Three-Dimensional Modeling of Spontaneous and Triggered Slow-Slip Events at the Hikurangi Subduction Zone, New Zealand

At the southern Hikurangi margin, New Zealand, long-term (duration of >1 year), deep (>25 km) slow-slip events (SSEs) occur at intervals of approximately 5 years. In contrast, along the northern and central Hikurangi margin, short-term and shallow (<10 km) SSEs (2- to 3-week duration) occur at 1- to 2-year intervals. However, it is not clear what controls these differences in SSE behavior. In this study, we simulate SSEs along the entire Hikurangi subduction zone using a 3-D model of the plate boundary incorporating a rate-and-state friction law with a cutoff velocity to the evolution effect. Overall, our models are able to replicate the along-strike and depth-dependent variations in SSE behavior at the Hikurangi margin. Our models also show that the slip velocities of the deep, long-term SSEs increase when the slip velocities at the deeper extension of the seismogenic zone increase during the latter half of the interseismic period. By imposing static stress changes induced by the Kaikoura earthquake in the SSE regions, we also reproduce patterns of triggering of deep, long-term SSEs that followed the earthquake. Our results further suggest that the propensity for SSE triggering via static stress transfer depends on the amplitude of perturbation and the timing with respect to the SSE cycle. Overall, both spontaneous and triggered SSEs require larger values of effective stress for deep, long-term events (5 MPa) compared to the shallow, short-term events (1 MPa). Such low effective stresses can be explained by the existence of high pore fluid pressure in these SSE source regions.