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

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.

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

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>

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

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.

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

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>

Shear Wave Splitting and the Features of the Crustal Stress Field in the Capital Circle

2006 ◽  
Vol 49 (1) ◽  
pp. 157-166 ◽  
Author(s):  
Yuan-Gen LAI ◽  
Qi-Yuan LIU ◽  
Jiu-Hui CHEN ◽  
Jie LIU ◽  
Shun-Cheng LI ◽  
...  
Keyword(s):  
Stress Field ◽  
Shear Wave ◽  
Shear Wave Splitting ◽  
Crustal Stress ◽  
Wave Splitting ◽  
Crustal Stress Field

Features of the Shear Wave Splitting and Stress Field in the Xinjiang-Jiashi Strong Earthquake Region

2002 ◽  
Vol 45 (1) ◽  
pp. 78-86 ◽  
Author(s):  
Yuan-Gen LAI ◽  
Qi-Yuan LIU ◽  
Jiu-Hui CHEN ◽  
Biao GUO ◽  
Shun-Cheng LI
Keyword(s):  
Stress Field ◽  
Shear Wave ◽  
Strong Earthquake ◽  
Shear Wave Splitting ◽  
Wave Splitting

Upper-Crustal Seismic Anisotropy in the Cantabrian Mountains (North Spain) from Shear-Wave Splitting and Ambient Noise Interferometry Analysis

2020 ◽  
Vol 92 (1) ◽  
pp. 421-436 ◽  
Author(s):  
Jorge Acevedo ◽  
Gabriela Fernández-Viejo ◽  
Sergio Llana-Fúnez ◽  
Carlos López-Fernández ◽  
Javier Olona
Keyword(s):  
Shear Wave ◽  
Ambient Noise ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Clear Correlation ◽  
Cantabrian Mountains ◽  
Noise Interferometry ◽  
Delay Times ◽  
Wave Splitting ◽  
Fast Polarization

Abstract The upper-crustal anisotropy of the Cantabrian Mountains (North Spain) has been investigated using two independent but complementary methodologies: (a) shear-wave splitting and (b) ambient seismic noise interferometry. For this purpose, we have processed and compared seismic data from two networks with different scales and recording periods. The shear-wave splitting results show delay times between 0.06 and 0.23 s and spatially variable fast-polarization directions. We calculate that the anisotropic layer has a maximum effective thickness of around 7.5 km and an average anisotropy magnitude of between 4% and 8%. Consistently, our ambient noise observations point to an anisotropy magnitude between 4% and 9% in the first 10 km of the crust. Our results show a clear correlation between the fast directions from both methods and the orientations of the local faults, suggesting that the anisotropy is mainly controlled by the structures. Furthermore, in the west of the study area, fast-polarization directions tend to align parallel to the Variscan fabric in the crust, whereas to the east, in which the Alpine imprint is stronger, many fast directions are aligned parallel to east–west-oriented Alpine features.

An upper crust shear-wave splitting study for the period 2013-2014 in the Western Gulf of Corinth (Greece)

2021 ◽  
Author(s):  
George Kaviris ◽  
Vasilis Kapetanidis ◽  
Georgios Michas ◽  
Filippos Vallianatos
Keyword(s):  
Stress Field ◽  
Shear Wave ◽  
Shear Wave Splitting ◽  
Double Difference ◽  
Extension Rate ◽  
Upper Crust ◽  
Inspection Method ◽  
Gulf Of Corinth ◽  
Waveform Cross Correlation ◽  
Wave Splitting

&lt;p&gt;Seismic anisotropy is investigated by performing an upper crust shear-wave splitting study in the Western Gulf of Corinth (WGoC). The study area, which is a tectonic rift located in Central Greece, is one of the most seismically active regions in Europe, characterized by a 10 to 15 mm/year extension rate in a NNW-SSE direction and E-W normal faulting. Intense seismic activity has been recorded in the WGoC during 2013-2014, including the 2013 Helike swarm, at the southern coast, and the offshore 2014 seismic sequence between Nafpaktos and Psathopyrgos, including an Mw 4.9 event on 21 September 2014. The largest event of the study period was an Mw 5.0 earthquake that occurred in November 2014, offshore Aigion, followed by an aftershock sequence. Seismicity was relocated using the double-difference method, including waveform cross-correlation differential travel-time data, yielding a high-resolution earthquake catalogue of approximately 9000 local events. This dataset was utilized in order to determine the shear-wave splitting parameters in seven stations installed at the WGoC, using a fully automatic technique based on the eigenvalue method and cluster analysis. A smaller subset was analyzed with the visual inspection method (polarigrams and hodograms) for verification of the automatic measurements. All selected station-event pairs were within the shear-wave window and had adequately high signal-to-noise ratio. The orientation of the seismometers of all stations used in the present study has been measured and verified in order to ensure the validity of the obtained fast shear-wave polarization directions and to apply corrections for borehole instruments. Mean anisotropy directions are in general agreement with the horizontal component of the dominant stress field, with some deviations, likely related to mapped faults and local stress anomalies. Temporal variations of time-delays between the two split shear-waves are examined in order to investigate their connection to possible stress field variations, related either to the occurrence of moderate to strong events or to fluid migration.&lt;/p&gt;&lt;p&gt;Acknowledgements&lt;/p&gt;&lt;p&gt;We would like to thank the personnel of the Hellenic Unified Seismological Network (http://eida.gein.noa.gr/) and the Corinth Rift Laboratory Network (https://doi.org/10.15778/RESIF.CL) for the installation and operation of the stations used in the current article. The present research is co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Programme &amp;#171;Human Resources Development, Education and Lifelong Learning 2014-2020&amp;#187; in the context of the project &amp;#8220;The role of fluids in the seismicity of the Western Gulf of Corinth (Greece)&amp;#8221; (MIS 5048127).&lt;/p&gt;

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