Investigations into the Factors Controlling Estimates of Slip During Slow Slip Events: Lessons Learned from the Hikurangi Subduction Margin, New Zealand

Abstract:

Understanding of slow slip events (SSEs) at subduction margins is critical to our understanding of seismic and aseismic fault behavior in these regions. The Hikurangi subduction margin adjacent to the North Island, New Zealand, provides an ideal natural laboratory to study SSEs, since both shallow and deep SSEs are observed there, and they are also correlated with the locking behavior, which varies from south to north. In our initial work, we included the effects of material property variations using a New-Zealand-wide seismic velocity model (Eberhart-Phillips et al., 2010), as well as an improved interface geometry (Williams et al., 2013), to provide improved slip estimates for portions of two SSEs: the Manawatu event from September - December, 2010, and the Kapiti event from January - July, 2013.

We have now improved our original technique, in which we use the finite element code PyLith (Aagaard et al., 2013) to generate Green’s functions using material properties from the seismic velocity model, and then use these Green’s functions in the DEFNODE inversion program (McCaffrey, 1995; 2002). Using our improved method, we re-examine a number of Hikurangi SSEs to provide revised slip estimates. We find that the inclusion of the more realistic material properties reduces the estimated slip by approximately one third, in comparison to models assuming a uniform elastic half-space. This highlights the necessity to account for realistic physical property variations of the crust in studies of slow slip—accounting for this can greatly impact the estimated slip budget accommodated by SSEs at subduction zones, as well as the equivalent moment magnitude of SSEs. In addition, we have developed an alternative inversion code that we apply to the same SSEs. By doing this, we are able to estimate the sensitivity of our results to: 1) Assumed interface geometry, 2) Variations in elastic structure, and 3) Inversion technique. By quantifying the sensitivity of our solution to these effects, we should be able to provide much more robust slip estimates for SSEs at the Hikurangi margin and elsewhere.