An official website of the United States government
Abstract We use the deterministic earthquake simulator RSQSim to generate complex sequences of ruptures on fault systems used for hazard assessment. We show that the source motions combined with a wave propagation code create surface ground motions that fall within the range of epistemic uncertainties for the Next Generation Attenuation-West2 set of empirical models. We show the model is well calibrated where there are good data constraints, and has good correspondence in regions with fewer data constraints. We show magnitude, distance, and mechanism dependence all arising naturally from the same underlying friction. The deterministic physics-based approach provides an opportunity for better understanding the physical origins of ground motions. For example, we find that reduced stress drops in shallow layers relative to constant stress drop with depth lead to peak ground velocities in the near field that better match empirical models. The simulators may also provide better extrapolations into regimes that are poorly empirically constrained by data because physics, rather than surface shaking data parameterizations, is underlying the extrapolations. Having shown the model is credible, we apply it to a problem where observations are lacking. We examine the case of crustal faults above a shallow subduction interface seen to break coseismically in simulations of the New Zealand fault system. These types of events were left out of consideration in the most recent New Zealand national seismic hazard model due to the modeling complexity and lack of observational data to constrain ground-motion models (GMMs). Here, we show that in the model, by breaking up the coseismic crustal and interface rupturing fault motions into two separate subevents, and then recombining the resulting ground-motion measures in a square-root-of-sum-of-squares incoherent manner, we reproduce well the ground-motion measures from the full event rupture. This provides a new method for extrapolating GMMs to more complex multifault ruptures.
more »
« less
Engineering characteristics of ground motions recorded in the 2019 Ridgecrest Earthquake Sequence
We present a database and analyze ground motions recorded during three events that occurred as part of the July 2019 Ridgecrest earthquake sequence: a moment magnitude (M) 6.5 foreshock on a left‐lateral cross fault in the Salt Wells Valley fault zone, an M 5.5 foreshock in the Paxton Ranch fault zone, and the M 7.1 mainshock, also occurring in the Paxton Ranch fault zone. We collected and uniformly processed 1483 three‐component recordings from an array of 824 sensors spanning 10 seismographic networks. We developed site metadata using available data and multiple models for the time‐averaged shear‐wave velocity in the upper 30 m (VS30) and for basin depth terms. We processed ground motions using Next Generation Attenuation (NGA) procedures and computed intensity measures including spectral acceleration at a number of oscillator periods and inelastic response spectra. We compared elastic and inelastic response spectra to seismic design spectra in building codes to evaluate the damage potential of the ground motions at spatially distributed sites. Residuals of the observed spectral accelerations relative to the NGA‐West2 ground‐motion models (GMMs) show good average agreement between observations and model predictions (event terms between about −0.3 and 0.5 for peak ground acceleration to 5 s). The average attenuation with distance is also well captured by the empirical NGA‐West2 GMMs, although azimuthal variations in attenuation were observed that are not captured by the GMMs. An analysis considering directivity and fault‐slip heterogeneity for the M 7.1 event demonstrates that the dispersion in the near‐source ground‐motion residuals can be reduced.
more »
« less
- Award ID(s):
- 1826458
- PAR ID:
- 10292292
- Date Published:
- Journal Name:
- Bulletin of the Seismological Society of America
- Volume:
- 110
- Issue:
- 4
- ISSN:
- 0037-1106
- Page Range / eLocation ID:
- 1474-1494
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Seismic faults are surrounded by damaged rocks with reduced rigidity and enhanced attenuation. These damaged fault zone structures can amplify seismic waves and affect earthquake dynamics, yet they are typically omitted in physics‐based regional ground motion simulations. We report on the significant effects of a shallow, flower‐shaped fault zone in foreshock‐mainshock 3D dynamic rupture models of the 2019 Ridgecrest earthquake sequence. We find that the fault zone structure both amplifies and reduces ground motions not only locally but at distances exceeding 100 km. This impact on ground motions is frequency‐ and magnitude‐dependent, particularly affecting higher frequency ground motions from the foreshock because its corner frequency is closer to the fault zone's fundamental eigenfrequency. Within the fault zone, the shallow transition to a velocity‐strengthening frictional regime leads to a depth‐dependent peak slip rate increase of up to 70% and confines fault zone‐induced supershear transitions mostly to the fault zone's velocity‐weakening roots. However, the interplay of fault zone waves, free surface reflections, and rupture directivity can generate localized supershear rupture, even in narrow velocity‐strengthening regions, which are typically thought to inhibit supershear rupture. This study demonstrates that shallow fault zone structures may significantly affect intermediate‐ and far‐field ground motions and cause localized supershear rupture penetrating into velocity‐strengthening regions, with important implications for seismic hazard assessment.more » « less
-
Abstract The ShakeOut scenario of an M 7.8 northwestward rupture on the southern San Andreas fault (SSAF) (Jones et al., 2008) predicted significant long-period ground-motion amplification in the greater Los Angeles, California, area, caused by a waveguide from interconnected sedimentary basins. However, the early ShakeOut ground-motion simulations omitted important model features with immature versions of the velocity structure and fault geometry. Here, we present 0–1 Hz 3D numerical wave propagation simulations for the ShakeOut scenario including surface topography, as well as updated high-resolution velocity structures and SSAF geometry. Spectral accelerations at 3 s are increased by the local high-resolution basin models (25%–45%) but decreased from complexity in velocity and density updates outside the basins (65%–100%) and inclusion of surface topography (∼30%). The updated model reduces the simulated long-period ground motions in the waveguide by 60%–70%, bringing the predictions significantly closer to the values by a leading Next Generation Attenuation-West2 ground-motion model.more » « less
-
The SCEC CyberShake platform implements a repeatable scientific workflow to perform 3D physics-based probabilistic seismic hazard analysis (PSHA). Earlier this year we calculated CyberShake Study 24.8 for the San Francisco Bay Area. Study 24.8 includes both low-frequency and broadband PSHA models, calculated at 315 sites. This study required building a regional velocity model from existing 3D models, with a near-surface low-velocity taper and a minimum Vs of 400 m/s. Pegasus-WMS managed the execution of Study 24.8 for 45 days on the OLCF Frontier and TACC Frontera systems. 127 million seismograms and 34 billion intensity measures were produced and automatically transferred to SCEC storage. Study 24.8 used a HIP language implementation of the AWP-ODC wave propagation code on AMD-GPU Frontier nodes to produce strain Green tensors, which were convolved with event realizations to synthesize seismograms. Seismograms were processed to derive data products such as intensity measures, site-specific hazard curves and regional hazard maps. CyberShake combines 3D low-frequency deterministic (≤1 Hz) simulations with high-frequency calculations using stochastic modules from the Broadband Platform to produce results up to 25 Hz, with validation performed using historical events. New CyberShake data products from this study include vertical seismograms, vertical response spectra, and period-dependent significant durations. The presented results include comparisons of hazard estimates between Study 24.8, the previous CyberShake study for this region (18.8), and the NGA-West2 ground motion models (GMMs). We find that Study 24.8 shows overall lower hazard than 18.8, likely due to changes in rupture coherency, with the exception of a few regions: 24.8 shows higher hazard than both the GMMs and 18.8 at long periods in the Livermore area, due to deepening of the Livermore basin in the velocity model, as well as higher hazard east of San Pablo Bay and south of San Jose. At high frequencies, Study 24.8 hazard is lower than that of the GMMs, reflecting reduced variability in the stochastic components. We are also using CyberShake ground motion data to investigate the effects of preferred rupture directions on site-specific hazard. By default, PSHA hazard products assume all events on a given fault and magnitude are equally likely, but by varying these probabilities we can examine the effects of preferred rupture directions on given faults on CyberShake hazard estimates.more » « less
-
Abstract Dynamic rupture simulations generate synthetic waveforms that account for nonlinear source and path complexity. Here, we analyze millions of spatially dense waveforms from 3D dynamic rupture simulations in a novel way to illuminate the spectral fingerprints of earthquake physics. We define a Brune-type equivalent near-field corner frequency (fc) to analyze the spatial variability of ground-motion spectra and unravel their link to source complexity. We first investigate a simple 3D strike-slip setup, including an asperity and a barrier, and illustrate basic relations between source properties and fc variations. Next, we analyze >13,000,000 synthetic near-field strong-motion waveforms generated in three high-resolution dynamic rupture simulations of real earthquakes, the 2019 Mw 7.1 Ridgecrest mainshock, the Mw 6.4 Searles Valley foreshock, and the 1992 Mw 7.3 Landers earthquake. All scenarios consider 3D fault geometries, topography, off-fault plasticity, viscoelastic attenuation, and 3D velocity structure and resolve frequencies up to 1–2 Hz. Our analysis reveals pronounced and localized patterns of elevated fc, specifically in the vertical components. We validate such fc variability with observed near-fault spectra. Using isochrone analysis, we identify the complex dynamic mechanisms that explain rays of elevated fc and cause unexpectedly impulsive, localized, vertical ground motions. Although the high vertical frequencies are also associated with path effects, rupture directivity, and coalescence of multiple rupture fronts, we show that they are dominantly caused by rake-rotated surface-breaking rupture fronts that decelerate due to fault heterogeneities or geometric complexity. Our findings highlight the potential of spatially dense ground-motion observations to further our understanding of earthquake physics directly from near-field data. Observed near-field fc variability may inform on directivity, surface rupture, and slip segmentation. Physics-based models can identify “what to look for,” for example, in the potentially vast amount of near-field large array or distributed acoustic sensing data.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/https://doi.org/10.1785/0120200036
