Dec. 7 quake: larger aftershocks possible

December 08, 2012

THE ASAHI SHIMBUN

The magnitude-7.3 temblor that rocked northeastern Japan on Dec. 7 is now considered an aftershock of last year's Great East Japan Earthquake--and it may not be the last.

At least one expert has warned of a potentially larger temblor still to come.

"The 2011 magnitude-9.0 earthquake could be followed by a magnitude-8 aftershock," said Yoshimitsu Okada, president of the National Research Institute for Earth Science and Disaster Prevention. "The Dec. 7 event should be taken as a warning for that."

Last year's giant quake was followed by many aftershocks, but these gradually diminished in frequency. The Dec. 7 quake was the first magnitude-7 or greater seismic event since July 2011. It shows that major aftershocks can lag the main event by more than 20 months.

Seismic activity in the area has yet to return to normal levels.

The Dec. 7 earthquake arose from what is known as a "normal-fault" mechanism, which involves slippage along a geological fault line under tensile stress. Experts said it could have occurred at any time.

The stress arose because the March 2011 quake caused tectonic plates to shift against each other, turning compressive stress in the oceanic plate into tensile stress.

Parts of the crust in the "outer rise," a topographical protuberance farther off the coast from the plate boundary, were under this tensile stress when they slipped Dec. 7.

An earthquake of the same type occurred in January 2007, following a November 2006 quake along the oceanic trench off the Kuril Islands, near Hokkaido. And the same mechanism was seen in the Showa Sanriku earthquake of 1933, which some experts described as an aftershock of the 1896 Meiji Sanriku earthquake.

A normal-fault earthquake in the outer rise carries a high tsunami risk. Slippage in pieces of crust under tensile stress generally results in large vertical movement. The seabed is deformed and much water is displaced. The tsunami risk remains even if the seismic source happens to be far from land.

The Dec. 7 quake resulted in a 1-meter tsunami in Ishinomaki, Miyagi Prefecture.

The quake made itself felt in Tokyo with slow and sustained shaking, despite the capital's considerable distance from the seismic source.

"That was presumably because the shallow seismic source gave rise to long-period 'surface waves,' which travel close to the Earth's surface," said Hiroshi Tsuruoka, an associate professor of seismology at the Earthquake Research Institute of the University of Tokyo.

Giant earthquakes of magnitude 8 or greater may involve ground movement of long oscillation periods, causing high-rise buildings to sway, experts say.

The Japan Meteorological Agency has called for continued vigilance.

THE ASAHI SHIMBUN
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  • A Japan Meteorological Agency spokesman speaks to journalists in Tokyo following the magnitude-7.3 earthquake that rocked eastern Japan on Dec. 7. (Yosuke Fukudome)

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