RELATION BETWEEN SEISMIC INTENSITY AND PREDOMINANT FREQUENCY OF THE GROUND IN IBARAKI CITY DURING THE 2018 NORTHERN OSAKA EARTHQUAKE

Abstract

 An earthquake (Mj 6.1) occurred in northern Osaka Prefecture on June 18, 2018. Immediately after the earthquake, the Japan Meteorological Agency (JMA) and other organization released the map of estimated seismic intensity distribution, which showed that the estimated intensity is 6-lower on the JMA scale over a wide area of Ibaraki and Takatsuki cities. On the other hand, in these areas, there were many cases of partial damage such as roof damage, but very few completely and partially destroyed houses. The seismic intensity estimated by the tombstone behavior varied from 5 lower to 6 upper in Ibaraki City, but was week as 5 lower or weaker in Takatsuki City. In this paper, we describe the results of dense single-point microtremor measurements carried out to clarify the characteristics of ground vibrations in terms of differences in detailed topography, focusing the area where the tombstone-behavior based seismic intensity varied in Ibaraki City. We clarify that the predominant period of vibration of the ground is approximately 0.1 to 0.2 seconds in the area where the tombstone seismic intensity is 6 upper, approximately 0.07 to 0.1 seconds in the area where the tombstone intensity was 6 lower, and equal to or longer than 0.2 seconds in the areas where the tombstone intensity was 5 upper or weaker.