shear wave splitting
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Geodynamics of the Patagonian Slab Window constrained by Shear Wave Splitting and Seismic Imaging
<p>This thesis involves the study of crustal seismic anisotropy through shear wave splitting. For the past three decades, shear wave splitting (SWS) measurements from crustal earthquakes have been utilized as a technique to characterize seismic anisotropic structures and to infer in situ crustal properties such as the state of the stress and fracture geometry and density. However, the potential of this technique is yet to be realized in part because measurements on local earthquakes are often controlled and/or affected by physical mechanisms and processes which lead to variations in measurements and make interpretation difficult. Many studies have suggested a variety of physical mechanisms that control and/or affect SWS measurements, but few studies have quantitatively tested these suggestions. This thesis seeks to fill this gap by investigating what controls crustal shear-wave splitting (SWS) measurements using empirical and numerical simulation approaches, with the ultimate aim of improving SWS interpretation. For our empirical approach, we used two case studies to investigate what physical processes control seismic anisotropy in the crust at different scales and tectonic settings. In the numerical simulation test, we simulate the propagation of seismic waves in a variety of scenarios. We begin by measuring crustal anisotropy via SWS analysis around central New Zealand, where clusters of closely-spaced earthquakes have occurred. We used over 40,000 crustal earthquakes across 36 stations spanning close to 5.5 years between 2013 and 2018 to generate the largest catalog of high-quality SWS measurements (~102,000) around the Marlborough and Wellington region. The size of our SWS catalog allowed us to perform a detailed systematic analysis to investigate the processes that control crustal anisotropy and we also investigated the spatial and temporal variation of the anisotropic structure around the region. We observed a significant spatial variation of SWS measurements in Central New Zealand. We found that the crustal anisotropy around Central New Zealand is confined to the upper few kilometers of the crust, and is controlled by either one mechanism or a combination of more than one (such as structural, tectonic stresses, and gravitational stresses). The high correspondence between the orientation of the maximum horizontal compressive stress calculated from gravitational potential energy from topography and average fast polarization orientation around the Kaikōura region suggests that gravitationally induced stresses control the crustal anisotropy in the Kaikōura region. We suggest that examining the effect of gravitational stresses on crustal seismic anisotropy should not be neglected in future studies. We also observed no significant temporal changes in the state of anisotropy over the 5.5 year period despite the occurrence of significant seismicity. For the second empirical study, we characterized the anisotropic structure of a fault approaching failure (the Alpine Fault of New Zealand). We performed detailed SWS analysis on local earthquakes that were recorded on a dense array of 159 three-component seismometers with inter-station spacing about 30 m around the Whataroa Valley, New Zealand. The SWS analysis of data from this dense deployment enabled us to map the spatial characteristics of the anisotropic structure and also to investigate the mechanisms that control anisotropy in the Whataroa valley in the vicinity of the Alpine Fault. We observed that the orientation of the fast direction is parallel to the strike of the Alpine Fault trace and the orientations of the regional and borehole foliation planes. We also observed that there was no significant spatial variation of the anisotropic structure as we move across the Alpine Fault trace from the hanging wall to the footwall. We inferred that the geological structures, such as the Alpine Fault fabric and foliations within the valley, are the main mechanisms that control the anisotropic structure in the Whataroa valley. For our numerical simulation approach, we simulate waveforms propagating through an anisotropic media (using both 1-D and 3-D techniques). We simulate a variety of scenarios, to investigate how some of the suggested physical mechanisms affect SWS measurements. We considered (1) the effect on seismic waves caused by scatterers along the waves' propagation path, (2) the effect of the earthquake source mechanism, (3) the effect of incidence angle of the incoming shear wave. We observed that some of these mechanisms (such as the incidence angle of the incoming shear wave and scatterers) significantly affect SWS measurement while others such as earthquake source mechanisms have less effect on SWS measurements. We also observed that the effect of most of these physical mechanisms depends on the wavelength of the propagating shear wave relative to the size of the features. There is a significant effect on SWS measurements if the size of the physical mechanism (such as scatterers) is comparable to the wavelength of the incoming shear wave. With a larger wavelength, the wave treats the feature as a homogeneous medium.</p>
<p>This thesis involves the study of crustal seismic anisotropy through shear wave splitting. For the past three decades, shear wave splitting (SWS) measurements from crustal earthquakes have been utilized as a technique to characterize seismic anisotropic structures and to infer in situ crustal properties such as the state of the stress and fracture geometry and density. However, the potential of this technique is yet to be realized in part because measurements on local earthquakes are often controlled and/or affected by physical mechanisms and processes which lead to variations in measurements and make interpretation difficult. Many studies have suggested a variety of physical mechanisms that control and/or affect SWS measurements, but few studies have quantitatively tested these suggestions. This thesis seeks to fill this gap by investigating what controls crustal shear-wave splitting (SWS) measurements using empirical and numerical simulation approaches, with the ultimate aim of improving SWS interpretation. For our empirical approach, we used two case studies to investigate what physical processes control seismic anisotropy in the crust at different scales and tectonic settings. In the numerical simulation test, we simulate the propagation of seismic waves in a variety of scenarios. We begin by measuring crustal anisotropy via SWS analysis around central New Zealand, where clusters of closely-spaced earthquakes have occurred. We used over 40,000 crustal earthquakes across 36 stations spanning close to 5.5 years between 2013 and 2018 to generate the largest catalog of high-quality SWS measurements (~102,000) around the Marlborough and Wellington region. The size of our SWS catalog allowed us to perform a detailed systematic analysis to investigate the processes that control crustal anisotropy and we also investigated the spatial and temporal variation of the anisotropic structure around the region. We observed a significant spatial variation of SWS measurements in Central New Zealand. We found that the crustal anisotropy around Central New Zealand is confined to the upper few kilometers of the crust, and is controlled by either one mechanism or a combination of more than one (such as structural, tectonic stresses, and gravitational stresses). The high correspondence between the orientation of the maximum horizontal compressive stress calculated from gravitational potential energy from topography and average fast polarization orientation around the Kaikōura region suggests that gravitationally induced stresses control the crustal anisotropy in the Kaikōura region. We suggest that examining the effect of gravitational stresses on crustal seismic anisotropy should not be neglected in future studies. We also observed no significant temporal changes in the state of anisotropy over the 5.5 year period despite the occurrence of significant seismicity. For the second empirical study, we characterized the anisotropic structure of a fault approaching failure (the Alpine Fault of New Zealand). We performed detailed SWS analysis on local earthquakes that were recorded on a dense array of 159 three-component seismometers with inter-station spacing about 30 m around the Whataroa Valley, New Zealand. The SWS analysis of data from this dense deployment enabled us to map the spatial characteristics of the anisotropic structure and also to investigate the mechanisms that control anisotropy in the Whataroa valley in the vicinity of the Alpine Fault. We observed that the orientation of the fast direction is parallel to the strike of the Alpine Fault trace and the orientations of the regional and borehole foliation planes. We also observed that there was no significant spatial variation of the anisotropic structure as we move across the Alpine Fault trace from the hanging wall to the footwall. We inferred that the geological structures, such as the Alpine Fault fabric and foliations within the valley, are the main mechanisms that control the anisotropic structure in the Whataroa valley. For our numerical simulation approach, we simulate waveforms propagating through an anisotropic media (using both 1-D and 3-D techniques). We simulate a variety of scenarios, to investigate how some of the suggested physical mechanisms affect SWS measurements. We considered (1) the effect on seismic waves caused by scatterers along the waves' propagation path, (2) the effect of the earthquake source mechanism, (3) the effect of incidence angle of the incoming shear wave. We observed that some of these mechanisms (such as the incidence angle of the incoming shear wave and scatterers) significantly affect SWS measurement while others such as earthquake source mechanisms have less effect on SWS measurements. We also observed that the effect of most of these physical mechanisms depends on the wavelength of the propagating shear wave relative to the size of the features. There is a significant effect on SWS measurements if the size of the physical mechanism (such as scatterers) is comparable to the wavelength of the incoming shear wave. With a larger wavelength, the wave treats the feature as a homogeneous medium.</p>
<p>In order to investigate the cracks/fractures in the geothermal fields of Rotokawa and Ngatamariki, we measure seismic anisotropy across both fields and interpret the results in the context of stress aligned microcracks. Cracks aligned perpendicular to the direction of maximum horizontal stress close and their fluid is forced into cracks aligned with maximum horizontal stress (SHmax). Seismic anisotropy is the directional dependence of a seismic wave's velocity and provides a measure of crack orientation and density. To measure seismic anisotropy we conduct shear wave splitting measurements on 52,000 station-earthquake pairs across both Rotokawa and Ngatamariki from earthquakes recorded during 2015. Both fields are the subject of other geophysical and geological studies. Thus they are excellent subjects for studying seismic anisotropy. We cluster our measurements by their station-event path and fit the parameters from these clusters to those from theoretical crack planes. We also apply 2-D tomography to shear wave splitting time delays (𝛿t) and spatial averaging to shear wave splitting fast polarisations (∅). In addition, we compare time delays with P-wave to S-wave velocity ratios (νP / vS). Local measurements of stress within Rotokawa and regional measures of stress within the Taupo Volcanic Zone provide a comparison for the shear wave splitting measurements. We measure ∅ which agrees with the NE-SW regional direction of SHmax across Ngatamariki and parts of Rotokawa. Within Rotokawa, we observe a rotation of ∅ away from NE-SW toward N-S that agrees with borehole measurements of direction of SHmax of 023° and 030°. Spatial averaging of ∅ reveals mean orientations close to the strike of nearby active faults. The theoretical crack planes, that fit best to the shear wave splitting measurements, correspond to aligned cracks striking 045° outside of both fields, 035° within Ngatamariki, and 035° through to 0° within Rotokawa. The average percent anisotropy for the full dataset, approximately 4%, is close to the upper bound for an intact rock. Delay time tomography shows regions of higher delay time per kilometre of path length (s=km) within both fields and possibly associated with the production field fault in Rotokawa. vP =vS shows a wide range of normally distributed values, from 1.1 through to 2.4 with a mean of 1.6, indicating a mixture of gas filled and saturated cracks. A positive correlation between delay time per kilometre (𝛿tpkm) and νP /νS indicates that the majority of the cracks are saturated.</p>
<p>In order to investigate the cracks/fractures in the geothermal fields of Rotokawa and Ngatamariki, we measure seismic anisotropy across both fields and interpret the results in the context of stress aligned microcracks. Cracks aligned perpendicular to the direction of maximum horizontal stress close and their fluid is forced into cracks aligned with maximum horizontal stress (SHmax). Seismic anisotropy is the directional dependence of a seismic wave's velocity and provides a measure of crack orientation and density. To measure seismic anisotropy we conduct shear wave splitting measurements on 52,000 station-earthquake pairs across both Rotokawa and Ngatamariki from earthquakes recorded during 2015. Both fields are the subject of other geophysical and geological studies. Thus they are excellent subjects for studying seismic anisotropy. We cluster our measurements by their station-event path and fit the parameters from these clusters to those from theoretical crack planes. We also apply 2-D tomography to shear wave splitting time delays (𝛿t) and spatial averaging to shear wave splitting fast polarisations (∅). In addition, we compare time delays with P-wave to S-wave velocity ratios (νP / vS). Local measurements of stress within Rotokawa and regional measures of stress within the Taupo Volcanic Zone provide a comparison for the shear wave splitting measurements. We measure ∅ which agrees with the NE-SW regional direction of SHmax across Ngatamariki and parts of Rotokawa. Within Rotokawa, we observe a rotation of ∅ away from NE-SW toward N-S that agrees with borehole measurements of direction of SHmax of 023° and 030°. Spatial averaging of ∅ reveals mean orientations close to the strike of nearby active faults. The theoretical crack planes, that fit best to the shear wave splitting measurements, correspond to aligned cracks striking 045° outside of both fields, 035° within Ngatamariki, and 035° through to 0° within Rotokawa. The average percent anisotropy for the full dataset, approximately 4%, is close to the upper bound for an intact rock. Delay time tomography shows regions of higher delay time per kilometre of path length (s=km) within both fields and possibly associated with the production field fault in Rotokawa. vP =vS shows a wide range of normally distributed values, from 1.1 through to 2.4 with a mean of 1.6, indicating a mixture of gas filled and saturated cracks. A positive correlation between delay time per kilometre (𝛿tpkm) and νP /νS indicates that the majority of the cracks are saturated.</p>
<p>This thesis is concerned with scrutinising the source, distribution and detectability of seismic velocity phenomena that may be used as proxies to study conditions in the crust. Specifically, we develop modelling techniques in order to analyse the directional variation of seismic wave speed in the crust and test them at Mt. Asama in Japan and Canterbury, New Zealand. We also implement both active source and noise interferometry to identify velocity variations at Mt. Ruapehu, New Zealand. Observations of temporal variation of anisotropic seismic velocity parameters at Asama volcano in Japan indicate that there is some process (or processes) affecting anisotropy, attributed to closure of microcracks in the rock as it is subjected to volcanic stress in the crust. To test this assertion, a 3D numerical model is created incorporating volcanic stress, ray tracing and estimation of the anisotropy to produce synthetic shear wave splitting results using a dyke stress model. Anisotropy is calculated in two ways; by considering a basic scenario where crack density is uniform and a case where the strength of anisotropy is related to dry crack closure from deviatoric stress. We find that the approach is sensitive to crack density, crack compliance, and the regional stress field. In the case of dry crack closure, modelled stress conditions produce a much smaller degree of anisotropy than indicated by measurements. We propose that the source of anisotropy changes at Asama is tied to more complex processes that may precipitate from stress changes or other volcanic processes, such as the movement of pore fluid. We develop a generalised anisotropy inversion model based on the linearised, iterative least-squares inversion technique of Abt and Fischer [2008]. The model is streamlined for use with results from the MFAST automatic shear wave splitting software [Savage et al., 2010]. The method iteratively solves for the best fitting magnitude and orientation of anisotropy in each element of the model space using numerically calculated partial derivatives. The inversion is applied to the Canterbury plains in the region surrounding the Greendale fault, using shear-wave splitting data from the 2010 Darfield earthquake sequence. Crustal anisotropy is resolved down to a depth of 20 km at a spatial resolution of 5 km, with good resolution near the Greendale fault. We identify a lateral variation in anisotropy strength across the Greendale fault, possibly associated with post-seismic stress changes. We perform active source and noise interferometry at Ruapehu in order to investigate potential seismic velocity changes and assess their use as a possible eruption forecasting method. Six co-located 100 kg ammonium nitrate fuel oil explosives were set off serially at Lake Moawhango, situated approximately 20 km south-east of Mount Ruapehu. Two methods of interferometry, using moving window cross correlation in the time and frequency domains, respectively, were applied to the recorded signal from each explosion pair in order to determine velocity changes from the signal coda waves. We identify possible diurnal velocity variations of ~ 0:7% associated with strain caused by the solid Earth tide. Synthetic testing of velocity variation recoverability was also performed using both methods. Interferometry of noise cross-correlations during the period was also performed using moving window cross correlation in the frequency domain. Analysis of velocity variations in the ZZ, RR and TT component pairs show little coherency. This, combined with results from synthetic testing that show that the frequency domain interferometry technique employed is unstable above velocity variations of 0.1%, indicate that the method may not be suitable for determining velocity variations at Ruapehu.</p>
<p>This thesis is concerned with scrutinising the source, distribution and detectability of seismic velocity phenomena that may be used as proxies to study conditions in the crust. Specifically, we develop modelling techniques in order to analyse the directional variation of seismic wave speed in the crust and test them at Mt. Asama in Japan and Canterbury, New Zealand. We also implement both active source and noise interferometry to identify velocity variations at Mt. Ruapehu, New Zealand. Observations of temporal variation of anisotropic seismic velocity parameters at Asama volcano in Japan indicate that there is some process (or processes) affecting anisotropy, attributed to closure of microcracks in the rock as it is subjected to volcanic stress in the crust. To test this assertion, a 3D numerical model is created incorporating volcanic stress, ray tracing and estimation of the anisotropy to produce synthetic shear wave splitting results using a dyke stress model. Anisotropy is calculated in two ways; by considering a basic scenario where crack density is uniform and a case where the strength of anisotropy is related to dry crack closure from deviatoric stress. We find that the approach is sensitive to crack density, crack compliance, and the regional stress field. In the case of dry crack closure, modelled stress conditions produce a much smaller degree of anisotropy than indicated by measurements. We propose that the source of anisotropy changes at Asama is tied to more complex processes that may precipitate from stress changes or other volcanic processes, such as the movement of pore fluid. We develop a generalised anisotropy inversion model based on the linearised, iterative least-squares inversion technique of Abt and Fischer [2008]. The model is streamlined for use with results from the MFAST automatic shear wave splitting software [Savage et al., 2010]. The method iteratively solves for the best fitting magnitude and orientation of anisotropy in each element of the model space using numerically calculated partial derivatives. The inversion is applied to the Canterbury plains in the region surrounding the Greendale fault, using shear-wave splitting data from the 2010 Darfield earthquake sequence. Crustal anisotropy is resolved down to a depth of 20 km at a spatial resolution of 5 km, with good resolution near the Greendale fault. We identify a lateral variation in anisotropy strength across the Greendale fault, possibly associated with post-seismic stress changes. We perform active source and noise interferometry at Ruapehu in order to investigate potential seismic velocity changes and assess their use as a possible eruption forecasting method. Six co-located 100 kg ammonium nitrate fuel oil explosives were set off serially at Lake Moawhango, situated approximately 20 km south-east of Mount Ruapehu. Two methods of interferometry, using moving window cross correlation in the time and frequency domains, respectively, were applied to the recorded signal from each explosion pair in order to determine velocity changes from the signal coda waves. We identify possible diurnal velocity variations of ~ 0:7% associated with strain caused by the solid Earth tide. Synthetic testing of velocity variation recoverability was also performed using both methods. Interferometry of noise cross-correlations during the period was also performed using moving window cross correlation in the frequency domain. Analysis of velocity variations in the ZZ, RR and TT component pairs show little coherency. This, combined with results from synthetic testing that show that the frequency domain interferometry technique employed is unstable above velocity variations of 0.1%, indicate that the method may not be suitable for determining velocity variations at Ruapehu.</p>
<p>Seismic anisotropy in the transpressional plate-boundary zone in New Zealand is investigated with shear-wave splitting to gain insights into lithospheric deformation and mantle flow. Constraints on plate-boundary deformation in the lithosphere of the oblique-collision and subduction regimes in South Island have been estimated from the local and regional shear-wave splitting parameters that are made at both inland and offshore seismographs. Mantle and lithospheric anisotropy of the southernmost Hikurangi subduction zone in the southern North Island is examined from SKS, ScS and teleseismic S-phases. The splitting of these phases measured on a recent transect crossing the Wellington region is analyzed to understand the lateral anisotropic structure of the fore-arc Hikurangi subduction zone. Local and regional splitting reveal both laterally and depth varying anisotropy in South Island. The scatter in splitting parameters at individual stations suggests the splitting of high-frequency S-phases is mainly controlled by heterogeneous anisotropic structure and S-wave propagation direction within those heterogeneities. When the average results are examined as a whole through 2-D delay time tomographic inversion and spatial averaging, consistent patterns in delay times and fast azimuths exist. Spatially averaged fast azimuths indicate a localized high strain zone in the southern central region of the South Island. Based on fast azimuths observed above 100 km depth, we suggest that the plate-boundary sub-parallel anisotropy that is produced by pervasive shear is mainly distributed within a zone extending ~130 km SE of the Alpine fault in the southern South Island and is widely distributed (at least 200 km wide) in the northern South Island. Average station delay times (δt) of ~0.1 - 0.4 s compared to 1.7 s SKS δt from previous studies in South Island further suggest a deep seated anisotropic zone or sensitivity of S-wave splitting to the layered and/or heterogeneous anisotropic structure of the plate-boundary zone in the inland South Island. The heterogeneous anisotropic structure further suggests that the lithosphere is not only characterized by the plate-boundary parallel shear related to Cenozoic deformation, but is also affected by anisotropic imprints from the other tectonic episodes and anisotropy that is governed by the contemporary stress. A shear-wave splitting anisotropy investigation in the offshore South Island regions is an extended study of the inland experiment and aims to provide a broad-scale understanding of the plate-boundary deformation. Individual splitting of local and regional S-phases yield a range of δt that varies between very small δt (~0.02 s), which may represent a nearly isotropic medium, and large δt (~0.6 s), which corresponds to lithospheric anisotropy. The average station δt of ~0.25 s and variable delays of the individual splitting measurements imply that the observed splitting is most likely controlled by the geometry of the ray paths. Long ray paths that are detected at the stations further away from the plate-boundary appear to penetrate to deeper lithosphere and capture a significant portion of the upper-mantle anisotropy to produce fast azimuths parallel to the plate-boundary shear (NE-SW). Thus, the long and deep ray paths respond to the deeper structure, but may not be re-split by the upper-most crustal structures. However, the observed variable delays suggest that changes in ray propagation direction with respect to the orientation of symmetry axes of the anisotropic media may have an effect on the measured anisotropy. Offshore measurements that are close to the land are consistent with the inland measurements and appear to be controlled by the regional stress field. This implies that short and shallow ray paths are mostly sensitive to the crustal anisotropy. The uneven distribution of ray paths from the shallow and deep events, therefore, plays a dominant role in controlling the observed splitting depending on their depth sensitivity and/or extent of anisotropy. Consequently, when fast directions are spatially averaged along with the inland measurements consistent patterns appear to correlate with the possible depth contribution of anisotropy in the region. We are unable to provide accurate constraints on the offshore extent of plate-boundary parallel shear because of the shallow stress-controlled anisotropy that likely overlies the mantle-shear zone. However, the splitting parameters from long and deep ray paths suggest a deep-seated, plate-boundary sub-parallel shear in a broad zone at least in the northern and upper-central South Island. Mantle anisotropy detected from teleseismic earthquakes recorded across the southern North Island displays NE-SW fast axis alignment, consistent with the strike of the Hikurangi trench and the predominant upper-plate faulting trends, with a range of δt (~0.5 - 3.0 s) and small-scale variation in NE-SW fast azimuths. When combined with the previous measurements in the western side of the array, δt from long period (>7 s) S-phases indicate an abrupt lateral variation across the fore-arc Hikurangi subduction zone. This lateral variation together with frequency dependence suggest that the shear wave splitting in the fore-arc of the Hikurangi subduction zone in the southern North Island is governed in part by the laterally varying crustal contribution of anisotropy or isotropic velocity variations within the shallow crust. Frequency dependent splitting also suggests that the anisotropic structure is governed by either multilayer or more complex anisotropy perhaps due to the combined effects of laterally varying multilayer structure. If the variations are due to lateral changes in crustal anisotropy, then mantle and crustal deformation are most likely coupled in the east of the Wairarapa fault where there is a possibility of strong crustal contribution.</p>
