A government institution says the magnitude 6.1 quake that hit Osaka Prefecture on June 18th may have been a result of movement along 3 fault zones running near the epicenter.
But the Headquarters for Earthquake Research Promotion adds that further studies are necessary before determining its cause.
Analysis has also shown that strain is building up across these fault lines. A researcher studying active faults says areas in and around Osaka may have become more prone to major earthquakes as a result of the recent trembler.
The magnitude 6.1 earthquake hit northern Osaka Prefecture on the morning of June 18th. Jolts with an intensity of 6-minus on the Japanese intensity scale of 0 to 7 were registered in the cities of Takatsuki, Hirakata, Ibaraki, and Minoh, as well as in Osaka City's Kita Ward. Those with intensity of 5-plus were observed in Nakagyo Ward in Kyoto City, Kameoka City in Kyoto Prefecture, Higashi-Yodogawa Ward in Osaka city, and Suita City in Osaka Prefecture.
There are 3 fault zones around the quake's epicenter. One is the Arima-Takatsuki fault zone that runs east-west. The other 2 are the Ikoma and Uemachi fault zones that run north-south.
The Arima-Takatsuki fault zone stretches about 55 kilometers from Kita Ward in Kobe City to Takatsuki City in Osaka Prefecture. The Headquarters for Earthquake Research Promotion says a quake with a magnitude of up to 7.5 could occur if a slip occurs along the eastern part or the entire stretch of this fault zone. It estimates the probability of such a quake occurring within 30 years is 0 to 0.03 percent.
The Uemachi fault zone stretches about 42 kilometers between Toyonaka and Kishiwada Cities, both in Osaka Prefecture. It runs through the busy Osaka City. The institution says a quake with a magnitude of up to 7.5 could occur if the entire length of this fault zone slips. It estimates the probability of such a quake occurring within 30 years at 2 to 3 percent.
The Ikoma fault zone stretches about 38 kilometers between Hirakata and Habikino Cities in Osaka. The institution says a quake with a magnitude of 7 to 7.5 could occur if the entire stretch of this fault zone slips. It estimates the probability of such a quake occurring within 30 years at 0 to 0.2 percent.
It says the recent quake may have been caused by activity of the 3 fault zones. But as no obvious changes were observed on the ground surface, it is difficult to determine which of them slipped. It says there is a need to conduct more surveys before reaching a conclusion.
Tohoku University Professor Shinji Toda, an expert on the mechanism of fault movements, has analyzed the impact of the quake on the 3 fault zones. He found strain is building up along each of them. Strain has built up in large parts of the Arima-Takatsuki and Uemachi fault zones, and in some part of the Ikoma fault zone.
Toda warns that a major earthquake may have become more likely in and around Osaka after the June 18th trembler, though it's not known when it will strike. He says local residents should be prepared.
The earthquake in Osaka has killed 5 people and injured at least 420 others. One of the victims, a 9-year-old girl, was killed when a concrete block wall collapsed on top of her. Her death shocked the country and sparked investigations.
The quake registered 6-minus on the Japanese seismic intensity scale of 0 to 7. The Meteorological Agency says jolts of such intensity make it difficult for people to remain standing. Unsecured furniture may move and topple over. Damaged wall tiles and windows may fall.
The agency says the last time a 6-minus quake hit Japan was in December 2016. It rocked northern Ibaraki Prefecture and injured 2 people.
The June 18th quake was the first with an intensity of 6-minus or higher to hit Osaka Prefecture since record-taking begin in 1923.
Osaka experienced tremors with an intensity of 5 when a magnitude 6.4 quake hit Nara Prefecture on February 21st, 1936, and those with an intensity of 5-minus when a magnitude 6.3 quake hit Hyogo Prefecture on April 13th, 2013.
The Meteorological Agency says that after the June 18th earthquake, more than 30 quakes with intensities of one or higher were observed by 6 PM on June 20th. They are believed to be aftershocks.
The agency warns of a growing risk of landslides or buildings collapsing in the hardest-hit areas, and calls on residents to stay away from risky areas.
Its officials are urging residents to stay on the alert for another trembler with an intensity of around 6-minus for at least one week after the first quake.
They also warn that even a small amount of rain could trigger mudslides in the affected areas, as powerful jolts have loosened the ground.
Mega-quakes have repeatedly occurred in the Nankai Trough off Japan's Pacific coast at intervals of about 100 to 200 years. The last time such a quake hit was in 1946. The magnitude 8 Showa-Nankai quake devastated wide areas of Shikoku and other regions.
The Headquarters for Earthquake Research Promotion says there is a 70 to 80 percent chance that a powerful quake with a magnitude of 8 or 9 will occur in Japan within the next 30 years. It says such a quake will inflict serious damage mainly on the Shikoku, Kinki and Tokai regions.
The government estimates that it will lead to the deaths of 323,000 people and more than 2.35 million buildings will collapse or burn down.
The government and the Meteorological Agency say huge quakes could occur anywhere in Japan, and are calling on the public to be prepared.