13:30 〜 13:45
[J05-5-01] Early recurrence of an M6 intraplate earthquake (5.8 years) observed in northern Kanto region, Japan, after the 2011 Tohoku-oki earthquake
The March 2011 Mw9.0 Tohoku-oki earthquake caused significant stress changes in the eastern half of Honshu Island, resulting in the excitation of widespread seismic activity. In northern Kanto region, swarm-like normal faulting activities were induced. One of such events was an M6 event on 19 March 2011 (hereafter called event A).
Approximately 5.8 years later on 28 December 2016, another M6 normal fault earthquake occurred (hereafter called event B). The epicenter of this event determined by the Japan Meteorological Agency is located a few kilometers away from that of the event A.
Performing InSAR processing using Japanese ALOS and ALOS-2 satellite data, it was found that the coseismic displacements for the two events were remarkably similar when the interferograms obtained using data acquired from the same direction were compared. Specifically, the locations of displacement discontinuity lines directing NW-SE, which should correspond to surface rupture, were almost identical, and the displacement patters were also similar, suggesting that the same fault ruptured.
The slightly larger displacement for the event A (maximum displacement discontinuity of ~45cm compared to ~30cm for the event B) indicates that this event was associated with larger slip on the fault at least close to the ground. A preliminary inversion for the event B found a dip angle of 42 degrees with fault slip confined in the upper-most 5km in the crust.
Two questions arise from our result: 1) How can only the upper part of the seismogenic layer (within 5km) rupture? 2) How can an M~6 earthquake re-rupture with such an extremely short time interval of 5.8 years? Two hypotheses can be proposed for the latter question: either a rapid loading of the fault occurred after the event A, possibly associated with the postseismic deformation due to the 2011 Tohoku-oki earthquake, or the stress level on the fault remained high after the event A, enabling further slip on the fault without significant loading.
Approximately 5.8 years later on 28 December 2016, another M6 normal fault earthquake occurred (hereafter called event B). The epicenter of this event determined by the Japan Meteorological Agency is located a few kilometers away from that of the event A.
Performing InSAR processing using Japanese ALOS and ALOS-2 satellite data, it was found that the coseismic displacements for the two events were remarkably similar when the interferograms obtained using data acquired from the same direction were compared. Specifically, the locations of displacement discontinuity lines directing NW-SE, which should correspond to surface rupture, were almost identical, and the displacement patters were also similar, suggesting that the same fault ruptured.
The slightly larger displacement for the event A (maximum displacement discontinuity of ~45cm compared to ~30cm for the event B) indicates that this event was associated with larger slip on the fault at least close to the ground. A preliminary inversion for the event B found a dip angle of 42 degrees with fault slip confined in the upper-most 5km in the crust.
Two questions arise from our result: 1) How can only the upper part of the seismogenic layer (within 5km) rupture? 2) How can an M~6 earthquake re-rupture with such an extremely short time interval of 5.8 years? Two hypotheses can be proposed for the latter question: either a rapid loading of the fault occurred after the event A, possibly associated with the postseismic deformation due to the 2011 Tohoku-oki earthquake, or the stress level on the fault remained high after the event A, enabling further slip on the fault without significant loading.