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The 6.4 quake near Ridgecrest has produced 1,400-plus aftershocks – so far

by in News

The 6.4-magnitude earthquake near Ridgecrest July 4  produced more than 1,400 aftershocks as of mid-Friday afternoon, including a 5.4 magnitude quake that struck at about 4 a.m., scientists said.

There’s a 6% chance that another magnitude 6 quake or more will hit the area, though the likelihood will decrease over time, said seismologist Zachary Ross during a press conference at the California Institute of Technology.

Overall, the number and movement of the aftershocks are expected following a quake of this magnitude, said Susan Hough, a seismologist for the United States Geological Survey.

But even though most of Southern California felt Thursday’s temblor, it had no significant bearing on future earthquakes for Californians outside of the small desert town, the researchers said.

Most of California’s fault lines, including the San Andreas Fault, were already back to where they were before the Fourth of July quake, Hough said.

“The odds that this earthquake is going to influence significantly faults away from the immediate area are very, very low,” she said. “We don’t even try to do a forecast because the odds are so low.”

However, since earthquakes of that magnitude, where the surface of the earth breaks and ruptures, are rare in the state, the quake has a lot of scientific value.

Researchers who flew over the area in helicopters were able to spot the ruptures in the ground, along the fault lines.

“It literally looks like a scar across the desert floor,” Hough said, explaining how they are at times hard to spot, due to shifting sand and tumble weeds.

Most of the broken surface was found in the U.S. Navy air base in China Lake, which is restricted to public access, Hough said. The base’s air control tower was damaged in the quake.

Some of Highway 178 was also effected by the surface break. Researchers were immediately sent in the area to study the rare breaks.

Ridgecrest is home to a cluster of fault lines, which researchers have dubbed the Little Lake Fault Zone. Thursday’s quake was produced from two fault lines within the zone that meet at their ends, forming an “L” shape.

Historically, it has not been common for two fault lines to be involved in a single quake, as was the case with the “L” fault lines, Ross said. Understanding the relationship between multiple fault lines will be on his radar for future research.

“It’s just important for hazard purposes that you consider all different kinds of scenarios. When multiple faults are involved it becomes more complicated,” he said.

 

In Ridgecrest, the damage was notably minimal.

The Navy base and the surrounding town development that followed came on the heels of World War II, noted Hough, whose grandparents moved to the area in 1946.

Most of the structures in the town are single family homes and department store buildings with wooden frames, all of which were built after California passed seismic provisions in the statewide building codes.

The building code revisions were spurred by the Long Beach quake in 1933 that killed at least 115 people, causing $40 million in damage.

Californians who fear the looming “Big One” can find solace in that the USGS shake alert warning system was functioning during Thursday’s quake, sending alerts to beta testers within 6.9 seconds after the quake began.

While the system is still in its developmental stages, it will eventually have the ability to warn residents of an incoming quake. Exactly how big the quake has to be to warn residents is still being debated.

“There’s a trade off,” Hough said. “If you’re sending off alerts when light shaking is expected, there’s a concern that people might start to kind of dismiss the alert and not pay as much attention if stronger shaking is predicted.”

So far, the threshold is at what experts label “intensity 4,” which is a level of shaking that will rattle your windows. “Intensity 5” will start to knock things off your desk. For most Californians on Thursday, neither occurred.