The 3‐D Spatial Distribution of Shear Strain Energy Changes Associated With the 2016 Kumamoto Earthquake Sequence, Southwest Japan. Issue 3 (3rd February 2020)

• Record Type: Journal Article
• Authors: Noda, Akemi; Saito, Tatsuhiko; Fukuyama, Eiichi; Terakawa, Toshiko; Tanaka, Sachiko; Matsu'ura, Mitsuhiro
• Is Part Of: Geophysical research letters. Volume 47:Issue 3(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-02-03
• Subjects: shear strain energy -- 2016 Kumamoto earthquake sequence -- GNSS displacement data -- coseismic stress change -- background stress level; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2019GL086369
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Shear strain energy is an essential physical quantity governing earthquake generation. To calculate the shear strain energy changes due to an earthquake, we need both coseismic stress changes and background crustal stress. The orientation of the background stress can be estimated from earthquake focal mechanisms, but its absolute level is still uncertain. Assuming the level of the background deviatoric stress to be frictional strength in the crust, we evaluated the three‐dimensional distribution of shear strain energy changes associated with the 2016 Kumamoto earthquake sequence, southwest Japan. Its spatial patterns strongly depend on the background stress level. From the energy balance of shear faulting, we proposed that the volume integral of the shear strain energy changes could constrain the background deviatoric stress level. It should be >14 MPa at 10‐km depth at the very least. We showed that approximately 75% of aftershocks occurred where the shear strain energy increased. Plain Language Summary: Since shear strain energy accumulated in the Earth's crust is necessary for earthquake generation, it is essential for understanding the earthquake generation to reveal the distribution of shear strain energy in seismogenic zones. In this study, we calculated the three‐dimensional distribution of shear strain energy changes caused by the 2016 Kumamoto earthquake sequence, southwest Japan. To calculate the shear strain energy changes, we need not only stress changes due to the earthquake but also the crustal stress state before the earthquake (background stress). In general, the orientations of the background stress can be estimated from earthquake observations, but its magnitude (background stress level) is generally unknown. Therefore, we calculated the distribution of the shear strain energy changes for several candidates of the background stress level, and found that its spatial patterns drastically change depending on the background stress level. Furthermore, from the energy balance of shear faulting, we estimated the lower limit of the background stress level to be greater than 14 MPa at 10‐km depth. We also found that local increase in shear strain energy activated aftershock generation there, while the total amount of shear strain energy decreased. Key Points: Coseismic shear strain energy changes strongly depend on the level of background deviatoric stress in the Earth's crust The volume integral of shear strain energy change constrained the background stress level to be >14 MPa at 10‐km depth at the very least A statistically significant number of aftershocks, approximately 75% of the total, occurred where the shear strain energy increased