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Quakes in the Gulf are infrequent and Sunday's was the largest of more than a dozen shocks that have been recorded in the area in the past 30 years.
Before the Sunday quake, the most recent in the area were a 5.2 tremor in February and a 4.4 in October 2003.
Sigala said the quake area is not located on a geological fault but rather in the middle of the North American plate. The quakes could be caused by the release of long-term stress under the plate, she said