11:15 AM - 11:30 AM
[SSS08-07] Visualization of Seismic Quiescence and Activation - A Case Study in the Tohoku Earthquake -
Keywords:Seismic activity, Quiescence, Activation
It is considered meaningful to monitor the space-temporal distribution of quiescence and activation of small seismicity to detect seismic anomalies such as quiescence and doughnut patterns in the preparation process of large earthquakes. Since we have reported the results of using eMAP (Hayashimoto and Akitagawa, 2010; Yoshikawa et al., 2021) for seismic activity in southwestern Japan (Yoshikawa, 2022 and 2023), we investigate the seismic activity before the 2011 Tohoku earthquake (M9.0) and confirm the effectiveness of this method.
The quiescence before the Tohoku earthquake has been investigated by Katsumata (2011). According to the results, quiescence was most pronounced in and around the offshore area of Miyagi Prefecture from the end of 1987 to 2003. We have tried to confirm the quiescence and activation in the area. The process of eMAP is as follows. First, the number of events that occurred within a certain distance (50 km) from the epicenter of each event during the entire period is counted for the reference and the evaluation periods. Next, the number of events during the evaluation period is converted into Poisson probability based on the average rate during the reference period, which is displayed on a map using GMT (Wessel and Smith) or other methods.
Quiescence and activation were analyzed by eMAP using the same reference and evaluation periods as Katsumata (2011). In this study, the analysis was performed on (a) JMA data and (b) transformed data by the ETAS model (Ogata, 1992; 2006), without any smoothing process by de-clustering. Fig. 1 shows that the activation area locates approximately 100 km off the coast of Fukushima Prefecture with a diameter of approximately 100 km, and the quiescent area surrounds the activation area for both cases. On the other hand, the opposite result was seen for the 1989-1996 activity of far offshore of Sanriku, which may be due to handling of aftershock activity. Fig. 2 shows the temporal changes in the cumulative number of events in the quiescent and active areas. In both (a) and (b), a decreasing trend is observed from 1988 in the quiescent area, while an increasing trend is observed from 1992-1993 in the active area.
The above results of eMAP are consistent with Katsumata (2011) regarding the quiescence prior to the Tohoku earthquake, except for the extent, and clearly indicate the existence of an active area including the main shock.
The quiescence before the Tohoku earthquake has been investigated by Katsumata (2011). According to the results, quiescence was most pronounced in and around the offshore area of Miyagi Prefecture from the end of 1987 to 2003. We have tried to confirm the quiescence and activation in the area. The process of eMAP is as follows. First, the number of events that occurred within a certain distance (50 km) from the epicenter of each event during the entire period is counted for the reference and the evaluation periods. Next, the number of events during the evaluation period is converted into Poisson probability based on the average rate during the reference period, which is displayed on a map using GMT (Wessel and Smith) or other methods.
Quiescence and activation were analyzed by eMAP using the same reference and evaluation periods as Katsumata (2011). In this study, the analysis was performed on (a) JMA data and (b) transformed data by the ETAS model (Ogata, 1992; 2006), without any smoothing process by de-clustering. Fig. 1 shows that the activation area locates approximately 100 km off the coast of Fukushima Prefecture with a diameter of approximately 100 km, and the quiescent area surrounds the activation area for both cases. On the other hand, the opposite result was seen for the 1989-1996 activity of far offshore of Sanriku, which may be due to handling of aftershock activity. Fig. 2 shows the temporal changes in the cumulative number of events in the quiescent and active areas. In both (a) and (b), a decreasing trend is observed from 1988 in the quiescent area, while an increasing trend is observed from 1992-1993 in the active area.
The above results of eMAP are consistent with Katsumata (2011) regarding the quiescence prior to the Tohoku earthquake, except for the extent, and clearly indicate the existence of an active area including the main shock.