How Two Big Earthquakes Triggered 16,000 More in Southern California

The two powerful earthquakes that rocked the Mojave Desert this month were part of a swarm of thousands of other earthquakes, many too weak to be felt, that continue to hit the area every few minutes.

Although the remote area was known to be seismically active and geologically complex, the earthquakes were sprinkled around a pair of neighboring geologic faults that scientists had not specifically known about before.

“I think if you’d talked to most geologists before these earthquakes, they’d have said, ‘Any fault capable of a 7, we should have seen,’ ” said Ross Stein, an adjunct professor of geophysics at Stanford University, referring to the July 5 earthquake.

Mapping aftershocks in the area reveals how such large quakes change the stresses in the ground around them and set off other earthquakes nearby. It also sketches the outlines of two faults running roughly perpendicular to each other.

The 6.4-magnitude earthquake on July 4 was a precursor, or foreshock, to the much larger 7.1-magnitude quake the next evening, which released about 11 times more energy than the first. It is not unusual for two strong quakes to occur on related faults within a few days of each other, but it is relatively uncommon for the second to be larger than the first.

“Once a large earthquake has happened, the odds of another large earthquake are as high as you’re ever going to see,” said Susan Hough, a seismologist at the United States Geological Survey.

That’s because powerful earthquakes often trigger more quakes nearby. Each of the thousands of aftershocks triggered by the July 5 earthquake has a tiny chance of being strong but is much more likely to be weak. More aftershocks mean more rolls of the dice, increasing the odds that at least one aftershock will be severe. Some scientists are concerned that the continuing aftershocks from the earthquake on July 5 could set off stronger quakes on other important faults in the area.

The frequency of aftershocks slows down over time, which means the chances of another intense earthquake also fall. Scientists can use this data to forecast the chances of another large earthquake.

While many of the aftershocks have been too small to feel, the 7.1-magnitude earthquake on July 5 caused sporadic power failures in Los Angeles and swayed the stands in Dodger Stadium, about 125 miles away.

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