Wednesday, June 15, 2005

file under geology, seismology and plate tectonics

Last night's earthquake was on the Juan de Fuca plate and was a strike-slip fault movement rather than a thrust or other type.

The LA Times story is here. Kate Hutton said that "the tsunami warning came from the West Coast and Alaska Tsunami Warning Network in Palmer, Alaska, and was issued as a precaution.

"I'm not sure about their thinking, but probably the reason they issued it is because it's so close to shore," she said. "There was no time. They didn't take any chances."

This little bit in the story struck me as interesting (probably because it is something I have been paying attention to for a while though hadn't read any recently published research):

"Tuesday's quake interested seismologists studying so-called trigger earthquakes because it came after several significant quakes struck in recent days. On Sunday, a 5.2 magnitude quake hit 20 miles south of Palm Springs in Anza. On Monday night, a 7.8 magnitude quake hit Chile, killing at least 11 people. On Tuesday morning, a magnitude 6.8 quake struck the Aleutian Islands, both preceded and followed by smaller quakes. [insert by me: so was Sunday's quake as near as I can tell, according to their data listed.]

"One of the things that is very exciting in seismology now is trigger earthquakes," Hutton said. "The seismic waves shake loose earthquakes in neighboring areas, or not-so-neighboring areas."

Hutton said, however, that given its relatively small size, it was unlikely the Anza quake was a true trigger."

One fault moves and all the others have to readjust - over a large scale and for months and sometimes years afterwards. I don't know what constitutes a "true trigger", but even small quakes in the 3 and 4 magnitude range, can set off the chain of 'readjusting'.

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