3:30 PM - 4:30 PM
[J06-P-11] Seismic anisotropy monitoring and detection of tremor activity in the southwest Japan subduction zone
We have been investigating the source region of Nankai trough mega-thrust earthquake from a view of seismic anisotropy. In this study, a seismic anisotropy monitoring, which is simultaneous continuous measurements using shear (S) wave splitting analysis and polarization analysis, was performed during tremor episodes at Hi-net stations developed in eastern Shikoku, southwest Japan. The method is similar to Bostock and Christensen [2012]. We inferred parameters of S-wave anisotropy (polarization direction of fast S wave and delay time between fast and slow S waves) and those of incoming wave (back azimuth and incident angle) using a 1 min long sliding window with 50% overlap. They reveal a close relationship among temporal variations of anisotropy, polarizations of incoming wave and spatial-temporal development of tremor activities. They show systematic changes during located tremor activities [e.g., world tremor database, Idehara et al., 2014].
We thus interpret monitoring the relationship among temporal variations of anisotropy, incoming wave, and tremor activity as a tremor detection method. That is, we assumed that the systematic change indicates occurrence of tremor. If so, it may be possible to detect occurrence of tremor by analyzing seismograms at a single station as well as detect occurrence of tremor, whereas commonly-used source location determination methods [e.g., Obara, 2002] require observation of coherent seismic signals at several stations.
Recently, we started to apply the monitoring method to ocean bottom seismometer data in order to investigate offshore tremor activity. Since the quality and quantity of marine observation data is limited in various aspects compared with those of land-based observation data, this will eventually discover some unknown phenomena. In this presentation, we show preliminary result of the monitoring using an offshore seismic network, DONET, off southwest Japan along the Nankai Trough.
We thus interpret monitoring the relationship among temporal variations of anisotropy, incoming wave, and tremor activity as a tremor detection method. That is, we assumed that the systematic change indicates occurrence of tremor. If so, it may be possible to detect occurrence of tremor by analyzing seismograms at a single station as well as detect occurrence of tremor, whereas commonly-used source location determination methods [e.g., Obara, 2002] require observation of coherent seismic signals at several stations.
Recently, we started to apply the monitoring method to ocean bottom seismometer data in order to investigate offshore tremor activity. Since the quality and quantity of marine observation data is limited in various aspects compared with those of land-based observation data, this will eventually discover some unknown phenomena. In this presentation, we show preliminary result of the monitoring using an offshore seismic network, DONET, off southwest Japan along the Nankai Trough.