The locations of aftershocks are often observed to be on the same fault plane as the mainshock and used as proxies for its rupture area. Recent developments in earthquake relocation techniques have led to great improvements in the accuracy of earthquake locations, offering an unprecedented opportunity to quantify both the aftershock distribution and the mainshock rupture area. In this study, we design a consistent approach to calculate the area enclosed by aftershocks of 12 Mw≥5.4 mainshocks in California, normalized by the mainshock rupture area derived from slip contours. We also investigate the Coulomb stress change from mainshock slip and compare it with the aftershock zone. We find that overall, the ratios of aftershock zone area to mainshock rupture area, hereinafter referred to as “aftershock ratio”, lie within a range of 0.5–5.4, with most values being larger than 1. Using different slip-inversion models for the same mainshock can have a large impact on the results, but the ratios estimated from both the relocated catalogs and Advanced National Seismic System catalog have similar patterns. The aftershock ratios based on relocated catalogs of southern California fall between 0.5 and 4.3, whereas they exhibit a wider range from 1 to 5.4 for northern California. Aftershock ratios for the early aftershock window (within one-day) show a similar range but of smaller values than using the entire aftershock duration, and we propose that continuing afterslip could contribute to the expanding aftershock zone area following several mainshocks. Our results show that areas with positive Coulomb stress change scale with aftershock zone areas, and spatial distribution of aftershocks represents stress release from mainshock rupture and continuing postseismic slip.
The recent 2019 Ridgecrest earthquake sequence in Southern California jostled the seismological community by revealing a complex and cascading foreshock series that culminated in a M7.1 mainshock. But the central Garlock fault, despite being located immediately south of this sequence, did not coseismically fail. Instead, the Garlock fault underwent post-seismic creep and exhibited a sizeable earthquake swarm. The dynamic details of the rupture process during the mainshock are largely unknown, as is the amount of stress needed to bring the Garlock fault to failure. We present an integrated view of how stresses changed on the Garlock fault during and after the mainshock using a combination of tools including kinematic slip inversion, Coulomb stress change, and dynamic rupture modeling. We show that positive Coulomb stress changes cannot easily explain observed aftershock patterns on the Garlock fault, but are consistent with where creep was documented on the central Garlock fault section. Our dynamic model is able to reproduce the main slip asperities and kinematically estimated rupture speeds (≤ 2 km/s) during the mainshock, and suggests the temporal changes in normal and shear stress on the Garlock fault were greatest near the end of rupture. The largest static and dynamic stress changes on the Garlock fault we observe from our models coincide with the creeping region, suggesting that positive stress perturbations could have caused this during or after the mainshock rupture. This analysis of near-field stress change evolution gives insight into how the Ridgecrest sequence influenced the local stress field of the northernmost Eastern California Shear Zone.
Back-projection has proven useful in imaging large earthquake rupture processes. The method is generally robust and does not require many assumptions about the fault geometry or the Earth velocity model. It can be applied in both the time and frequency domain. However, back-projection images are often obtained from records filtered in a narrow frequency range, limiting our ability to uncover the whole rupture process. Here we develop and apply a novel frequency-difference backprojection (FDBP) technique to image large earthquakes, which imitates frequencies below the bandwidth of the signal. The new approach originates from frequency-difference beamforming, which was initially designed to locate acoustic sources. The method stacks the phase-difference of frequency pairs, given by the autoproduct, and is less affected by multipathing and structural inhomogeneities. Additionally, it can potentially allow us to locate sources more accurately even in the presence of strong near-source scattering, albeit with lower resolution. In this study, we first develop the FDBP algorithm and then validate it by performing synthetic tests. We further compare two different stacking techniques of the FDBP method and their effects in the back-projection images. We then apply both the FDBP and conventional time-domain back-projection methods to the 2015 M 7.8 Gorkha earthquake as a case study. The back-projection results from the two methods agree well with each other, and we find that the peak radiation loci have standard error of less than 0.2° through a bootstrapping test. The FDBP method shows promise in resolving complex earthquake rupture processes in tectonically complex regions.
Summary Backprojection has proven useful in imaging large earthquake rupture processes. The method is generally robust and requires relatively simple assumptions about the fault geometry or the Earth velocity model. It can be applied in both the time and frequency domain. Backprojection images are often obtained from records filtered in a narrow frequency band, limiting its ability to uncover the whole rupture process. Here, we develop and apply a novel frequency-difference backprojection (FDBP) technique to image large earthquakes, which imitates frequencies below the bandwidth of the signal. The new approach originates from frequency-difference beamforming, which was initially designed to locate acoustic sources. Our method stacks the phase-difference of frequency pairs, given by the autoproduct, and is less affected by scattering and travel-time errors from three-dimensional Earth structures. It can potentially locate sources more accurately, albeit with lower resolution. In this study, we first develop the FDBP algorithm and then validate it by performing synthetic tests. We further compare two stacking techniques of the FDBP method, Band Width Averaged Autoproduct and its counterpart (BWAP and non-BWAP), and their effects in the backprojection images. We then apply both the FDBP and conventional backprojection methods to the 2015 M7.8 Gorkha earthquake as a case study. The backprojection results from the two methods agree well with each other, and we find that the peak radiation loci of the FDBP non-BWAP snapshots have standard error of less than 0.33○ during the rupture process. The FDBP method shows promise in resolving complex earthquake rupture processes in tectonically complex regions.
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