TY - JOUR T1 - Continuous Tremor Activity With Stable Polarization Direction Following the 2014 Large Slow Slip Event in the Hikurangi Subduction Margin Offshore New Zealand JF - Journal of Geophysical Research: Solid Earth Y1 - 2022 A1 - Iwasaki, Yuriko A1 - Mochizuki, Kimihiro A1 - Ishise, Motoko A1 - Todd, Erin K. A1 - Schwartz, Susan Y. A1 - Zal, Hubert A1 - Savage, Martha K. A1 - Henrys, Stuart A1 - Sheehan, Anne F. A1 - Ito, Yoshihiro A1 - Wallace, Laura M. A1 - Webb, Spahr C. A1 - Yamada, Tomoaki A1 - Shinohara, Masanao KW - New Zealand KW - polarization KW - S-wave splitting KW - Seamount KW - slow slip KW - tremor AB - Many types of slow earthquakes have been discovered at subduction zones around the world. However, the physical process of these slow earthquakes is not well understood. To monitor offshore slow earthquakes, a marine seismic and geodetic experiment was conducted at the Hikurangi subduction margin from May 2014 to June 2015. During this experiment, a large slow slip event (Mw 6.8) occurred directly beneath the ocean bottom seismometer (OBS) network. In this study, S-wave splitting and polarization analysis methods, which have been previously used on onshore data to investigate tremor and anisotropy, are applied to continuous OBS waveform data to identify tremors that are too small to detect by the envelope cross correlation method. Continuous tremor activity with stable polarization directions is detected at the end of the 2014 slow slip event and continued for about 2 weeks. The tremors are generated around a southwest bend in the slow slip contours and at the landward edge of a subducted seamount. Our findings corroborate a previous interpretation, based on burst-type repeating earthquakes and intermittent tremor, that localized slow slip and tremor around the seamount was triggered by fluid migration following the large plate boundary slow slip event and indicate tremor occurred continuously rather than as isolated and sporadic individual events. VL - 127 UR - https://onlinelibrary.wiley.com/doi/abs/10.1029/2021JB022161 N1 - _eprint: https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2021JB022161 ER - TY - JOUR T1 - Temporal velocity variations in the northern Hikurangi margin and the relation to slow slip JF - Earth and Planetary Science Letters Y1 - 2022 A1 - Wang, Weiwei A1 - Savage, Martha K. A1 - Yates, Alexander A1 - Zal, Hubert J. A1 - Webb, Spahr A1 - Boulton, Carolyn A1 - Warren-Smith, Emily A1 - Madley, Megan A1 - Stern, Tim A1 - Fry, Bill A1 - Mochizuki, Kimihiro A1 - Wallace, Laura KW - ambient noise KW - seismic velocity variations KW - slow slip event KW - the Hikurangi subduction zone AB - Slow slip events (SSE) have been studied in increasing detail over the last 20 years, improving our understanding of subduction zone processes. Although the relationship between SSEs and the physical properties of their surrounding materials is still not well-understood, the northern Hikurangi margin in New Zealand is the site of relatively shallow (<10 km deep), frequent SSEs, providing excellent opportunities for near-field investigations. From September to October 2014, an SSE occurred with more than 250 mm slip, and was recorded successfully by the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip (HOBITSS) deployment. This study applies scattered wave interferometry to ambient noise data acquired by nine HOBITSS ocean bottom seismometers (OBS) to study the seismic velocity variations related to the SSE. Single station cross-component correlations are computed within a period band that focuses on the upper plate in our study region. The average velocity variations display a decrease on the order of 0.05% during the SSE, followed by an increase of similar magnitude afterwards. We suggest two possibilities. The first possibility, which has been suggested by other seismological observations, is that the SSE causes a low-permeability seal on the plate boundary to break. The break allows fluid to migrate into the upper plate, causing a seismic velocity decrease during the SSE because of increased pore fluid volume in the upper plate. Under this model, after the SSE, the fluids in the upper plate diffuse gradually and the velocity increases again. The second possibility is the velocity changes are related to changes in crustal strain during the slow slip cycle, whereby elastic strain accumulates prior to the SSE, causing contraction and reduction of porosity and therefore increase of velocity above the SSE source (the seismic velocity increases between SSEs). During the SSE the upper plate goes into extension as the elastic strain is released, which results in dilation and a porosity increase (seismic velocity reduction). After the SSE, stress and strain accumulate again, causing a porosity decrease and a velocity increase. VL - 584 UR - https://www.sciencedirect.com/science/article/pii/S0012821X22000796 ER - 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 - TY - JOUR T1 - Investigations of Shallow Slow Slip Offshore of New Zealand JF - Eos Y1 - 2016 A1 - Harris, Robert A1 - Wallace, Laura A1 - Webb, Spahr A1 - Ito, Yoshihiro A1 - Mochizuki, Kimihiro A1 - Ichihara, Hiroshi A1 - Henrys, Stuart A1 - Tréhu, Anne A1 - Schwartz, Susan A1 - Sheehan, Anne A1 - Saffer, Demian A1 - Lauer, Rachel AB - Recent and upcoming studies of the Hikurangi margin east of New Zealand shed light on previously undetectable tectonic movements. VL - 97 UR - https://eos.org/project-updates/investigations-of-shallow-slow-slip-offshore-of-new-zealand ER -