Abstract
On December 7, 2012, an earthquake occurred within the Pacific Plate near the Japan Trench, which was composed of deep reverse- and shallow normal-faulting subevents (Mw 7.2 and 7.1, respectively) with a time interval of ~10 s. It had been known that the stress state within the plate was characterized by shallow tensile and deep horizontal compressional stresses due to the bending of the plate (bending stress). This study estimates the fault model of the doublet earthquake utilizing tsunami, teleseismic, and aftershock data and discusses the stress state within the incoming plate and spatiotemporal changes seen in it after the 2011 Tohoku-Oki earthquake. We obtained the vertical extents of the fault planes of deep and shallow subevents as ~45–70 km and ~5 (the seafloor)–35 km, respectively. The down-dip edge of the shallow normal-faulting seismic zone (~30–35 km) deepened significantly compared to what it was in 2007 (~25 km). However, a quantitative comparison of the brittle strength and bending stress suggested that the change in stress after the Tohoku-Oki earthquake was too small to deepen the down-dip end of the seismicity by ~10 km. To explain the seismicity that occurred at a depth of ~30–35 km, the frictional coefficient in the normal-faulting depth range required would have had to be ~0.07 ≤ μ ≤ ~0.2, which is significantly smaller than the typical friction coefficient. This suggests the infiltration of pore fluid along the bending faults, down to ~30–35 km. It is considered that the plate had already yielded to a depth of ~35 km before 2011 and that the seismicity of the area was reactivated by the increase in stress from the Tohoku-Oki earthquake.
Highlights
It is well known that the stress state within the incoming Pacific Plate near the Japan Trench is characterized by shallow tensile and deep horizontal compressional stresses along a direction perpendicular to the trench axis, separated by a thin aseismic “elastic core”
Fault modeling of subevent 2 Because we found that the subsidence of the tsunami source was generated by the shallow subevent 2, we first estimate a fault model for subevent 2, which best explains the subsidence region of the tsunami source model
Because the short-wavelength component disappeared in the tsunami source model due to the smoothing constraint imposed in the inversion and the spatial smoothing effect used in the deep-sea region during the tsunami generation, we consider the smoothing effect in fault modeling by the following procedure
Summary
We utilize the tsunami and aftershock data and the results of the teleseismic analysis to estimate the finite fault model of the 2012 doublet earthquake, focusing on the vertical extent of the fault planes of each subevent. The slip distribution of subevent 1 obtained by the teleseismic analysis (Lay et al 2013; Harada et al 2013) had a main rupture area with a dimension of L ~ 30 km and W ~ 20 km; we fix the fault length and width as 30 and 20 km, respectively (L1 = L2 = 15 km and W1 = W2 = 10 km).
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