3:30 PM - 4:30 PM
[J02-P-14] The seismotectonic implications of source models of M-7 class earthquakes before ad after the 2011 Tohoku-Oki Earthquake using offshore tsunami records
Some major (M ~7) earthquakes occurred in and around the rupture area of the 2011 Tohoku-Oki Earthquake. In this study, we investigated their source models using offshore tsunami records and discussed their relationships with the mainshock, which will give us new implications for subduction zone tectonics.
On March 9, 2011, Mw 7.2 and 6.6 interplate earthquakes preceded the mainshock at ~ 30 km northeast from the mainshock epicenter. The rupture area of the Mw 7.2 event was complementary to its postseismic slip area and to the epicenters of smaller seismicity before the mainshock. After the Mw 6.6 event, one of the aftershocks of the Mw 7.2, seismicity abruptly expanded to the area next to the mainshock epicenter. This migration of seismicity after the Mw 7.2 earthquake suggests the occurrence of the cascading propagation of the major earthquakes and aseismic slip towards the mainshock epicenter, which was likely to trigger the mainshock.
After the mainshock, some intraslab earthquakes occurred near the Japan Trench, associated with the slab bending stress perturbed by the 2011 mainshock. Based on a finite fault modeling of the intraslab doublet earthquake on Dec. 12, 2012, the shallow normal-faulting sub-event (Mw 7.2) extends down to ~ 35 – 40 km whereas the top of the deep thrust-faulting sub-event (Mw 7.2) was estimated at ~ 45 – 50 km, indicating an obvious deepening of the intraslab seismicity after the mainshock. The change is interpreted as the results of static stress change and of reduction of shear strength in the slab. Meanwhile, an intraslab strike-slip earthquake (Mw 7.0) in 2011 beneath the landward slope of the Japan Trench happened within the shallow portion of the slab under the downdip extensional field and its fault did not expand below the stress neutral depth estimated by the pre-2011 seismicity. This suggests that the enhancement of the downdip tensional stress after the mainshock was not large as compared to the near-trench area.
On March 9, 2011, Mw 7.2 and 6.6 interplate earthquakes preceded the mainshock at ~ 30 km northeast from the mainshock epicenter. The rupture area of the Mw 7.2 event was complementary to its postseismic slip area and to the epicenters of smaller seismicity before the mainshock. After the Mw 6.6 event, one of the aftershocks of the Mw 7.2, seismicity abruptly expanded to the area next to the mainshock epicenter. This migration of seismicity after the Mw 7.2 earthquake suggests the occurrence of the cascading propagation of the major earthquakes and aseismic slip towards the mainshock epicenter, which was likely to trigger the mainshock.
After the mainshock, some intraslab earthquakes occurred near the Japan Trench, associated with the slab bending stress perturbed by the 2011 mainshock. Based on a finite fault modeling of the intraslab doublet earthquake on Dec. 12, 2012, the shallow normal-faulting sub-event (Mw 7.2) extends down to ~ 35 – 40 km whereas the top of the deep thrust-faulting sub-event (Mw 7.2) was estimated at ~ 45 – 50 km, indicating an obvious deepening of the intraslab seismicity after the mainshock. The change is interpreted as the results of static stress change and of reduction of shear strength in the slab. Meanwhile, an intraslab strike-slip earthquake (Mw 7.0) in 2011 beneath the landward slope of the Japan Trench happened within the shallow portion of the slab under the downdip extensional field and its fault did not expand below the stress neutral depth estimated by the pre-2011 seismicity. This suggests that the enhancement of the downdip tensional stress after the mainshock was not large as compared to the near-trench area.