The powerful earthquake that rocked the seabed off the Northern California coast near Eureka on Saturday underscores the complexity of seismic dangers within the Earth's crust, and is likely to be followed by a large aftershock this week - but it is not expected to exceed the 6.5 magnitude of the temblor that was felt as far away as Reno, scientists said Sunday.
A "probability report" from the U.S. Geological Survey said there is a 65 percent chance for a "strong and possibly damaging aftershock" from the temblor in the next seven days. As many as 90 weaker aftershocks are expected to be felt in local communities, the report said, but it's not probable any will be larger than Saturday's mainshock.
More than 20 smaller aftershocks - some with magnitudes larger than 4 - churned the seabed throughout the day Sunday.
Although Californians are most conscious of the quakes that constantly hit the San Andreas Fault Zone, where its many offshoots include the dangerous Rodgers Creek and Hayward faults, offshore quakes are extremely common.
Saturday's quake was unrelated to the San Andreas, but struck within the southern end of an offshore geological feature of the Earth's crust called the Gorda Plate, according to David Oppenheimer, a seismologist with the Geological Survey's main research center in Menlo Park.
Scientists have long known that the entire crust of the Earth is composed of vast crustal plates that are constantly in slow movement. The familiar San Andreas Zone, for example, marks the boundary between the huge Pacific Plate and the North American Plate, and when these two plates suddenly slip after building up pressure grinding past each other, potentially deadly quakes are the result.
The Gorda Plate
NCR parts, with its eastern edge along the coasts of California and Oregon, is a much smaller slab of the crust, and above it lies a far larger segment of the crust called the Juan de Fuca Plate that extends along the coast well north of Seattle and Vancouver Island.
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