2:45 PM - 3:00 PM
[SCG55-15] Shear strain energy accumulation in a seismogenic layer by plastic flow: Driving force of large earthquakes in the San-in shear zone
Keywords:strain energy, plastic flow, San-in shear zone
Steady slip or creep on an infinitely long strike-slip fault beneath a seismogenic layer is the simplest model for generating a stress field that can drive strike-slip faults in the seismogenic layer (e.g., Savage and Burford 1970, 1973). In particular, if the earthquake fault and the deep creep are on the same plane, the slip at a large earthquake can be regarded as the release of the slip deficit accumulated during the interseismic period. This simple model can reproduce the displacement rate at the ground surface observed in the San-in shear zone, Japan. However, since the strike of large earthquakes is different from the strike of the deep creep (Nishimura & Takada 2017), it is difficult to apply the concept of the slip deficit to relate the coseismic slip amount. In addition, the deep creep was estimated as plastic flow with a finite width rather than a simple fault plane (Meneses-Gutierrez & Nishimura 2020), and the asthenosphere does not behave elastically in response to steady motions. Further consideration is necessary to discuss the quantitative relationship between surface deformation and large earthquake generation.
In this study, in order to understand the stress accumulation process for inland earthquakes, we formulate the displacement and stress caused by plastic flow in deep lithosphere in a 2D space. Also, applying this model to the observations, we estimate the accumulation/release of the shear strain energy in the San-in shear zone. We consider a layer of elastic lithosphere (thickness H) overlying a fully relaxed asthenosphere and set plastic flow with the shear strain rate v/2T, the thickness T, and the top depth D in the deep lithosphere. We derived the analytical solutions of the displacement rate and stress rate. By setting H = 30km, T = 30km, D = 15km, and v = 11 mm/year, we can reproduce the observed surface displacement rate. Furthermore, by using the background stress field around the San-in shear zone obtained by a stress tensor inversion analysis, we estimated the shear strain energy density distribution generated by the plastic flow by the method proposed by Saito et al (2018JGR). In the seismogenic layer above the plastic body, the shear strain energy density increases by ~4 J/m3/year (assuming the differential stress is 50 MPa). We also estimated the release of the shear strain energy density on the fault plane at the 2000 Western Tottori earthquake (MW 6.6) as 1.2 kJ/m3(assuming the differential stress is 50 MPa). This indicates that this large earthquake released the energy accumulated for ~300 years. This is shorter than the recurrence intervals of general inland earthquakes (> ~1000 years). To fully discuss the energy budget in earthquake cycles, it would be important to quantitatively compare strain energy release by aftershocks, after slips, viscoelastic deformations, and other seismic/non-seismic events in addition to large earthquakes.
In this study, in order to understand the stress accumulation process for inland earthquakes, we formulate the displacement and stress caused by plastic flow in deep lithosphere in a 2D space. Also, applying this model to the observations, we estimate the accumulation/release of the shear strain energy in the San-in shear zone. We consider a layer of elastic lithosphere (thickness H) overlying a fully relaxed asthenosphere and set plastic flow with the shear strain rate v/2T, the thickness T, and the top depth D in the deep lithosphere. We derived the analytical solutions of the displacement rate and stress rate. By setting H = 30km, T = 30km, D = 15km, and v = 11 mm/year, we can reproduce the observed surface displacement rate. Furthermore, by using the background stress field around the San-in shear zone obtained by a stress tensor inversion analysis, we estimated the shear strain energy density distribution generated by the plastic flow by the method proposed by Saito et al (2018JGR). In the seismogenic layer above the plastic body, the shear strain energy density increases by ~4 J/m3/year (assuming the differential stress is 50 MPa). We also estimated the release of the shear strain energy density on the fault plane at the 2000 Western Tottori earthquake (MW 6.6) as 1.2 kJ/m3(assuming the differential stress is 50 MPa). This indicates that this large earthquake released the energy accumulated for ~300 years. This is shorter than the recurrence intervals of general inland earthquakes (> ~1000 years). To fully discuss the energy budget in earthquake cycles, it would be important to quantitatively compare strain energy release by aftershocks, after slips, viscoelastic deformations, and other seismic/non-seismic events in addition to large earthquakes.