Experts are cautiously warning that Monday morning’s powerful earthquake in Osaka and surrounding areas could be a sign that greater seismic activity is due in western Japan in the near future.
While details of the fault movements behind the fatal magnitude 6.1 quake have yet to be determined, the inland temblor occurred at a depth of about 13 km, registered as a lower 6 on the Japanese seismic intensity scale to 7, and struck in an area where active faults are concentrated, experts say.
“The quake was shallow, so there is a possibility that it has something to do with the active faults, but a magnitude 6 class quake itself is not big enough to have been caused directly by an active fault,” explained Yasuhiro Suzuki, a professor at Nagoya University’s Disaster Mitigation Research Center.
“A quake of this level in terms of magnitude and intensity can happen anywhere in Japan, even in places where there are no known active faults,” he said. “But this one occurred where there are big active faults nearby, so we need to think about how it could affect them.”
Suzuki said that while the probability is low, the quake, which killed three people, could trigger a larger seismic movement in the coming days as seen two years ago in Kumamoto Prefecture and surrounding areas of Kyushu when a magnitude 6.5 earthquake was followed two days later by a magnitude 7.3 temblor. At least 50 died from the Kumamoto quakes.
The Meteorological Agency warned the public to brace for potential aftershocks of around the same intensity in the coming week or so.
“There’s no need to be excessively worried,” Suzuki said. “But people should take simple measures like not sleeping close to big pieces of furniture.”
Takuya Nishimura, an associate professor of geodesy at Kyoto University, said it is possible Monday’s quake occurred at the eastern end of a fault zone extending from northern Osaka Prefecture to Awaji Island in adjacent Hyogo.
This is the same fault line that produced the Great Hanshin Earthquake, which claimed more than 6,400 lives in 1995.
“It might be an aftershock resulting from distorted geological formations caused by the 1995 quake,” Nishimura said, adding that another fault further north might be behind Monday’s quake.
He said recent observations have found a high density of underground deformations along the Arima-Takatsuki fault zone. These fault zones near the quake’s epicenter in northern Osaka make the area susceptible to major quakes, experts say.
Shinji Toda, a professor of earthquake geology at Tohoku University, theorized that a different fault running north to south just under the city of Osaka might be associated with the quake.
Toda said experts have regarded the fault as dangerous because it has been inactive for more than 10,000 years, whereas active faults often release energy every once in a while by creating earthquakes.
The experts denied there are direct links between the Osaka quake and a feared quake expected to occur in the Nankai Trough, a long, deep sea canyon off the Pacific coast. A Nankai Trough quake is expected to have enough energy to cause seismic and tsunami damage that could devastate extensive areas of Japan.
“Earthquakes occur very easily and frequently nowadays, so some people have this image that these events will eventually be followed by the Nankai Trough quake,” Nagoya University’s Suzuki said.
“The latest quake could be one of the phenomena related to moves that indicate the Nankai Trough quake may be nearing, so it doesn’t hurt to take this opportunity to take precautions,” he said.