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

Abstract

The 15 April 2015 Mw 7.8 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 the S wave speed) that is controlled by 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.

Impulsive Source of the 2017 MW=7.3 Ezgeleh, Iran, Earthquake

“…This pulse‐like behavior is therefore unlikely to result from healing phases emanating from the along‐dip finiteness of the fault. A rapid crack‐pulse transition is in agreement with early observations by Heaton () and later studies (e.g., Beroza & Mikumo, ; Meier et al, ; Wang et al, ). Such a pulse may result from a number of mechanisms such as frictional self‐healing, fault strength or stress heterogeneities, bimaterial effects, and wave reflections within low‐velocity fault zones (e.g., Andrews & Ben‐Zion, ; Huang & Ampuero, ; Perrin et al, ).…”

Nonlinear Suppression of High‐Frequency S Waves by the Near‐Field Velocity Pulse With Reference to the 2002 Denali Earthquake

“…Generic three‐dimensional numerical calculations indicate that the near‐field velocity pulse of strong earthquakes likely causes near‐fault nonlinear failure (Roten et al, , , Roten, Olsen, Cui, & Day, ; Wang et al, ). In this case, dynamic stresses at depth scale with particle velocity and approximately with PGV is measured at the surface.…”

“…There is some expectation from generic numerical models of earthquake rupture that brought the crystalline rock beneath PS10 into inelastic failure (Roten et al, 2014(Roten et al, , 2016Roten, Olsen, Cui, & Day, 2017;Wang et al, 2019). In addition, dimensional analysis of the seismic signal recorded at PS10 makes it plausible that such nonlinear failure did occur beneath PS10.…”

“…The assumption of isotropy seems reasonable for grain‐grain contacts in gravel and maybe for pervasively cracked crystalline rock in the uppermost basement, and we lack information on inelastic anisotropy beneath PS10. The S ‐wave Roten et al (, ; Roten, Olsen, Cui, & Day, ; Roten, Olsen, & Day, ; Wang et al, ) used this rheology in three‐dimensional numerical models.…”

“…We discuss this previously described process (Sleep, and references therein). Such nonlinearity is a general feature of three‐dimensional numerical models of earthquake rupture (Roten et al, , ; Roten, Olsen, Cui, & Day, ; Wang et al, ). There is thus some presumption that this process occurred beneath PS10.…”