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A Trial Application of Geodetic Data for Inland Fault Assessment – Coulomb Stress Changes Estimated from GNSS Surface Displacements

We examine a method to calculate changes in Coulomb failure stress (ΔCFS) from observed GNSS displacements. The method assumes no stress changes on a horizontal plane and a linear elastic relation between strain and stress, represented by Hooke’s law. The ΔCFS distributions calculated using this method are applied to the 2003 Tokachi-oki and the 2016 Kumamoto earthquakes and they are compared with those using a standard dislocation model buried in an elastic half-space. The results suggest that the ΔCFS distribution at a depth of 10 km in a region far from a deformation source can give a first-order approximation using observations of surface displacements. However, ΔCFS distributions near the source cannot be reproduced by the examined method and need to be evaluated using the standard method. We apply the examined method and GNSS displacement data to calculate ΔCFS on major active faults as well as source faults of large inland earthquakes in southwest Japan for the period 1996-2017. ΔCFS from five large earthquakes, including the 2016 Kumamoto earthquake are separately calculated using the standard method with published fault models. Calculated ΔCFS increases by an order of 10 KPa at most faults over the past 21 years. ΔCFS on the source faults for the 2000 Western Tottori, the 2016 Kumamoto, and the 2016 Central Tottori earthquakes reached a maximum just before their rupture. Coseismic and postseismic deformation of the 2011 Tohoku-oki earthquake accelerated an increase of ΔCFS at some faults, including the source fault of the 2016 Central Tottori earthquake and the Arima-Takatsuki fault zone. The examined method can provide information on the activity of inland earthquakes using contemporarily observed deformation, and can hopefully improve the preparedness for earthquakes.