Anticipating a major Nankai Trough quake

The Meteorological Agency has launched a system in which it will release information on a much-feared major earthquake in the Nankai Trough off the Pacific coast if the probability of such a quake occurring is deemed higher than normal. Given the current status of seismological data and knowledge, a precise forecast of an earthquake is impossible. Although the new system may be of some help, it is more important for both the government and the private sector to be prepared for an earthquake on the assumption that one can strike at any time and at any place without warning.

The idea for the new system arose after the government’s Central Disaster Management Council issued a report in September. Noting the difficulty of predicting the occurrence of an earthquake with a high degree of certainty, the report called for a review of anti-disaster measures that are based on the assumption that quake forecasts are possible, the notion presumed in the Law on Special Measures Concerning Countermeasures for Large-Scale Earthquakes, which empowers the prime minister to issue an official alert when a large-scale quake is forecast as imminent in and around Suruga Bay off Shizuoka Prefecture. As a result, the system for predicting the Tokai quake — with the prime minister issuing an alert on its basis — is effectively no longer in place.

The government is now studying how the national and local authorities should respond when the chances of a large-scale quake occurring in the whole Nankai Trough become high. The Meteorological Agency decided to launch the new system before the government study is concluded.

The Nankai Trough at the bottom of the Pacific Ocean stretches from the Tokai region to Kyushu. As the Philippine Sea plate is subducting beneath the continental Eurasian plate in the Nankai Trough, tectonic strain is accumulating. Large-scale quakes have happened at an interval of 100 to 150 years in the trough. The government estimates a roughly 70 percent chance that a magnitude 8 or 9 temblor will strike within 30 years. It is feared that if seismological activities in three hypocentral regions in the trough — Tokai, Tonankai and Nankai — interlock, a mega-quake will hit, triggering tsunami more than 30 meters high.

Under the new system, an experts’ committee will hold an evaluation meeting if an abnormal phenomenon occurs in the area where the big quake is feared to hit. These include an earthquake of magnitude 7 or more, an unusual change in the measurement of strain meters observing the condition of plate boundaries following a magnitude 6 quake or significant changes in the measurement of strain meters without a quake. The experts will assess whether the phenomenon could lead to a mega-quake. If the committee concludes that the chances of a big quake have increased, it will issue a warning and urge people to remain on alert for subsequent information.

But since the information to be released is not to be deemed a quake forecast with a high degree of certainty, the committee will not call for an immediate evacuation of residents. Because of the nature of the information, it is possible that a big quake won’t take place. On the other hand, the opposite could happen — the Meteorological Agency and the committee may fail to notice signs linked to an imminent major quake. It is important once again to remember that it is impossible for seismologists to predict when or with how much intensity an earthquake will occur.

That does not mean the national and local governments can slacken their efforts to beef up the network for observing earthquake-related phenomena such as crustal movements and tsunami. The network plays an important role in issuing immediate earthquake reports and tsunami warnings, thereby contributing to preventing or reducing damage from such disasters.

Currently there is a regional imbalance in the observation network. The Meteorological Agency and the Shizuoka Prefecture have strain meters several hundred meters underground in 27 locations in the prefecture and the eastern part of neighboring Aichi Prefecture — a result of the government project to predict the occurrence of a big Tokai quake. The strain observation in the area is carried out around the clock. But on the Kii Peninsula and in Shikoku, the agency has no strain meters. Although it receives observation data from the National Institute of Advanced Industrial Science and Technology, the observation does not occur 24 hours a day. The agency needs to take steps to improve the monitoring network.

While the Japanese archipelago occupies less than 1 percent of the Earth’s surface, more than 10 percent of the world’s major earthquakes occur here. Large quakes hitting without advance signs are a possibility not just in the Nankai Trough area, but everywhere. Local authorities, businesses and people in general can’t be too careful in taking precautionary measures to minimize damage from big quakes and tsunami.