@article {wang_temporal_2022,
	title = {Temporal velocity variations in the northern Hikurangi margin and the relation to slow slip},
	journal = {Earth and Planetary Science Letters},
	volume = {584},
	year = {2022},
	month = {04/2022},
	pages = {117443},
	abstract = {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.},
	keywords = {ambient noise, seismic velocity variations, slow slip event, the Hikurangi subduction zone},
	issn = {0012-821X},
	doi = {10.1016/j.epsl.2022.117443},
	url = {https://www.sciencedirect.com/science/article/pii/S0012821X22000796},
	author = {Wang, Weiwei and Savage, Martha K. and Yates, Alexander and Zal, Hubert J. and Webb, Spahr and Boulton, Carolyn and Warren-Smith, Emily and Madley, Megan and Stern, Tim and Fry, Bill and Mochizuki, Kimihiro and Wallace, Laura}
}
@article {117,
	title = {Temporal and spatial variations in seismic anisotropy and V P /V S ratios in a region of slow slip},
	journal = {Earth and Planetary Science Letters},
	volume = {532},
	year = {2020},
	pages = {115970},
	type = {Journal Article},
	abstract = {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.},
	doi = {10.1016/j.epsl.2019.115970},
	url = {https://app.dimensions.ai/details/publication/pub.1123727644},
	author = {Zal, Hubert Jerzy and Jacobs, Katrina and Savage, Martha Kane and Yarce, Jefferson and Mroczek, Stefan and Graham, Kenny and Todd, Erin K. and Nakai, Jenny and Iwasaki, Yuriko and Sheehan, Anne and Mochizuki, Kimihiro and Wallace, Laura and Schwartz, Susan and Webb, Spahr and Henrys, Stuart}
}
