How two big quakes triggered 16,000 more

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.

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

The earthquakes could help researchers understand how multiple strong quakes can happen on connected faults. Fault maps of the area that were last updated in the mid-1990s show many short, disconnected faults. But the pattern of aftershocks, along with new cracks and surface ruptures that cover about 30 miles, revealed the short faults to be more connected than depicted on the maps.