Studies of Seismic Velocities in Subduction Zones from Continuous Ocean Bottom Seismometer Data

<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>

Seismic Evidence of Pop-up Tectonics Beneath the Shillong Plateau Area of Northeast India

Our detailed analysis of high-quality arrival time data recorded by the local seismographic network using three-dimensional seismic tomography of the Shillong Plateau region using high-quality arrival times of the body wave phases recorded at a dense temporary seismic network. This technique is used to understand the heterogeneities of the crust and its implications for pop-up tectonics characterizing evaluation the of the Shillong Plateau. We investigated an area covering ~150 ×100 km2 that revealed seismicity to be confined in a depth range ≤ 60 km. High - velocity anomalies in the upper crust appear to be responsible for intense small to moderate seismic activity in the region. Crustal seismic velocities inferred from 3-D seismic tomography showed significant lateral heterogeneities beneath the lithosphere of the Shillong Plateau. High-velocity anomalies in the uppermost crust, interpreted as the Shillong Plateau act as a geometric asperity where interseismic strain may accumulate. Low-velocity anomalies in the lower crust probably play a major role to accommodating the stresses generated due to plate separation, culminating in future sources of great earthquakes. The geological faults are well imaged in the cross-sections and support the concept of Pop-up tectonics beneath the Shillong of NE India.

Studies of Seismic Velocities in Subduction Zones from Continuous Ocean Bottom Seismometer Data

<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>

Studies of Seismic Velocities in Subduction Zones from Continuous Ocean Bottom Seismometer Data

<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>

Leveraging active-source seismic data in mining tailings: Refraction and MASW analysis, elastic parameters, and hydrogeological conditions

We applied active-source seismic method for the interpretation of elastic parameters in tailings facilities which is essential for evaluating stability and seismic response. The methodology uses different analysis methods on the same dataset, i.e., conventional seismic refraction (SR) to determine compressional-wave velocity (Vp) and multichannel analysis of surface wave (MASW) to estimate shear-wave velocity (Vs). Seismic velocities in conjunction with tailings physics approach revealed interpretable data in terms of elastic parameters and hydrogeological conditions. The results determined the empirical linear relationships between Vp and Vs that are particular to an unconsolidated media such as tailings and showed that variability of hydrogeological conditions influences the elastic seismic response (Vp and Vs) and the elastic parameters. The analysis of the elastic parameters identified the state condition of the tailings at the time of the survey. The Bulk modulus K that relates the change in hydrostatic stress to the volumetric strain was predominant between 1.0−2.0 GPa. The Young’s modulus E in the tailings media was in the low range of 0.15−0.23 GPa. Poisson’s ratio values in all sections were in the upper limit in the range of 0.37−0.49, meaning that the tailings media is highly susceptible to transverse deformation under axial compression.

Rock Acoustics of Diagenesis and Cementation

We simulate the effects of diagenesis, cementation and compaction on the elastic properties of shales and sandstones with four different petro-elastical theories and a basin-evolution model, based on constant heating and sedimentation rates. We consider shales composed of clay minerals, mainly smectite and illite, depending on the burial depth, and the pore space is assumed to be saturated with water at hydrostatic conditions. Diagenesis in shale (smectite/illite transformation here) as a function of depth is described by a 5th-order kinetic equation, based on an Arrhenius reaction rate. On the other hand, quartz cementation in sandstones is based on a model that estimates the volume of precipitated quartz cement and the resulting porosity loss from the temperature history, using an equation relating the precipitation rate to temperature. Effective pressure effects (additional compaction) are accounted for by using Athy equation and the Hertz-Mindlin model. The petro-elastic models yield similar seismic velocities, despite the different level of complexity and physics approaches, with increasing density and seismic velocities as a function of depth. The methodology provides a simple procedure to obtain the velocity of shales and sandstones versus temperature and pressure due to the diagenesis-cementation-compaction process.

A novel principle to localize the sensitivity of waveform tomography using wave interferences at the observation boundary

When using waveform tomography to perform high-resolution imaging of a medium, it is vital to calculate the sensitivity in order to describe how well a model fits a given set of data and how the sensitivity changes with the spatial distribution of the heterogeneities. The traditional principle behind calculating the sensitivity—for detecting small changes—suffers from an inherent limitation in case other structures, not of interest, are present along the wave propagation path. We propose a novel principle that leads to enhanced localization of the sensitivity of the waveform tomography, without having to know the intermediate structures. This new principle emerges from a boundary integral representation which utilizes wave interferences observed at multiple points. When tested on geophysical acoustic wave data, this new principle leads to much better sensitivity localization and detection of small changes in seismic velocities, which were otherwise impossible. Overcoming the insensitivity to a target area, it offers new possibilities for imaging and monitoring small changes in properties, which is critical in a wide range of disciplines and scales.

Seismic Velocity Structure of the Shallow Part of the Alpine Fault and Gravity Study of the Basement Features in the Whataroa River Flood Plain, Central Westland, South Island

<p>The deep and middle sections of the Alpine fault have extensively been studied, however, the shallow part has had relatively minor geophysical attention. This study focuses on the basement geometry and the determination of the upper-crustal velocity structure of the Alpine fault in the vicinity of the Whataroa River flood plain in Central Westland, South Island. Data from a temporary gravity survey collected in November 2006, the GNS gravity database and four of the westernmost shot gathers from the SIGHT96's transect 1 were used for this project. A ray-tracing software was used to establish the velocity structure of the shallow part of the Alpine fault. Seismic velocities decrease to 3.8 km/s immediately southeast of the mylonite strip, which is adjacent to the Alpine fault's ramp heading towards the fault's surface trace from the southeast or from depth. Velocities of 5 km/s reach 2 km depth to the southeast of the Alpine fault's ramp. Results of the gravity and seismic models coincide in the positions and the dimensions of two northwest-orientated glacial overdeepings. The strike of their alignment is offset to the northeast by 3.5 km and is sub-parallel to the mouth of the Whataroa River. We propose that these kettle holes, thought to have been carved successively during the Waimea and Otira glaciations, are the beheaded river mouth of the Whataroa river. By supposing that the furthest kettle hole was carved during the Waimea glaciation, the 3.5 km offset thus corresponds to 140 Ka of dextral slip on the Alpine fault, we could approximate the mean displacement rate over the time interval of 140-18 Ka of 25 mm/yr.</p>