| 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 7.3as the mainshock and 6.4 as a foreshock. The near surface survey comprised “Hybrid Surface WaveSurvey” (HSWS), capacitively coupled resistivity (CCR) measurement using OhmMapper, and GPRmeasurements using Utility Scan DF or 350 HS tools. A total of 5 short survey lines were set to intersecta branched surface rupture, or placed in the downtown area of Mashiki Town. The purposes of the surveywere to assess the usefulness of the above geophysical methods for the delineation of near-surfaceconditions of such earthen structures as levees and road embankments attacked by strong earthquakes, andto provide high-resolution subsurface profiles of the sites where surface structures were sporadicallydamaged. Notable results of our study were as follows. First, GPR successfully imaged detailed structureson the surficial zones form 2 m to in case to 5 m in depth. It was characteristic that a number of step-likesharp dislocations were identified around the surface rupture. Layered resistivity structures, concordantwith 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, HSWSrecords were processed through an ordinary seismic reflection data processing flow. As a result, reversefaulting structure was clearly imaged in concordance with the other near-surface survey results. |
