ALBERT

Description: We present rupture details of the Mw 6.3 April 6, 2009 L’Aquila earthquake derived by back‐projecting teleseismic P waves. This technique has previously been applied to large magnitude earthquakes, but this is the first application to a moderate size event. We processed vertical‐component seismograms for 60 broadband stations obtained from the Incorporated Research Institutions for Seismology (IRIS) data center. The traces were aligned and normalized using a multi‐channel cross‐correlation algorithm and 4th root stacking was used to image the rupture. We found that the L’Aquila earthquake ruptured towards the south and that a second discrete pulse of energy occurred 20–25 km east of the epicenter about 17–18 s after the nominal origin time. The spatial extent of the rupture image correlates well with a post‐seismic survey of damage in the region. Because the technique is potentially very fast (images can be produced within 20–30 minutes of the origin time), it may be useful to governmental agencies tasked with emergency response and rescue.

Description: Published

Description: L03301

Description: 4.2. TTC - Modelli per la stima della pericolosità sismica a scala nazionale

Description: JCR Journal

Description: reserved

Description: Based only on weak-motion data, we carried out a combined study on region-specific source scaling and crustal attenuation in the Central Apennines (Italy). Our goal was to obtain a reappraisal of the existing predictive relationships for the ground motion, and to test them against the strong-motion data [peak ground acceleration (PGA), peak ground velocity (PGV) and spectral acceleration (SA)] gathered during the Mw 6.15 L’Aquila earthquake (2009 April 6, 01:32 UTC). The L’Aquila main shockwas not part of the predictive study, and the validation test was an extrapolation to one magnitude unit above the largest earthquake of the calibration data set. The regional attenuation was determined through a set of regressions on a data set of 12 777 high-quality, high-gain waveforms with excellent S/N ratios (4259 vertical and 8518 horizontal time histories). Seismograms were selected from the recordings of 170 foreshocks and aftershocks of the sequence (the complete set of all earthquakes with ML ≥ 3.0, from 2008 October 1 to 2010 May 10). All waveforms were downloaded from the ISIDe web page (http://iside.rm.ingv.it/iside/standard/index.jsp), a web site maintained by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). Weak-motion data were used to obtain a moment tensor solution, as well as a coda-based moment-rate source spectrum, for each one of the 170 events of the L’Aquila sequence (2.8 ≤ Mw ≤ 6.15). Source spectra were used to verify the good agreement with the source scaling of the Colfiorito seismic sequence of 1997–1998 recently described by Malagnini et al. (2008). Finally, results on source excitation and crustal attenuationwere used to produce the absolute site terms for the 23 stations located within ∼80 km of the epicentral area. The complete set of spectral corrections (crustal attenuation and absolute site effects) was used to implement a fast and accurate tool for the automatic computation of moment magnitudes in the Central Apennines.

Description: Published

Description: 325-337

Description: 4.1. Metodologie sismologiche per l'ingegneria sismica

Description: JCR Journal

Description: reserved

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: Modern tectonic studies often use regional moment tensors (RMTs) to interpret the seismotectonic framework of an earthquake or earthquake sequence; however, despite extensive use, little existing work addresses RMT parameter uncertainty. Here, we quantify how network geometry and faulting style affect RMT sensitivity. We examine how data-model fits change with fault plane geometry (strike and dip) for varying station configurations. We calculate the relative data fit for incrementally varying geometries about a best-fitting solution, applying our workflow to real and synthetic seismograms for both real and hypothetical station distributions and earthquakes. Initially, we conduct purely observational tests, computing RMTs from synthetic seismograms for hypothetical earthquakes and a series of well-behaved network geometries. We then incorporate real data and station distributions from the International Maule Aftershock Deployment (IMAD), which recorded aftershocks of the 2010 M W 8.8 Maule earthquake, and a set of regional stations capturing the ongoing earthquake sequence in Oklahoma and southern Kansas. We consider RMTs computed under three scenarios: (1) real seismic records selected for high data quality; (2) synthetic seismic records with noise computed for the observed source-station pairings and (3) synthetic seismic records with noise computed for all possible station-source pairings. To assess RMT sensitivity for each test, we observe the ‘fit falloff’, which portrays how relative fit changes when strike or dip varies incrementally; we then derive the ranges of acceptable strikes and dips by identifying the span of solutions with relative fits larger than 90 per cent of the best fit. For the azimuthally incomplete IMAD network, Scenario 3 best constrains fault geometry, with average ranges of 45° and 31° for strike and dip, respectively. In Oklahoma, Scenario 3 best constrains fault dip with an average range of 46°; however, strike is best constrained by Scenario 1, with a range of 26°. We draw two main conclusions from this study. (1) Station distribution impacts our ability to constrain RMTs using waveform time-series; however, in some tectonic settings, faulting style also plays a significant role and (2) increasing station density and data quantity (both the number of stations and the number of individual channels) does not necessarily improve RMT constraint. These results may be useful when organizing future seismic deployments (e.g. by concentrating stations in alignment with anticipated nodal planes), and in computing RMTs, either by guiding a more rigorous data selection process for input data or informing variable weighting among the selected data (e.g. by eliminating the transverse component when strike-slip mechanisms are expected).

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.