California overdue for high-magnitude quake, researchers say

ASU and University of California-Irvine researchers discovered evidence suggesting a high-magnitude earthquake along the San Andreas Fault is overdue.

The research team discovered a pattern of major earthquakes happening along the fault line about 100 miles northwest of Los Angeles at approximately 100-year intervals, said Ramon Arrowsmith, professor in the School of Earth and Space Exploration, who was part of the research team.

A major earthquake has not occurred since the 7.8-magnitude Fort Tejon earthquake in 1857, Arrowsmith said. The Fort Tejon quake occurred 454 miles northeast of San Luis Obispo, which is halfway between San Francisco and Los Angeles, according to the Southern California Earthquake Data Center.

“The individual intervals between the earthquakes we studied were as short as 45 years and as long as 145 years,” Arrowsmith said. “It has now been 154 years since the great 1857 earthquake, so that interval is longer than it has been in the past.”

The San Andreas Fault consists of a series of fractured tectonic plates that are slowly but constantly moving. This motion creates earthquakes, according to the United States Geological Survey. The fault line runs about 800 miles through California.

Even though an earthquake along the San Andreas Fault is destined to occur, the estimates of the quake’s magnitude vary significantly.

“The one small consolation … [is] if the earthquakes are recurring more rapidly, they might at least occasionally be smaller than the great ones,” Arrowsmith said. “These smaller events are still big and would cause lots of shaking and problems in south-central California if they recurred.”

The research team analyzed the Earth’s layers by looking for evidence of past movements and recording current movements, Arrowsmith said.  The layers contained small pieces of charcoal and other organic materials that were used for high-precision radiocarbon dating, a process of estimating an organic substance’s age by evaluating carbon levels.

“The radiocarbon dates can date the layers immediately above and below [the ground surface during an earthquake],” Arrowsmith said.

The results of the study, which will be featured in September’s issue of the journal Geology, will change the way earthquakes are predicted along the San Andreas Fault, said Olaf Zielke, a research associate in the School of Earth and Space Exploration who also participated in the study.

Forecasting the recurrence interval of earthquakes on the San Andreas Fault is difficult because the range of magnitudes varies, Zielke said.

This discovery will help prevent mass casualties by forecasting when a high magnitude earthquake may occur, according to a press release from the University of California-Irvine team. This would allow for an organized evacuation plan.

“This observation suggests that earthquake forecasting for the San Andreas and other faults around the world is even more difficult than previously thought,” Zielke said.

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