Saturday, August 16, 2008

Study could help with Pacific earthquake preparation

Thomas Guengerich New Mexico Tech

New Mexico Tech geophysics professor Dr. Glenn Spinelli has discovered that the area where earthquakes may occur off the coast of Washington and Oregon probably extends further inland than previously believed.

Spinelli's research could impact estimates of the hazard due to ground shaking, and therefore earthquake preparedness plans for the cities of Seattle and Portland — and similar locations around the Pacific, where one tectonic plate moves under another.

Spinelli is combining marine geology research off the coasts of Japan and Costa Rica to estimate the extent of the area capable of producing earthquakes in subduction zones. His studies off the coast of Costa Rica will soon be published in the prestigious journal Nature Geoscience. Another paper by Spinelli — based on his research off the coast of Japan — will be published in an upcoming edition of the journal Geology. The Tech professor has also been invited to be guest lecturer by the Consortium for Ocean Leadership for its Integrated Ocean Drilling Program.

Spinelli is studying deep earth temperatures to estimate the potential rupture area of future earthquakes. Seismologists widely believe that earthquakes in subduction zones occur in the region where temperatures are between 150  and 350 C (300  to 650  F). Spinelli's research shows that water rapidly circulating in an aquifer in seafloor rocks moves a tremendous amount of heat in subduction zones and affects where those key temperatures for earthquake generation occur — they are much further landward than previously thought.

Spinelli's studies focus on water flowing through large areas of rock below the ocean floor. Much of that rock is basalt — lava that erupted on the seafloor at mid-ocean ridges, cooled fast and cracked. The numerous cracks in the basalt are pathways for water to rapidly flow through the rock. Water flowing through the basalt formations moves heat out of subduction zones.

"The important implication relates to seismic hazards," he said. "The 350 degree boundary is shifted landward. Based on our research in Japan, we believe the 350 degree boundary moves from 50 to 100 kilometers further inland than previously thought."

In the subduction zone off Japan, that heat transfer cools the underground rock, thus shifting the area where earthquakes can be generated more than 50 kilometers (30 miles) farther landward, and closer to coastal cities, than previously believed.

"We're trying to understand the plumbing of the largest aquifer on Earth," Spinelli said. "Water is redistributing the heat from deep in the subduction zone."

In Spinelli's marine geology research, he and his fellow researchers sank specialized thermometers into the sediment at the bottom of the Pacific Ocean. There, they systematically gathered temperature readings from sediment at varying distances from basaltic seamounts where seawater enters or leaves the aquifer below the seafloor.

Sea water that is about 2 degrees C (35.6 degrees F) enters the basaltic formation at one mound, travels underground to another mound. While flowing through the basalt, the water is heated up to 40 degrees C (104 degrees F), thus transferring heat from the Earth to the water.

"We know the Earth is cooling," Spinelli said. "However, it's happening differently than you might expect. Water moving through the basaltic formations is a really efficient and effective heat transfer mechanism."

Spinelli's the next phase of research is to examine heat-transfer offshore the Pacific Northwest to determine the eastern boundary of the seismic zone, which may result in a re-evaluation of the seismic hazard for the greater Seattle metropolitan area.

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