Seismic Anisotropy and Vp/Vs Changes Measured on Ocean Bottom Seismometers Suggest Fluid Pressure Changes Occur With Slow Slip off the Hikurangi Margin, New Zealand

As part of the HOBITSS (Hikurangi Ocean Bottom Investigations of Tremor and Slow Slip) study, we deployed 9 ocean bottom seismometers (OBS) from May 2014-June 2015, which included a two-week long slow slip episode in September 2014. Up to 20 cm of slip occurred directly below the HOBITSS array. We supplemented the OBS array with data from nine permanent onshore GeoNet seismic stations. We determined the orientations of the horizontal components of the OBSs using P and Rayleigh waves. We consider shear wave splitting (SWS) and V_P/V_S ratios calculated for single event-station pairs on ∼3000 local earthquakes during the OBS deployment. Weak temporal changes in delay time and V_P/V_S are observed on moving averages of measurements on some OBS stations. Rotations in fast polarization directions are observed at three stations and two of these stations are located on the border between a subducted seamount and a high reflectivity zone. Stacking measurements recorded on OBSs reveals a decrease in V_P/V_S initiating ∼100 days before the SSE (from 2.0+-0.01 to 1.9+-0.01), with a sharp increase (up to ∼2.05) during the 2 weeks of the SSE. This is followed by a decrease back to the initial V_P/V_S of ∼2 over the next ∼150 days. Delay time trends are inversely related to V_P/V_S,exhibiting an initial increase, from ∼0.15+-0.01 to ∼0.2+-0.01 seconds, over ∼60 days before the SSE, followed by a sharp decrease, from ∼0.2 to ∼0.14 during the SSE, and an increase back to the initial ∼0.15 seconds during the next ∼60 days. Because SWS results are sensitive to variations in earthquake locations, we analysed results from individual spatial earthquake distributions and conclude that the temporal variations are not driven by spatial variations in earthquake locations.

We interpret these results in terms of changing shapes of liquid-filled cracks. We suggest the cracks are rounder when differential stresses are lower, leading to smaller anisotropy and thus smaller delay times, while the rounder cracks can hold more liquid and yield higher Vp/Vs ratios. When stresses build up the cracks become more elongated, yielding lower Vp/Vs ratios and higher delay times. The SSE temporarily decreases the differential stress and thus changes the cracks' shapes. The gradual changes before the SSE suggest that there may be recognizable precursory signals to slow slip events.