5:15 PM - 6:45 PM
[SSS08-P05] Detection of immediate aftershocks following the 2011 Tohoku Earthquake using a seismic array in the Kanto region
Keywords:2011 Tohoku earthquake, Matched Filter Method
Very intense aftershock activity occurred following the 2011 Tohoku earthquake (Mw9.0). During this activity, the wavefields generated by large events prevented the detection of medium and small events. For this reason, many events were missed from existing earthquake catalogs. Therefore, we tried to detect missed events using the Matched Filter Method which has an advantage of being able to separate the overlapping wavefields of multiple events and detect each one separately. This method detects the events that have similar waveforms to known events (template events). Specifically, the correlation coefficients (CC) between the template and the continuous waveforms were calculated, and we considered that the events occurred when the CC values exceeded the preset threshold values.
Lengliné et al. (2012) applied the Matched Filter Method to detect missed the immediate aftershocks following the 2011 Tohoku earthquake to waveform data observed by Hi-net stations located in Tohoku and Kanto regions. However, they could only detect the event up to approximately 12 hours after the mainshock due to the loss of data caused by the power outage. Therefore, we performed the Matched Filter Method using the waveform data almost completely recorded by MeSO-net, which has been deployed in the Kanto region near the aftershock area and was not affected by the power outage. As a result, we detected about 2.6 times as many events as listed in the JMA catalog for M>=4.5, and the completeness magnitude was improved by 0.5 compared with the JMA catalog.
The detected event catalog showed a clearer expansion of the southern edge of the aftershock area than the JMA or previous work catalog, and the edge reached the contact zone between the Pacific Plate and the Philippine Sea Plate beneath the Kanto region. We interpret that the aftershock expansion was driven by afterslip propagation, which may have caused stress loading on the upper surface of the Philippine Sea Plate and contributed on triggering of a slow-slip event off the Boso peninsula.
The expansion of aftershock area from the landward side to the trench axis side was also observed off the coast of Iwate. Following the arrival of the aftershock zone near the trench axis, seismicity outside of the trench axis increased. During the 1968 Tokachi earthquake or the 1994 offshore Sanriku earthquake, the dynamic rupture started from the shallow depth of the subduction zone, and then propagated landward and caused the largest slip area at the deep depths. Based on the rupture processes of these two large earthquakes and the aftershock expansion observed in this study, it is likely that this area has a characteristic that the slip tends to propagate along the direction of subduction of the Pacific Plate.
Lengliné et al. (2012) applied the Matched Filter Method to detect missed the immediate aftershocks following the 2011 Tohoku earthquake to waveform data observed by Hi-net stations located in Tohoku and Kanto regions. However, they could only detect the event up to approximately 12 hours after the mainshock due to the loss of data caused by the power outage. Therefore, we performed the Matched Filter Method using the waveform data almost completely recorded by MeSO-net, which has been deployed in the Kanto region near the aftershock area and was not affected by the power outage. As a result, we detected about 2.6 times as many events as listed in the JMA catalog for M>=4.5, and the completeness magnitude was improved by 0.5 compared with the JMA catalog.
The detected event catalog showed a clearer expansion of the southern edge of the aftershock area than the JMA or previous work catalog, and the edge reached the contact zone between the Pacific Plate and the Philippine Sea Plate beneath the Kanto region. We interpret that the aftershock expansion was driven by afterslip propagation, which may have caused stress loading on the upper surface of the Philippine Sea Plate and contributed on triggering of a slow-slip event off the Boso peninsula.
The expansion of aftershock area from the landward side to the trench axis side was also observed off the coast of Iwate. Following the arrival of the aftershock zone near the trench axis, seismicity outside of the trench axis increased. During the 1968 Tokachi earthquake or the 1994 offshore Sanriku earthquake, the dynamic rupture started from the shallow depth of the subduction zone, and then propagated landward and caused the largest slip area at the deep depths. Based on the rupture processes of these two large earthquakes and the aftershock expansion observed in this study, it is likely that this area has a characteristic that the slip tends to propagate along the direction of subduction of the Pacific Plate.