Nucleation of Characteristic Earthquakes in Simulated Cycles Involving Deep Huge Slow Slip Events

Abstract

The 1944 Tonankai and 1946 Nankai megathrust earthquakes are likely to have been immediately preceded by a huge slow slip event (SSE) in the deeper brittle-to-ductile transition zone, which accumulates slip of a meter or more within several days. The present study investigates the interaction between such huge SSEs and the characteristic earthquakes through earthquake cycle simulations obeying the rate- and state-dependent friction. To simulate huge SSEs, we employ a cutoff velocity (V cx ) above which the friction changes from velocity weakening to velocity strengthening. We assume that V cx decreases from 10 to 10 −9  m/s with depth over the transition zone located between 20- and 35-km depths. Depending on the characteristic slip distance L of rate and state friction, simulation results showed three patterns of earthquake nucleation. If L is small, an SSE in the transition zone directly grows into an earthquake without experiencing a separate nucleation at the brittle zone (brittle nucleation). If L is moderate, an SSE triggers brittle nucleation, which develops into an earthquake. If L is large, an SSE occurs but ceases without triggering an earthquake nor brittle nucleation, then slip deficit accumulates further to finally start brittle nucleation without the direct help of SSE, and an earthquake occurs without a large precursor. In addition, isolated huge SSEs, resembling precursory ones, occur in all cases, at most a few times per cycle.