Japan braces for major quake amid moderate tremors ahead of 2011 Tohoku earthquake, Fukushima disaster anniversary

The Jama Meteorological Agency (JMA) has recorded 28 quakes since the start of March this year, and 30 similar tremors last month. Most have been relatively small, but there were several level-four quakes on the agency’s seven-level seismic intensity scale. A quake on February 29 had a magnitude of 4.9 and another the following day registered a magnitude of 5.2.

The quakes are occurring both onshore and off the coast, with the JMA warning that tectonic plates are experiencing a “slow-slip event” between the continental plate and the Philippine Sea plate to the east of Chiba Prefecture.

“This area off Chiba Prefecture has seen this sort of slow-slip activity several times over the last few decades, with a series of small to medium-sized quakes coming and going over time,” said Yoshiaki Hisada, a professor who specialises in the study of earthquakes at Japan’s Kogakuin University.

“People sometimes think that these slow-slip quakes are safer because they are releasing smaller amounts of energy more often, but that is incorrect and the energy being released is much smaller than if there was a big quake,” he told This Week in Asia.

While the subterranean activity taking place off Chiba involves different tectonic plates from the faults that shifted so dramatically on March 11, 2011 – causing a magnitude-9 quake that immediately resulted in a tsunami that was in places almost 40 metres high – Hisada warns of more danger.

“Yes, it is a different fault, but this line is closer to the coast of Chiba Prefecture,” he pointed out. “A tsunami may not be as big as the one that struck Tohoku, but it will hit the coast much faster and with less warning.”

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