Detailed Near Surface Geophysical Survey at an Area Stricken by the 2016 Kumamoto Earthquakes

Abstract

We conducted a detailed near surface geophysical survey at Mashiki Town, Kumamoto Prefecture, western Japan, where was severely damaged by the 2016 Kumamoto twin earthquakes of magnitudes 7.3 as the mainshock and 6.4 as a foreshock. The near surface survey comprised "Hybrid Surface Wave Survey" (HSWS), capacitively coupled resistivity (CCR) measurement using OhmMapper, and GPR measurements using Utility Scan DF or 350 HS tools. A total of 5 short survey lines were set to intersect a branched surface rupture, or placed in the downtown area of Mashiki Town. The purposes of the survey were to assess the usefulness of the above geophysical methods for the delineation of near-surface conditions of such earthen structures as levees and road embankments attacked by strong earthquakes, and to provide high-resolution subsurface profiles of the sites where surface structures were sporadically damaged. Notable results of our study were as follows. First, GPR successfully imaged detailed structures on the surficial zones form 2 m to in case to 5 m in depth. It was characteristic that a number of step-like sharp dislocations were identified around the surface rupture. Layered resistivity structures, concordant with inferred geological structure of the area, were imaged by CCR surveys up to 10 m in depth. HSWS, recently proposed by the authors, reconstructed S-wave structures up to 40 m in depth. In addition, HSWS records were processed through an ordinary seismic reflection data processing flow. As a result, reverse faulting structure was clearly imaged in concordance with the other near-surface survey results.