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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 Mw 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 attenu-ation 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 geo-metrical spreading g(r) characterized by a crossover distance at 40 km: g(r) ɑ -10 fits our results at short distances (r < 40 km), whereas g(r) ɑ -0.3 is better at larger distances (40 < r < 200 km). A frequency-dependent quality factor, Q(f) = 100(f/fref)0.43 (in which fref = 1.0 Hz), is coupled to the geometrical spreading. Because of the inherent trade-off of the excitation/attenuation parameters (δ), their specific values strongly depend on the choice made for the stress drop of the smaller earthquakes. After choosing a Brune stress drop δBrune = 4 MPa at Mw = 3.5, we were able to define (1) an effective high frequency, distance- and mag-nitude-independent roll-off spectral parameter, eff = 0.03 s and (2) a size-dependent stress-drop parameter, which increases with moment magnitude, from δBrune = 4 MPa at Mw 3.5 to δBrune = 20 MPa at Mw 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. [ABSTRACT FROM AUTHOR]

On 6 April 2009, an earthquake of Mw 6.13 (Herrmann et al, 2011) occurred in central Italy, close to the town of L'Aquila. Although the earthquake is considered to be a moderate-size event, it caused extensive damage to the surrounding area. The earthquake is identified with significant directivity effects: high-amplitude, short-duration motions are observed at the stations that are oriented along the rupture direction, whereas low-amplitude, long-duration motions are observed at the stations oriented in the direction opposite to the rupture. The complex nature of the earthquake combined with its damage potential brings the need for studies that assess the seismological characteristics of the 2009 L'Aquila mainshock. In this study, we present the strong-ground-motion simulation of this particular earthquake using a stochastic finite-fault model with a dynamic corner frequency approach. For modeling the resulting ground motions, we choose two finite-fault source models that take into account the source complexity of the L'Aquila mainshock. In order to test the sensitivity of ground-motion parameters to the seismic wave attenuation parameters, we use two different attenuation models obtained in the study region using weak-motion and strong-motion databases. Comparisons are made between the attenuation of synthetics and ground-motion prediction equations (GMPEs). Synthetic ground motions are further compared with the observed ones in terms of Fourier amplitude and response spectra at 21 strong-ground-motion stations that recorded the mainshock within an epicentral distance of 100 km. The spatial distribution of shaking intensity obtained from the "Did You Feel It?" project and site survey results are compared with the spatial distributions of simulated peak ground-motion intensity parameters. Our results show that despite the limitations of the method in simulating the directivity effects, the stochastic finite-fault model seems an effective and fast tool to simulate the high-frequency portion of ground motions. [ABSTRACT FROM AUTHOR]

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° 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. [ABSTRACT FROM AUTHOR]

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 M 6.15 L'Aquila earthquake (2009 April 6, 01:32 UTC). The L'Aquila main shock was 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 M≥ 3.0, from 2008 October 1 to 2010 May 10). All waveforms were downloaded from the ISIDe web page (), 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 ≤ M≤ 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 . Finally, results on source excitation and crustal attenuation were 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. [ABSTRACT FROM AUTHOR]

The determination of regional attenuation (Q-1) can depend upon the analysis method employed. The discrepancies between methods are due to differing parameterizations (e.g., geometrical spreading rates), employed datasets (e.g., choice of path lengths and sources), and the nature of the methodologies themselves (e.g., measurement in the frequency or time domain). Here we apply five different attenuation methodologies to a Northern California dataset. The methods are (1) coda normalization (CN), (2) two station (TS), (3) reverse two station (RTS), (4) source pair/ receiver pair (SPRP), and (5) coda-source normalization (CS). The methods are used to measure Q of the regional phase, Lg (QLg), and its power-law dependence on the frequency of the form Q0fη with controlled parameterization in the well-studied region of Northern California using a high-quality dataset from the Berkeley Digital Seismic Network. We investigate the difference in power-law Q calculated among the methods by focusing on the San Francisco Bay area, where knowledge of attenuation is an important part of seismic hazard mitigation. All methods return similar power-law parameters, though the range of the joint 95% confidence regions is large (Q0 = 85 ± 40; η = 0:65 ± 0:35). The RTS and TS methods differ the most from the other methods and from each other. This may be due to the removal of the site term in the RTS method, which is shown to be significant in the San Francisco Bay area. In order to completely understand the range of power-law Q in a region, we advise the use of several methods to calculate the model. We also test the sensitivity of each method to changes in geometrical spreading, the Lg frequency bandwidth, the distance range of data, and the Lg measurement window. For a given method, there are significant differences in the power-law parameters, Q0 and η, due to perturbations in the parameterization when evaluated using a conservative pairwise comparison. The CN method is affected most by changes in the distance range, which is most likely due to its fixed coda-measurement window. Because the CS method is best used to calculate the total path attenuation, it is very sensitive to the geometrical spreading assumption. The TS method is most sensitive to the frequency bandwidth, which may be due to its incomplete extraction of the site term. The RTS method is insensitive to parameterization choice, whereas the SPRP method as implemented here in the time domain for a single path has great error in the power-law model parameters, and η is strongly affected by changes in the method parameterization. When presenting results for a given method we suggest calculating Q0fη for multiple parameterizations using some a priori distribution. [ABSTRACT FROM AUTHOR]

A 1D coda method was proposed by Mayeda et al. (2003) in order to obtain stable seismic source moment-rate spectra using narrowband coda envelope measurements. That study took advantage of the averaging nature of coda waves to derive stable amplitude measurements taking into account all propagation, site, and S-to-coda transfer function effects. Recently, this methodology was applied to microearthquake data sets from three subregions of northern Italy (i.e., western Alps, northern Apennines, and eastern Alps). Because the study regions were small, ranging between local-to-near-regional distances, the simple 1D path assumptions used in the coda method worked very well. The lateral complexity of this region would suggest, however, that a 2D path correction might provide even better results if the data sets were combined, especially when paths traverse larger distances and complicated regions. The structural heterogeneity of northern Italy makes the region ideal to test the extent to which coda variance can be reduced further by using a 2D Q tomography technique. The approach we use has been developed by Phillips et al. (2005) and is an extension of previous amplitude ratio techniques to remove source effects from the inversion. The method requires some assumptions, such as isotropic source radiation, which is generally true for coda waves. Our results are compared against direct S-wave inversions for 1/Q and results from both share very similar attenuation features that coincide with known geologic structures. We compare our results with those derived from direct waves as well as some recent results from northern California obtained by Mayeda et al. (2005) that tested the same tomographic methodology applied in this study to invert for 1/Q. We find that 2D coda path corrections for this region significantly improve upon the 1D corrections, in contrast to California where only a marginal improvement was observed. We attribute this difference to stronger lateral variations in Q for northern Italy relative to California. [ABSTRACT FROM AUTHOR]

By using small-to-moderate earthquakes located within ∼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(ƒ), 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(-πfr/ßQ(ƒ), where: Q(ƒ) = 180 ƒ 0.42. A body-wave geometric spreading, g(r) = r-1.0, is used at short hypocentral distances (r < 30 km), whereas g(r) = r-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 (δML) for each of the stations. (2) Moment magnitudes (Mw) 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. [ABSTRACT FROM AUTHOR]

What can be learned about absolute site effects on ground motions, with no geotechnical information available, in a very poorly instrumented region? In addition, can reliable source spectra be computed at a temporary deployment? These challenges motivated our current study of aftershocks of the 2001 Mw 7.6 Bhuj earthquake, in western India, where we decouple the ambiguity between absolute source radiation and site effects by first computing robust estimates of coda-derived moment-rate spectra of about 200 aftershocks in each of two depth ranges. Crustal attenuation and spreading relationships, based on the same data used here, were determined in an an earlier study. Using our new estimates of source spectra, and our understanding of regional wave propagation, for direct S waves we isolate the absolute site terms for the stations of the temporary deployment. Absolute site terms for each station were determined in an average sense for the three components of the ground motion via an L1-norm minimization. Results for each site were averaged over wide ranges of azimuths and incidence angles. The Bhuj deployment is characterized by a variable shallow geology, mostly of soft sedimentary units. Vertical site terms in the region were observed to be almost featureless (i.e., flat), with amplifications slightly < 1.0 within wide frequency ranges. As a result, the horizontal-to-vertical (H/V) spectral ratios observed at the deployment mimic the behavior of the corresponding absolute horizontal site terms, and they generally overpredict them. This differs significantly from results for sedimentary rock sites (limestone, dolomite) obtained by Malagnini et al. (2004) in northeastern Italy, where the H/V spectral ratios had little in common with the absolute horizontal site terms. Spectral ratios between the vector sum of the computed horizontal site terms for the temporary deployment with respect to the same quantity computed at the hardest rock station available, BAC1, are seriously biased by its nonflat, nonunitary site response. This indicates that, occasionally, the actual behavior of a rock outcrop may be far from that of an ideal, reference site (Steidl et al., 1996). [ABSTRACT FROM AUTHOR]

We describe the characteristics of crustal wave propagation in eastern Sicily by using the background seismicity of the area. We follow the approach described by Malagnini, Hermann, and Di Bona (2000) and Malagnini et al. (2002). Our data set consists of 106 earthquakes recorded by nine three-component digital seismic stations between 1994 and 2001. We used only crustal events (depths shallower than 25 km), with local magnitudes ranging from 1.0 to 4.3, and hypocentral distances from 10 to 130 km. Peak ground velocities from 1311 narrow bandpass-filtered waveforms are measured in the frequency range 1.0-16.0 Hz, and regressed to define crustal propagation, excitation, and site characteristics, with respect to a reference station. A subsequent modeling effort is carried out, through the use of random vibration theory (RVT), for obtaining a quantitative evaluation of the apparent geometrical spreading g(r), and of the crustal quality factor Q(f). An attenuation parameter, κ0, is also evaluated relative to a reference rock site. The attenuation and source parameters estimated in this study are used through the RVT in order to predict the peak horizontal ground acceleration (PGA), and the 5% damping pseudoacceleration spectra (PSA). [ABSTRACT FROM AUTHOR]

We studied the excitation, propagation, and site effects in the Kachchh basin of India by using ground-motion recordings from a temporary seismograph network deployed to study aftershocks of the MW 7.6 Bhuj earthquake of 26 January 2001. The Kachchh basin has been proposed as a useful analog region for studying hazard in other earthquake-prone but slowly deforming regions, such as the central United States. The earthquakes we studied ranged in size from about M 2 to M 5.2, and travel paths ranged from a few kilometers to about a hundred kilometers. There was a broad range of focal depths among the aftershocks, so the data were divided into two overlapping subsets to test the sensitivity of the derived propagation and source parameters to focal depth. Parameters we constrained include the source excitation terms (related to stress drop), a frequency-dependent attenuation operator, a geometric spreading function, and an operator to account for site effects. Our results indicate that seismic-wave attenuation in Kachchh crust is very low, similar to other continental intraplate areas such as central and eastern North America. We also estimated seismic moments and stress drops for the earthquakes by fitting single-corner-frequency source-model spectra to the observed spectra, corrected for propagation by using our derived parameters. Stress drops were found to scale with seismic moment and to be rather high overall. By using a stochastic point-source model to estimate mainshock ground motions, we found that the distance decay of expected peak ground motions, assuming a stress drop of 15-20 MPa, compare well with the scant observations for the Bhuj earthquake. Ground-motion predictions for Kachchh, based on Bhuj aftershock data, support the idea that the region may have similar hazard to proposed analog areas in North America. [ABSTRACT FROM AUTHOR]

We use previously determined direct-wave attenuation functions as well as stable, coda-derived source excitation spectra to isolate the absolute S-wave site effect for the horizontal and vertical components of weak ground motion. We use selected stations in the seismic network of the eastern Alps. A detailed regional attenuation function derived by Malagnini et al. (2002) for the region is used to correct the vertical and horizontal S-wave spectra. These corrections account for the gross path effects (i.e., all distance-dependent effects), although the source and site effects are still present in the distance-corrected spectra. The main goal of this study is to isolate the absolute site effect (as a function of frequency) by removing the source spectrum (moment-rate spectrum) from the distance-corrected S-wave spectra. Typically, removing the S-wave source spectrum is difficult because of inadequate corrections for the source radiation pattern, directivity, and random interference. In addition to complexities near the source, 2D and 3D structure beneath the recording site will result in an azimuth-dependent site effect. Since the direct wave only samples a narrow range in takeoff and backazimuth angles, multistation averaging is needed to minimize the inherent scatter. Because of these complicating effects, we apply the coda methodology outlined by Mayeda et al. (2003) to obtain stable moment-rate spectra. This methodology provides source amplitude and derived source spectra that are a factor of 3-4 times more stable than those derived from direct waves. Since the coda is commonly thought of as scattered energy that samples all ray parameters and backazimuths, it is not very sensitive to the source radiation pattern and 3D structure. This property makes it an excellent choice for use in obtaining average parameters to describe the source, site, and path effects in a region. Due to the characteristics of the techniques used in this study... [ABSTRACT FROM AUTHOR]

In this study we aim to assessing the nature of stochastic uncertainties in ground-motion predictions, by including the variability of region-specific crustal attenuation in time and space in the Central/Northern Apennines (Italy), using the events occurred during 2016-2017 earthquake sequence. Spectral characteristics of excitation, attenuation and duration of ground motion are derived through a regression analysis of the peak ground velocities in the frequency range of 0.25–22 Hz. Regressions are carried out over thousands time windows before and after the Amatrice (M6.0), Visso (M5.9) and Norcia (M6.3) earthquakes, in order to evaluate the fluctuations in seismic wave attenuation induced by the largest mainshocks of the seismic sequence. Propagation terms are modeled using random vibration theory, through a grid search over the attenuation parameters. Here we show that crustal attenuation is strongly affected by transients triggered by the main events, and quantify the impact of the seismic wave attenuation variability on the ground-motion hazard in the Central/Northern Apennines. We also determine the effect of spatial variability of crustal attenuation and its contribution to stochastic uncertainties in ground motion predictions. [ABSTRACT FROM AUTHOR]