The Mainichi answers some common questions readers may have about the type of fault behind the April 2016 Kumamoto Earthquake.
Question: What are "faults"?
Answer: Faults are fractures in underground strata resulting from the release of built-up energy. Those that have the possibility of future activity are called active faults, and these usually do not move. When stress builds up, however, they can suddenly slip -- causing earthquakes. The 1995 Great Hanshin Earthquake is an example of an earthquake caused by an active fault.
Q: How many active faults are there in Japan?
A: There are said to be at least 2,000 active faults across the country. The latest Kumamoto quakes are believed to have been caused by the Hinagu and Futagawa fault zones. The Japan Meteorological Agency believes that each of the massive earthquakes in Kumamoto and surrounding areas was caused by an active fault slipping.
Q: How was the Kumamoto Earthquake mechanism different from the Great East Japan Earthquake?
A: The earth's surface is covered by tectonic plates believed to be between several dozen to 200 kilometers thick. Japan sits on the border of four plates, and the Great East Japan Earthquake happened when pressure between two plates built up and one plate thrust upward. Its mechanism was therefore different from the Kumamoto earthquakes, which were caused by active faults.
Q: Is it possible to predict when active faults will move?
A: Japan's Earthquake Research Committee had previously estimated that the chances within the next 30 years of an earthquake of magnitude 6.8 or greater in Kyushu's southern region -- which includes the Hinagu and Futagawa fault zones -- were 7 to 18 percent. Because active faults move in cycles of 1,000 to several tens of thousands of years, however, it is impossible to accurately predict when fault-induced quakes will take place. This means that we must always be prepared for the next earthquake, and continue to review whether our disaster countermeasures are adequate.