 10 17 N·m) occurred near Mineral, Virginia, within the central Virginia seismic zone and was felt by more people than any other earthquake in United States history. The U.S. Geological Survey (USGS) received 148,638 felt reports from 31 states and 4 Canadian provinces. The USGS PAGER system estimates as many as 120,000 people were exposed to shaking intensity levels of IV and greater, with approximately 10,000 exposed to shaking as high as intensity VIII. Both regional and teleseismic moment tensor solutions characterize the earthquake as a northeast-striking reverse fault that nucleated at a depth of approximately 7±2 km. The distribution of reported macroseismic intensities is roughly ten times the area of a similarly sized earthquake in the western United States ( Horton and Williams, 2012 ). Near-source and far-field damage reports, which extend as far away as Washington, D.C., (135 km away) and Baltimore, Maryland, (200 km away) are consistent with an earthquake of this size and depth in the eastern United States (EUS). Within the first few days following the earthquake, several government and academic institutions installed 36 portable seismograph stations in the epicentral region, making this among the best-recorded aftershock sequences in the EUS. Based on modeling of these data, we provide a detailed description of the source parameters of the mainshock and analysis of the subsequent aftershock sequence for defining the fault geometry, area of rupture, and observations of the aftershock sequence magnitude–frequency and temporal distribution. The observed slope of the magnitude–frequency curve or b -value for the aftershock sequence is consistent with previous EUS studies ( b =0.75), suggesting that most of the accumulated strain was released by the mainshock. The aftershocks define a rupture that extends between approximately 2–8 km in depth and 8–10 km along the strike of the fault plane. Best-fit modeling of the geometry of the aftershock sequence defines a rupture plane that strikes N36°E and dips to the east-southeast at 49.5°. Moment tensor solutions of the mainshock and larger aftershocks are consistent with the distribution of aftershock locations, both indicating reverse slip along a northeast–southwest striking southeast-dipping fault plane. Online Material: Tables of regional moment tensor source parameters and aftershock location.

Description: A detailed study of the 3D variation of shear-wave velocities in the southern part of the Korean Peninsula is made by combining high-frequency surface- wave tomography results of Cho et al. (2006b) with teleseismic P-wave receiver functions at 80 locations on the peninsula. Receiver functions were derived from high-gain acceleration, short-period, and broadband digital data streams of the Korea Meteorological Administration (KMA) and Korean Institute for Geosciences and Mineral Resources (KIGAM) networks. Vertical cross sections trace the lateral variation in the depth to the Moho, the variation of low velocities near the surface, and the variable thickness of the transition from surface velocities to midcrustal velocities. The derived crustal structure provides new insights on the evolution of the Korean crust.

Description: Cross correlation of seismic-background motions (Campillo and Paul, 2003; Shapiro et al., 2005) is applied to observations from the Korean Meteorological Administration seismic network to estimate the short-period Rayleigh and Love wave dispersion characteristics of the region. Standard processing procedures are applied to the cross correlation, except that signal whitening is used in place of one-bit sampling to equalize power in signals from different times. Multiple-filter analysis is used to extract the group velocities from the estimated Green"s functions, which are then used to image the spatially varying dispersion at periods between 0.5 and 20 sec. The tomographic inversion technique used inverts all periods simultaneously to provide a smooth dispersion curve as a function of period in addition to the usual smooth spatial image for a given period. The Gyeongsang Basin in the southeastern part of the peninsula is clearly resolved with lower group velocities.

Description: Surface waves were generated by the North Korean nuclear explosion of 9 October 2006 and were recorded at epicentral distances up to 34 degrees , from which we estimated a surface wave magnitude (M (sub s) ) of 2.94 with an interstation standard deviation of 0.17 magnitude units. The International Data Center estimated a body-wave magnitude (m (sub b) ) of 4.1. This is the only explosion we have analyzed that was not easily screened as an explosion based on the differences between the M (sub s) and m (sub b) estimates. Additionally, this M (sub s) predicts a yield, based on empirical M (sub s) /yield relationships, that is almost an order of magnitude larger than the 0.5-1 kt reported for this explosion. We investigate how emplacement medium effects on surface wave moment and magnitude may have contributed to the yield discrepancy.

Description: Large data sets of vertical and horizontal seismograms from the Pacific Northwest Seismic Network, Northern California Seismic Network, and Berkeley Digital Seismic Network are used to study the high-frequency (0.25-16 Hz) ground-motion scaling characteristics in Washington-Oregon, northern California (39 degrees N to 42 degrees N and 119 degrees W to 124 degrees W), and central California (35 degrees N to 39 degrees N and 118 degrees W to 123 degrees W). We used peak filtered ground velocities to characterize the propagation, excitation, and site terms. The regression results for propagation were modeled using a geometrical spreading function, g(r), and a frequency-dependent attenuation, Q(f) = Q (sub 0) f (super eta ) . For the Pacific Northwest, the best Q model that fits the observation is expressed by Q(f) = 280f (super 0.55) . The eastern central California and western central California results are parameterized with Q(f) = 280f (super 0.50) and Q(f) = 240f (super 0.35) , respectively. The northern California results are not easy to model and require using a frequency-dependent eta and a frequency-dependent geometrical spreading. The geometrical spreading effect for the frequencies higher than 5 Hz is very strong in that region. The excitation terms for the small events studied were modeled using a Brune's source model. An average stress drop of 30 bars was obtained for the Pacific Northwest. The northern California average value for stress drop is 90 bars. The observations of eastern central California were modeled with a stress drop of 49 bars, while the best fit for the western part required Delta sigma = 80 bars. The range of the values obtained for Q, g(r), and Delta sigma indicates that the ground-motion parameters for one region should not be used for another. As an example, our results show that the ground-motion amplitude due to an M (sub w) 5.0 earthquake at a distance of 50 km is different by a factor of 1.5 between the Pacific Northwest and eastern central California.

Description: Cong et al. (2000) used P-wave dispersion from small earthquakes in the New Madrid Seismic Zone to estimate QP through the use of a continuous-relaxation model. This technique was used as an alternative to the usual spectral-slope techniques and employed an innovative step of comparing group delays relative to those at a reference frequency in order to reduce the bias in the dispersion due to errors in the assumed distance and velocity. The resultant Qm values showed an increasing trend with distance, which they interpreted as due to an increase of crustal Q with depth and the portion of the wave propagation through an active seismic zone...

Description: By using small-to-moderate earthquakes located within approximately 200 km of San Francisco, we characterize the scaling of the ground motions for frequencies ranging between 0.25 and 20 Hz, obtaining results for geometric spreading, Q(f), and site parameters using the methods of Mayeda et al. (2005) and Malagnini et al. (2004). The results of the analysis show that, throughout the Bay Area, the average regional attenuation of the ground motion can be modeled with a bilinear geometric spreading function with a 30-km crossover distance, coupled to an anelastic function exp(-pi fr/[capital greek beta]Q(f), where: Q(f) = 180 f (super 0.42) . A body-wave geometric spreading, g(r) = r (super -1.0) , is used at short hypocentral distances (r〈30 km), whereas g(r) = r (super -0.6) fits the attenuation of the spectral amplitudes at hypocentral distances beyond the crossover. The frequency-dependent site effects at twelve of the Berkeley Digital Seismic Network stations were evaluated in an absolute sense using coda-derived source spectra. Our results show the following. (1) The absolute site response for frequencies ranging between 0.3 Hz and 2.0 Hz correlate with independent estimates of the local magnitude residuals (delta M (sub L) ) for each of the stations. (2) Moment magnitudes (M (sub w) ) derived from our path and site-corrected spectra are in excellent agreement with those independently derived using full-waveform modeling as well as coda-derived source spectra. (3) We use our weak-motion-based relationships to predict motions regionwide for the Loma Prieta earthquake, well above the maximum magnitude spanned by our data set, on a completely different set of stations. Results compare well with measurements taken at specific National Earthquake Hazards Reduction Program site classes. (4) An empirical, magnitude-dependent scaling was necessary for the Brune stress parameter to match the large-magnitude spectral accelerations and peak ground velocities with our weak-motion-based model.