We study the effects of pore fluid pressure (
To better quantify how injection, prior seismicity, and fault properties control rupture growth and propagation of induced earthquakes, we perform a finite‐fault slip inversion on a
- NSF-PAR ID:
- 10444647
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Solid Earth
- Volume:
- 127
- Issue:
- 5
- ISSN:
- 2169-9313
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract P f ) on the pre‐earthquake, near‐fault stress state, and 3‐D earthquake rupture dynamics through six scenarios utilizing a structural model based on the 2004M w 9.1 Sumatra‐Andaman earthquake. As pre‐earthquakeP f magnitude increases, effective normal stress and fault shear strength decrease. As a result, magnitude, slip, peak slip rate, stress drop, and rupture velocity of the scenario earthquakes decrease. Comparison of results with observations of the 2004 earthquake support that pre‐earthquakeP f averages near 97% of lithostatic pressure, leading to pre‐earthquake average shear and effective normal tractions of 4–5 and 22 MPa. The megathrust in these scenarios is weak, in terms of low mean shear traction at static failure and low dynamic friction coefficient during rupture. Apparent co‐seismic principal stress rotations and absolute post‐seismic stresses in these scenarios are consistent with the variety of observed aftershock focal mechanisms. In all scenarios, the mean apparent stress rotations are larger above than below the megathrust. Scenarios with largerP f magnitudes exhibit lower mean apparent principal stress rotations. We further evaluate pre‐earthquakeP f depth distribution. IfP f follows a sublithostatic gradient, pre‐earthquake effective normal stress increases with depth. IfP f follows the lithostatic gradient exactly, then this normal stress is constant, shifting peak slip and peak slip rate updip. This renders constraints on near‐trench strength and constitutive behavior crucial for mitigating hazard. These scenarios provide opportunity for future calibration with site‐specific measurements to constrain dynamically plausible megathrust strength andP f gradients. -
Abstract The 12 November 2017
M w 7.3 Ezgeleh‐Sarpolzahab earthquake is the largest instrumentally recorded earthquake in the Zagros Simply Folded Belt by a factor of ∼10 in seismic moment. Exploiting local, regional, and teleseismic data and synthetic aperture radar interferometry imagery, we characterize the rupture, its aftershock sequence, background seismicity, and regional tectonics. The mainshock ruptured slowly (∼2 km/s), unilaterally southward, for ∼40 km along an oblique (dextral‐thrust) fault that dips ∼14°E beneath the northwestern Lurestan arc. Slip is confined to basement depths of ∼12–18 km, resolvably beneath the sedimentary cover which is ∼8 km thick in this area. The gentle dip angle and basement location allow for a broad slip area, explaining the large magnitude relative to earthquakes in the main Fars arc of the Zagros, where shallower, steeper faults are limited in rupture extent by weak sedimentary layers. Early aftershocks concentrate around the southern and western edges of the mainshock slip area and therefore cluster in the direction of rupture propagation, implying a contribution from dynamic triggering. A cluster of events ∼100 km to the south near Mandali (Iraq) reactivated the ∼50° dipping Zagros Foredeep Fault. The basement fault responsible for the Ezgeleh‐Sarpolzahab earthquake probably accounts for the ∼1 km elevation contrast between the Lurestan arc and the Kirkuk embayment but is distinct from sections of the Mountain Front Fault that define frontal escarpments elsewhere in the Zagros. It may be related to a seismic interface underlying the central and southern Lurestan arc, and a key concern is whether or not the more extensive regional structure is also seismogenic. -
Abstract The Raton Basin has been an area of injection induced seismicity for the past two decades. Previously, the reactivated fault zone structures and spatiotemporal response of seismicity to evolving injection have been poorly constrained due to sparse publicly available seismic monitoring. The application of a machine‐learning phase picker to 4 years of continuous seismic data from a local array enables the detection and location of ∼38,000 earthquakes. The events from 2016 to 2020 are ∼2.5–6 km below sea level and range from ML < −1 to 4.2. Most earthquakes occur within previously identified ∼N‐S zones of seismicity, however our new catalog illuminates that these zones are composed of many short faults with variable orientations. The two most active zones, the Vermejo Park and Tercio zones, are potentially linked by small intermediate faults. In total, we find ∼60 short (<3 km long) basement faults with strikes from WNW to NNE. Faulting mechanisms are predominantly normal but some variability, including reverse dip‐slip and oblique‐slip, is observed. The Trinidad fault zone, which previously hosted a Mw5.3 earthquake in 2011, is quiescent during 2016–2020, likely in response to both slow accumulation of tectonic strain after the 2011 sequence, and the significant decrease (80% reduction) in nearby wastewater injection from 2012 to 2016. Unlike some other regions, where induced seismicity was triggered in response to higher injection rates, the Raton Basin's frequency‐magnitude and spatiotemporal statistics are not distinguishable from tectonic seismicity. The similarity suggests that seismicity in the Raton Basin is predominantly releasing tectonic stress.
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Abstract We develop finite element models of the coseismic displacement field accounting for the 3D elastic structures surrounding the epicentral area of the 2019 Ridgecrest earthquake sequence containing two major events of Mw7.1 and Mw6.4. The coseismic slip distribution is inferred from the surface displacement field recorded by interferometric synthetic aperture radar. The rupture dip geometry is further optimized using a novel nonlinear‐crossover‐linear inversion approach. It is found that accounting for elastic heterogeneity and fault along‐strike curvilinearity improves the fit to the observed displacement field and yields a more accurate estimate of geodetic moment and Coulomb stress changes. We observe spatial correlations among the locations of aftershocks and patches of high slip, and rock anomalous elastic properties, suggesting that the shallow crust's elastic structures possibly controlled the Ridgecrest earthquake sequence. Most of the coseismic slip with a peak slip of 7.4 m at 3.6 km depth occurred above a zone of reduced
S ‐wave velocity and significant post‐Mw7.1 afterslip. This implies that viscous materials or fluid presence might have contributed to the low rupture velocity of the mainshock. Moreover, the zone of high slip on the northwest‐trending fault segment is laterally bounded by two aftershock clusters, whose location is characterized by intermediate rock rigidity. Notably, some minor orthogonal faults consistently end above a subsurface rigid body. Overall, these observations of structural controls improve our understandings of the seismogenesis within incipient fault systems. -
Abstract In September and October 2015, three
M 4+ earthquakes occurred as a sequence along a fault northwest of the Cushing city, Oklahoma, followed by anotherM 5 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 fourM 4‐5 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 fourM 4‐5 earthquakes were very complex, and each of them had several subevents with different rupture directivities. The 2016M 5 earthquake started near the region where threeM 4+ events initiated, but the majority of the slip occurred a few kilometers away in the northeast direction. In comparison, the 2015M 4.3 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 twoM 4+ events. The foreshocks 3 days before the firstM 4+ 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.