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Tiny foreshocks can help signal large earthquakes, new Southern California research shows

30 years of earthquakes strike California in colorful time lapse

California earthquakes from 1989 to 2019, including July's Ridgecrest quake and the Loma Prieta quake that struck the San Francisco Bay Area, are shown in a colorful time lapse based on US Geological Survey data.
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California earthquakes from 1989 to 2019, including July's Ridgecrest quake and the Loma Prieta quake that struck the San Francisco Bay Area, are shown in a colorful time lapse based on US Geological Survey data.

Scientists are still a long way off from forecasting earthquakes, but a new study shows that they’re a little bit closer to that becoming a reality.

Small foreshocks, tiny earthquakes that precede larger earthquakes, can signal that a larger earthquake is coming within days to weeks nearly three-quarters of the time, according to research from the Los Alamos National Laboratory published a study in “Geophysical Research Letters.”

This marks an important, though incremental step, in earthquake forecasting, according to a laboratory statement released Wednesday.

“We are progressing toward statistical forecasts, though not actual yes or no predictions, of earthquakes,” said Daniel Trugman, Los Alamos seismologist and study coauthor. “It’s a little like the history of weather forecasting, where it has taken hundreds of years of steady progress to get where we are today.”

The study found that 72 percent of the time, tiny earthquakes — smaller than magnitude 1 — preceded a “mainshock” earthquake, defined as the largest earthquake in the sequence.

Researchers studied 1.8 million such earthquakes in Southern California, using sophisticated new technology produced by Caltech in order to detect the hard-to-spot temblor.

Scientists are still a long way off from predicting earthquakes, however.

“The small foreshocks may be too difficult to discern in real time to be of use in earthquake forecasting,” according to a Los Alamos statement. “Another important issue is that quakes run in packs: they cluster in both space and time, so sorting the foreshocks of a particular quake out from the family of preliminary, main and aftershock rumbles of its fellow earth adjustments is no simple task.”

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Andrew Sheeler covers California’s unique political climate for McClatchy. He has covered crime and politics from Interior Alaska to North Dakota’s oil patch to the rugged coast of southern Oregon. He attended the University of Alaska Fairbanks.
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