Monday, March 14, 2011

NSC Seismograph and the Japan Earthquake

The 8.9 magnitude Japan quake recorded by the NSC seismograph
by Dr. Edwin Price, Ph.D. - Assistant Professor of Environmental Science

The 8.9 magnitude earthquake in Japan on Friday, March 11, was detected by the NSC seismograph, located deep underground at the Liberal Arts and Sciences building.  It was still Thursday evening when the seismic waves passed through the Las Vegas area at 9:46 pm., March 10.  It took only 12 minutes for the seismic waves to travel through the Earth’s crust from Japan to Las Vegas.  That is a travel speed of approximately 26,700 miles per hour.
Why was the Japan quake so large?
Northern Japan sits literally on top of the tectonic plate boundary between the Eurasian Plate, which includes most of the Asian continent, and the Pacific Plate, which consists of most of the Pacific Ocean.  Here these two plates are moving toward each other at a rate of about 3.5 inches per year.  Typically, the Pacific Plate crushes and slides underneath the Eurasian Plate, right beneath northern Japan.  This sliding is along a large inclined fault called a subduction zone. The high amount of compression between the two plates is normally taken up by earthquake-generating fault slippage along the subduction zone.
The subduction zone fault, which comes to the surface on the ocean floor just east of northern Japan, had not slipped in many decades.  A consistent 3.5 inches of compression per year had been building up in the rock over this time.  It finally snapped along the fault on March 11, sending seismic shock waves in all directions. 
What caused the tsunami and why was it so large?
Tsunamis are caused by a sudden shift of the sea floor either up or down.  In the case of the devastating Indonesian tsunami of 2004 and as is thought to have happened in the Japanese tsunami, the sea floor popped up several feet along one side of fault zone exposed in the ocean floor.  If a part of the sea floor suddenly moves up, the ocean surface directly above also is suddenly pushed up creating a mound of water in the ocean.  As the mound of water settles down, water flows out in all directions across the ocean in the form of low, but very long-wavelength waves.  As the waves encounter shallow water they build in height, and if the land is flat, can flow up onto the land.  Once on land the waves then return as backwash toward the ocean, carrying debris out to sea.

Image courtesy of Jim O'Donnell at BC-Geophysics, Geophysical Consultant/Contractor.
World mapping of the distance between Japan and the seismograph at NSC.

Image courtesy of Jim O'Donnell at BC-Geophysics, Geophysical Consultant/Contractor.
Actualy activity captured by the NSC seismograph.

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