A 7.2 Foreshock Preceeded the 8.9 Earthquake in Japan

Here's one for the We Like Science category.  This is regarding the recent 8.9 earthquake in Japan.

This earthquake was unique primarily for its size.  It was one of the top five ever recorded at the time of its impact.  In all other aspects that scientists measure (location, depth, mechanism), this earthquake was typical for its location and fault type.

The Woods Hole Oceanography Institute (WHOI) is the largest private non-profit oceanographic institution in the world, based in Woods Hole, Massachusetts on Cape Cod.  WHOI issued a statement about the earthquake east of Honchu on March 11, 2011.  Part of the statement dealt with how "pre-quakes" foreshadow larger earthquakes.  Thus, these are called forequakes or more properly, "foreshocks."  

“What is noticeable about the March 11, 2011 quake is that there was a magnitude 7.2 foreshock [on March 9], which is only 40 kilometers (km) away from the epicenter of the mainshock,” said Senior Scientist Jian Lin, who has studied large quakes extensively, including last year’s major quakes in Haiti and Chile.

A 7.2 foreshock?   7.2 is a pretty serious earthquake in itself. The October 17, 1989 earthquake in the San Francisco Bay Area was "only" a 6.9 and was truly devastating.

And yet, says WHOI, a magnitude 7.2 earthquake had hit near Japan on March 9.  It had an ongoing aftershock sequence which may have helped trigger the March 11 quake.

However, for perspective, there were similar events of magnitude 7.8 in 1994 and 7.7 in 1978 also nearby that did not result in a follow-on event the size of the March 11 earthquake.

“Foreshocks have been used previously as a tool for forecasting larger earthquakes,” he said. “I am sure that we will be looking closely at this and other foreshocks.”

Lin is on a boat in the Pacific that started its mission in Christchurch, New Zealand, which experienced its own damaging 6.3 earthquake last February (2011). The ship ventured on a research voyage and is now heading back to Christchurch.  The ship will likely encounter the remnants of the Japan tsunami, although it will be barely noticeable. “Our cruise started with the Christchurch earthquake in New Zealand, and ends in sailing through a tsunami caused by the great Japan earthquake,” Lin said.

For more from Woods Hole Institute on the Honshu earthquake, see: http://www.whoi.edu/page.do?pid=12463&tid=282&cid=93649
Kathleen Hurleyscience