9:30 AM - 9:45 AM
[SSS12-03] Repeated Long-term Slow Slip Events beneath the Hamanako-lake area and seismic activity in the crust
Keywords:Seismic activity, Slow slip event, Hamanako-lake
The eMAP is used for showing seismic activation and quiescence (Aketagawa and Ito, 2008; Hayashimoto and Aketagawa, 2010; Yoshikawa et al., 2017). The method is based on the occurrence probability of the seismicity for evaluation period calculated from the Poisson distribution with the average seismicity for reference period, which is counted in every circular or spherical regions centered at all hypocenters. We have analyzed seismic activity in the Tokai area for checking applicability of the eMAP by NIED catalogue of hypocenters, and could confirm that the spatial distribution of activated zone was similar to that of the asperities obtained by Matsumura (2007).
We investigated the correlation between LSSE and the crustal seismic activity by means of the seismic quiescence and activation observed in Tokai area using de-clustered data from JMA catalogue of hypocenters. In the figure, spatial distribution of the occurrence probability for the hypocenters shallower than the plate boundary is shown by plane views and that for the hypocenters in all depths to 50km is shown by vertical views beneath the rectangular zone. The occurrence probability is evaluated in the following periods: (a) LSSE1 (2001–2005), (b) resting (2006-2012), (c) LSSE2 (2013-2015) and (d) resting (2016-2017), compared with the period (1997.10-2000). We can see three bands of activated zones perpendicularly to the major axis of rectangle in the plane view and those inclined to SE in the vertical view for the period of LSSE1. Then we can see expanding of the activated zone both in the plane and vertical views for the resting period (b). The distribution of the activated and quiescent zones for LSSE2 becomes similar to that for LSSE1. And the activated zone extends to NW from the middle of the rectangle for the resting period (d). By the number of the earthquakes in the NW part of the rectangle zone, we could confirm that the seismicity becomes lower for LSSE and higher for resting of LSSE and that the activity becomes remarkable after the latter of 2017. However the activated zone in the resting period (d) seems too small for detection of changes in the crustal seismic activity distinctly.
The spatial distribution of the contraction vectors around the Hamanako-lake area inferred from the GNSS data of GSI shows that maximum axes of contraction indicate a clear NW–SE direction for the periods of resting of LSSE (1997-2001; 2006-2012; 2016-2017), whereas they are not clear for the periods of LSSE (2001-2005; 2013-2015). Since active crustal seismicity seems correlating to large contraction for resting of LSSE, it may represent recovery of the stress state to the ordinary state caused by steady plate motion, which should be kept monitoring further.