TY - JOUR T1 - Temporal and spatial variations in seismic anisotropy and V P /V S ratios in a region of slow slip JF - Earth and Planetary Science Letters Y1 - 2020 A1 - Zal, Hubert Jerzy A1 - Jacobs, Katrina A1 - Savage, Martha Kane A1 - Yarce, Jefferson A1 - Mroczek, Stefan A1 - Graham, Kenny A1 - Todd, Erin K. A1 - Nakai, Jenny A1 - Iwasaki, Yuriko A1 - Sheehan, Anne A1 - Mochizuki, Kimihiro A1 - Wallace, Laura A1 - Schwartz, Susan A1 - Webb, Spahr A1 - Henrys, Stuart AB - In September 2014, a five week long slow slip event (SSE) occurred near Gisborne at the northern Hikurangi subduction zone, New Zealand, and was recorded by offshore instruments deployed by the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) project. Up to 25 cm of slip occurred directly below the HOBITSS array. We calculate shear wave splitting (SWS) and V P / V S ratios for event-station pairs on HOBITSS ocean bottom seismometers and onshore GeoNet seismic stations to determine the relationship in time and space between slow slip and these seismic properties. Spatial averaging of SWS fast azimuths yields trench-perpendicular fast azimuths in some areas, suggesting that compressive stress from plate convergence closes microcracks and controls anisotropy in the upper-plate. Variations from the trench perpendicular directions are observed near a subducting seamount, with directions closely resembling fracture and fault patterns created by subducting seamounts previously observed in both laboratory and field experiments. Temporal variations in fast azimuths are observed at three stations, two of which are located above the seamount, suggesting measurable variations in stress orientations. During the SSE, median V P / V S measurements across all offshore stations increase from 1.817 to 1.894 and SWS delay times decrease from 0.178 s to 0.139 s (both changes are significant within 95% confidence intervals). Temporal variations in V P / V S and delay time are consistent with fluid pressurization below a permeability barrier and movement of fluids during the rupture of a slow-slip patch. VL - 532 UR - https://app.dimensions.ai/details/publication/pub.1123727644 ER - TY - JOUR T1 - Earthquakes and Tremor Linked to Seamount Subduction During Shallow Slow Slip at the Hikurangi Margin, New Zealand JF - Journal of Geophysical Research: Solid Earth Y1 - 2018 A1 - Todd, Erin K. A1 - Schwartz, Susan Y. A1 - Mochizuki, Kimihiro A1 - Wallace, Laura M. A1 - Sheehan, Anne F. A1 - Webb, Spahr C. A1 - Williams, Charles A. A1 - Nakai, Jenny A1 - Yarce, Jefferson A1 - Fry, Bill A1 - Henrys, Stuart A1 - Ito, Yoshihiro KW - earthquake KW - New Zealand KW - Seamount KW - slow slip KW - subduction KW - tremor AB - Shallow slow slip events have been well documented offshore Gisborne at the northern Hikurangi subduction margin, New Zealand, and are associated with tectonic tremor downdip of the slow slip patch and increases in local microseismicity. Tremor and seismicity on the shallow subduction interface are often poorly resolved due to their distance from land-based seismic and geodetic networks. To address this shortcoming, the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip experiment deployed 24 absolute pressure gauges and 15 ocean bottom seismometers on the seafloor above the Gisborne slow slip patch to investigate the spatial and temporal extent of slow slip and associated tremor and earthquake activity. We present a detailed spatiotemporal analysis of the seismic signatures of various interplate slip processes associated with the September/October 2014 Gisborne slow slip event. Tectonic tremor begins toward the end and continues after the geodetically constrained slow slip event and is localized in the vicinity of two subducted seamounts within and updip of the slow slip patch. The subsequent, rather than synchronous occurrence of tremor suggests that tremor may be triggered by stress changes induced by slow slip. However, Coulomb failure stress change models based on the slow slip distribution fail to predict the location of tremor, suggesting that seamount subduction plays a dominant role in the stress state of the shallow megathrust. This and the observed interplay of seismic and aseismic interplate slip processes imply that stress changes from slow slip play a secondary role in the distribution of associated microseismicity. VL - 123 UR - https://onlinelibrary.wiley.com/doi/abs/10.1029/2018JB016136 N1 - _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2018JB016136 ER -