On 4 and 6 July 2019, two large strike‐slip earthquakes with W‐phase moment magnitudes MWW 6.5 (foreshock) and MWW 7.1 (mainshock) struck the Eastern California Shear Zone, northeast of Ridgecrest. The faulting geometry and kinematic coseismic slip distribution of both events are determined by jointly inverting seismological and geodetic observations guided by aftershock and surface rupture locations. The foreshock ruptured two orthogonal faults with a prominent L‐shaped geometry with maximum slip of ~1.1 m on the NE‐SW segment. The mainshock faulting extended NW‐SE along several primary fault segments that straddle the foreshock slip. The surface rupture and slip model indicate mostly near‐horizontal strike‐slip motion with maximum slip of ~3.7 m, but there is a localized vertical dip‐slip motion. Both the foreshock and mainshock ruptures terminate in regions of complex surface offsets. High aftershock productivity and low rupture velocity may be the result of rupture of a relatively immature fault system.
The number of aftershocks increases with mainshock size following a well‐defined scaling law. However, excursions from the average behavior are common. This variability is particularly concerning for large earthquakes where the number of aftershocks varies by factors of 100 for mainshocks of comparable magnitude. Do observable factors lead to differences in aftershock behavior? We examine aftershock productivity relative to the global average for all mainshocks ( MW>6.5) from 1990 to 2019. A global map of earthquake productivity highlights the influence of tectonic regimes. Earthquake depth, lithosphere age, and plate boundary type correspond well with earthquake productivity. We investigate the role of mainshock attributes by compiling source dimensions, radiated seismic energy, stress drop, and a measure of slip heterogeneity based on finite‐fault source inversions for the largest earthquakes from 1990 to 2017. On an individual basis, stress drop, normalized rupture width, and aspect ratio most strongly correlate with aftershock productivity. A multivariate analysis shows that a particular set of parameters (dip, lithospheric age, and normalized rupture area) combines well to improve predictions of aftershock productivity on a cross‐validated data set. Our overall analysis is consistent with a model in which the volumetric abundance of nearby stressed faults controls the aftershock productivity rather than variations in source stress. Thus, we suggest a complementary approach to aftershock forecasts based on geological and rupture properties rather than local calibration alone.
Recent earthquakes have demonstrated that rupture may propagate through geometrically complex networks of faults. Ancient exhumed faults have the potential to reveal the details of complex rupture at seismogenic depths. We present a new set of field observational criteria for determining which of a population of pseudotachylyte fault veins formed in the same earthquake and apply it to map rupture networks representing single earthquakes. An exceptional exposure of an exhumed ancient strand of the Norumbega Shear Zone preserves evidence of multistranded earthquake rupture in the deep seismogenic zone of a continental transform fault. Individual fault strands slipped at least 2–18 cm, so significant slip is represented by each rupture network. Our data show that synchronously slipped faults intersect at angles of 0 to ∼55°, with the opening angles of fault intersections directed toward the dilational quadrants for dextral slip. Multistranded rupture on a fault network instead of rupture of a single fault may result in greater and/or more variable slip and cause slip rake to vary spatially and temporally. Slip on intersecting faults unequivocally means that there will be motion perpendicular to the average fault plane. Modern earthquakes displaying non‐double‐couple components to focal mechanism solutions and spatially varying rake, slip, and anomalous stress drop may be explained by rupture across fault networks that are too close (spatially and temporally) to be resolved seismically as separate events.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.