Local earthquake analysis in a region of slow slip and mud volcanism: The onshore NZ3D Hikurangi margin experiment

In December 2017 to October 2018 a dense grid of seismometers was deployed in the Gisborne area of the Hikurangi subduction margin, New Zealand. The primary aim of this deployment was to detect air-gun shots from the R/V Marcus Langseth which shot a 3D multi-channel seismic volume offshore in January 2018. However, this dense array with a rather unconventional nodal design, also offers the opportunity to use local seismicity to decipher the physical properties of the subsurface.

The location of the 3D array lies above a part of the Hikurangi plate boundary fault which experiences slow slip events every 18 -24 months at a depth of 15-20 km. The Waimata Valley region in the western part of the array also experiences extensive mud-volcanism. Large mud volcano eruptions occurred in the 1930's and we have anecdotal reports from landowners of increasing intensity of mud pool bubbling following the November 2016 Kaikoura earthquake, 550 km to the south.

Our array consists of 49 CMG-6TD broadband seismometers from the NERC Geophysical Equipment Facility (SEIS-UK) operating with a frequency range of 30 s to 100 Hz. These instruments were deployed roughly every 2 km over a 60 km x 15 km area. These data were supplemented by 119 DATA-CUBE3 recorders from the Geophysical Instrument Pool Potsdam and 25 GSX which both use 4.5 Hz geophone sensors. These short-period instruments were used to fill gaps in the broadband array where CMG-6TDs could not be deployed due to the terrain. A very dense grid of DATA-CUBE3 seismometers was deployed in the zone of known mud volcanism. In December 2018 a significant mud volcano eruption occurred in the study area, meaning that we have captured data which characterises the subsurface imminent to a mud eruption.

Our aims in this study are to precisely locate local and regional seismicity and determine earthquake magnitudes. Given the likely unusual attenuation properties of the sub-surface in this area of probable high-fluid pressure, standard Richter local magnitude distance corrections for southern California will not apply. In this study we use our dense nodal array of instruments to determine the local magnitude distance correction bespoke for the northern Hikurangi margin. We use our local earthquake catalogue to conduct local earthquake shear-wave splitting to investigate anisotropy within the slow slip zone and over-riding plate, providing further constraints on the geophysical environment of slow slip.