New insight into the frequency-dependent megathrust rupture from dynamic simulations

Abstract:

The recent observation that megathrust earthquake radiation exhibits frequency-depth dependence seems ubiquitous: most the low frequency seismic radiation is shallow while the high frequency seismic radiation occurs deeper. The most cited explanation is that there are systematic depth variations in stress or frictional properties of the slab surface. In this study, we build dynamic rupture models for megathrust earthquakes in realistic 2D elastic structure. We use P-wave velocity Vp of the Tohoku area from Miura et al. (2005) and scale the shear-wave velocity Vs by imposing a Vp/Vs ratio, which we vary in the upper plate and in the slab to the reported relatively high values of wet conditions. We also explore a large parameter space for on-fault frictional and pre-stress properties.

All modeled ruptures present the common patterns of their source function: the slip motions near the trench are crack-like with smooth slip rate functions, while the slip motions on the downdip are pulse-like with sharp slip rate functions. Fitting their spectral shapes with a Brune-type spectral model with a single corner frequency and a spectral falloff rate, our results show the same patterns of the spectral parameters for all those dynamic models: the falloff rate monotonically decreases from the updip to downdip. Our modeling exercise shows that simple geometrical effects explain the frequency-dependence in radiation. Our modeling also highlights that high Vp/Vs ratio, as inferred by tomographic studies or measured in offshore coring, energize the rupture by increasing up-dip rupture velocities.