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March 11, 2011 / Dave Gorham

Why So Much Tsunami Damage From This Particular Quake?

[Geology expert and ImpactWeather StormWatch Manager Fred Schmude guest posts about the terrible events in Japan today.]

Last night’s massive earthquake occurred near the northeast coast of Honshu Island in Japan at 0546Z (11:46pm CST) on Friday March 11, 2011. The epicenter of the quake was located at 38.32N and 142.37E or about 81 miles east of Sendai, Japan or about 234 miles northeast of Tokyo.   The quake measured about 8.9 on the Richter scale and that number may increase based on how much of the crust ruptured off the east coast of Japan.   If this magnitude is confirmed, this could turn out to be 5th or 6th strongest earthquake in recorded history.

Here is a map identifying where the initial powerful earthquake occurred:

Here is a shake map describing how much ground movement was observed during the powerful shaking.  Note that most of the east coast of Honshu Island from Tokyo through Sendai experienced strong to severe ground shaking during the event with damage potential ranging from light to heavy.

In addition to the shaking activity, a large tsunami also resulted from the initial powerful quake affecting parts of the east coast of Honshu Island.

Strong earthquakes that occur over large bodies of water are always a huge concern for geologists since only a slight break in the ocean floor near the epicenter of a large quake will in essence push the water up resulting in a tsunami.  The size of the rupture is directly proportional to the amount of displaced water and resultant tsunami. The shallower the focal depth of the quake the greater the risk of ground rupture on the ocean floor.   Latest data indicates the focal depth of this quake was estimated at 24 kilometers (~15 miles) which is relatively shallow.

Typically quakes with focal depths less than 10 miles are the greater concern, however since this was a much larger quake affecting a large part of the earth’s crust,  the risk of ground breakage on the ocean floor was higher even though the focal depth was a little deeper.

Here is a map detailing the travel time of the initial tsunami wave generated by the massive quake:

Already Hawaii has experienced some of the higher waves associated with the tsunami during the past hour.  It looks like the west coast of North America will be threatened over the next 1 to 3 hours where there is currently either a tsunami advisory or warning in place:

In quakes like this, most of the deaths that occur just inland of the coast are due to the tsunami wave rather than the actual ground sharking.  The death toll from this event could easily reach into the thousands and will probably not be fully known for several weeks.   Expect numerous aftershocks over this region of the Pacific over the next several weeks with peak intensities ranging from 7.0 to 8.0.  Aftershocks of this size could produce more tsunami threats depending on how shallow the focal depths (break levels) are.

UPDATE: As of 10:50CST, the U.S. Geological Survey reports as many as 80 aftershocks to the Honshu quake have already occurred.

Donations for recovery can be made at the American Red Cross’ donations site.

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