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[SSS08-22] Intraplate repeating earthquakes in the Fukushima-Ibaraki border region following the 2011 Tohoku-Oki earthquake
Keywords:repeated earthquake, intraplate fault
Since the 2011 M9 Tohoku-Oki earthquake, intense seismicity has continued around the Fukushima-Ibaraki prefectural border. Particularly in the northern part of Ibaraki prefecture, about M6 earthquakes occurred on March 19, 2011, and December 28, 2016. It was pointed out that the two earthquakes may have repeatedly ruptured the same location on the same fault (Fukushima et al., 2018; Uchide, 2017, JpGU). It is extremely rare for intraplate faults to have repeated earthquakes in such a short interval. It is important to investigate these cases to understand the intraplate earthquake cycle and the stress accumulation and release process. If repeated earthquakes were present in this seismicity, they would provide information about the aseismic slip that may occur within the earthquake cycle (Nadeau & Johnson, 1998). Therefore, the present study detected repeated earthquakes included in the small and medium-sized earthquakes during seismicity around the Fukushima-Ibaraki prefectural border. Many previous studies have used waveform cross-correlation to detect repeated earthquakes, but this method may mistake a group of nearby earthquakes for repeaters. In this study, we investigate the existence of repeated earthquakes based on the precise relocating hypocenters.
Using the Double-Difference method (Waldhauser & Ellsworth, 2002), we relocated hypocenters. Target is 45,988 earthquakes with MJMA>1 that occurred from 2003 to October 31, 2018. The obtained hypocenter distribution showed a complex fault structure. However, there is one dominant planar structure with west-dipping in northern Ibaraki prefecture, on which the M6 earthquake probably occurred.
We detected repeated earthquakes by comparing the distances between the hypocenters of all earthquake pairs with their fault sizes. Specifically, we regarded an earthquake pair as a repeating earthquake pair when (1) the distance is less than the average fault radius, and (2) the magnitude difference is 0.5. The fault size was calculated assuming a circular fault with a stress drop of 20 MPa (Ikeda et al., 2020, JpGU). As a result, 181 seismic pairs were found, most of which were located in northern Ibaraki prefecture. The seismic waveforms of these possible repeating earthquake pairs were very similar even at nearby stations. Besides, the S-P arrival times of the earthquake pairs were very similar among different stations; the standard deviation was within 0±0.01 seconds for 112 pairs. Therefore, it is likely that these seismic pairs are repeated earthquakes.
Based on the empirical relationship by Nadeau & Johnson (1998), we estimated the amount of aseismic slip around repeated earthquakes and obtained a value of about 7 to 15 cm. Fukushima et al. (2018) estimated an anomalously high amount of afterslip, about several tens of cm, caused by the M6 earthquake in 2011. The value is about the same as those estimated from repeated earthquakes. The repeated earthquakes detected in this study may have been caused by the afterslip of the M6 earthquake.
Using the Double-Difference method (Waldhauser & Ellsworth, 2002), we relocated hypocenters. Target is 45,988 earthquakes with MJMA>1 that occurred from 2003 to October 31, 2018. The obtained hypocenter distribution showed a complex fault structure. However, there is one dominant planar structure with west-dipping in northern Ibaraki prefecture, on which the M6 earthquake probably occurred.
We detected repeated earthquakes by comparing the distances between the hypocenters of all earthquake pairs with their fault sizes. Specifically, we regarded an earthquake pair as a repeating earthquake pair when (1) the distance is less than the average fault radius, and (2) the magnitude difference is 0.5. The fault size was calculated assuming a circular fault with a stress drop of 20 MPa (Ikeda et al., 2020, JpGU). As a result, 181 seismic pairs were found, most of which were located in northern Ibaraki prefecture. The seismic waveforms of these possible repeating earthquake pairs were very similar even at nearby stations. Besides, the S-P arrival times of the earthquake pairs were very similar among different stations; the standard deviation was within 0±0.01 seconds for 112 pairs. Therefore, it is likely that these seismic pairs are repeated earthquakes.
Based on the empirical relationship by Nadeau & Johnson (1998), we estimated the amount of aseismic slip around repeated earthquakes and obtained a value of about 7 to 15 cm. Fukushima et al. (2018) estimated an anomalously high amount of afterslip, about several tens of cm, caused by the M6 earthquake in 2011. The value is about the same as those estimated from repeated earthquakes. The repeated earthquakes detected in this study may have been caused by the afterslip of the M6 earthquake.