[S-CG39] Science of slow earthquakes: Toward unified understandings of whole earthquake process

[SCG39-29] Temporal velocity variations in the northern Hikurangi margin and their relation to slow slip

*Weiwei Wang¹、Martha Savage¹、Alec Yates^1,2、Hubert Zal¹、Carolyn Boulton¹、Megan Kortink³、Tim Stern¹、Bill Fry³、Kimihiro Mochizuki⁴、Spahr Webb⁵、Wallace Laura³ (1.Victoria University of Wellington、2.Université Grenoble Alpes, Université Savoie Mont Blanc、3.GNS Science、4.Earthquake Research Institute, University of Tokyo、5.Lamont-Doherty Earth Observatory, Columbia University)

キーワード：slow slip event, the Hikurangi subduction zone, seismic velocity variations, ambient noise

Slow Slip Events (SSE) have been studied in increasing detail over the last 20 years, improving our understanding of subduction zone behavior. 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 coda wave interferometry to the 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 and a period band of 2.5-14 s is used to calculate velocity variations, which focuses on the upper plate in our study region. The average velocity variations display a velocity decrease of approximate 0.05% during the SSE, followed by a velocity increase of approximate 0.05% afterward. We suggest that two possibilities exist to explain this: The first 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 above the SSE source (seismic velocity increase 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. The second possibility, which has been suggested by other seismological observations is that fluid migration into the upper plate during the SSE, due to breakage of a low-permeability seal on the plate boundary caused by the SSE, explains the seismic velocity decrease during the SSE. Under this model, after the SSE, the fluids in the upper plate diffuse gradually and the velocity increases again.