Offshore tremor activity over a subducted seamount accompanying SSE in the northern Hikurangi subduction zone revealed by polarization and shear-wave splitting analysis of OBS data

Abstract:

A wide spectrum of fault slip behavior ranging from slow slip to regular earthquakes reflects variation of the frictional properties of the fault. Spatiotemporal relationships among these different types of slips would provide important information for better understanding of generation mechanisms of fault slips.

A marine seismic and geodetic experiment was conducted from May 2014 to June 2015 in the northern Hikurangi subduction zone, where slow-slip events have been regularly observed at relatively constant intervals of ~1.8 years. Fifteen ocean bottom seismometers (OBSs) and 24 absolute pressure gauges (APGs) were deployed in a region of 100 km x 100 km. During the observation, a large SSE with Mw 6.8 occurred in September and October 2014 directly beneath the APG network, and its slip distribution that reached near the Hikurangi Trough axis circumventing the existing subducted seamounts was precisely determined. Offshore tremor activity in Hikurangi was first identified located by an envelope correlation method at a rate of about 10 events per day. The activity, that lasted for two weeks following the SSE, appeared to be spatially limited around one of the subducted seamounts up-dip of the slow slip distribution. Further investigation was made to better resolve the nature of this tremor activity by applying a polarization and shear-wave splitting (SWS) analysis in which cross-correlation (CC) of two horizontal components of OBS was calculated. The CC coefficient values stayed high only during this two-week period of tremor activity through the one year observation that showed continuous arrival of polarized signals from the tremors.

We conducted another offshore observation in the same region from October 2018 to October 2019 using 5 OBSs around the subducted seamount. A large SSE occurred from March to May 2019, and accompanying intense tremor activity was observed. The envelope correlation analysis located this tremor activity around the subducted seamount directly beneath our network. We also applied the polarization and SWS method and found that the CC coefficient values and polarization directions stayed continuously high and stable during the activity. We further investigate relationships between the directions of polarization and the plate convergence to infer the generation mechanisms of tremors.