Study on Temporal Variations of Shear-Wave Splitting in the Chiayi Area, Aftershock Zone of 1999 Chichi Earthquake, Taiwan

2008 ◽  
Vol 51 (1) ◽  
pp. 115-124 ◽  
Author(s):  
Xiu-Fen ZHENG ◽  
Chau-Huei CHEN ◽  
Chun-He ZHANG
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Temporal Variations ◽  
Wave Splitting ◽  
Chichi Earthquake

scholarly journals Discriminating Between Spatial and  Temporal Variations in Seismic  Anisotropy at Active Volcanoes

2021 ◽  
Author(s):  
◽  
Jessica Helen Johnson
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Temporal Variations ◽  
Temporal Changes ◽  
Stable Region ◽  
Strong Anisotropy ◽  
Wave Splitting ◽  
Okmok Volcano ◽  
Active Volcanoes

<p>This thesis addresses the measurement and interpretation of seismic anisotropy around active volcanoes via shear wave splitting analysis. An overpressured magma reservoir will exert a stress on the surrounding country rock that may or may not be manifest as observable strain. Shear wave splitting analysis can be a useful indicator of stress in the crust and hence, the pressure induced by magma movement. Changes in shear wave splitting have already been observed at Mt. Ruapehu following eruptions in 1995/1996 and are inferred to be caused by changes in local stress in response to magma pressure. One of the main problems with the interpretation of temporal changes in shear wave splitting is the possibility of spatial variations being sampled along differing raypaths and being interpreted as temporal changes. Using a dense observational network and an automated shear wave splitting analysis, we examine local earthquakes occurring in 2008 within 100 km of Mt. Ruapehu. We note a strong azimuthal dependence of the fast direction of anisotropy (phi) and so introduce a spatial averaging technique and a two-dimensional tomography of recorded delay times (dt), to observe the spatial variation in more detail. Using this new method of mapping shear wave splitting parameters, we have created a benchmark of spatial variations in shear wave anisotropy around Mt. Ruapehu, against which future temporal changes may be measured. The observed anisotropy is used to define regions in which phi agrees with stress estimations from focal mechanism inversions, suggesting stress-induced anisotropy, and those in which phi aligns with structural features such as fault strikes, suggesting structural anisotropy. Data from past deployments of three-component seismometers have been analysed in the same way as those recorded during the 2008 experiment and the results compared. We identify a stable region of strong anisotropy, interpreted to be caused by schistose mineral alignment, and a transient region of strong anisotropy centred on the volcano during the major magmatic eruption of 1995. We also introduce a method of analysing temporal variations in seismic anisotropy at active volcanoes by using tight clusters of earthquakes and highly correlated multiplets. At Mt. Ruapehu, changes in shear wave splitting parameters associated with the 2006 and 2007 phreatic eruptions are detected using a cluster of earthquakes to the west of the volcano. Similar analyses using another cluster and multiplets from the stable region of strong anisotropy do not reveal temporal changes, although examination of the waveform codas of the repeating earthquakes reveals systematic changes that we interpret as being caused by seismic scatterers associated with the 2006 and 2007 eruptions. These scatterers appear to contaminate the shear wave coda and so inhibit the detection of any subtle changes in shear wave splitting parameters. Finally, we apply some of these methods to data from the 2008 eruption of Okmok volcano, Alaska. Shear wave splitting analysis at Okmok reveals a change in anisotropy associated with the 2008 eruption. This change however, is attributed to a change in dominant hypocentre location. Multiplet analysis at Okmok volcano reveals a similar scatterer contamination of the shear wave arrival. This spurious phase is interpreted to be an S to P conversion from interaction with the magma reservoir.</p>

scholarly journals Monitoring increases in fracture connectivity during hydraulic stimulations from temporal variations in shear wave splitting polarization

2013 ◽  
Vol 195 (2) ◽  
pp. 1120-1131 ◽  
Author(s):  
A. F. Baird ◽  
J.- M. Kendall ◽  
J. P. Verdon ◽  
A. Wuestefeld ◽  
T. E. Noble ◽  
...  
Keyword(s):  
Shear Wave ◽  
Shear Wave Splitting ◽  
Temporal Variations ◽  
Wave Splitting

scholarly journals Discriminating Between Spatial and  Temporal Variations in Seismic  Anisotropy at Active Volcanoes

2021 ◽  
Author(s):  
◽  
Jessica Helen Johnson
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Temporal Variations ◽  
Temporal Changes ◽  
Stable Region ◽  
Strong Anisotropy ◽  
Wave Splitting ◽  
Okmok Volcano ◽  
Active Volcanoes

<p>This thesis addresses the measurement and interpretation of seismic anisotropy around active volcanoes via shear wave splitting analysis. An overpressured magma reservoir will exert a stress on the surrounding country rock that may or may not be manifest as observable strain. Shear wave splitting analysis can be a useful indicator of stress in the crust and hence, the pressure induced by magma movement. Changes in shear wave splitting have already been observed at Mt. Ruapehu following eruptions in 1995/1996 and are inferred to be caused by changes in local stress in response to magma pressure. One of the main problems with the interpretation of temporal changes in shear wave splitting is the possibility of spatial variations being sampled along differing raypaths and being interpreted as temporal changes. Using a dense observational network and an automated shear wave splitting analysis, we examine local earthquakes occurring in 2008 within 100 km of Mt. Ruapehu. We note a strong azimuthal dependence of the fast direction of anisotropy (phi) and so introduce a spatial averaging technique and a two-dimensional tomography of recorded delay times (dt), to observe the spatial variation in more detail. Using this new method of mapping shear wave splitting parameters, we have created a benchmark of spatial variations in shear wave anisotropy around Mt. Ruapehu, against which future temporal changes may be measured. The observed anisotropy is used to define regions in which phi agrees with stress estimations from focal mechanism inversions, suggesting stress-induced anisotropy, and those in which phi aligns with structural features such as fault strikes, suggesting structural anisotropy. Data from past deployments of three-component seismometers have been analysed in the same way as those recorded during the 2008 experiment and the results compared. We identify a stable region of strong anisotropy, interpreted to be caused by schistose mineral alignment, and a transient region of strong anisotropy centred on the volcano during the major magmatic eruption of 1995. We also introduce a method of analysing temporal variations in seismic anisotropy at active volcanoes by using tight clusters of earthquakes and highly correlated multiplets. At Mt. Ruapehu, changes in shear wave splitting parameters associated with the 2006 and 2007 phreatic eruptions are detected using a cluster of earthquakes to the west of the volcano. Similar analyses using another cluster and multiplets from the stable region of strong anisotropy do not reveal temporal changes, although examination of the waveform codas of the repeating earthquakes reveals systematic changes that we interpret as being caused by seismic scatterers associated with the 2006 and 2007 eruptions. These scatterers appear to contaminate the shear wave coda and so inhibit the detection of any subtle changes in shear wave splitting parameters. Finally, we apply some of these methods to data from the 2008 eruption of Okmok volcano, Alaska. Shear wave splitting analysis at Okmok reveals a change in anisotropy associated with the 2008 eruption. This change however, is attributed to a change in dominant hypocentre location. Multiplet analysis at Okmok volcano reveals a similar scatterer contamination of the shear wave arrival. This spurious phase is interpreted to be an S to P conversion from interaction with the magma reservoir.</p>

Upper crust seismic anisotropy study and temporal variations of shear-wave splitting parameters in the western Gulf of Corinth (Greece) during 2013

2017 ◽  
Vol 269 ◽  
pp. 148-164 ◽  
Author(s):  
George Kaviris ◽  
Ioannis Spingos ◽  
Vasileios Kapetanidis ◽  
Panayotis Papadimitriou ◽  
Nicholas Voulgaris ◽  
...  
Keyword(s):  
Shear Wave ◽  
Seismic Anisotropy ◽  
Shear Wave Splitting ◽  
Temporal Variations ◽  
Upper Crust ◽  
Gulf Of Corinth ◽  
Wave Splitting

Location and temporal variations of shear wave splitting in a South African gold mine

2017 ◽  
pp. 185-198 ◽  
Author(s):  
N. Nagai ◽  
M. Ando ◽  
H. Ogasawara ◽  
T. Ohkura ◽  
Y. Iio ◽  
...  
Keyword(s):  
South African ◽  
Shear Wave ◽  
Shear Wave Splitting ◽  
Temporal Variations ◽  
Gold Mine ◽  
Wave Splitting ◽  
South African Gold
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