An earthquake with a preliminary magnitude of 7.3 (M7.3) occurred off the east coast of Japan on 7 December 2012, causing a small tsunami and sparking evacuations in the areas devastated by the major tsunami of 2011.
The tremor occurred in the same area as the earlier, larger, event with an epicentre offshore approximately 450km from the Japanese capital, Tokyo, at a depth of 36 km (United States Geological Survey).
Shaking is said to have lasted for three minutes in the area, although at the time of writing no significant damage or injuries had been reported.
A tsunami of around 1m in height struck the coastline of Miyagi Prefecture but residents, alerted by official warnings and their own previous experience, were able to move to higher ground.
Tectonic Setting and the 2011 Honshu Earthquake
The western Pacific has a complicated tectonic setting with several of the Earth’s tectonic plates coming together. To the north, a wedge of the North American plate (which is divided, it is believed, into numerous microplates) tapers down to meet the northern point of the Philippines Plate, while the Pacific Plate moves westwards against both, and against the continental mass of the Eurasian Plate which lies further to the west.
The early USGS reports attribute the earthquake of 7 December to reverse (or thrust) faulting within the Pacific Plate just to the east of the zone where it subducts beneath the North American Plate. This is in contrast to the March 2011 tremor, which occurred at the plate boundary itself.
The memory of the damage and devastation caused by the March 2011 event, which killed an estimated 20,000 people and displaced many more, is still fresh, and the December 2012 tremor inevitably invites comparison. Although the location and tectonic setting of the two events is similar, however, the difference in scale is considerable.
The magnitude scale used to measure earthquake size is logarithmic. The December 2012 event, at M7.3, is considered large but that of March 2011, at M9.0 was 50 times larger in magnitude terms and released a staggering 354 times more energy. This is reflected in the difference in the size of the tsunamis generated, with the 2011 waves reaching almost 30 meters in height. (Other factors also affect the occurrence and size of tsunamis.)
Historic Regional Earthquakes
Given the tectonic forces at play, Japan is particularly vulnerable to earthquakes, and in fact the M7.3 is not out of the ordinary for the region. The USGS list of significant earthquakes for the country includes over 20 tremors of at least M6.0 since the beginning of the century. This excludes any of the hundreds of aftershocks which continued to shake the region following the 2011 earthquake.
The 2011 earthquake demonstrated the potential damage which can occur following significant earth movement. Although the December 2012 tremor is certainly considered significant, it is nowhere near as large as the 2011 event and, in consequence, considerably less damaging.
BBC News. Japan earthquake sparks tsunami scare. (2012). Accessed December 7, 2012.
United States Geological Survey. Historic earthquakes list. (2012). Accessed December 7, 2012.
United States Geological Survey. M7.3 – 245km SE of Kamaishi, Japan. (2012). Accessed December 7, 2012.
United States Geological Survey. Magnitude 9.0 Near the east coast of Honshu, Japan. (2012). Accessed December 7, 2012.
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