Earthquake Seismic Moment, Rupture Radius and Stress-drop from P-wave Displacement Amplitude vs Time Curves

The reliable determination of earthquake source parameters is a relevant task of seismological investigations which ground nowadays on high quality seismic waveforms collected by near-source dense arrays of ground motion sensors. Here we propose a parametric modelling technique which analyzes the time-domain P-wave signal recorded in the near-source range of small-to-large size earthquakes. Assuming a triangular moment-rate function and a uniform speed, circular rupture model, we develop the equations to estimate the seismic moment, rupture radius and stress-drop from the corner-time and plateau level of the average logarithm of the P-wave displacement vs time curves (LPDT). The constant-Q, anelastic attenuation effect is accounted by a post-processing procedure that evaluates the Q-unperturbed moment-rate triangular shape.<br>The methodology has been validated through the application to the acceleration records of the 2016-2017 Central Italy and 2007-2019 Japan earthquake sequences covering a wide moment magnitude range (Mw 2.5 - 6.5) and recording distance < 100 km. After correcting for the anelastic attenuation function, the estimated average stress-drop and the confidence interval (〈∆σ〉=0.60 (0.42-0.87) MPa and 〈∆σ〉=1.53 (1.01-2.31) for crustal and subcrustal events of Japan and 〈∆σ〉=0.36(0.30-0.44) MPa for Central Italy) show, for both regions, a self-similar, constant stress-drop scaling of the rupture duration/radius with seismic moment. The smaller sensitivity of the spatially averaged, time-varying peak displacement amplitude to the radiation from localized high slip patch on the fracture surface, could explain the retrieved smaller average stress-drops for sub-crustal earthquakes in Japan and M>5.5 events in Central Italy relative to previous estimates using spectral methods.<br><br>

Earthquake Seismic Moment, Rupture Radius and Stress-drop from P-wave Displacement Amplitude vs Time Curves

The reliable determination of earthquake source parameters is a relevant task of seismological investigations which ground nowadays on high quality seismic waveforms collected by near-source dense arrays of ground motion sensors. Here we propose a parametric modelling technique which analyzes the time-domain P-wave signal recorded in the near-source range of small-to-large size earthquakes. Assuming a triangular moment-rate function and a uniform speed, circular rupture model, we develop the equations to estimate the seismic moment, rupture radius and stress-drop from the corner-time and plateau level of the average logarithm of the P-wave displacement vs time curves (LPDT). The constant-Q, anelastic attenuation effect is accounted by a post-processing procedure that evaluates the Q-unperturbed moment-rate triangular shape.<br>The methodology has been validated through the application to the acceleration records of the 2016-2017 Central Italy and 2007-2019 Japan earthquake sequences covering a wide moment magnitude range (Mw 2.5 - 6.5) and recording distance < 100 km. After correcting for the anelastic attenuation function, the estimated average stress-drop and the confidence interval (〈∆σ〉=0.60 (0.42-0.87) MPa and 〈∆σ〉=1.53 (1.01-2.31) for crustal and subcrustal events of Japan and 〈∆σ〉=0.36(0.30-0.44) MPa for Central Italy) show, for both regions, a self-similar, constant stress-drop scaling of the rupture duration/radius with seismic moment. The smaller sensitivity of the spatially averaged, time-varying peak displacement amplitude to the radiation from localized high slip patch on the fracture surface, could explain the retrieved smaller average stress-drops for sub-crustal earthquakes in Japan and M>5.5 events in Central Italy relative to previous estimates using spectral methods.<br><br>

Regional attenuation models in Central and Eastern North America using the NGA-East database

The anelastic attenuation term found in ground motion prediction equations (GMPEs) represents the distance dependence of the effect of intrinsic and scattering attenuation on the wavefield as it propagates through the crust and contains the frequency-dependent quality factor, [Formula: see text], which is an inverse measure of the effective anelastic attenuation. In this work, regional estimates of [Formula: see text] in Central and Eastern North America (CENA) are developed using the NGA-East regionalization. The technique employed uses smoothed Fourier amplitude spectrum (FAS) data from well-recorded events in CENA as collected and processed by NGA-East. Regional [Formula: see text] is estimated using an assumption of average geometrical spreading applicable to the distance ranges considered. Corrections for the radiation pattern effect and for site response based on [Formula: see text] result in a small but statistically significant improvement to the residual analysis. Apparent [Formula: see text] estimates from multiple events are combined within each region to develop the regional models. Models are provided for three NGA-East regions: the Gulf Coast, Central North America, and the Appalachian Province. Consideration of the model uncertainties suggests that the latter two regions could be combined. There were not sufficient data to adequately constrain the model in the Atlantic Coastal Plain region. Tectonically stable regions are usually described by higher [Formula: see text] and weaker frequency dependence ([Formula: see text]), while active regions are typically characterized by lower [Formula: see text] and stronger frequency dependence, and the results are consistent with these expectations. Significantly different regional [Formula: see text] is found for events with data recorded in multiple regions, which supports the NGA-East regionalization. An inspection of two well-recorded events with data both in the Mississippi embayment and in southern Texas indicates that the Gulf Coast regionalization by Cramer in 2017 may be an improvement to that of NGA-East for anelastic attenuation. The [Formula: see text] models developed serve as epistemic uncertainty alternatives in CENA based on a literature review and a comparison with previously published models.

A Ground-Motion Database for Israel with Its Corresponding Point-Source Parameters, for Engineering Seismology Applications

The seismic activity in Israel and its surrounding neighbors originates mainly from the active Dead Sea fault system. The historical and archeological records suggest a recurrence interval of approximately 102 and 103 yr for earthquakes of M 6 and 7, respectively. Despite the existing hazard, local advancements on this topic have been slow and incremental, partly due to inherent challenges, such as the limited number of significant recorded earthquakes and a limited azimuthal coverage resulting from geopolitical constraints. Annual earthquake bulletins and an earthquake catalog have been published over the years by the Geophysical Institute of Israel, which operated the network until January 2020. In this article, we summarize a joint effort aimed at standardizing the catalog and creating the first of its kind, publicly available ground-motion database for Israel, which is open to the scientific and engineering communities. The database is composed of three main components: event database, ground-motion recording database, and site database. Once compiled, we use this database to derive and calibrate the source, path, and site parameters required to execute ground-motion simulations, namely, the regional stress drop, anelastic attenuation factor Q, geometrical spreading, and the near-surface high-frequency attenuation, kappa. The parameters are optimized to fit the measurement database, and once they do, a synthetic dataset can be simulated, which will compensate for the lack of measurements in the large-magnitude range.

Embedding Spectral Decomposition Results in Broadband Simulation: Application to Rhine Graben Area

&lt;p&gt;Over the past decade, there is growing consensus that physics-based simulations can be utilized in engineering applications. However, for the simulations to be accepted, they need to be calibrated and validated. This study presents the results of ground motion simulation calibration and validation using earthquakes that occurred in the Upper Rhine Graben with a modified version of the Graves-Pitarka (GP) hybrid ground-motion simulation methodology implemented on the Southern California Earthquake Center Broadband Platform, which uses an improved high-frequency computation.&amp;#160;To calibrate the HF simulation, we take advantage of the growth of seismological data (including weak motions) in the region and the ability to evaluate critical seismic parameters such as anelastic attenuation, stress drop, and site effects through spectral decomposition methods (separate site-source-propagation from the datasets).&amp;#160; Hence in the simulation, the adopted anelastic attenuation and stress parameter are defined based on the spectral decomposition results. The additional modification of the standard GP method is the incorporation of compressional wave in the HF motion.&lt;/p&gt;&lt;p&gt;Results are compared with observations and simulations from the unmodified GP approach; we also use a range of ground motion intensity measures as summary statistics. We found that in general, the modification in the HF part (e.g., incorporation of compressional waves) was necessary to improve the fit with observations. Our findings also validate the fact that parameters from the spectral decomposition are giving well-calibrated time-histories (in terms of frequency and amplitude) when used as input parameters of the broadband simulations. The findings in this study support the incorporation of scenario-based ground motion simulations for use in the characterization of seismic hazard and other engineering applications. For simulation of future earthquakes, instead of using event-specific stress-drops, we use the average stress-drops taken from the distribution of the stress drops derived from spectral decomposition.&lt;/p&gt;

Local and regional P-wave spectral attenuation model for Iran

SUMMARY A robust frequency-dependent local and regional P-wave attenuation model is estimated for continental paths in the Iranian Plateau. In order to calculate the average attenuation parameters, 46 337 vertical-component waveforms related to 9267 earthquakes, which are recorded at the Iranian Seismological Center (IRSC) stations, have been selected in the distance range 10–1000 km. The majority of the event's magnitudes are less than 4.5. This collection of records provides high spatial ray path coverage. Results indicate that the shape of attenuation P-wave curve versus distance is not uniform and has three distinct sections with hinges at 90 and 175 km. A trilinear model for attenuation of P-wave amplitude in the frequency range 1–10 Hz is proposed in this study. Fourier spectral amplitudes are found to decay as R−1.2 (where R is hypocentral distance), corresponding to geometric spreading within 90 km from the source. There is a section from 90 to 175 km, where the attenuation is described as R0.8, and the attenuation is described well beyond 175 km by R−1.3. Moreover, the average quality factor for Pg and Pn waves (QPg and QPn), related to anelastic attenuation is obtained as Qpg= (54.2 ± 2.6)f(1.0096±0.07) and Qpn= (306.8 ± 7.4)f (0.51±0.05). There is a good agreement between the results of the model and observations. Also, the attenuation model shows compatibility with the recent regional studies. From the results it turns out that the amplitude of P waves attenuates more rapidly in comparison with the global models in local distances.

Seismic quality factor estimation using prestack seismic gathers: A simulated annealing approach

Fluid movement and grain boundary friction are the two main factors affecting the anelastic attenuation of seismic data. The quality factor ([Formula: see text]) quantifies the degree of anelastic attenuation and is commonly used in assisting the identification of gas reservoirs. We can accurately compute [Formula: see text] if we obtain the accurate amplitude spectrum of seismic wavelets at refereed and at target time indexes of the seismic profile. However, it is very difficult to obtain the accurate wavelets at the referred and target time indexes. Instead, we usually compare the changes of the amplitude spectrum of refereed and target seismic waveform. The seismic waveform is the convolution result between the seismic wavelet and the reflectivity series. Thus, the reflectivity series would affect our [Formula: see text] computation. We have assumed that the changes of the reflectivity series are negligible within one common-midpoint gather. Then the effect of relativity to the seismic spectrum is the same for the same seismic waveform at different offsets. The seismic waveforms at near offsets are treated as referred seismic signals, and the seismic waveforms at medium and far offsets are regarded as target seismic signals. We use simulated annealing to simultaneously obtain [Formula: see text] values at the entire time index of seismic traces. The method is applied to synthetic and real seismic data to demonstrate its validity and effectiveness. Both applications illustrate the effectiveness of the method in estimating seismic attenuation.

Non-ergodic FAS Ground-Motion Model for France

&lt;p&gt;In this study, we use an ergodic ground motion model (GMM) of California of Bayless and Abrahamson (2019) as a backbone and incorporate the varying-coefficient model (VCM) to develop a new French non-ergodic GMM based on the French RESIF data set (1996-2016). Most of the magnitudes of this database are small (Mw = 2.0 &amp;#8211; 5.2), so we adopt the Fourier amplitude spectral GMM rather than the spectral acceleration model, which allows the use of small magnitude data to constrain path and site effects without the complication of the scaling being affected by differences in the response spectral shape. For the VCM, the coefficients of GMPE can vary by geographical location and they are estimated using Gaussian process regression. That is, there is a separate set of coefficients for each source and site coordinate, including both the mean coefficients and the epistemic uncertainty in the coefficients. Moreover, the epistemic uncertainty associated with the predicted ground motions also varies spatially: it is small in locations where there are many events or stations and it is large in sparse data regions. Finally, we modify the anelastic attenuation term of a GMM by the cell-specific approach of Kuehn et al. (2019) to allow for azimuth-dependent attenuation for each source which reduces the standard deviation of residuals at long distances. The results show that combining the above two methods (VCM &amp; cell-specific) to lead an aleatory standard deviation of residuals for the GMM that is reduced by ~ 47%, which can have huge implications for seismic-hazard calculations.&lt;/p&gt;