scholarly journals Distinguishing between Stress-induced and Structural Anisotropy at Mount Ruapehu Volcano

2021 ◽  
Author(s):  
J Johnson ◽  
Martha Savage ◽  
John Townend
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Horizontal Stress ◽  
Structural Features ◽  
Spatial Variations ◽  
Induced Anisotropy ◽  
Structural Anisotropy ◽  
Mount Ruapehu ◽  
Stress Induced Anisotropy ◽  
Maximum Horizontal Stress

We have created a benchmark of spatial variations in shear wave anisotropy around Mount Ruapehu, New Zealand, against which to measure future temporal changes. Anisotropy in the crust is often assumed to be caused by stress-aligned microcracks, and the polarization of the fast quasi-shear wave (φ) is thus interpreted to indicate the direction of maximum horizontal stress, but can also be due to aligned minerals or macroscopic fractures. Changes in seismic anisotropy have been observed following a major eruption in 1995/96 and were attributed to changes in stress from the depressurization of the magmatic system. Three-component broadband seismometers have been deployed to complement the permanent stations that surround Ruapehu, creating a combined network of 34 three-component seismometers. This denser observational network improves the resolution with which spatial variations in seismic anisotropy can be examined. Using an automated shear wave splitting analysis, we examine local earthquakes in 2008. We observe a strong azimuthal dependence of φ and so introduce a spatial averaging technique and two-dimensional tomography of recorded delay times. The anisotropy can be divided into regions in which φ agrees with stress estimations from focal mechanism inversions, suggesting stress-induced anisotropy, and those in which φ is aligned with structural features such as faults, suggesting structural anisotropy. The pattern of anisotropy that is inferred to be stress related cannot be modeled adequately using Coulomb modeling with a dike-like inflation source. We suggest that the stress-induced anisotropy is affected by loading of the volcano and a lithospheric discontinuity. Copyright 2011 by the American Geophysical Union.

scholarly journals Distinguishing between Stress-induced and Structural Anisotropy at Mount Ruapehu Volcano

2021 ◽  
Author(s):  
J Johnson ◽  
Martha Savage ◽  
John Townend
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Horizontal Stress ◽  
Structural Features ◽  
Spatial Variations ◽  
Induced Anisotropy ◽  
Structural Anisotropy ◽  
Mount Ruapehu ◽  
Stress Induced Anisotropy ◽  
Maximum Horizontal Stress

We have created a benchmark of spatial variations in shear wave anisotropy around Mount Ruapehu, New Zealand, against which to measure future temporal changes. Anisotropy in the crust is often assumed to be caused by stress-aligned microcracks, and the polarization of the fast quasi-shear wave (φ) is thus interpreted to indicate the direction of maximum horizontal stress, but can also be due to aligned minerals or macroscopic fractures. Changes in seismic anisotropy have been observed following a major eruption in 1995/96 and were attributed to changes in stress from the depressurization of the magmatic system. Three-component broadband seismometers have been deployed to complement the permanent stations that surround Ruapehu, creating a combined network of 34 three-component seismometers. This denser observational network improves the resolution with which spatial variations in seismic anisotropy can be examined. Using an automated shear wave splitting analysis, we examine local earthquakes in 2008. We observe a strong azimuthal dependence of φ and so introduce a spatial averaging technique and two-dimensional tomography of recorded delay times. The anisotropy can be divided into regions in which φ agrees with stress estimations from focal mechanism inversions, suggesting stress-induced anisotropy, and those in which φ is aligned with structural features such as faults, suggesting structural anisotropy. The pattern of anisotropy that is inferred to be stress related cannot be modeled adequately using Coulomb modeling with a dike-like inflation source. We suggest that the stress-induced anisotropy is affected by loading of the volcano and a lithospheric discontinuity. Copyright 2011 by the American Geophysical Union.

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 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 Earthquake-induced debris flows at Popocatépetl Volcano, Mexico

2020 ◽  
Author(s):  
Velio Coviello ◽  
Lucia Capra ◽  
Gianluca Norini ◽  
Norma Dávila ◽  
Dolores Ferrés ◽  
...  
Keyword(s):  
Debris Flows ◽  
Horizontal Stress ◽  
Structural Features ◽  
Volcanic Edifice ◽  
Western Slope ◽  
Slope Failures ◽  
Ground Shaking ◽  
Seismic Records ◽  
Maximum Horizontal Stress ◽  
First Time

Abstract. The M7.1 Puebla-Morelos earthquake that occurred on 19 September 2017, with epicenter located ∼ 70 km SW from Popocatépetl volcano, severely hit central Mexico. Seismic shaking of the volcanic edifice induced by the earthquake triggered hundreds of shallow landslides on the volcanic flanks, remobilizing loose pyroclastic deposits and saturated soils. The largest landslides occurred on the slopes of aligned ENE-WSW-trending ravines on opposite sides of the volcanic cone, roughly parallel to the regional maximum horizontal stress and local volcanotectonic structural features. This configuration may suggest transient reactivation of local faults and extensional fractures as one of the mechanisms that has weakened the volcanic edifice and promoted the largest slope failures. The seismic records from a broadband station located at few kilometers from the main landslides are used to infer the intensity of ground shaking that triggered the slope failures. The material involved in the larger landslides, mainly ash and pumice fall deposits from late Holocene eruptions with a total volume of about 106 cubic meters, transformed into two large debris flows on the western slope of the volcano and one on its eastern side. The debris flows were highly viscous and contained abundant large woods (about 105 cubic meter). Their peculiar rheology is reconstructed by field evidences and analyzing the grain size distribution of samples from both landslide scars and deposits. This is the first time that such flows were observed at this volcano. Our work provides new insights to constrain a multi-hazard risk assessment for Popocatépetl and other continental active volcanoes.

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 An analytic model for the stress-induced anisotropy of dry rocks

Geophysics ◽  
2011 ◽  
Vol 76 (3) ◽  
pp. WA125-WA133 ◽  
Author(s):  
Boris Gurevich ◽  
Marina Pervukhina ◽  
Dina Makarynska
Keyword(s):  
Seismic Anisotropy ◽  
Uniaxial Stress ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Analytic Model ◽  
Induced Anisotropy ◽  
S Wave ◽  
Isotropic Rock ◽  
Model Predictions ◽  
Stress Induced Anisotropy

One of the main causes of azimuthal anisotropy in sedimentary rocks is anisotropy of tectonic stresses in the earth’s crust. We have developed an analytic model for seismic anisotropy caused by the application of a small anisotropic stress. We first considered an isotropic linearly elastic medium (porous or nonporous) permeated by a distribution of discontinuities with random (isotropic) orientation (such as randomly oriented compliant grain contacts or cracks). The geometry of individual discontinuities is not specified. Instead, their behavior is defined by a ratio B of the normal to tangential excess compliances. When this isotropic rock is subjected to a small compressive stress (isotropic or anisotropic), the number of cracks along a particular plane is reduced in proportion to the normal stress traction acting on that plane. This effect is modeled using the Sayers-Kachanov noninteractive approximation. The model predicts that such anisotropic crack closure yields elliptical anisotropy, regardless of the value of the compliance ratio B. It also predicts the ratio of Thomsen’s anisotropy parameters [Formula: see text] as a function of the compliance ratio B and Poisson’s ratio of the unstressed rock. A comparison of the model predictions with the results of laboratory measurements indicates a reasonable agreement for moderate magnitudes of uniaxial stress (as high as 30 MPa). These results can be used for differentiating stress-induced anisotropy from that caused by aligned fractures. Conversely, if the cause of anisotropy is known, then the anisotropy pattern allows one to estimate P-wave anisotropy from S-wave anisotropy.

scholarly journals Anisotropy analysis for optimized gas production in Coalbed Methane reservoir, Bokaro field, India

2021 ◽  
Author(s):  
Abir Banerjee ◽  
Rima Chatterjee ◽  
Dip Kumar Singha
Keyword(s):  
Hydraulic Fracturing ◽  
Shear Wave ◽  
Coalbed Methane ◽  
Horizontal Stress ◽  
Polarization Angle ◽  
Wave Polarization ◽  
Low Resistivity ◽  
Fracture Orientation ◽  
Maximum Horizontal Stress ◽  
Resistivity Image

Abstract The efficient production of Coalbed Methane (CBM) gas is facing challenges due to the larger dewatering period from fracture connectivity to the aquifer zone. Also, commingled production from well makes it more difficult to identify the coal seam-wise problem. Therefore, prior knowledge of sub-surface fractures in coal seams is necessary to execute an accurate simulation model for planning hydraulic fracturing treatment. This paper highlights the studies in Bokaro CBM reservoir to mitigate challenges in few wells by characterizing anisotropy, determining fast shear wave polarization angle, maximum horizontal stress direction, fracture orientation, and analysis of low resistivity signature. Both the fast shear wave polarization angle and fracture orientation in resistivity image are observed in the same direction (N26°-35°E) in coal. The fast and slow shear slowness versus frequency plot concludes stress-induced anisotropy resulting from fractures that are supported by resistivity image and drilling core. Processing of the resistivity image log shows the maximum horizontal stress is along NE-SW direction, as identified from drilling-induced fractures. The observation of low resistivity signature with resistivity ranging from 0.4 to 0.8 ohm-m in few wells confirms the presence of conducting minerals such as siderite and pyrite from the x-ray diffraction studies of sidewall core. The present work guides in making production, drilling, and hydraulic fracturing design strategies to better understand the fluid propagation for optimized CBM production and will also help in future geomechanical studies.

scholarly journals Modeling shear wave splitting due to stress-induced anisotropy, with an application to Mount Asama Volcano, Japan

2014 ◽  
Vol 119 (5) ◽  
pp. 4269-4286 ◽  
Author(s):  
Adrian Shelley ◽  
Martha Savage ◽  
Charles Williams ◽  
Yosuke Aoki ◽  
Boris Gurevich
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Induced Anisotropy ◽  
Asama Volcano ◽  
Wave Splitting ◽  
Stress Induced Anisotropy

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.

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