What controls the along-strike segmentation of shallow slow slip events? Insights from 3D numerical modeling of slow slip events along the Hikurangi margin, New Zealand

Abstract:

Over the last two decades, geodetic and seismic observations have revealed a spectrum of slow earthquakes along the Hikurangi margin in New Zealand. Of those, shallow slow slip events (SSEs) that occur at depths of less than 15 km show a strong along-strike segmentation in the location of slip patches and recurrence intervals, which vary from ~1 to 5 years from offshore Tolaga Bay in the northeast to Cape Turnagain ~300 km to the southeast, respectively. To understand the factors that control this segmentation, we conduct numerical simulations of SSEs incorporating laboratory-derived rate-and-state friction laws and either a planar or a non-planar fault geometry. Among a wide range of parameters considered here, we find that a relatively simple model assuming non-planar fault geometry given by a recent model of the plate interface can reproduce the observed segmentation of shallow SSEs. Our preferred model shows a spatial pattern of magnitudes and durations of SSEs that is consistent with observations, and captures the northward decrease of their recurrence interval. The longer recurrence interval of SSEs offshore Cape Turnagain is favored by the shallower dipping angle of the fault and the lower convergence rate, whereas in the northern part of the margin, the higher plate convergence rate and steeper dip angle lead to shorter recurrence times of SSEs. Our results indicate that the segmentation of SSEs is mainly controlled by both the rate of plate convergence and along-strike changes in the downdip extent of the low effective normal stress region. We also find that, although the first-order segmentation of SSEs could also be reproduced using the planar fault geometry, the model with the more realistic non-planar geometry fits better the recurrence intervals of SSEs and gives rise to slip patches that are more persistent over time. These results may help explain the segmentation of SSEs observed in other subduction zones as well, such as Nankai and Mexico.