Detection of Small Earthquakes Using the Matched Filter Method Immediately After the Tohoku-Oki Earthquake

<p>Before the 2011 Tohoku-Oki earthquake, the Off-Kamaishi M∼5 repeating earthquake sequence occurs with a nearly constant recurrence interval of ∼5 years. The seismicity in the Kamaishi area becomes active in the later part of their M∼5 earthquake cycles. Just after the 2011 Tohoku-Oki earthquake, however, the recurrence interval has become as short as 9 days and small earthquakes have become undetectable. The decrease of detectability is mainly due to successive occurrence of large aftershocks. It has not been verified that there is no missing event even for the relatively large Off-Kamaishi mainshock sequence (5≦M≦6). In this study, we collect continuous waveform data of the NIED Hi-net and the microearthquake observation network of Tohoku University near the Off-Kamaishi sequence. We used the repeating sequences as template events and applied Matched Filter (MF) method to detect earthquakes for ∼1 month period (March 11, 2011 to April 13, 2011) after the Tohoku-Oki earthquake. We detected 25 earthquakes including 18 earthquakes that are not listed in the Japan Meteorological Agency (JMA) catalog. These detected earthquakes are highly likely to be located in the vicinity of the Off-Kamaishi mainshock sequence. Further, we found that the sequences are almost completely detectable from two minutes after the Tohoku-oki earthquake. This enabled us to evaluate immediate afterslip in Off-Kamaishi region from the cumulative slip of the mainshock sequence. The result shows the afterslip amount was 1.0∼1.4m in a period from 1 hour to 34 days after the Tohoku-Oki earthquake. For the post Tohoku-oki smaller events down to M2, almost all earthquakes occurred in the latter part of the two Off-Kamaishi mainshock earthquake cycles, which is a similar pattern observed before the Tohoku-oki earthquake. The investigation of undetectable time of earthquakes also supports that this tendency is not an artifact. This result indicates that the temporal change of microseismicity is preserved even when the repeating interval is extremely short (10 to 23 days) due to the afterslip of the Tohoku-Oki earthquake and suggests similar process is occurring in the earthquake cycle of the Off-Kamaishi sequence before and after the Tohoku-Oki earthquake.</p>