Estimation of the Stress Breakdown Slip from Strong-Motion Seismograms

Session: What Can We Infer About the Earthquake Source Through Analyses of Strong Ground Motion?
Type: Oral
Date: 4/29/2020
Time: 03:00 PM
Room: 115
Description: 

We use 3D simulations of strike-slip spontaneous rupture propagation to test the method by Mikumo et al. (2003) for extracting information about the friction controlling earthquake rupture directly from near-field seismograms. This method computes a parameter (Dc”) approximating the stress breakdown slip (Dc) from the rake-parallel displacement at the time of the peak particle velocity. Our results show very different prospects for estimating Dc for subshear and supershear ruptures. For subshear ruptures, Dc” is almost always controlled by rupture and wave propagation effects, rather than the fault friction process. Only if rupture reaches the Earth’s surface and within a short distance from the fault (Rc, less than a few 100 meters) is it possible to extract information about Dc from strong-motion data, due to the fast distance decay from the fault of the seismic energy related to the stress breakdown process. Beyond Rc, Dc” is controlled by the dynamic stress drop from the earthquake rupture without information about Dc (Cruz-Atienza et al., BSSA, 2009). Rc is comparable to the length of the fault cohesive zone where the stress breakdown process takes place during rupture and is approximately equal to 80% of the wavelength associated with the breakdown frequency, defined as the reciprocal of the time span needed by the stress to drop to the dynamic level. When the rupture propagates with supershear speeds, on the other hand, this energy is carried much farther away from the fault by (particularly Rayleigh) Mach waves, when rupture reaches the Earth's surface, effectively enabling the method by Mikumo et al. to estimate Dc. We use the method via an asymptotic approximation of the slip and slip-rate time histories from the Mach waves in a viscoelastic half-space model and show that Dc can be estimated within an error of 40% from Mach waves that have propagated at least 3 km from the fault (Cruz-Atienza and Olsen, Tectonophysics, 2010). The method estimates a Dc of ~1.5 m for the 2002 Mw7.9 Denali, Alaska, earthquake and ~1.7 m for the 1999 Mw7.6 Izmit, Turkey, event.

Presenting Author: Kim Olsen

Authors