3:30 PM - 4:30 PM
[J06-P-12] Low frequency tremor activity in the Tohoku subduction zone based on ocean bottom seismograms
Various kinds of slow earthquakes have been found along the plate boundary zones in the world. In the Tohoku subduction zone, where slow event activities have been considered insignificant, low frequency tremors prior to the 2011 Tohoku-Oki earthquake and very low frequency earthquakes were identified by recent studies based on geodetic signals from seafloor pressure observations and onshore broad-band seismographs. In the Nankai subduction zone, records of ocean bottom seismometers(OBS) are proved to be effective for detecting low frequency tremors beneath the OBS networks. In this study, we try to detect low frequency tremors and to locate their sources using the OBS records deployed in the Tohoku-Oki.
As the initial trial, we focused on the continuous records on Dec. 2007 when VLFEs were previously detected by onshore broad band seismograms. We applied a envelope correlation method (ECM) to the short period OBS records obtained near the epicenters of VLFEs to detect tremors. Majority of the events detected by ECM seemed to be ordinary earthquakes containing a lot of high frequency signals. In order to identify tremors from the detected events, we calculated an amplitude ratio of the envelope in low frequency band (2- 8 Hz) to that in high frequency band (10- 20 Hz). As a result, we detected an event having significantly larger ratio than those of ordinary earthquakes at all OBS stations, as a possible candidate of a low frequency tremor event. After making visual inspection of the waveforms, we confirmed evident prevalence of low frequency contents and depletion of high frequency signals, as usually observed for low frequency tremors in general. The source was located just beneath the OBS network based on the relative arrival times, the larger amplitudes were observed at stations nearer to the located source. Therefore, the identified event is likely to be a low frequency event occurred in the vicinity of the location of the OBS network.
As the initial trial, we focused on the continuous records on Dec. 2007 when VLFEs were previously detected by onshore broad band seismograms. We applied a envelope correlation method (ECM) to the short period OBS records obtained near the epicenters of VLFEs to detect tremors. Majority of the events detected by ECM seemed to be ordinary earthquakes containing a lot of high frequency signals. In order to identify tremors from the detected events, we calculated an amplitude ratio of the envelope in low frequency band (2- 8 Hz) to that in high frequency band (10- 20 Hz). As a result, we detected an event having significantly larger ratio than those of ordinary earthquakes at all OBS stations, as a possible candidate of a low frequency tremor event. After making visual inspection of the waveforms, we confirmed evident prevalence of low frequency contents and depletion of high frequency signals, as usually observed for low frequency tremors in general. The source was located just beneath the OBS network based on the relative arrival times, the larger amplitudes were observed at stations nearer to the located source. Therefore, the identified event is likely to be a low frequency event occurred in the vicinity of the location of the OBS network.