Inverse Method for Static Stress Drop and Application to the 2011 M_w9.0 Tohoku‐Oki Earthquake

Abstract

Seismic stress drop is a fundamental parameter for the investigation of earthquake mechanism. In general, it is indirectly predicted by fault slip based on the dislocation source model or seismic moment; inversion for the stress change on faults has not received the deserved attention. In this study, we propose a finite element method to invert the stress drop on fault, constrained by the observed coseismic deformation. Rupture termination and displacement on fault are automatically predicted from the model. Applying the method to the 2011 Mw9.0 Tohoku‐Oki earthquake, we find that the fault consists of two asperities with maximum shear stress drops of 11.7 and 10.1 MPa, respectively. The predicted maximum horizontal and vertical displacements on the hanging wall at the Japan Trench are 55.2 and 10.8 m, respectively, in good agreement with observation. The predicted total static moments of the mainshock and the Mw7.9 aftershock, 29 min after the mainshock, are 4.48 × 1022 and 1.46 × 1021 Nm, corresponding to moment magnitudes of Mw9.0 and Mw8.0, respectively, again in excellent agreement with the observationally determined Mw9.0 and Mw7.9 by the U.S. Geological Survey.

Fault stress inversion reveals seismogenic asperity of the 2011 Mw 9.0 Tohoku-Oki earthquake

“…Furthermore, the agreement between the two smaller seismogenic asperities predicted in this study also compare well with the areas of the two largest aftershocks. In addition, the predicted depth of the seismogenic asperity of the mainshock by the contour of zero-shear stress is consistent with that of the rupture termination identified 13 , marked by black-dish line in Fig. 2a.…”

“…But the centers of both predicted areas were much deeper (35–40 km) than that of our seismogenic asperity (~15 km). The agreement of the latter with the rupture area of the mainshock 13 is strong evidence for the reliability of the present method. Furthermore, the agreement between the two smaller seismogenic asperities predicted in this study also compare well with the areas of the two largest aftershocks.…”

“…Figure 2a shows that three prominent zones of high shear-stress accumulation (positive for stress increase) occur near the trench, each centers about 15 km below the seafloor and is bounded by a contour of zero-shear stress. The zone of the fastest shear-stress accumulation, amounting to 0.3 MPa over the seven years, hosts the hypocenter and main rupture zone of the future M w 9.0 Tohoku-Oki earthquake 13 . The normal stress also increases in this zone over the seven years (Fig.…”

“…An inversion method based on an earthquake stress model (Methods) was proposed 13 for determining not only the shear stress change on fault surface but also the normal stress change on the ruptured fault, by which they directly determined the coseismic stress drop of the 2011 M w 9.0 Tohoku-Oki earthquake from inverting the coseismic GNSS displacements. In this study we use the same method that inverts the displacements (see Table S1) at 163 GPS onshore stations of the GEONET in the period from 2004 to 2011 to determine stress accumulation on the megathrust over a period of seven years prior to the earthquake.…”

Spatial and Temporal Variations in Earthquake Stress Drops between the 2008 Wenchuan and 2013 Lushan Earthquakes

“…In real earthquake sequences, stress drops most likely exhibit spatial variations, creating a stress drop distribution. Recent studies have reported spatial and temporal variations in stress drop in California (Shearer et al, 2006;Allmann and Shearer, 2007;Abercrombie, 2014), Japan (Oth et al, 2011;Yoshida et al, 2017;Xie and Cai, 2018), and New Zealand (Oth and Kaiser, 2014). In these papers the stress drops associated with major earthquakes cannot be treated as a mere progression from low to high values between major events; rather, they demonstrate subtler changes in the character of a stress field on a fault (Hardebeck and Aron, 2009;Candela et al, 2009).…”