Group B, Poster #238, Ground Motions

Fault damage zone effects on near-field ground motion parameters and plastic strain in a multi-scale dynamic rupture model of the 2019 Ridgecrest sequence

Nico Schliwa, & Alice-Agnes Gabriel
Poster Image: 

Poster Presentation

2022 SCEC Annual Meeting, Poster #238, SCEC Contribution #12443 VIEW PDF
We analyze near-field ground motion parameters and plastic deformation of two multi-scale dynamic rupture models of the 2019 Ridgecrest sequence, including the Mw 6.4 Searles Valley foreshock, followed by the Mw 7.1 Ridgecrest mainshock 34 hours later. Both models incorporate a complex 3D non-vertical fault geometry, 3D velocity structure, high-resolution topography, 3D initial stress distribution, off-fault plasticity, and viscoelastic attenuation. Frictional failure is described by a strong velocity weakening rate-and-state friction law. We compare models with and without a pre-existing shallow flower-shaped fault damage zone, representing the 0.5-1.5 km wide “intensely damaged inner core”... identified by Qiu et al. (2021).

We find that close to the dynamically propagating foreshock and mainshock ruptures, the fault damage zone amplifies median ground motion parameters by 20-100%. Foreshock ground motions and spectral accelerations of higher frequencies (> 0.5 Hz) feature larger amplifications since they match the eigenfrequencies of the fault zone. Conversely, at distances between 1-10 km from the fault system, median ground motion parameters of the mainshock are reduced due to fault zone shielding effects. The fault zone only mildly affects dynamic rupture characteristics, and median ground motion parameters of models with and without accounting for a pre-existing damage zone overall converge at distances > 80 km to the rupture. However, wave field deflections due to the fault zone still heavily impact ground motions locally at such large distances, highlighting the relevance of fault zone complexity for accurate ground motion modeling and seismic hazard assessment.

Next, we examine co-seismic fault-zone plastic deformation. The median values of the dynamically induced absolute off-fault plastic strain concentrate within a distance of 1-2 km to the rupture, which coincides with the observed fault damage zone widths by Qiu et al. (2021). However, the impact of the pre-existing shallow flower structure fault zone on median plastic strain is weak. In the future, we aim to generate and resolve very high frequencies within the extreme near-field, including pre-existing and co-seismic inelastic deformation zones and fault roughness, to analyze the rapidly decaying (near-field) source effects.