Offshore Seismic Attenuation Heterogeneity and Implications for Pore-Fluid Pressure in Gisborne Slow-Slip Region, Northern Hikurangi Margin, North Island, New Zealand

Date: 4/24/2019

Time: 02:30 PM

Room: Cascade II

The Hikurangi subduction margin offshore Gisborne, North Island, New Zealand hosts large slow-slip events (SSEs) every 18-24 months. The May 2014–June 2015 HOBITSS (Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip) deployment of ocean-bottom seismometers (OBS) and pressure gauges was designed to overlie the source region of the Gisborne SSEs, and successfully recorded two SSEs with maximum displacement of ~20 cm. Hypotheses on the cause of the SSEs focus on high interface pore-fluid pressure, a fluid-filled and fractured overriding plate, and frictional properties of the plate interface. The degree of heterogeneity of these properties is important for understanding why slip varies spatially and why the depth of the brittle-viscous transition zone (where many SSEs are presumed to occur) varies widely from north (shallow) to south (deep). We aim to resolve the attenuation of P- and S-waves from local earthquakes. We determine ~2,700 t* (integrated path attenuation) values for 153 earthquakes (> M_L 2.5), mostly within the subducting plate, located with 12 OBS and 24 land stations. The addition of OBS expands the resolvable model region of attenuation with additional raypaths, which has thus far been limited by measurements from onshore stations. Initial results suggest ~90% of the path-averaged Qs values for earthquakes shallower than 25 km are <500, which agrees with existing onshore Q tomography (Eberhart-Phillips et al., 2017). The majority of path-averaged Qs from shallow earthquakes at OBS are <300. These regions of lower Qs lie within and updip of high Vp/Vs (Eberhart-Phillips and Bannister, 2015), inferred to be associated with high pore-fluid pressure in the overlying plate. Variable Qs/Qp ratios (Eberhart-Phillips et al., 2017) suggest heterogeneity in pore-fluid pressures in and above the interface and spatially coincides with variable slip rate deficit and topography of the incoming plate. We aim to further resolve the offshore attenuation structure to determine whether significant changes in seismic attenuation occur along the margin strike.

Presenting Author: Jenny Nakai

Authors