Numerical modeling of shallow slow slip events along the Hikurangi margin, New Zealand.

Slow slip events (SSEs) have transformed our conception of strain accumulation and release along plate boundaries. SSEs are now considered an integral part of the earthquake cycle and discussed as potential precursors of the associated earthquake hazard. The Hikurangi subduction zone in the North Island of New Zealand is exceptional in the diversity of SSE characteristics. Thus, it is an excellent natural laboratory to study this phenomenon. In this thesis, we study SSEs that occur along the shallow (<15 km depth) portion of the Hikurangi subduction zone. In particular, we investigate the mechanism behind two poorly understood observations of these events, namely the along-strike change in their recurrence interval and the inferred pore-pressure cycles accompanying shallow Hikurangi SSEs. To do so, we conduct numerical simulations under the framework of rate-and-state friction laws.

Analysis of GPS data revealed that the recurrence interval of shallow SSEs varies along the Hikurangi margin. In the northern part of the margin, SSEs occur every ~1 yr, while in the central and southern regions, they occur every ~2 yrs and ~5 yrs, respectively. To understand the factors that control this segmentation, we conduct numerical models of SSEs based on continuum elasticity and the non-planar geometry of the Hikurangi plate interface. Our model considers velocity-weakening friction properties and elevated pore fluid pressure at the source depths of shallow Hikurangi SSEs. After exploring different model parameters, we find that a relatively simple model reproduces the characteristics of shallow SSEs as constrained by geodetic observations. The preferred model captures the magnitudes and durations of these events and the southward increase in their recurrence intervals. Our model results indicate that the segmentation of SSE recurrence intervals is favored by along-strike changes in both the plate convergence rate and the downdip width of the SSE source region. Modeled SSEs with longer recurrence intervals concentrate in the southern part of the fault, where the plate convergence rate is the lowest, and the SSE source region is the widest due to the shallower slab dip angle. The observed segmentation of shallow SSEs is not reproducible with a simple planar fault model, indicating that a realistic plate interface is an important factor to account for in modeling SSEs. These results may give insight into the segmentation of SSEs in other subduction zones, such as Mexico and Nankai.

Recent geophysical observations additionally suggest that temporal pore fluid pressure changes correlate with slow slip events (SSEs) occurring along the shallow portion of the Hikurangi margin and in different subduction zones. These fluctuations in pore fluid pressure are attributed to fluid migration before and during SSEs, which may modulate SSE occurrence. To examine the effect of pore fluid pressure changes on SSE generation, we develop numerical models in which periodic pore-pressure perturbations are applied to a stably sliding, velocity-strengthening fault. By varying the physical characteristics of the pore-pressure perturbations (amplitude, characteristic length and period), we find models that reproduce shallow Hikurangi SSE properties (duration, magnitude, slip, recurrence) and SSE moments and durations from different subduction zones. The stress drops of modeled SSEs range from ~20–120 kPa while the amplitudes of pore-pressure perturbations is several MPa, broadly consistent with those inferred from observations. Our results indicate that large permeability values of ~10^14 to 10^10 m^2 are needed to reproduce the observed SSE properties. Such high values could be due to transient and localized increases in fault zone permeability in the shear zone where SSEs occur. Our results suggest that SSEs may arise on faults with velocity-strengthening frictional conditions subject to pore-pressure perturbations.

The results of this thesis help to understand the factors governing SSE behavior along the Hikurangi margin and other subduction zones. We find that the geometry of the plate interface, the tectonic loading, the fault zone properties, and the pore pressure conditions may influence SSE characteristics. In addition, we find that two different modelling approaches can reproduce SSEs comparable to observations. Further constraints on the SSE environment will be required to test the viability of each approach. The models, approaches and results of this thesis motivate future investigations of the generation mechanism of SSEs, the interaction between SSEs and earthquakes, and the temporal changes in fault zone hydraulic properties during SSEs.