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Old 08-29-2012, 01:45 PM
 
Location: Wasilla, Alaska
17,824 posts, read 20,775,493 times
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Quote:
Originally Posted by cjg5 View Post
There have been a few off the coast of Washington right on our subduction zone recently too.
I am not entirely sure what constitutes an "earthquake swarm." Wikipedia defines it as "where a local area experiences sequences of many earthquakes striking in a relatively short period of time. The length of time used to define the swarm itself varies, but the United States Geological Survey points out that an event may be on the order of days, weeks, or months."

If we use Wikipedia's criteria for an "earthquake swarm," then there is an "earthquake swarm" every day in Alaska. We average 50 quakes per day, 350 per week, 1,500 per month, 18,000 per year. The vast majority are too small to be felt (somewhere in the 1.0 to 4.0 magnitude range) , but at least once a month we can expect a 5.0 or larger quake.
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Old 09-02-2012, 11:03 AM
 
2,920 posts, read 9,050,986 times
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I guess I came late to this thread. I am in the Imperial Valley, where the earthquake swarm ocurred. The swarms have quite often, every year there are several, but usually very weak, maybe a 3.5 at the highest. So this one was more robust. A year ago from December there was another one with several 4s. So swarms aren't unusual. They also ocurr across the border about 20 miles into Mexico. They happen on transition zones (spreading centers) between faults that don't line up. This is the same process that creates the Gulf of California.

With that said, Brawley had several trailers knocked off their foundations, ceiling tiles fell down in businesses, one pane windows shattered, things fell off shelves...Wal Mart and the supermarkets closed. So it was a big event and being ontop of the epicenter made it feel stronger as well. School was also cancelled for two days afterwards. There are cracks in the older buildings as well.

I am about 20 miles south of the quake epicenters, here there was just an initial strong jolt that rocked everything hard and then it was a swaying motion. We only felt the 5s...the 4s were too weak to be felt, even at this distance.

All is back to normal now...till the next one!
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Old 09-02-2012, 11:24 PM
 
Location: Wasilla, Alaska
17,824 posts, read 20,775,493 times
Reputation: 6506
Quote:
Originally Posted by Chelito23 View Post
I guess I came late to this thread. I am in the Imperial Valley, where the earthquake swarm ocurred. The swarms have quite often, every year there are several, but usually very weak, maybe a 3.5 at the highest. So this one was more robust. A year ago from December there was another one with several 4s. So swarms aren't unusual. They also ocurr across the border about 20 miles into Mexico. They happen on transition zones (spreading centers) between faults that don't line up. This is the same process that creates the Gulf of California.

With that said, Brawley had several trailers knocked off their foundations, ceiling tiles fell down in businesses, one pane windows shattered, things fell off shelves...Wal Mart and the supermarkets closed. So it was a big event and being ontop of the epicenter made it feel stronger as well. School was also cancelled for two days afterwards. There are cracks in the older buildings as well.

I am about 20 miles south of the quake epicenters, here there was just an initial strong jolt that rocked everything hard and then it was a swaying motion. We only felt the 5s...the 4s were too weak to be felt, even at this distance.

All is back to normal now...till the next one!
We experience these kinds of quake "swarms" all the time in Alaska. So often that they do not bother calling them "swarms." We experience well over a thousand quakes per month. The vast majority are small, under 4.0. It is when they reach 5.0 or above that they become a concern, and Brawley was only a couple miles from the epicenter of two quakes that were above 5.0. Fortunately for us, most of our earthquake swarms occur around our active volcanoes and not around population centers. I am glad to hear no one was hurt.
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