Megaquake more likely now than ever? Oh dear

Slow-motion tremors make Tokyo megaquake more likely

14:00 16 April 2014 by Jeff Hecht
The people of Tokyo have long lived in fear of another great earthquake, and those fears are increasingly justified. Slow-motion earthquakes have become more common beneath the city in the last few years, causing tectonic stresses to build up. The after-effects of the 2011 Tōhoku megaquake are also prodding the area in the direction of a big quake, but seismologists cannot predict when it might occur, nor which part of the region’s complex fault system will break.

Shinzaburo Ozawa of the Geospatial Information Authority of Japan in Tsukuba used GPS sensors to track the surface motion of the Bōsō peninsula, the eastern side of Tokyo Bay. Between 28 December 2013 and 10 January 2014, he detected centimetre-scale shifts. These were caused by two tectonic plates, kilometres below the surface, slipping by about 10 centimetres. The motion released as much energy as a magnitude-6.5 earthquake, but it caused no damage because it was spread over two weeks.

Seismographs do not record such slow slips, so they went unnoticed until GPS came along, says Heidi Houston of the University of Washington in Seattle, who was not involved in the research.

The concern is that the slow-slip quakes seem to be coming more frequently, a sign of increasing tectonic stress in the region. The latest slip came only 2.2 years after the previous one, a month-long slip in October and November 2011. The first slips detected, beginning in 1996, were 6.4 years apart.

The earlier-than-expected Bōsō slip is a reminder that “it is essential to keep a close eye on the deformation and seismicity in this region,” says Roland Burgmann of the University of California, Berkeley, who was not involved in the study.

Shifting plates

Ozawa’s research adds to the evidence that a big Tokyo quake is on the way. After the 2011 Tōhoku quake, seismicity in the Tokyo area initially jumped tenfold, then levelled off at three times the earlier rate.

Based on that increase, a study last year estimated a 17 per cent probability of a large shock under Tokyo between March 2013 and March 2018. That is two-and-a-half-times higher than if the Tōhoku quake had not happened (Geophysical Research Letters, doi.org/scz).

The events after the Tōhoku quake have “completely rearranged the whole system in north-east Japan”, says Burgmann. “They definitely point to the very complicated area around Tokyo becoming a zone of greater hazard.”

Four tectonic plates meet in the Tokyo area, and as a result it has suffered several quakes above magnitude 7 in the past four centuries. The largest in the past 1000 years was the Genroku quake, estimated to have been magnitude 8.2, that killed 2300 people on 31 December 1703 and produced a tsunami that killed several thousand more.

However, the deadliest was the magnitude-7.9 Great Kantō earthquake of 1 September 1923 (pictured, above right), which killed 100,000 people – with some help from a typhoon. Since 1960, 1 September has been Disaster Prevention Day across Japan.

Journal reference: Geophysical Research Letters, DOI: 10.1002/2014GL060072
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