Seismic Anisotropy at the Rotokawa and Ngatamariki Geothermal Fields in the Taupo Volcanic Zone

Mapping Intimacies ◽

10.26686/wgtn.17141978 ◽

2021 ◽

Author(s):

◽

Stefan Mroczek

Keyword(s):

Shear Wave ◽

Delay Time ◽

Seismic Anisotropy ◽

Horizontal Stress ◽

Shear Wave Splitting ◽

Taupo Volcanic Zone ◽

Volcanic Zone ◽

Geothermal Fields ◽

Wave Splitting ◽

Maximum Horizontal Stress

<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>

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Seismic Anisotropy at the Rotokawa and Ngatamariki Geothermal Fields in the Taupo Volcanic Zone

10.26686/wgtn.17141978.v1 ◽

2021 ◽

Author(s):

◽

Stefan Mroczek

Keyword(s):

Shear Wave ◽

Delay Time ◽

Seismic Anisotropy ◽

Horizontal Stress ◽

Shear Wave Splitting ◽

Taupo Volcanic Zone ◽

Volcanic Zone ◽

Geothermal Fields ◽

Wave Splitting ◽

Maximum Horizontal Stress

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Stress filed data visualization by using earthquake focal mechanism and shear wave splitting results in the Makran region northern Iran

Nexo Revista Científica ◽

10.5377/nexo.v34i02.11570 ◽

2021 ◽

Vol 34 (02) ◽

pp. 825-834

Author(s):

Shahrokh Pourbeyranvand

Keyword(s):

Shear Wave ◽

Data Visualization ◽

Focal Mechanism ◽

Horizontal Stress ◽

Shear Wave Splitting ◽

Northern Iran ◽

Earthquake Focal Mechanism ◽

Study Results ◽

Wave Splitting ◽

Maximum Horizontal Stress

In this study, a new method is introduced for stress data visualization. As an example, the SHmax variations in the Makran region are mapped by using this new approach. Maximum horizontal stress directions in the study area are extracted from earthquake focal mechanism data. The anisotropy study results in terms of shear wave splitting fast direction axes are added to the dataset to show its effect. The results show substantial variation in SHmax directions, which reflects the complicated tectonic nature of the region. Adding the shear wave splitting data improved the results' accuracy and showed the correlation between the two quantities in the study area.

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Seismic Anisotropy in the Northern Sumatra Region from Shear Wave Splitting

Indonesian Journal of Physics ◽

10.5614/itb.ijp.2015.26.2.3 ◽

2015 ◽

Vol 26 (2) ◽

pp. 31-35

Author(s):

Arya Dwi Candra ◽

Bagus Jaya Santosa

Keyword(s):

Shear Wave ◽

Delay Time ◽

Seismic Anisotropy ◽

Lower Layer ◽

Epicentral Distance ◽

Correlation Method ◽

Shear Wave Splitting ◽

Polarization Direction ◽

Wave Splitting ◽

Anisotropic Layers

The northern Sumatra consists of several tectonic segments, such as subduction zone, the Sumatra and Mentawai faults. An analysis that can be used to identify the tectonic segments, located beneath the northern Sumatra, is shear wave- splitting. The purpose of the analysis of shear-wave splitting is to monitor the anisotropic characteristics of the structure of the earth layers located beneath the northern Sumatra. The monitoring data were collected from 4 BMKG stations with the magnitude was more than 6.3 Mw and the the range of the epicentral distance was about 85̊-140̊. The data analysis was conducted by using Splitlab program based on rotation-correlation method. The result of the shear-wave splitting analysis shows that there are two anisotropic layers. The delay time found in the first layer is about 0,5-0,9 s, which is assumed that it occurs due to the Sumatran fault. Meanwhile, the delay time found in the second layer is about 1,4-1,8 s, which is assumed that it occurs due to the subduction plate movement on the upper mantle layer. The results of TPTI, TSI and TRSI stations has shown congruence, that is the polarization direction is parallel to the Sumatra fault on the upper layer and the polarization direction is perpendicular to the Sumatra fault on the lower layer. The PSI station shows the polarization direction is different from the other stations, in which they show the polarization direction is perpendicular to the Sumatra fault on the upper layer and the polarization direction is parallel to the Sumatra fault on the lower layer. The difference of the data processing in the PSI station, we assume, is caused by the presence of a complex layer beneath Toba caldera.

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Spatially Distinct Tectonic Zones across Oklahoma Inferred from Shear-Wave Splitting

Seismological Research Letters ◽

10.1785/0220200237 ◽

2021 ◽

Author(s):

Angie D. Ortega-Romo ◽

Jacob I. Walter ◽

Xiaowei Chen ◽

Brett M. Carpenter

Keyword(s):

Stress Field ◽

Shear Wave ◽

Structural Control ◽

Horizontal Stress ◽

Shear Wave Splitting ◽

Wave Splitting ◽

Stress Orientations ◽

Maximum Horizontal Stress ◽

Direction Of Polarization ◽

Fast Polarization

Abstract To better understand relationships among crustal anisotropy, fracture orientations, and the stress field in Oklahoma and southern Kansas, we conduct shear-wave splitting analysis on the last 9 yr of data (2010–2019) of local earthquake observations. Rather than a predominant fast direction (ϕ), we find that most stations have a primary fast direction of polarization (ϕpri) and a secondary fast direction of polarization (ϕsec). At most stations, either the primary fast direction of polarization (ϕpri) or the secondary fast direction of polarization (ϕsec) is consistent with the closest estimated maximum horizontal stress (σHmax) orientation in the vicinity of the observation. The general agreement between fast directions of polarization (ϕ) and the maximum horizontal stress orientations (σHmax) at the regional level implies that the fast polarization directions (ϕ) are extremely sensitive to the regional stress field. However, in some regions, such as the Fairview area in western Oklahoma, we observe discrepancies between fast polarization directions (ϕ) and maximum horizontal stress orientations (σHmax), in which the fast directions are more consistent with local fault structures. Overall, the primary fast direction of polarization (ϕpri) is mostly controlled and influenced by the stress field, and the secondary fast direction of polarization (ϕsec) likely has some geologic structural control because the secondary direction is qualitatively parallel to some mapped north-striking fault zones. No significant changes in fast directions over time were detected with this technique over the 5 yr (2013–2018) of measurements, suggesting that pore pressure may not cause a significant enough or detectable change above the magnitude of the background stress field.

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Seismic anisotropy and mantle flow constrained by shear wave splitting in central Myanmar

Journal of Geophysical Research Solid Earth ◽

10.1029/2021jb022144 ◽

2021 ◽

Author(s):

Enbo Fan ◽

Yumei He ◽

Yinshuang Ai ◽

Stephen S. Gao ◽

Kelly H. Liu ◽

...

Keyword(s):

Shear Wave ◽

Seismic Anisotropy ◽

Shear Wave Splitting ◽

Mantle Flow ◽

Wave Splitting

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Seismic anisotropy of the crust and upper mantle beneath western Tibet revealed by shear wave splitting measurements

Geophysical Journal International ◽

10.1093/gji/ggy448 ◽

2018 ◽

Vol 216 (1) ◽

pp. 535-544 ◽

Cited By ~ 4

Author(s):

Changhui Ju ◽

Junmeng Zhao ◽

Ning Huang ◽

Qiang Xu ◽

Hongbing Liu

Keyword(s):

Shear Wave ◽

Upper Mantle ◽

Seismic Anisotropy ◽

Shear Wave Splitting ◽

Crust And Upper Mantle ◽

Wave Splitting

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Measuring Crustal Seismic Anisotropy through Shear Wave Splitting

10.26686/wgtn.17151191 ◽

2021 ◽

Author(s):

◽

Kenny Graham

Keyword(s):

Numerical Simulation ◽

New Zealand ◽

Shear Wave ◽

Seismic Anisotropy ◽

Shear Wave Splitting ◽

Anisotropic Structure ◽

Crustal Anisotropy ◽

Physical Mechanisms ◽

Alpine Fault ◽

Wave Splitting

<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>

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