How Japan predicts earthquakes. Hint: It’s all about numbers.

Anyone who has experienced a major earthquake, such as the Kobe Earthquake that struck western Japan in 1995, or the March 2011 earthquake and tsunami that devastated northeastern Japan, is aware of their frightening power. What is much harder to know is when the next big quake will occur near you.

In seismically active Japan, the government calculates and publishes the odds of a major temblor occurring in any one place using various methods, but the predictions are not always consistent — with good reason.

Margin of error

In the early hours of April 16, a magnitude-7.3 earthquake hit the southern Japanese prefecture of Kumamoto. The temblor occurred along the Futagawa fault. Active faults — cracks in the earth’s crust — are constantly being pushed and pulled by the surrounding tectonic plates. When the stress builds beyond the breaking point, the plates slip suddenly, causing an earthquake.

The government’s Headquarters for Earthquake Research Promotion put the likelihood of a magnitude-7 or -8 earthquake occurring in the next 30 years in the area where Kumamoto earthquake took place at between “nearly zero and 0.9%.” The research group publishes long-term predictions for big earthquakes by studying 97 major active faults across the country. Their predictions are based on the frequency of earthquakes at a given point, and when the most recent quake has occurred.

An active fault causes a major earthquake once every 1,000 to tens of thousands of years. Thus, a probability of a big temblor shaking any one spot over a 30-year time span is quite low. Although this is a convenient, human-scale time span, it can give people a false sense of security. When the Kumamoto quake struck, the group’s predictions for the area were criticized for not properly communicating the risk to residents.

In response to that criticism, the team in August decided to use a probability ranking to indicate the level of risk along active faults. It labels faults with a 3% or higher chance of having a big earthquake within 30 years as “S” (highly probable), those with a probability between 0.1% and 3% as “A” (somewhat probable), and those with a less-than 0.1% chance as “Z” (other). Those that the probability cannot be or has not been determined are classified as “X.”

But there are other active faults near the designated faults and movement along any of them can trigger a temblor. So researchers began publishing the probability of fault quakes by region in 2013. To calculate the likelihood of a magnitude-6.8 or stronger quake taking place somewhere in a given region within 30 years, the probability of an occurrence is calculated for different active faults within a region.

In determining the overall probability, the team also takes into consideration earthquakes that may occur along smaller active faults, since these caused significant damage in earthquakes that hit central Japan’s Niigata Prefecture in 2004 and the Noto Peninsula in Ishikawa Prefecture in 2007.

This method led researchers to conclude, before the Kumamoto earthquake, that there was an 18-27% chance of a large quake in central Kyushu, the island in the southern Japan where the Futagawa fault runs.

Although regional probabilities provide more realistic numbers than predictions for quakes along specific faults, the numbers change depending on where the fault lines are drawn. The larger the area, the higher the probability of an earthquake occurring somewhere in the zone. In their July report, for example, the researchers found a 40% probability of a big quake for the northern part of the Chugoku region in western Japan.

Feel the shake

The regional forecasts, however, exclude the risk of another type of earthquake common in Japan: subduction-zone earthquakes. These occur where an oceanic plate sinks below a continental plate along an undersea trench. The plate can suddenly spring back, releasing pent-up tectonic stress. This type of earthquake of magnitude 7-8 occurs once in several decades to a few hundred years, making them much more frequent than quakes along active faults.

The research group also determines the probability of a large subduction-zone earthquake in terms of risk areas. For instance, it thinks there is a nearly 70% chance of a magnitude-8 or -9 earthquake occurring within 30 years along the Nankai Trough, which stretches off the Pacific coast of Japan’s main island of Honshu from the Tokai region in the east to the western island of Shikoku.

The team also publishes a national seismic hazard map to warn people of both fault and subduction-zone earthquakes across the country. Unlike the team’s other predictions, this map focuses on the scale of a potential earthquake rather than its magnitude, which measures how powerful it is.

The map shows the likelihood of an earthquake of a particular scale hitting an area within 30 years. In the latest estimate, published in June, Tokyo had a 47% chance of an earthquake, while Osaka had a 55% chance. The Greater Tokyo area, and areas along the Nankai Trough, which have both seen increased seismic activity recently, were assigned high probabilities: The cities of Mito, Chiba and Yokohama near Tokyo each had probabilities estimated at 80%. In Shikoku, the cities of Kochi and Tokushima had probabilities above 70%.

The map is “the most important source of information for disaster prevention,” said Naoshi Hirata, head of the Earthquake Research Committee, a subgroup of the body that publishes the quake predictions. “Nowhere in Japan is free from earthquakes,” he said, calling on people to use the map to prepare for the next shake-up.

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