April 10, 2009

2007 Quake Still Puzzling Experts

Geoscientists remain somewhat perplexed as they work to understand exactly what happened during an earthquake and subsequent tsunami that rocked the Solomon Islands in 2007.

The tsunami that followed the quake was considerably larger than scientists had anticipated, causing some 52 deaths and extensive property damage all across the Melanesian island chain.  They were particularly surprised by the amount of seismic activity in an area that had been previously relatively quake-free.

The geological process known as subduction is typically responsible for most of the planet's earthquakes.  In subduction, one of the tectonic plates that make up the Earth's crust essentially begins to slide beneath another plate.  In so-called subduction zones, an oceanic plate is forced beneath a continental plate, frequently causing volcano or mountain formation, earthquakes and tsunamis.

What's peculiar about the Solomon Island quakes is that there were three plates involved, all of them moving at different rates, says geoscientist Kevin P. Furlong of Penn State University.  Both the Australian Plate and the Solomon Plate are gradually sliding beneath the Pacific Plate "“ the Australian Plate at a rate of about 5.5 inches per year while the neighboring Solomon Plate is only moving at about 4 inches per year.  Further complicating researchers' studies, is that fact that the plates are not moving in the same direction.

The 2007 quake initially started where the Australian and Pacific plates meet but then suddenly moved to the Solomon-Pacific boundary. 

On the uniqueness of the geoactivity in the Solomon Island area, Furlong said: "Normally we think earthquakes stop at the plate boundaries"¦I do not know of any other place where we have observed that behavior during an earthquake before, but it most certainly has happened here before."

As the earthquake hopped from one plate boundary to the other, it changed its direction to match that of the moving plates.  This quick change of motion caused the edges of the Pacific Plate to buckle upwards by a few yards which was likely the cause of the larger-than-expected tsunamis.

Furlong mentioned that the area is also home to numerous coral islands which probably have their origin in similar seismic activity.

The Solomon subduction zone consists of some of the youngest oceanic crust in the world.  Seismologists usually associate older regions of crust with major earthquakes, adding another level of peculiarity this geological puzzle.


Image 2: This is a cartoon of the tectonic plates in the Solomon Islands area showing subduction beneath the Pacific plate. The Pacific plate is not shown. Credit: Kevin Furlong, Penn State 


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