Analysis of Causes of Differences in Tremors Affecting Hamaoka Nuclear Power Station during Suruga Bay Earthquake
April 05, 2010
Chubu Electric Power Co.,Inc.
By means of a survey of the characteristics of underground structures (additional survey) conducted in response to the Suruga Bay earthquake in August 2009 and analyses of seismological monitoring records, Chubu Electric is analyzing the causes of differences in tremors that have affected its power generation facilities during the earthquake. As the survey results and evaluations are compiled, they are reported to a government Working Group*. The present report compiles details of this analysis of causes of differences in tremors affecting the facilities that have already been reported.
The results obtained to date can be summarized as follows:
- The results of part of the survey of the characteristics of underground structures (additional survey) have indicated the existence in the Sagara layers (alternating layers of sandstone and mudstone) under the facilities, as a localized structure, of alternating sandstone-dominant layers in which the proportion of sandstone exceeds 50%. Subsequent survey results indicated the existence of a "low-velocity layer," sitting atop the alternating sandstone-dominant layers, that slowed the velocity of the S-waves by approximately 30% against the surrounding bedrock, from 300 to 500 m below Reactor No. 5.
- Based on the characteristics of a reflection surface profile obtained from a seismic reflection survey, among other data, it is believed that this "low-velocity layer" is locally distributed in an eastward direction from the vicinity of Reactor No. 5 to the exterior of the facility site.
- The results of an analysis using a ground model that reflected the distribution of this "low-velocity layer" suggested that, given that a difference was observed in the vicinity of Reactor No. 5 in the case of the tremors from the direction of the main shock of the Suruga Bay earthquake (a northeasterly direction, as viewed from the facility), the "low-velocity layer" was the main cause of the greater intensity of the tremors at Reactor No. 5 during the Suruga Bay earthquake.
Chubu Electric will continue to conduct further studies, for example increasing the accuracy of the "low-velocity layer" analytic model, and will report results to the government Working Group.
* The government Working Group is a Joint Working Group on Earthquake, Tsunami, Geology, and Ground Foundation under the Seismic and Structural Design Subcommittee, Nuclear and Industrial Safety Subcommittee, Advisory Committee for Natural Resources and Energy.