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Katherine Woods

and 11 more

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.

Andrea Perez-Silva

and 5 more

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.

Noel Bartlow

and 1 more

Episodic Tremor and Slip, or ETS, occurs frequently in Cascadia with recurrence intervals of roughly 8-22 months for large ETS. Characterizing these events is critical to our understanding of subduction plate interface mechanics, plate motion budgets, and the potential for damaging earthquakes. Here we combine a novel technique for separating ETS and inter-ETS velocities with the Network Inversion Filter [Segall and Matthews,1997; McGuire and Segall, 2003; Miyazaki et al., 2006] to fully characterize ETS slip using daily GPS time series. The velocity separation technique allows for an inversion of the stacked, time-averaged ETS velocities to obtain a time-averaged ETS slip rate on the plate interface for the last 10-20 year time period. These time-averaged velocities are directly comparable to plate rate to characterize the overall slip budget. We use time dependent NIF inversions with our newly derived inter-ETS velocities to create a catalog of ETS events. Slip and tremor track closely in all ETS events, consistent with prior results [Bartlow et al., 2011; Wech and Bartlow, 2014]. We generate heterogenous elastic Green’s functions for both of our inversions using the PyLith finite element code [Aagaard et al., 2013], based on the velocity model of Stephenson [2007] to better estimate slip amplitudes. We find that while 75-100% of the plate rate is accommodated in the northern segment, consistent with prior results [Chapman and Melbourne, 2009], in the central segment and parts of the southern segment ETS accommodates only 0-50% of the plate rate, leaving additional slip to be released as inter-ETS creep, in an earthquake, as posteseismic relaxation, or as ETS slip at other points in the megathrust earthquake cycle. Currently published locking models [Schmalzle et al., 2014; Pollitz and Evans, 2017] indicate that inter-ETS creep is likely to take up most of the remaining slip budget, but some coupling may remain in the ETS zone.