6/14/2010 Aftershock of 4/4/2010 quake big enough for aftershocks of its own

Posted on June 16, 2010


A magnitude 5.7 earthquake that rumbled through Orange County Monday night left a trail of questions in its wake: Why so many quakes? Are we having more than normal? But geophysicists say despite the larger size of Monday’s quake, it is still part of a continuing series of aftershocks from the magnitude 7.2 quake in northern Mexico on April 4. There were no reports of serious injury or damage from Monday’s quake.

        Although over time the number of aftershocks diminishes, the Easter Sunday quake has generated more than 10,000 so far, including many large enough to be felt in Orange County. They’ve been concentrated in a more northerly group near the city of Ocotillo and the Mexican border, and a second group farther south. All are taking place along the Laguna-Salada fault system, which includes a variety of interconnected faults. And while the odds drop for larger aftershocks, they’re still possible.

“There’s probably about a 20 to 25 percent chance there will be another magnitude 5 in this shock sequence in the next week or so,” said U.S. Geological Survey geophysicist Bob Dollar. “Also we stand about a five percent chance that there could be a larger one.”

Aftershocks, he said, “are usually smaller, but sometimes they’re bigger.”

And Monday’s aftershock was large enough to generate aftershocks of its own. The region has been rumbling with aftershocks ever since. Dollar said since 8 p.m. Monday, about an hour and a half before the 5.7 quake, the area has seen one quake of magnitude 5 or higher, five magnitude 4s, 39 magnitude 3s, and and more than 400 smaller quakes.

Of the more than 10,000 quakes since the big 7.2 quake in April, about 900 have been magnitude 3 or greater, he said.


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