09:30 〜 09:45
[SSS04-03] 稠密OBS観測から検出された日本海溝―千島海溝会合部における地震学的構造の時間変化と地震活動の関係
キーワード：海底地震計、地震波干渉法、地震波速度
Recent ocean-bottom seismogram (OBS) studies have detected temporal seismic velocity variations associated with regular and slow earthquakes by applying seismic interferometry (SI) to OBS data; the coseismic velocity drop and postseismic healing associated with the 2016 Off-Mie earthquake (Mw 5.9) and its recovery (Ikeda and Tsuji, 2018), and transient changes in seismic heterogeneity occurring before and during shallow slow earthquakes (Tonegawa et al., 2022).
We have applied SI to OBS data collected around the Japan-Kuril trenches junction in 2006 - 2007 and examined temporal seismic velocity change related to seismic activities (Sato et al., AGU, 2022); a coseismic velocity drop of up to about 0.2% and healing on the occasion of a local moderate earthquake (Mj 6.2), and no seismic velocity changes during and near three tectonic tremor activity periods (Kawakubo et al., 2021) and two M8-class earthquakes off the Kuril Islands (Lay et al., 2009). However, the detailed relationship between velocity changes and the strong ground motion remains unknown because of the saturation of seismograms at nearfield sites. In this presentation, we investigate the relationship between the detected velocity drops and the peak ground velocities (PGV) at the timings of significant velocity changes, namely during the Mj 6.2 earthquake. The PGVs at saturated stations were estimated by extrapolating the unsaturated seismograms of smaller earthquakes near the Mj6.2 earthquake. We found a positive correlation between coseismic velocity drops and PGV but of which several sites refer to extrapolated values. A similar correlation has been reported in the Nankai subduction zone (Ikeda and Tsuji, 2018) but a degree of velocity drops against amplitude of the PGV seems to be smaller than reported in the previous study.
In addition to the temporal changes in seismic velocity, those of correlation coefficients (CC) are also an important scope. A CC drop would be caused by a temporal disturbance in the wave field or change in the scattering property. Here, we estimated the variations of CC by a stretching method (Sens-Schönfelder & Wegler, 2006). We rearranged the temporal relation of seismic velocity and CC with seismic activities. At first, we found no CC change in addition to the coseismic velocity drop related to the Mj 6.2 earthquake. Next, we newly identified CC decreases related to two large earthquakes off the Kuril Islands. These CC changes tend to appear definitely at sites near the trench, which could be attributed to the distribution of the sedimentary prism. Finally, we confirmed no changes in CC in addition to seismic velocity related to tectonic tremor activities, different from the case of slow earthquake activities in the Nankai subduction zone. We consider the difference would result from differences in the location and/or magnitude of slow earthquake activities between the Nankai and Japan subduction zones. We will discuss the seismological meanings of the presented CC variation in more detail.
We have applied SI to OBS data collected around the Japan-Kuril trenches junction in 2006 - 2007 and examined temporal seismic velocity change related to seismic activities (Sato et al., AGU, 2022); a coseismic velocity drop of up to about 0.2% and healing on the occasion of a local moderate earthquake (Mj 6.2), and no seismic velocity changes during and near three tectonic tremor activity periods (Kawakubo et al., 2021) and two M8-class earthquakes off the Kuril Islands (Lay et al., 2009). However, the detailed relationship between velocity changes and the strong ground motion remains unknown because of the saturation of seismograms at nearfield sites. In this presentation, we investigate the relationship between the detected velocity drops and the peak ground velocities (PGV) at the timings of significant velocity changes, namely during the Mj 6.2 earthquake. The PGVs at saturated stations were estimated by extrapolating the unsaturated seismograms of smaller earthquakes near the Mj6.2 earthquake. We found a positive correlation between coseismic velocity drops and PGV but of which several sites refer to extrapolated values. A similar correlation has been reported in the Nankai subduction zone (Ikeda and Tsuji, 2018) but a degree of velocity drops against amplitude of the PGV seems to be smaller than reported in the previous study.
In addition to the temporal changes in seismic velocity, those of correlation coefficients (CC) are also an important scope. A CC drop would be caused by a temporal disturbance in the wave field or change in the scattering property. Here, we estimated the variations of CC by a stretching method (Sens-Schönfelder & Wegler, 2006). We rearranged the temporal relation of seismic velocity and CC with seismic activities. At first, we found no CC change in addition to the coseismic velocity drop related to the Mj 6.2 earthquake. Next, we newly identified CC decreases related to two large earthquakes off the Kuril Islands. These CC changes tend to appear definitely at sites near the trench, which could be attributed to the distribution of the sedimentary prism. Finally, we confirmed no changes in CC in addition to seismic velocity related to tectonic tremor activities, different from the case of slow earthquake activities in the Nankai subduction zone. We consider the difference would result from differences in the location and/or magnitude of slow earthquake activities between the Nankai and Japan subduction zones. We will discuss the seismological meanings of the presented CC variation in more detail.