Geophysical observations indicate 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, rate-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 m2 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 in rate-strengthening frictional conditions subject to pore-pressure perturbations.

The spatiotemporal characteristics of infrequent back-arc rifting events, and their relationships to volcanic unrest and other transient processes within the subduction system are not well known. We report 10 spatio-temporal swarms of earthquakes that occurred along ~300 km of the northern and central Taupō Volcanic Zone (TVZ) between March-September 2019. The swarms lack clear mainshock-aftershock distributions, suggesting involvement of magmatic or pressurized fluids. The most energetic swarms occurred beneath the southwestern flank of Whakaari/White Island volcano (WI) and were accompanied by elevated SO2 emissions and ~8 mm southwest displacement of the edifice. Focal mechanisms of the WI swarms suggest horizontal dilation in the direction of tectonic extension achieved by slip along arrays of closely spaced NE-striking normal faults linked by oblique-slip to strike-slip faults. Given the small GNSS displacements, we favor the interpretation that magmatic volatiles were released by changes in crustal stress. Based on the punctuated, cascading nature of swarms along the northern TVZ, we hypothesize that a rapid reduction in the minimum horizontal stress occurred along the entire northern TVZ over ~5 months, leading to widespread unrest. The probable release of magmatic fluids from shallow magma bodies in the vicinity of WI between May and June argues against a causal relation to the December 9, 2019 WI eruption.

Detecting crustal deformation during transient deformation events at offshore subduction zones remains challenging. The spatiotemporal evolution of slow slip events (SSEs) on the offshore Hikurangi subduction zone, New Zealand, during February–July 2019, is revealed through a time-dependent inversion of onshore and offshore geodetic data that also account for spatially varying elastic crustal properties. Our model is constrained by seafloor pressure time series (as a proxy for vertical seafloor deformation), onshore continuous Global Navigation Satellite System (GNSS) data, and Interferometric Synthetic Aperture Radar (InSAR) displacements. Large GNSS displacements onshore and uplift of the seafloor (10-33 mm) require peak slip during the event of 150 to >200 mm at 6-12 km depth offshore Hawkes Bay and Gisborne, comparable to maximum slip observed during previous seafloor pressure deployments at north Hikurangi. The onshore and offshore data reveal a complex evolution of the SSE, over a period of months. Seafloor pressure data indicates the slow slip may have persisted longer near the trench than suggested by onshore GNSS stations in both the Gisborne and Hawkes Bay regions. Seafloor pressure data also reveal up-dip migration of SSE slip beneath Hawke Bay occurred over a period of a few weeks. The SSE source region appears to coincide with locations of the March 1947 Mw 7.0–7.1 tsunami earthquake offshore Gisborne and estimated Great earthquake rupture sources from paleoseismic investigations offshore Hawkes Bay, suggesting that the shallow megathrust at north and central Hikurangi is capable of both seismic and aseismic rupture.

Over the last two decades, geodetic and seismic observations have revealed a spectrum of slow earthquakes along the Hikurangi subduction zone in New Zealand. Of those, shallow slow slip events (SSEs) that occur at depths of less than 15 km along the plate interface show a strong along-strike segmentation in their recurrence intervals, which vary from ~1 year from offshore Tolaga Bay in the northeast to ~5 years offshore Cape Turnagain ~300 km to the southeast. To understand the factors that control this segmentation, we conduct numerical simulations of SSEs incorporating laboratory-derived rate-and-state friction laws with both planar and non-planar fault geometries. We find that a relatively simple model assuming a realistic non-planar fault geometry can reproduce the characteristics of shallow SSEs as constrained by geodetic observations. Our preferred model captures the magnitudes and durations of SSEs, as well as the northward decrease of their recurrence intervals. Our results indicate that the segmentation of SSEs’ 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 interval concentrate in the southern part of the fault (offshore Cape Turnagain), where the plate convergence rate is lowest and the source region of SSEs is widest due to the shallower slab dip angle. Notably, the observed segmentation of shallow SSEs cannot be reproduced with a simple planar fault model, which indicates that a realistic plate interface is an important factor to account for in modeling SSEs.