Expect 34,000 aftershocks from Ridgecrest earthquakes. But seismic activity is slowing down

Aftershocks from the recent earthquakes near Ridgecrest, Calif., are decreasing in both frequency and magnitude, and seismologists say they expect the pattern to continue.

The earthquakes on July 4 and 5 — one a magnitude 6.4 and the other a 7.1 — were the strongest to hit the area in 20 years. Thousands of aftershocks have already been reported, and scientists have said they expect thousands more — about 34,000 over the next six months.

But since an initial cluster of magnitude 5 and above quakes that struck in the hours following the 7.1 temblor, the aftershocks have been subsiding in intensity and striking less often, an analysis of seismological data shows.

Caltech seismologist Egill Hauksson said late Monday the earthquake sequence seemed to be quieting down faster than was initially thought.

One possible reason: The quakes are occurring in an area of Earth’s crust that is relatively warm. Aftershock sequences in these areas can be particularly intense but also fade more quickly — as has been seen in relatively hotter rock in the Imperial Valley.

Scientists had earlier said the probability of another earthquake of magnitude 7 or greater resulting from the July 5 quake was less than 1%.

“Now it’s probably quite a bit less than that,” Hauksson said. “It’s extremely unlikely by now to have another magnitude 7” quake resulting from that sequence.

The U.S. Geological Survey said Tuesday the aftershocks would continue to taper off in the coming days and weeks, and the probability of another large quake — magnitude 4 or higher — also will decrease.

No deaths or major injuries were reported as a result of last week’s temblors, but the earthquakes damaged homes and roads and sparked widespread anxiety, causing scores of residents to question their earthquake preparedness.

On Tuesday, the Trump administration declared a federal emergency for areas of Southern California devastated by the Ridgecrest quakes.

The declaration, which was requested by Gov. Gavin Newsom, authorizes the Federal Emergency Management Agency and the Department of Homeland Security to assist with disaster relief in Kern and San Bernardino counties in the aftermath of the quakes.

“California is grateful for President Trump’s approval of our request and the ongoing efforts of local, state and federal emergency responders to protect communities across the region,” Newsom said in a statement.

The federal assistance adds to the state and local relief efforts already underway.

Times staff writers Phil Willon and Rong-Gong Lin II contributed to this report.