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[SSS07-P04] Measurement of seismic velocity anisotropy in the Tohoku region, NE Japan by shear wave splitting analysis (3)
Keywords:anisotropy
Shear wave polarization anisotropy in the Tohoku region was measured using shear wave splitting, in which shear wave splits into fast and slow shear waves when a seismic wave passes through an anisotropic elastic medium in the crust. The method used was the Multiple Filter Automatic Splitting Technique (MFAST; Savage et al., 2010). In this method, by inputting the three components of the seismic waveform, the optimum filter is selected from 14 prepared filters, and the orientation and magnitude of the anisotropy in multiple time windows are measured by the method of SC91 (Silver and Chan, 1991). Then, quality evaluation is performed using cluster analysis (Teanby et al., 2004) from values measured from multiple time windows. MFAST is a program that automates this sequence, so it can process large amounts of data automatically, and can obtain objective and reliable measurements.
The analysis was carried out throughout the Tohoku region, and further detailed analysis was performed in the area where earthquakes are swarming in Akita Prefecture. We used temporary earthquake observation data from the Group for the aftershock observations of the 2011 off the Pacific coast of Tohoku Earthquake (the Tohoku-oki earthquake) and others. After the 2011 off the Pacific coast of Tohoku Earthquake, the orientations of anisotropy measured at each station in the whole Tohoku region can be seen as either N-S orientation or E-W orientation. In the Pacific coast from Iwate to Miyagi, the N-S orientation dominated, and in the central inland area, the E-W orientation had a stronger tendency. In addition, at some stations, the observed orientations were different for the deep earthquakes in the subducting plate and for the shallow earthquakes in the crust. The delay time, which indicates the magnitude of anisotropy, tended to increase in the regions where earthquake swarms occurred.
In the inland area of Akita prefecture, the analysis was performed before and after the 2011 Tohoku-oki earthquake. Overall, NE-SW to N-S orientation was predominant, and the orientation did not change much before and after the Tohoku-oki earthquake. Especially around the region where earthquakes swarms occurred, the tendency of the orientation differs for each station, suggesting that the anisotropy changes on a small scale. The delay time was larger overall before and after the Tohoku-oki earthquake than in the whole Tohoku region. In comparison before and after the Tohoku-oki earthquake, both areas with increased delay time and with decreased were observed.
The analysis was carried out throughout the Tohoku region, and further detailed analysis was performed in the area where earthquakes are swarming in Akita Prefecture. We used temporary earthquake observation data from the Group for the aftershock observations of the 2011 off the Pacific coast of Tohoku Earthquake (the Tohoku-oki earthquake) and others. After the 2011 off the Pacific coast of Tohoku Earthquake, the orientations of anisotropy measured at each station in the whole Tohoku region can be seen as either N-S orientation or E-W orientation. In the Pacific coast from Iwate to Miyagi, the N-S orientation dominated, and in the central inland area, the E-W orientation had a stronger tendency. In addition, at some stations, the observed orientations were different for the deep earthquakes in the subducting plate and for the shallow earthquakes in the crust. The delay time, which indicates the magnitude of anisotropy, tended to increase in the regions where earthquake swarms occurred.
In the inland area of Akita prefecture, the analysis was performed before and after the 2011 Tohoku-oki earthquake. Overall, NE-SW to N-S orientation was predominant, and the orientation did not change much before and after the Tohoku-oki earthquake. Especially around the region where earthquakes swarms occurred, the tendency of the orientation differs for each station, suggesting that the anisotropy changes on a small scale. The delay time was larger overall before and after the Tohoku-oki earthquake than in the whole Tohoku region. In comparison before and after the Tohoku-oki earthquake, both areas with increased delay time and with decreased were observed.