[S-CG56] Dynamics in mobile belts

[SCG56-P05] Estimating the background stress fields in the source region of the 2016 Kumamoto earthquake based on shear strain energy

*Toshiko Terakawa¹, Kimiyuki Asano², Yumi Urata³ (1.Earthquake and Volcano Research Center, Graduate School of Environmental Studies, Nagoya University, 2.Disaster Prevention Research Institute, Kyoto University, 3.National Institute of Advanced Industrial Science and Technology)

Keywords:stress, shear strain energy, The 2016 Kumamoto earthquake sequence

We aim to estimate the background stress level prior to the 2016 Kumamoto earthquake sequence, examining temporal changes in elastic strain energy from various aspects. Shear stress is equal to the frictional strength of the fault at the time of an earthquake. With the method of Terakawa & Hauksson (2018) we estimated the six components of the background stress fields by examining the fault orientations relative to the stress pattern, and by assuming the Coulomb failure criterion characterized by three representative apparent friction coefficients (m’ = 0.4, 0.2, and 0.1). We also estimated coseismic stress change fields due to the largest preshock (14 April) and the mainshock (16 April), using the slip distributions of the events and slip response function (Asano & Iwata, 2016; Fukahata & Matsu’ura, 2005). Then, we obtained three (absolute) stress fields immediately after the mainshock, superposing the coseismic stress changes fields on the background stress fields. Using all the six components of stress fields immediately before and after the earthquake sequence, we evaluated temporal changes in elastic strain energies (shear and volumetric strain energies).

We examined relationship between the total changes in shear strain energies and the final slip on the mainshock faults. In the case of m’ = 0.4, the larger slip contributed to release shear strain energies more. We found similar tendency in the case of m’ = 0.2 except for the shallow part with large slip on the Hinagu fault. In the case of m’ = 0.1, on the other hand, large slip in most shallow parts, which includes large slip area at Futagawa fault, contributed to increase shear strain energies. The results with m’ = 0.1 are inconsistent with that earthquakes are physical process to release shear strain energies. This perspective is supported by further analysis on the relation between temporal changes in slip and shear strain energies and on temporal changes in stress orientation. Considering these results, we conclude that the background stress fields with m’ = 0.4 can be the most rational, and that the source region of the Kumamoto earthquake sequence is in the Anderson-Byerlee stress condition.