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Title: A Comparison of Geodetic and Geologic Rates Prior to Large Strike-Slip Earthquakes: A Diversity of Earthquake-Cycle Behaviors?: DIVERSE EARTHQUAKE-CYCLE BEHAVIORS?
Author(s) / Creator(s):
Publisher / Repository:
DOI PREFIX: 10.1029
Date Published:
Journal Name:
Geochemistry, Geophysics, Geosystems
Page Range / eLocation ID:
4426 to 4436
Medium: X
Sponsoring Org:
National Science Foundation
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  1. Faults are usually surrounded by damage zones associated with localized deformation. Here we use fully dynamic earthquake cycle simulations to quantify the behaviors of earthquakes in fault damage zones. We show that fault damage zones can make a significant contribution to the spatial and temporal seismicity distribution. Fault stress heterogeneities generated by fault zone waves persist over multiple earthquake cycles that, in turn, produce small earthquakes that are absent in homogeneous simulations with the same friction conditions. Shallow fault zones can produce a bimodal depth distribution of earthquakes with clustering of seismicity at both shallower and deeper depths. Fault zone healing during the interseismic period also promotes the penetration of aseismic slip into the locked region and reduces the sizes of fault asperities that host earthquakes. Hence, small and moderate subsurface earthquakes with irregular recurrence intervals are commonly observed in immature fault zone simulations with interseismic healing. To link our simulation results to geological observations, we will use simulated fault slip at different depths to infer the timing and recurrence intervals of earthquakes and discuss how such measurements can affect our understanding of earthquake behaviors. We will also show that the maturity and material properties of fault damage zones have strong influence on whether long-term earthquake characteristics are represented by single events. 
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  3. Abstract

    In September and October 2015, threeM4+ earthquakes occurred as a sequence along a fault northwest of the Cushing city, Oklahoma, followed by anotherM5 earthquake in November 2016. While previous studies have shown that moderate‐size earthquakes in Oklahoma are likely induced by wastewater injections, it is still not clear what controls the rupture process and spatiotemporal evolutions of seismicity during individual sequences. In this study, we investigated the rupture process of these fourM4‐5 events in 2015–2016 with finite fault model (FFM) inversions, and computed the static stress changes during this sequence. We found that the rupture processes of fourM4‐5 earthquakes were very complex, and each of them had several subevents with different rupture directivities. The 2016M5 earthquake started near the region where threeM4+ events initiated, but the majority of the slip occurred a few kilometers away in the northeast direction. In comparison, the 2015M4.3 event mainly ruptured toward the southwest direction. Due to data limitation and inversion uncertainties, we were unable to constrain the rupture directivities for the other twoM4+ events. The foreshocks 3 days before the firstM4+ earthquake in 2015 occurred in a region of positive shear stress changes caused by previous earthquakes in 2014–2015 on unmapped faults several kilometers to the south. Our results suggest small‐scale heterogeneity in controlling complex seismicity and rupture patterns in the 2015–2016 Cushing sequence, and possible triggering of this sequence by a small stress perturbation on order of a few kilopascals.

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  4. Abstract

    Mature strike‐slip faults are usually surrounded by a narrow zone of damaged rocks characterized by low seismic wave velocities. Observations of earthquakes along such faults indicate that seismicity is highly concentrated within this fault damage zone. However, the long‐term influence of the fault damage zone on complete earthquake cycles, that is, years to centuries, is not well understood. We simulate aseismic slip and dynamic earthquake rupture on a vertical strike‐slip fault surrounded by a fault damage zone for a thousand‐year timescale using fault zone material properties and geometries motivated by observations along major strike‐slip faults. The fault damage zone is approximated asan elastic layer with lower shear wave velocity than the surrounding rock. We find that dynamic wave reflections, whose characteristics are strongly dependent on the width and the rigidity contrast of the fault damage zone, have a prominent effect on the stressing history of the fault. The presence of elastic damage can partially explain the variability in the earthquake sizes and hypocenter locations along a single fault, which vary with fault damage zone depth, width and rigidity contrast from the host rock. The depth extent of the fault damage zone has a pronounced effect on the earthquake hypocenter locations, and shallower fault damage zones favor shallower hypocenters with a bimodal distribution of seismicity along depth. Our findings also suggest significant effects on the hypocenter distribution when the fault damage zone penetrates to the nucleation sites of earthquakes, likely being influenced by both lithological (material) and rheological (frictional) boundaries.

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