We present initial findings from the ongoing Community Stress Drop Validation Study to compare spectral stress-drop estimates for earthquakes in the 2019 Ridgecrest, California, sequence. This study uses a unified dataset to independently estimate earthquake source parameters through various methods. Stress drop, which denotes the change in average shear stress along a fault during earthquake rupture, is a critical parameter in earthquake science, impacting ground motion, rupture simulation, and source physics. Spectral stress drop is commonly derived by fitting the amplitude-spectrum shape, but estimates can vary substantially across studies for individual earthquakes. Sponsored jointly by the U.S. Geological Survey and the Statewide (previously, Southern) California Earthquake Center our community study aims to elucidate sources of variability and uncertainty in earthquake spectral stress-drop estimates through quantitative comparison of submitted results from independent analyses. The dataset includes nearly 13,000 earthquakes ranging from M 1 to 7 during a two-week period of the 2019 Ridgecrest sequence, recorded within a 1 degrees radius. In this article, we report on 56 unique submissions received from 20 different groups, detailing spectral corner frequencies (or source durations), moment magnitudes, and estimated spectral stress drops. Methods employed encompass spectral ratio analysis, spectral decomposition and inversion, finite-fault modeling, ground-motion-based approaches, and combined methods. Initial analysis reveals significant scatter across submitted spectral stress drops spanning over six orders of magnitude. However, we can identify between-method trends and offsets within the data to mitigate this variability. Averaging submissions for a prioritized subset of 56 events shows reduced variability of spectral stress drop, indicating overall consistency in recovered spectral stress-drop values.

ABSTRACT We investigate the influence of the heterogeneous slip-weakening distance (DC) in dynamic rupture simulations, in which DC is proportional to the fault irregularities. Specifically, we compare a heterogeneous fractal DC distribution to a uniform DC over the entire fault when the initial shear stress is also heterogeneous. We find that even small changes in the average value of DC (<1 mm) can lead to significant differences in the rupture evolution; that is, the average DC and the way DC is distributed determines if the rupture is a runaway, self-arrested, or nonpropagating. We find that the self-arrested ruptures differ from runaway ruptures in the amount of area characterized by large slips (asperities). Self-arrested ruptures match the Somerville et al. (1999) asperity criteria in which ∼25% of ruptured area radiate ∼45% of the total seismic moment. This criterion is not satisfied for runaway ruptures. For runaway ruptures, ∼50% of the ruptured area radiates about 70% of the seismic moment, indicating that the ruptured area is not linearly proportional to the seismic moment. Self-arrested ruptures are characterized by dynamic shear stress drops (SDs) in the range ∼2.9–5.5 MPa, whereas for runaway ruptures the dynamic SDs increase to values between ∼12 and 20 MPa. Self-arrested ruptures generated by fractal distributed DC resemble the rupture properties of observed earthquakes. In addition, results show that the conditions for self-arrested ruptures are connected to the decrease of residual energy at rupture boundaries.

&lt;p&gt;Using S-wave records at epicentral distances less than 60 km we determine the apparent stress for 62 Mw&amp;#8805;4.5 earthquakes in southern California since 2000. All earthquakes have reliable network moment tensor solutions. We compute seismic radiated energy with two methods: a time domain method by Kanamori et al. (2020) and a frequency domain method by Boatwright et al. (2002). The Kanamori approach (GR) is a modified Gutenberg-Richter in which attenuation and near surface effects are not considered. The Boatwright method uses path attenuation, near surface kappa0, and a station specific radiation pattern. With Boatwright we compute seismic energy 1) with an average radiation pattern (F0) and 2) with station specific radiation pattern (F1). The geometric means of apparent stress are 0.48, 0.40 and 0.57 MPa for GR, F0 and F1, respectively. Apparent stress is independent of seismic moment for these earthquakes. Converting apparent stress to Brune&amp;#8217;s stress drop (Andrews, 1986), we find stress drops of 2.1, 1.7 and 2.5 MPa for GR, F0 and F1, respectively. From the perspective of seismic radiated energy, a Brune stress drop is nearly the same as that when using Madariaga (1976) and Kaneko and Shearer (2014) models (Ji, Archuleta and Wang, 2022). The standard deviation of stress drop (log10) is 0.35&amp;#8212;almost the same for GR, F0 and F1. Cotton et al. (2013) show the standard deviation from stochastic vibration theory used in ground motion prediction equations is 0.15 for Mw&gt;5.5 earthquakes. Seismic moment/corner frequency methods produce a standard deviation of 0.61, though the magnitude range is larger in some studies. Apparent stress (and consequently stress drop) shows a statistically significant depth dependence (~0.05 MPa/km).&lt;/p&gt;

We examine the source parameters of four Mw≥7.0 intraslab earthquakes that occurred near the Tohoku coast over the past two decades: 2003, 2011, 2021, and 2022. By analyzing the finite fault slip histories constrained by inland strong motion observations, we found that these earthquakes occurred within the upper plane of the subducted Pacific Plate due to downdip compression caused by plate unbending. These earthquakes have a more compact fault area and higher stress drop compared to shallow crustal earthquakes. Additionally, intraslab earthquakes have much slower relative rupture velocity than shallow crustal earthquakes. Good spatial correlations between the static stress drop and slip rate are found, which may suggest the compatibility between dynamic stress drop and static stress drop. The rupture area, average slip, asperity area, average static stress drop over the entire fault, and asperities are consistent with the reported scaling relationship for global intraslab earthquakes within a similar depth range. Using plate unbending, we found the recurrence intervals of these intraslab earthquakes are around 600 years, which is comparable with that of the 2011 Tohoku earthquake. A visual spatial-correlation between the locations of these earthquakes and seismicity in the lower plane is reported. These findings provide insights into the tectonic background and source parameters of intraslab earthquakes in the Tohoku region and contribute to better seismic hazard assessment.

This article summarizes the Next Generation Attenuation (NGA) Subduction (NGA-Sub) project, a major research program to develop a database and ground motion models (GMMs) for subduction regions. A comprehensive database of subduction earthquakes recorded worldwide was developed. The database includes a total of 214,020 individual records from 1,880 subduction events, which is by far the largest database of all the NGA programs. As part of the NGA-Sub program, four GMMs were developed. Three of them are global subduction GMMs with adjustment factors for up to seven worldwide regions: Alaska, Cascadia, Central America and Mexico, Japan, New Zealand, South America, and Taiwan. The fourth GMM is a new Japan-specific model. The GMMs provide median predictions, and the associated aleatory variability, of RotD50 horizontal components of peak ground acceleration, peak ground velocity, and 5%-damped pseudo-spectral acceleration (PSA) at oscillator periods ranging from 0.01 to 10 s. Three GMMs also quantified "within-model" epistemic uncertainty of the median prediction, which is important in regions with sparse ground motion data, such as Cascadia. In addition, a damping scaling model was developed to scale the predicted 5%-damped PSA of horizontal components to other damping ratios ranging from 0.5% to 30%. The NGA-Sub flatfile, which was used for the development of the NGA-Sub GMMs, and the NGA-Sub GMMs coded on various software platforms, have been posted for public use.

We investigate the relation between the kinematic double-corner-frequency source spectral model JA19_2S (Ji and Archuleta, 2020) and static fault geometry scaling relations proposed by Leonard (2010). We find that the nonself-similar low-corner-frequency scaling relation of JA19_2S model can be explained using the fault length scaling relation of Leonard's model combined with an average rupture velocity similar to 70% of shear-wave speed for earthquakes 5.3 < M < 6.9. Earthquakes consistent with both models have magnitude-independent average static stress drop and average dynamic stress drop around 3 MPa. Their scaled energy-e is not a constant. The decrease of -e with magnitude can be fully explained by the magnitude dependence of the fault aspect ratio. The high-frequency source radiation is generally controlled by seismic moment, static stress drop, and dynamic stress drop but is further modulated by the fault aspect ratio and the relative location of the hypocenter. Based on these two models, the commonly quoted average rupture velocity of 70%-80% of shear-wave speed implies predominantly unilateral rupture.

ABSTRACTA review of a collection of theoretical source spectral models revealed: (1) Despite the well-known variation in predicting static stress drop Δσs from the seismic moment and corner frequency, all models, especially the three conventional models, suggest that earthquakes radiate about half of the available strain energy into the surrounding medium. This similarity justifies a less model-dependent approach to estimate Δσs, though estimates for natural earthquakes rely on apparent seismic radiation efficiency (=2σa/Δσs; σa is apparent stress of an earthquake). (2) When one attempts to use Δσs and spectral models to make predictions, such as apparent stress σa, there is a model-dependent discrepancy between the σa inferred from theoretical energy partitioning and the σa predicted using spherical mean corner frequency. Their ratio cp varies significantly from 1.0 for the Brune (1970, 1971) model to 6.38 for the Madariaga (1976) model. If one uses spectral models to predict the ground motion, cp must be considered. (3) We infer that the constancy of the “stress parameter” (Δσ˜) found in engineering seismology (e.g., Boore, 1983; Atkinson and Beresnev, 1998) is similar to having constant apparent stress, σa (e.g., Ide and Beroza, 2001). The observation that Δσ˜ is generally larger than the average static stress drop Δσs for global M >5.5 shallow crustal earthquakes in active tectonic regions implies that these earthquakes radiate, on average, more seismic energy than predicted from the conventional dynamic crack models.

The best-known part of Brune’s (1970) spectral model is the single corner f–2 source spectrum. However, Brune noted that a more realistic heterogeneous rupture would have a source displacement spectrum with a low-frequency f–0 segment proportional to seismic momentM0, a segment with f–2 high-frequency decay, and an intermediate f–1 branch that connects the two. This “f–0–f–1–f–2” shape source spectrum features two corner frequencies fC1 and fC2>fC1. Brune (1970) associated the emergence of the fC2 with the partial stress drop over a fault considering a rupture with heterogeneous stress release. Here we introduce two double-corner source spectral models JA19 and JA19_2S for 3.3≤M≤7.3, constrained by stochastic modeling the mean PGA and mean PGV of the NGA West-2 database. JA19 is self-similar. Its two corner frequencies fC1 and fC2 scale with moment magnitude (M) as (1) log(fC1(M))=1.754– 0.5M and (2) log(fC2(M)) = 3.250 – 0.5M. We find that relation (1) is consistent with the known self-similar scaling relations of the rupture duration (Td) where Td= 1/(πfC1). Relation (2) may reflect scaling relation of the average rise time (TR), whereTR ~0.8/fC2. Stochastic simulations using JA19 cannot reproduce the sharp change in magnitude dependence of PGA and PGV at M5.3, suggesting a breakdown of self-similarity. To model this change, JA19_2S is found by perturbing the fC1 scaling relationship in JA19. For JA19_2S: log(fC1(M)) = 1.474 – 0.415M for M≤5.3 and log(fC1(M)) = 2.375 – 0.585M for M>5.3. In both models the relation fC2/fC1>>1 applies. Seismic radiated energy scales withM02fC12fC2. The ratiofC2/fC1 scales not only with the ratio of effective stress drop and static stress drop as Brune (1970) pointed out but also with the fault aspect ratio. The source spectral shape “f–0–f–1–f–2”, originally proposed by Brune (1970), provides a bridge to reconcile the known scaling relationships in source duration, static stress drop, seismic radiated energy, fault aspect ratio, and ground motion parameters within acceptable uncertainties. It also explains why the stress parameter would generally be larger than static stress drop which is related to the lower corner frequency.

ABSTRACTWe introduce double-corner-frequency (DCF) source spectral models JA19 and JA19_2S, which, in conjunction with a stochastic ground-motion model, can reproduce the mean peak ground acceleration (PGA) and mean peak ground velocity (PGV) of the Next Generation Attenuation-West 2 database for magnitudes 3.3–7.3. Their displacement amplitude spectrum remains constant for frequencies less than fc1, decays as f−1 between fc1 and fc2, and decays as f−2 for frequencies greater than fc2. The model JA19 is self-similar. Its two corner frequencies fc1 and fc2 scale with moment magnitude (M) as (1) log(fc1(M))=1.754−0.5M and (2) log(fc2(M))=3.250−0.5M. We find that relation (1) is consistent with the known self-similar scaling relations of the rupture duration (Td), in which Td=1/(πfc1). Relation (2) may reflect the scaling relation of the average rise time (TR), where TR∼0.8/(fc2). Stochastic simulations of ground motion using JA19 cannot reproduce the sharp change in magnitude dependence of PGA and PGV at M 5.3, suggesting a breakdown of self-similarity. The magnitude dependence of PGA and PGV and this change in slope is well explained by JA19_2S, which results from perturbing the fc1 scaling relationship in JA19. For JA19_2S: log(fc1(M))=1.474−0.415M for M≤5.3; log(fc1(M))=2.375−0.585M for M>5.3. The scaling relation for fc2 is unchanged. When fc1≪fc2, the scaled energy (ratio of radiated energy and seismic moment) scales with M0fc12fc2. The scaled energy of JA19 is 2.2×10−5, independent of magnitude. Because JA19_2S is not self-similar, its scaled energy is 2.2–4.7×10−5, increasing 2.2 times, when magnitude increases from 3.3 to 5.3, and, subsequently decreasing 2.2 times, as magnitude further increases from 5.3 to 7.3. Both agree with the global average (∼3×10−5) reported previously. Using our proposed empirical models, the standard deviation of average static stress drop from seismological studies can be significantly greater than the standard deviation of the stress parameter used to estimate PGA and PGV.

Source spectral models developed for strong ground motion simulations are phenomenological models that represent the average effect that the source processes have on near fault ground motion. Their parameters are directly regressed from the observations and often do not have clear meaning for the physics of the source process. We investigate the relation between the kinematic double-corner frequency (DCF) source spectral model JA19_2S (Ji and Archuleta, BSSA, 2020) and static fault geometry scaling relations proposed by Leonard (2010). We derive scaling relations for the low and high corner frequency in terms of static stress drop, dynamic stress drop, fault rupture velocity, fault aspect ratio, and relative hypocenter location. We find that the non-self-similar low corner frequency scaling relation of JA19_2S model for 5.3<M<6.9 earthquakes is well explained using the fault length scaling relation of Leonard’s model combined with a constant rupture velocity. Earthquakes following both models have constant average static stress drop and constant average dynamic stress drop. The high frequency source radiation is controlled by seismic moment, static stress drop and dynamic stress drop but strongly modulated by the fault aspect ratio and the hypocenter’s relative location. The mean, scaled energy (or apparent stress) decreases with magnitude due to the magnitude dependence of the fault aspect ratio. Based on these two models, the commonly quoted average rupture velocity of 70-80% of shear wave speed implies predominantly unilateral rupture.

The loss of life and economic consequences caused by several recent earthquakes demonstrate the importance of developing seismically safe building codes. The quantification of seismic hazard, which describes the likelihood of earthquake‐induced ground shaking at a site for a specific time period, is a key component of a building code, as it helps ensure that structures are designed to withstand the ground shaking caused by a potential earthquake. Geologic or geomorphic data represent important inputs to the most common seismic hazard model (probabilistic seismic hazard analyses, or PSHAs), as they can characterize the magnitudes, locations, and types of earthquakes that occur over long intervals (thousands of years). However, several recent earthquakes and a growing body of work challenge many of our previous assumptions about the characteristics of active faults and their rupture behavior, and these complexities can be challenging to accurately represent in PSHA. Here, we discuss several of the outstanding challenges surrounding geologic and geomorphic data sets frequently used in PSHA. The topics we discuss include how to utilize paleoseismic records in fault slip rate estimates, understanding and modeling earthquake recurrence and fault complexity, the development and use of fault‐scaling relationships, and characterizing enigmatic faults using topography. Making headway in these areas will likely require advancements in our understanding of the fundamental science behind processes such as fault triggering, complex rupture, earthquake clustering, and fault scaling. Progress in these topics will be important if we wish to accurately capture earthquake behavior in a variety of settings using PSHA in the future.

Obituary| March 04, 2020 Paul A. Spudich (1950–2019) Ralph J. Archuleta; Ralph J. Archuleta * 1Department of Earth Science, University of California, Santa Barbara, California, U.S.A. *Corresponding author: ralph.archuleta@ucsb.edu Search for other works by this author on: GSW Google Scholar Jon B. Fletcher Jon B. Fletcher 2U.S. Geological Survey, Menlo Park, California, U.S.A. Search for other works by this author on: GSW Google Scholar Seismological Research Letters (2020) 91 (3): 1341–1342. https://doi.org/10.1785/0220200024 Article history first online: 30 Apr 2020 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Ralph J. Archuleta, Jon B. Fletcher; Paul A. Spudich (1950–2019). Seismological Research Letters 2020;; 91 (3): 1341–1342. doi: https://doi.org/10.1785/0220200024 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietySeismological Research Letters Search Advanced Search You do not currently have access to this article.

The 2016 MW 7.8 Kaikoura earthquake in New Zealand may have involved co-seismic slip on more than 10 distinct faults. We attempt to assess the sequence of rupture on the segments as well as the contribution of the different segments to the recorded ground motion in New Zealand. First, we approximate the segments as points sources to determine the temporal sequence of faulting and to determine the relative contribution to the seismic moment. We reduce the overall number of segments to 10 crustal faults. We invert strong motion, geodetic and teleseismic body and surface wave data to provide a spatio-temporal map of slip and rupture time.

The UCSB method simulates earthquakes as heterogeneous kinematic ruptures to produce synthetic broadband ground motions (0-25Hz) for 5 M 8. A Kostrov-like slip-rate function is specified at a dense number of points on a finite fault. Each slip-rate function is specified by the total slip, time to reach the maximum slip-rate (peak-time), the total time of slipping (rise-time), and a rupture time, i.e., the time when the point first begins to slip. The rupture time is related to the local rupture velocity. The slip, peak time, rise time and rupture time are all characterized by their own marginal distribution (one-point statistic), and each parameter is correlated with the other. The heterogeneity of the slip distribution on the fault is determined by filtering white noise with a Von Karman wavenumber power spectrum. The Von Karman spectrum is determined from a correlation length and a spectral decay parameter for length scales shorter than the correlation length. The other kinematic parameters are also heterogeneous with different decay parameters--each functionally related to the decay of the slip. With a fault area and seismic moment (magnitude) the only remaining free parameter is average stress drop. The code will iterate on the kinematic parameters until the moment-rate spectrum of the simulated earthquake is similar to a Brune spectrum, with a low frequency level corresponding to the seismic moment and the corner frequency corresponding to the average stress drop.

Measurement of ground motion variability is essential to estimate seismic hazard. Over-estimation of variability can lead to extremely high annual hazard estimates of ground motion exceedance. We explore different parameters that affect the variability of ground motion such as the spatial correlations of kinematic rupture parameters on a finite fault and the corner frequency of the moment-rate spectra. To quantify the variability of ground motion, we simulate kinematic rupture scenarios on several vertical strike-slip faults and compute ground motion using the representation theorem. In particular, for the entire suite of rupture scenarios, we quantify the within-event and the between-events ground motion variability of peak ground acceleration (PGA) and response spectra at several periods, at 40 stations—all approximately at an equal distance of 20 and 50 km from the fault. Both within-event and between-events ground motion variability increase when the slip correlation length on the fault increases. The probability density functions of ground motion tend to truncate at a finite value when the correlation length of slip decreases on the fault, therefore, we do not observe any long-tail distribution of peak ground acceleration when performing several rupture simulations for small correlation lengths. Finally, for a correlation length of 6 km, the within-event and between-events PGA log-normal standard deviations are 0.58 and 0.19, respectively, values slightly smaller than those reported by Boore et al. (Earthq Spectra, 30(3):1057–1085, 2014 ). The between-events standard deviation is consistently smaller than the within-event for all correlations lengths, a feature that agrees with recent ground motion prediction equations.

We have determined a scalable apparent moment rate function (aMRF) that correctly predicts the peak ground acceleration (PGA), peak ground velocity (PGV), local magnitude, and the ratio of PGA/PGV for earthquakes 3.3M5.3. Using the NGA-West2 database for 3.0M7.7, we find a break in scaling of LogPGA and LogPGV versus M around M similar to 5.3 with nearly linear scaling for LogPGA and LogPGV for 3.3M5.3. Temporal parameters t(p) and t(d)related to rise time and total durationcontrol the aMRF. Both scale with seismic moment. The Fourier amplitude spectrum of the aMRF has two corners between which the spectrum decays similar to f(-1). Significant attenuation along the raypath results in a Brune-like spectrum with one corner f(C). Assuming that f(C)1/t(d), the aMRF predicts non-self-similar scaling M0fC3.3 and weak stress drop scaling sigma M00.091. This aMRF can explain why stress drop is different from the stress parameter used to predict high-frequency ground motion.