Studies of Seismic Velocities in Subduction Zones from Continuous Ocean Bottom Seismometer Data
Authors:   , Wang Weiwei 
Published: January 2022

<p><b>This thesis uses continuous ambient noise data recorded by Ocean Bottom Seismometers (OBSs) to study seismic velocities in the upper crust of the overriding plate. The first and second projects (Chapters 3 and 4) focus on temporal seismic velocity variations in the northern Hikurangi subduction zone offshore the North Island, New Zealand, while the third project (Chapter 5) focuses on shear wave velocities in the southwestern Okinawa Trough offshore northeastern Taiwan. In the first project (Chapters 3), we investigate a region of frequent slow slip events (SSEs) offshore Gisborne, North Island, New Zealand. From September to October 2014, an SSE occurred with a slip over 250 mm and was recorded successfully by the Hikurangi Ocean Bottom Investigation of Tremor and Slow Slip deployment II (HOBITSS II). We apply coda wave interferometry on the ambient noise data acquired by nine OBSs deployed by the HOBITSS II to study the seismic velocity variations related to the SSE. The average velocity variations display a decrease on the order of 0.05% during the SSE, followed by an increase of similar magnitude afterwards. Two hypotheses are proposed to explain our observation. The first hypothesis, which has been suggested by previous studies, considers that the velocity decrease during the SSE is caused by more fluids migrating into the upper plate as the SSE breaks a low-permeability seal on the plate boundary. After the SSE, the fluids in the upper plate diffuse gradually and the velocity increases; The second hypothesis is that before the SSE, elastic strain accumulates causing contraction and reduction of porosity and therefore increase of velocity (the velocity increase between SSEs). During the SSE, the velocity decrease is caused by increased porosity as the SSE relieves the accumulated elastic strain on the plate interface, which results in dilation. After the SSE, stress and strain accumulate again, causing a porosity decrease and a velocity increase back to the original value. This study demonstrates that the velocity variations related to SSEs are observable and provides evidence for slow slip mechanism hypotheses.</b></p>
<p>The second project (Chapter 4) focuses on the temporal seismic velocity variations associated with an SSE in 2019 offshore Gisborne, North Island, New Zealand. This is a later SSE in the same area as the first project (Chapters 3). Based on the success of the HOBITSS II, more ocean bottom instruments were deployed in the northern Hikurangi subduction zone from 2018 to 2019 (HOBITSS V). An SSE lasting approximately one month from the end of March to the beginning of May 2019 occurred during the deployment and was recorded by the network. The main slip was south of the deployment and the slip beneath the deployment was up to 150 mm. This study applies coda wave interferometry on the ambient noise data acquired by five OBSs and computes seismic velocity variations to investigate their relation to the SSE. A velocity decrease on the order of 0.015% during the SSE and an increase back to the original velocity value are observed at 1–2.5 s. This supports the two hypotheses proposed in Chapters 3: fluid migration and/or stain changes through the SSE cycle. In addition, velocity variations computed from individual stations show velocity increases before the SSE, which are destructively interfered in their average. Such a situation could occur if the SSE migrated across the network. If the velocity increases before the SSE from individual stations are real, they can be only explained by the hypothesis of crustal strain changes (the second hypothesis in project 1). However, fluid migration (the first hypothesis in project 1) may still happen concomitantly.</p>
<p>The third project focuses on the tectonics in southwestern Okinawa Trough offshore northeastern Taiwan. The southwestern Okinawa Trough is an active back-arc basin, extending and rifting within the continental lithosphere. The tectonic development of the back-arc basin is still not well-understood. This study uses continuous ambient noise data recorded by 34 OBSs deployed by Academia Sinica at various periods from 2010 to 2018. Cross-correlations on vertical seismic components and pressure gauges are computed to construct Rayleigh/Scholte waves to study the shear wave velocity structure in the southwestern Okinawa Trough. Phase velocities are measured from the Rayleigh/Scholte waves. Shear velocities are inverted from the phase velocities. Results show the velocity in the south of the back-arc rifting axis near the axis is slower than the velocity in the north of the rifting axis, suggesting the velocity structure in the southwestern Okinawa Trough is asymmetric along the rifting axis. Previous studies have shown high heat flows (about 110mW/m 2 on average) in the south of the rifting axis. The low velocity in the south could be caused by the high heat flow that may be related to asymmetric back-arc extension and/or rifting. This study presents the shear wave velocity structure in the southwest Okinawa Trough is asymmetric along the rifting axis, which implies the back-arc extending/rifting is asymmetric in the study region. This study also suggests effective techniques for OBS noise corrections and unwrapping the cycle skipping of phase velocity measurements.</p>
<p>In summary, this thesis represents three projects focusing on seismic velocities in two subduction zones using ambient noise data collected by OBSs. The first and second projects study the temporal velocity variations and the relation to SSEs. Both studies observe velocity decreases during the SSEs and increases after the SSEs, supporting two hypotheses of fluid migration and/or stain changes through the SSE cycle. The third project finds the shear velocity structure in the southwestern Okinawa Trough is asymmetric along the rifting center, which may imply the back-arc extension is asymmetric.</p>