EVALUATION OF SEISMIC INTENSITIES DURING THE NORTHERN OSAKA PREFECTURE EARTHQUAKE OF JUNE 18, 2018 BASED ON TOMBSTONE BEHAVIOR SURVEYS

Abstract

 The seismic intensity distributions estimated by the Japan Meteorological Agency (JMA) and the National Research Institute for Earth Science and Disaster Resilience (NIED), which were published immediately after the Northern Osaka Prefecture of June 18, 2018 earthquake (Mj6.1), shows that estimated intensities are 6-lower in the major areas of Takatsuki and Ibaraki Cities. However, the damage to houses and other structures was lighter than expected. To investigate the high-density seismic intensity distribution in this area, we conducted a high-density tombstone behavior survey, evaluated the tombstone fall rate, TFR, and the spatial distribution of tombstone-based seismic intensity (TSI). As a result, the TSI was estimated to be 5-upper to 6-upper in Ibaraki City, and 5-lower or lower than that in the southern part of Takatsuki City without a tombstone fall. In comparison with the topography, it was found that the TSI of 5-lower or lower than that is evaluated in the back marsh and that of 6-lower and 6-upper were concentrated in the fan beside Senri hill west of the Ai River. The MZO and SKU sites where the tombstone fall rate TFR was particularly high were located near the old river channel and on the old water area in Ibaraki City, respectively. It is probable that locally soft or changing micro-topography affected the fall rate. It was also found that the tombstones were less likely to fall after 1995 when the TSI increased up to 6-lower or 6-upper.