15:30 〜 16:30
[J05-P-04] Source processes of the M6-class repeating earthquakes which occurred in northern Ibaraki Prefecture, Japan, on 2011 and 2016
An M 6.3 crustal earthquake occurred on December 28, 2016 in northern Ibaraki Prefecture, Japan. Since just after the great 2011 Tohoku Earthquake M 9.0, a significant increase in the shallow normal fault type seismicity has been observed around this area, and the M 6.1 earthquake had occurred on March 19, 2011. The crustal deformations of the 2016 and the 2011 earthquakes, estimated by InSAR data, are almost identical (GSI, 2017), which means that comparable size earthquakes have repeated in the interval of only about 5.7 years. However, the resolution is insufficient to discuss precise source mechanics, so source process inversion analyses using near field strong motion data are performed in this study.
The waveforms of the K-NET and KiK-net (NIED) were used in these inversion analyses. Basically, identical stations were selected for the 2016's and 2011's events. The acceleration waveforms were filtered between 0.03 and 0.8 Hz, and were integrated to velocity waveforms for the inversion analyses. The source processes were inverted by the multi time window method. The size of sub-faults for the inversion analyses were set in 1 km.
The epicenter of the 2016's event is located at about 7 km south of the 2011's event. Though, it was revealed that the rupture of the 2016's event propagated toward the north and its large slip area (asperity) exists fairly near the asperity of the 2011's event. However, the strike and the dip of these events are different and it suggests that the fault planes of the events are not identical. Furthermore, the estimated slip distributions show that the asperities of two events scarcely overlap each other. According to these results, it is deduced that the dominant slip areas of the 2016's and 2011's events are different. The inversion analyses suggest a following conclusion: adjacent but alternate faults had been activated in the interval of about 5.7 years.
The waveforms of the K-NET and KiK-net (NIED) were used in these inversion analyses. Basically, identical stations were selected for the 2016's and 2011's events. The acceleration waveforms were filtered between 0.03 and 0.8 Hz, and were integrated to velocity waveforms for the inversion analyses. The source processes were inverted by the multi time window method. The size of sub-faults for the inversion analyses were set in 1 km.
The epicenter of the 2016's event is located at about 7 km south of the 2011's event. Though, it was revealed that the rupture of the 2016's event propagated toward the north and its large slip area (asperity) exists fairly near the asperity of the 2011's event. However, the strike and the dip of these events are different and it suggests that the fault planes of the events are not identical. Furthermore, the estimated slip distributions show that the asperities of two events scarcely overlap each other. According to these results, it is deduced that the dominant slip areas of the 2016's and 2011's events are different. The inversion analyses suggest a following conclusion: adjacent but alternate faults had been activated in the interval of about 5.7 years.