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Withstanding a megaquake

Appropriate fault assessment

Geological faults can be divided into those that could become active again in the future (active faults) and those that have become stable.
Active faults could cause an earthquake or large displacement of the ground surface.
There is no active fault underneath the site of the Hamaoka Nuclear Power Station.
Although there is a fault called the H Fault System at the site, detailed surveys have confirmed that it is not an active fault.

Faults, active faults

Faults are areas in which subterranean strata and bedrock have been displaced in the past. These displacements have been caused by the action of subterranean pushing and pulling forces generated by crustal movement (the movement of tectonic plates) and other geological activity over the course of extremely long periods. Geological surveys provide information about these faults, including information concerning their length, depth, and other aspects of their distribution, the direction of the forces on the faults, and the number of years in which there has been no motion in them.
Active faults are faults which geological science, based on the results of surveys, tells us have moved repeatedly in the past and have the potential to move again in the future. This can result in earthquakes or large displacements of the ground surface.

The H fault System

Past surveys has identified a geological fault at the site of the Hamaoka Nuclear Power Station (called the H Fault System).
Detailed surveys have examined the status and the fault characteristics of the H fault system, concluding that there is no possibility of it becoming active again in the future to cause an earthquake or large displacement of the ground surface (therefore not an active fault). We will continue working to further improve reliability and strive to reflect on the latest finding.

The results of surveys of the H fault system

An image of State of a fault on the site

About fracture zone

Although a fracture zone itself may not cause an earthquake, it has been pointed out that it could become dragged into the movement of a nearby active fault and cause large displacement on the ground surface.
A fracture zone is defined as a band of fractured bedrock, destroyed with force generated with the movement of a geological fault underground.

On the other hand, detailed surveys have confirmed that the H Fault System is already solidified in the pulled and stretched state, showing no characteristics of a fracture zone, described above. In assessing the safety of a nuclear power station, it is important to examine the possibility of such a geological feature moving again in the future.

As explained above, it has been concluded that the H Fault System has no possibility of becoming active again in the future.

Results of assessment of active faults existing in the vicinity of Hamaoka Nuclear Power Station

Surveys to date have demonstrated the existence of some 25 active faults on land and under the sea close to the Hamaoka Nuclear Power Station. We conducted detailed assessments of the active faults able to produce vibration of sufficient power to affect the station, and verified safety based on analysis and assessment of the most powerful potential seismic motion, including linked earthquakes with interplate earthquakes. The assessment methods we are employing are stipulated by the government, based on the opinions of seismological experts.

Active faults in the vicinity of Hamaoka Nuclear Power Station

Influence of crustal movements

The continental plate is lifted in an interplate earthquake. If the earthquake in the Nankai Trough actually occurs, the ground around Hamaoka Nuclear Power Station is expected to be elevated due to the upward movement of the land-side plate. However, this will be a gently sloping uplift over a wide area, and will not result in any partial large elevation that could affect structures and facilities.