20 Mai 2012
Several experts have questioned whether "fracture zones" directly underneath reactor buildings at the Japan Atomic Power Co.'s Tsuruga nuclear power plant in Tsuruga, Fukui Prefecture, are active faults. The Mainichi answers common questions readers may have about these fracture zones.
Question: What is a fracture zone?
Answer: When faults misalign with each other and cause an earthquake, the fault surfaces can crush rocks and create a cracked zone in the bedrock. This is a fracture zone. Most are said to be from a few centimeters to a few meters thick. Around 150 to 160 have been found on the grounds of the Tsuruga nuclear power plant.
Q: What are active faults?
A: Active faults are those that have moved many times within about the past 2 million years and have a possibility of moving again. Japan's earthquake-resistance schematic evaluation policies define active faults as those that have moved since 120,000 to 130,000 years ago.
Q: How is it known when a fault moved?
A: Generally older ground layers are below newer ground layers, and according to earthquake researcher Kunihiko Shimazaki, professor emeritus at the University of Tokyo, one way to date them is to dig up ground layers and compare them to dated volcanic ash. Another way is to use carbon-14 dating on the carbon in wooden fragments if they remain in the ground layers. Using such methods, the times of ground movements and their frequency can be estimated.
Q: How will the fracture zones at the nuclear plant be investigated?
A: The plan is to collect volcanic ash on the problem fracture zones from many different spots to improve the accuracy of dating estimates, and also to investigate minerals in the fracture zones. After carefully examining the faults' history of activity, technicians plan to submit a report to the national government by November. (Answers by Taku Nishikawa, Science & Environment News Department)