Quake study makes national headlines

Rebecca Neipp

News Review Staff Writer

Quake study makes national headlinesLast Thursday brought with it a trifecta of reminders about our earthquake risk — the observance of drills during the Great ShakeOut, a 3.8 aftershock and a headline in the Los Angeles Times that heightened the discussion of the potential impact of the Ridgecrest sequence on the Garlock Fault.

For the ShakeOut, local institutions joined organizations across the nation in an exercise aimed at improving earthquake planning and response. While the 3.8 tremor is dwarfed by those that happened immediately following our 6.4 and 7.1 quakes, it was the largest recorded in the last month.

But the greatest source of discussion was an Times article by Rong-Gong Lin II entitled “Unprecedented movement detected on California earthquake fault [Garlock] capable of 8.0 temblor.” The report, along with subsequent online speculation that followed it, raised the profile of looming questions about the prospect for future quakes.

“In the modern historical record, the 160-mile-long Garlock fault on the northern edge of the Mojave Desert has never been observed to produce either a strong earthquake or even to creep — the slow movement between earthquakes that causes a visible scar on the ground surface. But new satellite radar images now show that the fault has started to move, causing a bulging of land that can be viewed from space,” states Lin in his article.

Dr. Lucy Jones, a Caltech seismologist who rose to local prominence following the Ridgecrest sequence for her ability to convey complicated scientific concepts in everyday English, posted a series of tweets that same day regarding the issue.

“People are talking about the ‘unprecedented’ movement of the Garlock fault after the Ridgecrest quake,” she tweeted Oct. 18, the day the article was published. “It’s true we haven’t seen this in the 30 years of modern geodesy on the Garlock fault. But we’ve seen it many times on the San Andreas and it has never caused a quake.”

Jones said the that movement, called aseismic creep, occurs in the top few hundred meters of the fault. “No quake can occur in the shallow part because there’s no confining pressure. Big quakes begin 10-15 kilometers down.”

Big quakes triggered aseismic creep on the San Andreas fault in 1979, 1992 and 1999, she further wrote. “The creep never caused another quake. Ridgecrest was the first big quake near the Garlock since we have records so it’s the first time we’ve seen creep on the Garlock. But it’s not unprecedented.”

The Times article cited a report in the October 2019 research journal Science that suggests the Ridgecrest sequence destabilized the local geography following the July earthquakes.

The abstract of that study, penned by Zachary E. Ross along with several other scientists, states:

“A nearly 20-year hiatus in major seismic activity in Southern California ended on 4 July 2019 with a sequence of intersecting earthquakes near the city of Ridgecrest, California. This sequence included a foreshock with a moment magnitude (Mw) of 6.4 followed by a Mw 7.1 mainshock nearly 34 hours later. Geodetic, seismic, and seismicity data provided an integrative view of this sequence, which ruptured an unmapped multiscale network of interlaced orthogonal faults.

“This complex fault geometry persists over the entire seismogenic depth range. The rupture of the mainshock terminated only a few kilometers from the major regional Garlock fault, triggering shallow creep and a substantial earthquake swarm.

“The repeated occurrence of multifault ruptures, as revealed by modern instrumentation and analysis techniques, poses a formidable challenge in quantifying regional seismic hazards.”

(For the complete report see https://science.science mag.org/content/366/6463/

346.)

“We’ve known about the Garlock fault for a long time,” said Elizabeth Cochran, an observational seismologist with the U.S. Geological Survey. She said that while Garlock is not a fault to ignore, and is certainly capable of a large earthquake, “we have no reason to believe a large earthquake is imminent.”

She noted that the Garlock and San Andreas faults are similar in that they are both quite large with mostly horizontal motion. She said that the data she has indicates the fault is capable of a quake of up to a 7.8 magnitude.

“The study notes some creep along the southern end near the Ridgecrest events,” said Cochran

“The reason it’s not too concerning is that we’ve seen this kind of creep before, and it’s not unusual.”

Echoing Jones’ statements on Twitter, Cochran said that the activity at the surface does not necessarily indicate shifting at depth.

Cochran was part of the USGS team that was deployed here following the July 4-5 quakes.

“We are still in the middle of analyzing the data,” she said. “What I was doing in Ridgecrest was putting out additional seismometers. We do this because aftershock sequences are the best opportunity we have to record moderate to strong ground motion.”

The Ridgecrest sequence was quite complicated, she said, because there was slippage on multiple planes.

She likened the slippage to a row of books lined up that, when disturbed, are skewed slightly differently along the contact points of each book.

While the incidence of earthquakes is impossible to predict, the data collected during and after large incidents can help inform forecast models that can determine what will happen when the next big quake does hit.

“Essentially, what we are most interested in understanding is the seismic hazards to our communities across California and the U.S.,” said Cochran.

“By integrating this kind of complexity observed into the models, we can better forecast the seismic hazards across a large region.”

She said that the Eastern California Sheer Zone — which includes our high desert region — demonstrated similar complexity in the Landers earthquake of 1992 and the Hector Mine earthquake of 1999.

“In all three cases, the earthquake essentially linked up faults we had mapped, but did not expect to break in the same incident.”

Many seismologists are pointing to the recent quakes — the most significant in more than 20 years — as a good reminder to stay prepared.

“People can become a little complacent about seismic hazards,” Cochran said.

“We are always trying to remind people of the importance of being prepared. It’s easier to understand the risk when you feel this kind of shaking.”

Pictured: The surface of the Ridgecrest rupture shows a zone of disturbed ground about 10 feet wide along one of the main faults that broke after the 7.1-magnitude earthquake. Photo courtesy of USGS

Story First Published: 2019-10-25