Seismology, Geophysics

The Transition of Dynamic Rupture Styles in Elastic Media Under Velocity-Weakening Friction

by Alice - Agnes Gabriel

Co-authored with J.-P. Ampuero, L. A. Dalguer, and P. M. Mai

Although kinematic earthquake source inversions show dominantly pulse-like rupture behavior, seismological... more Although kinematic earthquake source inversions show dominantly pulse-like rupture behavior, seismological observations, laboratory experiments and theoretical models indicate that earthquakes can operate with two dierent rupture styles, either as pulses or as cracks, and can propagate at subshear or supershear speeds. The determination of rupture style and speed has important implications for ground motions and may inform about the state of stress and strength of active fault zones. Here we conduct 2D in-plane dynamic rupture simulations with a spectral element method to investigate the diversity of rupture styles on faults governed by velocity-and-state-dependent friction with dramatic velocity-weakening at high slip rate. Our rupture models assume uniform initial stresses and are articially initiated by prescribing time-weakening over a time-dependent nucleation area. We identify the conditions that lead to dierent rupture styles. In particular we investigate the transitions between decaying, steady-state and growing pulses,
cracks, sub-shear and super-shear ruptures as a function of background stress, nucleation duration and characteristic velocity at the onset of severe weakening. Our models show
that small changes of background stress or nucleation energy can lead to dramatic changes of rupture style. Each rupture style transition features a characteristic value of dynamic
stress at the point of transition. We characterize the asymptotic properties of steady state and self-similar pulses as a function of background stress, in particular their slip prole, peak slip rate, rupture speed and rise time. We show that an earthquake may not be restricted to a single rupture style, but that complex rupture patterns can emerge that consist of multiple rupture fronts, possibly involving dierent styles and back-propagating
fronts. For instance, growing pulses lead to re-activation of slip due to gradual stress build up near the hypocenter. We also demonstrate the possibility of a super-shear transition
for pulse-like ruptures. Identifying this diversity of rupture patterns in real earthquakes poses an interesting observational challenge.