Monitor Velocity Variations in the Northern Hikurangi Margin in 2014-2015

Slow slip events (SSE) have been a hot topic as they extend our understanding of subduction behaviour, but are difficult to observe offshore where they are most common. The Northern Hikurangi margin has the advantage of documented SSE events at relatively shallow depths, as well as onshore and offshore seismic sensors to collect crucial data. The behaviour of SSEs and how they influence, or are influenced by, the physical properties of surrounding materials are open to question. Monitoring velocity changes before, during and after SSEs provides opportunities to study the slow earthquake behaviour and its relation to the changes of material physical properties.

From May of 2014 to June of 2015, for the project "The Hikurangi Ocean Bottom Investigation into Tremor and Slow Slip (HOBITSS)", 15 OBSs were deployed offshore Gisborne, New Zealand on the Hikurangi margin. During this period, there were four main SSEs, with the second in October 2014, in particular, producing significant slip. Ambient noise data from the OBSs are used to study seismic velocity changes related to these SSEs. Horizontal components of the stations are rotated to be parallel and perpendicular to the coastline. Single station cross-component correlations are computed at a frequency range of 2.5-14 s. At this frequency range we focus on the upper plate. Coda waves retrieved from cross correlations are then used to compute velocity changes. The velocity changes averaged from all used stations display a 0.06% velocity decrease during the largest SSE in October, followed by a 0.075% velocity increase after. This suggests a fluid increase in the upper plate during the SSE followed by a fluid decrease. We propose that fluid migrated from the lower plate into the upper plate as a result of the SSE breaking the fluid reservoir seal on the plate interface, causing a velocity decrease in the upper plate. After the SSE, the fluid in the upper plate spread away and the velocity increased.