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1. Geometric Controls on Pulse‐Like Rupture in a Dynamic Model of the 2015 Gorkha Earthquake

   https://doi.org/10.1029/2018JB016602  

   Wang, Yongfei ; Day, Steven M. ; Denolle, Marine A. ( February 2019 , Journal of Geophysical Research: Solid Earth)

   The 15 April 2015M_w7.8 Nepal Gorkha earthquake occurred on a shallowly dipping portion of the Main Himalayan Thrust (MHT). Notable features of the event include (1) the dominance of a slip pulse of about 6‐s duration that unlocked the lower edge of the MHT and (2) the near‐horizontal fault geometry, which, combined with proximity of the free surface, allows surface‐reflected phases to break the across‐fault symmetries of the seismic wavefield. Our dynamic rupture simulations in an elastoplastic medium yield earthquake parameters comparable to those deduced from kinematic inversions, including seismic moment and rupture velocity. The simulations reproduce pulse‐like behavior predicting pulse widths in agreement with those kinematic studies and supporting an interpretation in which the pulse‐like time dependence of slip is principally controlled by rupture geometry. This inference is strongly supported by comparison of synthetic ground velocity with the near‐field high‐rate GPS recording at station KKN4, which shows close agreement in pulse width, amplitude, and pulse shape. That comparison also constrains the updip extent of rupture and disfavors significant coseismic slip on the shallow ramp segment. Over most of the rupture length, the simulated rupture propagates at a near‐constant maximum velocity (~90% of theSwave speed) that is controlled bymore »the antiplane geometry and off‐fault plastic yielding. Simulations also reveal the role of reflected seismic waves from the free surface, which may have contributed ~30% elongation of the slip pulse, and show the potential for significant free‐surface interaction effects in shallow events of similar geometry.

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2. Evaluating the Updip Extent of Large Megathrust Ruptures Using P _coda Levels

   https://doi.org/10.1029/2019GL082774  

   Lay, Thorne ; Rhode, Andrea ( May 2019 , Geophysical Research Letters)

   When slip at shallow depth occurs during large subduction zone thrust events,Pwave energy enters the water layer and establishespwP, the reverberating waves called “water bounces” that followpP. For water depths ≥5–6 km (i.e., near the trench) above the shallow slip,pwPmanifests in a strong ~10‐s period ringing that can persist for minutes into the teleseismicPwave coda at all azimuths. Deeper slip can generate shorter‐periodpwPringing at trenchward azimuths. At large distances,P_codawindows have several‐minute‐long intervals free of secondary arrivals. We consider rmsP_coda/rmsPamplitude ratios at distances from 80° to 120° as a potential proxy for occurrence of shallow slip for 39M_W7.5+ megathrust earthquakes from 1990 to 2016 with estimated slip distributions. Ratios for the 15‐ to 7‐s‐period band have a strong bimodal distribution, with higher averageP_coda/Pamplitudes observed for ruptures with slip extending to shallow depth.
3. Rupture Model for the 29 July 2021 M _W 8.2 Chignik, Alaska Earthquake Constrained by Seismic, Geodetic, and Tsunami Observations

   https://doi.org/10.1029/2021JB023676  

   Ye, Lingling ; Bai, Yefei ; Si, Daojun ; Lay, Thorne ; Cheung, Kwok Fai ; Kanamori, Hiroo ( July 2022 , Journal of Geophysical Research: Solid Earth)

   A great earthquake struck the Semidi segment of the plate boundary along the Alaska Peninsula on 29 July 2021, re‐rupturing part of the 1938 rupture zone. The 2021M_W8.2 Chignik earthquake occurred just northeast of the 22 July 2020M_W7.8 Simeonof earthquake, with little slip overlap. Analysis of teleseismicPandSHwaves, regional Global Navigation Satellite System (GNSS) displacements, and near‐field and far‐field tsunami observations provides a good resolution of the 2021 rupture process. During ∼60‐s long faulting, the slip was nonuniformly distributed along the megathrust over depths from 32 to 40 km, with up to ∼12.9‐m slip in an ∼170‐km‐long patch. The 40–45 km down‐dip limit of slip is well constrained by GNSS observations along the Alaska Peninsula. Tsunami observations preclude significant slip from extending to depths <25 km, confining all coseismic slip to beneath the shallow continental shelf. Most aftershocks locate seaward of the large‐slip zones, with a concentration of activity up‐dip of the deeper southwestern slip zone. Some localized aftershock patches locate beneath the continental slope. The surface‐wave magnitudeM_Sof 8.1 for the 2021 earthquake is smaller thanM_S = 8.3–8.4 for the 1938 event. Seismic and tsunami data indicate that slip in 1938 was concentrated in the eastern region of its aftershock zone, extending beyond the Semidimore »Islands, where the 2021 event did not rupture.

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4. Seismic and Geodetic Analysis of Rupture Characteristics of the 2020 Mw 6.5 Monte Cristo Range, Nevada, Earthquake

   https://doi.org/10.1785/0120200327  

   Liu, Chengli ; Lay, Thorne ; Pollitz, Fred F. ; Xu, Jiao ; Xiong, Xiong ( July 2021 , Bulletin of the Seismological Society of America)

   ABSTRACT The largest earthquake since 1954 to strike the state of Nevada, United States, ruptured on 15 May 2020 along the Monte Cristo range of west-central Nevada. The Mw 6.5 event involved predominantly left-lateral strike-slip faulting with minor normal components on three aligned east–west-trending faults that vary in strike by 23°. The kinematic rupture process is determined by joint inversion of Global Navigation Satellite Systems displacements, Interferometric Synthetic Aperture Radar (InSAR) data, regional strong motions, and teleseismic P and SH waves, with the three-fault geometry being constrained by InSAR surface deformation observations, surface ruptures, and relocated aftershock distributions. The average rupture velocity is 1.5  km/s, with a peak slip of ∼1.6  m and a ∼20  s rupture duration. The seismic moment is 6.9×1018  N·m. Complex surface deformation is observed near the fault junction, with a deep near-vertical fault and a southeast-dipping fault at shallow depth on the western segment, along which normal-faulting aftershocks are observed. There is a shallow slip deficit in the Nevada ruptures, probably due to the immature fault system. The causative faults had not been previously identified and are located near the transition from the Walker Lane belt to the Basin and Range province. The east–west geometry of the system is consistentmore »with the eastward extension of the Mina Deflection of the Walker Lane north of the White Mountains.« less
5. Anatomy of strike-slip fault tsunami genesis

   https://doi.org/10.1073/pnas.2025632118  

   Elbanna, Ahmed ; Abdelmeguid, Mohamed ; Ma, Xiao ; Amlani, Faisal ; Bhat, Harsha S. ; Synolakis, Costas ; Rosakis, Ares J. ( May 2021 , Proceedings of the National Academy of Sciences)

   Tsunami generation from earthquake-induced seafloor deformations has long been recognized as a major hazard to coastal areas. Strike-slip faulting has generally been considered insufficient for triggering large tsunamis, except through the generation of submarine landslides. Herein, we demonstrate that ground motions due to strike-slip earthquakes can contribute to the generation of large tsunamis (>1 m), under rather generic conditions. To this end, we developed a computational framework that integrates models for earthquake rupture dynamics with models of tsunami generation and propagation. The three-dimensional time-dependent vertical and horizontal ground motions from spontaneous dynamic rupture models are used to drive boundary motions in the tsunami model. Our results suggest that supershear ruptures propagating along strike-slip faults, traversing narrow and shallow bays, are prime candidates for tsunami generation. We show that dynamic focusing and the large horizontal displacements, characteristic of strike-slip earthquakes on long faults, are critical drivers for the tsunami hazard. These findings point to intrinsic mechanisms for sizable tsunami generation by strike-slip faulting, which do not require complex seismic sources, landslides, or complicated bathymetry. Furthermore, our model identifies three distinct phases in the tsunamic motion, an instantaneous dynamic phase, a lagging coseismic phase, and a postseismic phase, each of which maymore »affect coastal areas differently. We conclude that near-source tsunami hazards and risk from strike-slip faulting need to be re-evaluated.

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