BERKELEY (KCBS) – A 8.6-magnitude earthquake in the Indian Ocean this spring had a far reaching impact seismologists once considered unlikely, triggering aftershocks off the coasts of Oregon, Baja California, and Southern Mexico.

Analysis by University of California, Berkeley and the U.S. Geological Survey connected the April 11 quake to four quakes over the next four days all with magnitudes greater than 6, said USGS seismologist Fred Pollitz.

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“One of those was off the coast of Oregon. Two of them were in the Gulf of California, and one was in the Mexican trench,” he said.

KCBS’ Anna Duckworth Reports:

“Normally we get one magnitude 6 or greater event every three days. So it’s a large increase in the rate of large earthquakes.”

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Pollitz said the number of quakes magnitude 5.5 or stronger worldwide was five times higher than normal, while analysis of 300 major earthquakes over the last 30 years had shown no large aftershocks occurring far away.

“We didn’t think this sort of thing was possible.”

Pollitz said it remains true on average that large earthquakes between magnitude 7 and 8.4 do not create tremors outside of the immediate aftershock zone.

The Indian Ocean quake was unusual for two reasons, he said. It shook the middle of an ocean plate, and that plate moved horizontally instead of up and down, generating more waves beneath the Earth’s surface.

“The next time that something like this happens, and it probably won’t happen very often, we could ask ourselves if California would become part of the statistics the next time.”

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