Vault-housed extensometers recorded two earthquakes in the Hyuga-nada subduction zone, Japan, with similar hypocenter locations that generated strain with the opposite sign

Abstract Miyazaki Observatory in southern Kyushu Island, Southwest Japan, acquires continuous strain measurements using vault-housed extensometers at several sites located adjacent to the offshore Hyuga-nada subduction zone. Two thrust earthquakes occurred in this offshore region on 10 May 2019 with similar hypocenter locations and fault mechanisms; however, the extensometers installed in one direction at Miyazaki Observatory recorded strain with the opposite sign for these events. Strain calculations within an assumed uniform half-space medium show that the opposite signs of the coseismic strain can be due to differences in hypocenter locations of only a few kilometers. This result shows that strainmeters can effectively detect offshore events and determine their relative locations when other ground observations are unable to precisely locate such events.