Japan quake bumped up to 9.0

As if the March 11 quake off Japan's northeast coast needed any more historic cred, the USGS recalculated it upward in magnitude. "Independently, Japanese seismologists have also updated their estimate of the earthquake’s magnitude to 9.0," the U.S. Geological Survey announced today. "This magnitude places the earthquake as the fourth largest in the world since 1900 and the largest in Japan since modern instrumental recordings began 130 years ago." From USGS's tectonic summary of what it calls the Tohoku earthquake:

The magnitude 9.0 Tohoku earthquake on March 11, 2011, which occurred near the northeast coast of Honshu, Japan, resulted from thrust faulting on or near the subduction zone plate boundary between the Pacific and North America plates. At the latitude of this earthquake, the Pacific plate moves approximately westwards with respect to the North America plate at a rate of 83 mm/yr, and begins its westward descent beneath Japan at the Japan Trench....

The March 11 earthquake was preceded by a series of large foreshocks over the previous two days, beginning on March 9th with a M 7.2 event approximately 40 km from the epicenter of the March 11 earthquake, and continuing with another three earthquakes greater than M 6 on the same day.

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A predecessor may have occurred on July 13, 869, when the Sendai area was swept by a large tsunami that Japanese scientists have identified from written records and a sand sheet. Continuing readjustments of stress and associated aftershocks are expected in the region of this earthquake.

Uh, yeah. In the past 24 hours alone, USGS has registered about two dozen Japanese quakes of magnitude 5.0 or higher.