major California fault capable of producing a magnitude 8 earthquake
has begun moving for the first time on record, a result of this
year’s Ridgecrest earthquake sequence destabilizing nearby faults,
Caltech scientists say in a new study released in the journal Science
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.
is surprising, because we’ve never seen the Garlock fault do
anything. Here, all of a sudden, it changed its behavior,” said the
lead author of the study, Zachary Ross, assistant professor of
geophysics at Caltech. “We don’t know what it means.”
observations reported are another piece of evidence that illustrates
a widely persistent myth that circulates in California and beyond —
that quakes like the Ridgecrest temblors are somehow a good thing
that makes future quakes less likely. In fact, generally speaking,
earthquakes make future earthquakes more likely. Most of the time,
the follow-up quakes are smaller. But occasionally, they’re bigger.
creeping illustrates how the Ridgecrest quakes that began on the
Fourth of July have destabilized this remote desert region of
California between the state’s greatest mountain range, the Sierra
Nevada, and its lowest point, Death Valley. Not only has the Garlock
fault begun to creep in one section, but there’s also been a
substantial swarm of small earthquakes in another section of the
fault, and two additional clusters of earthquakes elsewhere — one
south of Owens Lake and the other in the Panamint Valley just west of
the destabilization will result in a major quake soon cannot be
predicted. In September, the U.S. Geological Survey said the most
likely scenario is that the Ridgecrest quakes probably won’t
trigger a larger earthquake. Nevertheless, the USGS said that the
July quakes have raised the chances of an earthquake of magnitude 7.5
or more on the nearby Garlock, Owens Valley, Blackwater and Panamint
Valley faults over the next year.
a creeping fault triggered by a nearby quake doesn’t necessarily
mean a big quake is coming. The southernmost tip of the San Andreas
fault has traditionally crept in response to distant quakes,
including the magnitude 8.2 quake off the coast of southern Mexico in
2017, nearly 2,000 miles away. “But that doesn’t mean the San
Andreas went off,” said USGS research geologist Kate Scharer, who
was not part of the study.
unusual now, Ross said, is that the Garlock fault has been
seismically quiet in the historical record until now. And while it’s
unclear what the creeping and aftershocks might mean for the near
future, the newly recorded movement highlights how much of a
potential risk the Garlock fault is to California, should it rupture
in a big way.
large quake on the Garlock fault has the potential to send strong
shaking to the San Fernando Valley, Santa Clarita, Lancaster,
Palmdale, Ventura, Oxnard, Bakersfield and Kern County, one of the
nation’s most productive regions for agriculture and oil. Important
military installations could also get strong shaking, such as Edwards
Air Force Base, Naval Air Weapons Station China Lake and Fort Irwin
National Training Center. The fault is crossed by two of Southern
California’s most important supplies of imported water — the
California and Los Angeles aqueducts — and critical roads like
Interstate 5, state routes 14 and 58 and U.S. 395.
major quake on the Garlock fault could then, in turn, destabilize the
San Andreas. A powerful earthquake on a stretch of the roughly
300-mile-long southern San Andreas fault could cause the worst
shaking the Southern California region has felt since 1857, and send
destructive tremors through Los Angeles and beyond.
research was authored by some of the nation’s leading experts in
earthquake science at Caltech in Pasadena and NASA’s Jet Propulsion
Laboratory in La Cañada Flintridge — which is operated by Caltech.
findings confirm what some scientists expected from the Ridgecrest
quakes. The largest quake of the sequence, the magnitude 7.1 event on
July 5, ruptured along 35 miles over a series of previously
unidentified faults over 22 seconds. Its southeastern ends terminated
just a few miles away from the Garlock fault.
Garlock fault accumulates seismic strain at among one of the faster
rates in California. According to USGS research geophysicist Morgan
Page, who was not involved with the study, the average time between
earthquakes of at least magnitude 7 on the central part of the fault
is about every 1,200 years. But there’s huge variation; sometimes,
only 200 years might pass between major quakes on the fault; then,
however, it could be 2,000 years before an encore. The last time a
big quake is believed to have hit the Garlock fault is about 465
years ago, give or take a century.
some scientists, the physics of the magnitude 7.1 quake on July 5
immediately suggested that the Garlock fault would be more likely to
rupture as a result. Here’s one possible explanation: The
southwestern side of the fault that ruptured on July 5 lurched
northwest. This had the effect of moving a block of land away from
the Garlock fault, unclamping it and making it easier for blocks of
land accumulating seismic strain on both sides of the Garlock fault
to move — as if a bicyclist had decided to loosen brakes that had
been gripping the tire tightly.
radar imagery shows that the part of the Garlock fault that has begun
to creep is about 20 miles long, with the land on the northern side
of the fault moving west, while the other side moves east. The radar
images show one side of the fault has moved at its largest extent
about four-fifths of an inch relative to the other.
scientists has been state-of-the-art observations with incredible
high-resolution details that haven’t been possible in any previous
major California earthquake.
Ridgecrest earthquakes struck in an area that has a particularly
extensive network of earthquake sensors near the seismically active
Coso Volcanic Field of Inyo County, which uses heat from magma to
fuel a power plant. More seismic stations have been installed since
the last big Southern California quake in 1999, and there is now
frequent satellite radar imagery taken of the Earth’s surface.
the Garlock fault, there is also reason to focus on risks from other
is a line of potentially ripening fault zones along the so-called
Eastern California Shear Zone, one of the state’s most significant
seismic zones, which carries a good chunk of the earthquake burden
needed to accommodate tectonic plate movement as the Pacific plate
slides northwest past the North American plate.
include, generally speaking, an unruptured segment about 30 miles
long between faults that ruptured in the 1872 Owens Valley quake and
the Ridgecrest quakes, and another 75-mile gap along the Blackwater
fault system between the faults the caused the Ridgecrest quakes and
the magnitude 7.3 Landers quake of 1992. Some day, those fault
segments will eventually need to rupture to catch up with the
movement of the tectonic plates, but it’s not known if that’ll
happen in our lifetime.
scientists not affiliated with the study called the discovery of the
triggered creep on the Garlock fault scientifically interesting that
should be understood better, but emphasize that its implications are
not clear. Although the Garlock hasn’t been observed to creep
before in response to big quakes, other faults that have crept
haven’t been seen to rupture in major quakes.
actually probably pretty common, and if that’s the case … that
doesn’t necessarily mean it’s portending something terrible,”
said Page of the USGS.
the creep observed was only probably in a relatively shallow area.
“What we’re really interested in is what happens at the depths of
where earthquakes occur,” said USGS seismologist Elizabeth Cochran,
who was not involved with the study. Earthquakes typically occur
between 1 mile and 10 miles deep; the creep calculated probably
occurred in the shallowest hundreds of feet below the surface.
research needs to be done on whether the releasing of seismic energy
in the form of a creeping fault near the surface advances or slows a
subsequent earthquake, said Scharer of the USGS. In this particular
case, the amount of creep and its shallowness would do little to
affect the timing of when the next earthquake strikes the Garlock
fault, Scharer said.
big quakes can lead to other ones; a classic example was in 1992,
when the magnitude 6.1 Joshua Tree temblor in April was followed up
two months later by the magnitude 7.3 Landers earthquake, which in
turn triggered just hours later the magnitude 6.3 Big Bear
earthquake; seven years later, a magnitude 7.1 quake hit Hector Mine.
But other times, a single big earthquake and its associated
aftershocks can lead to decades of seismic quiet, like the magnitude
6.9 Loma Prieta earthquake of 1989.
the uncertainty, what’s happening in this region bears close
scrutiny, given how the Garlock fault is an important major fault for
Southern California. There are few big earthquakes that have been
observed in California in modern times, and just because something
hasn’t been observed in the past doesn’t mean it can’t happen.