A Baseline for Temporal Monitoring of Azimuthal Anisotropy of the Hikurangi Megathrust in a Region of Shallow Slow Slip

In 2014, ocean bottom seismometers (OBS) and absolute pressure gauges (APG) from the HOBITSS deployment captured a relatively large and shallow slow slip event (SSE) offshore of the North Island, New Zealand. The event is one of the best recorded shallow SSE to date. Because the SSE occurred only kilometers ( 2-15) from the seafloor instrumentation, the dataset represents one of the best chances to date for observing changes in seismicity and physical parameters that govern seismic wave propagation before, during, and after the SSE. In this work, we focus on seismic anisotropy as determined from the inversion of ambient noise data. We have retrieved daily Green's Functions from ambient noise correlograms of 3-component OBS and 1-component APG throughout the recording period. The daily cross-correlation functions show high signal to noise ratios and remarkable mutual consistency, providing a promising approach to temporal monitoring. We will present results of a linearized inversion in which we solve a frequency-dependent ray-based inverse problem for the average isotropic and anisotropic surface wavespeeds between 0.2 and 1 Hz. In this frequency range, anisotropy should be sensitive to changes in the local stress tensor of the shallow crust and megathrust fault. Our results provide a background model in which to compare short-term estimates of anisotropy when searching for temporal variations.