Seismological observations and numerical modeling of slow earthquakes

Abstract

Slow earthquakes phenomena, including slow slip and tectonic tremor, are a type of fault slip that is in distinct contrast to stable creep and regular earthquakes. They represent a significant component of the complete seismic cycle. However, the underlying physics is unclear. In this thesis, I investigate the detailed spatiotemporal evolution of slow slip with seismological observations of tremor and numerical simulations.

In the observational part, we develop high-resolution methods based on cross station cross correlations to detect and locate tremor, a proxy for the underlying slow slip. Beneath northern Cascadia, we observe numerous rapid tremor migrations, which either propagate along the main front regardless of its orientation, or originate from the main front (when the main front is within the study region) and extend tens of kilometers back into areas that have already ruptured. Beneath Guerrero, Mexico, however, we find migrations that occur well behind the local main tremor front and are not connected to that front. The propagation speeds of these migrations increase with the recurrence intervals, possibly caused by change in slip speeds due to host rock permeability evolution.

Combining the tremor observations from northern Cascadia and Guerrero, as well as northern Kii Peninsula, Japan, we find that tremor tends to occur in short bursts that repeatedly occupy the same areas. Their recurrence intervals increasing with time during a slow slip event, from being too short to be tidally modulated to being close to tidal periods. This behavior is most conspicuous in regions with high tremor density. We propose that these bursts are likely to be driven by loading from slow slip in the areas surrounding the tremor zone. As the main front moves across the tremor zone, the loading rate at a given location decreases, leading to increasing recurrence intervals of the bursts. We design a numerical model of a heterogeneous fault in which the tremor zone covers a portion of the slow slip zone. The burst-like behavior and the increasing recurrence intervals are successfully reproduced. The simulated bursts tend to occur when tidal stress is high, consistent with the observations. We find that for the repetitive bursts to arise, the limit for the slip speeds within the tremor zone should be higher than that on the background fault.