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 - Upper mantle seismic anisotropy at a strike-slip boundary: South Island, New Zealand JF - Journal of Geophysical Research: Solid Earth Y1 - 2014 A1 - Zietlow, Daniel W. A1 - Sheehan, Anne F. A1 - Molnar, Peter H. A1 - Savage, Martha K. A1 - Hirth, Greg A1 - Collins, John A. A1 - Hager, Bradford H. KW - mantle lithosphere KW - MOANA KW - New Zealand KW - ocean bottom seismometers KW - seismic anisotropy KW - South Island AB - New shear wave splitting measurements made from stations onshore and offshore the South Island of New Zealand show a zone of anisotropy 100–200 km wide. Measurements in central South Island and up to approximately 100 km offshore from the west coast yield orientations of the fast quasi-shear wave nearly parallel to relative plate motion, with increased obliquity to this orientation observed farther from shore. On the eastern side of the island, fast orientations rotate counterclockwise to become nearly perpendicular to the orientation of relative plate motion approximately 200 km off the east coast. Uniform delay times between the fast and slow quasi-shear waves of nearly 2.0 s onshore continue to stations approximately 100 km off the west coast, after which they decrease to 1 s at 200 km. Stations more than 300 km from the west coast show little to no splitting. East coast stations have delay times around 1 s. Simple strain fields calculated from a thin viscous sheet model (representing distributed lithospheric deformation) with strain rates decreasing exponentially to both the northwest and southeast with e-folding dimensions of 25–35 km (approximately 75% of the deformation within a zone 100–140 km wide) match orientations and amounts of observed splitting. A model of deformation localized in the lithosphere and then spreading out in the asthenosphere also yields predictions consistent with observed splitting if, at depths of 100–130 km below the lithosphere, typical grain sizes are 6–7 mm. VL - 119 UR - https://onlinelibrary.wiley.com/doi/abs/10.1002/2013JB010676 N1 - _eprint: https://onlinelibrary.wiley.com/doi/pdf/10.1002/2013JB010676 ER -