Simultaneous long-term and short-term slow slip events at the Hikurangi subduction margin, New Zealand: Implications for processes that control slow slip event occurrence, duration, and migration

We document a sequence of simultaneous short-term and long-term slow slip events (SSEs) at the Hikurangi subduction zone during the 2010/2011 period. The sequence of short-term events (each similar to 2-3 weeks in duration) ruptured much of the shallow plate interface (< 15 km) at central and northern Hikurangi over a 6-month period, was accompanied by microseismicity and involved patchy, irregular migration of SSE slip. We suggest that the patchy migration of the short-term SSE is due to large-scale (similar to 100-3500 km(2)) heterogeneities on the plate interface related to seamount subduction and sediment subduction and/or underplating. This is in contrast to a 2010/2011 long-term SSE at the central Hikurangi margin, which evolved steadily over similar to 1.5 years and ruptured much of the plate interface between 20 and 70 km depth. We suggest that the occurrence of long-term versus short-term SSEs at Hikurangi is related to differences in effective normal stresses and relative heterogeneity of the subduction interface. The long-term SSE sequence began 1 year before the short-term sequence. Coulomb stress change models suggest that the long-term SSE may have triggered initiation of the subsequent short-term SSE sequence. Initiation of the short-term sequence occurred in a region just updip of or within an interseismically locked portion of the plate interface and may be located within the updip transition from seismic to aseismic behavior. Alternatively, it could be characteristic of a region undergoing partial interseismic coupling. This is in contrast to SSEs observed elsewhere in the world that typically occur within the downdip transition from seismic to aseismic behavior.