By Tommy Hough
I have a layperson interest in seismology, and if I was capable of competently executing math and logarithm formulas, I'm pretty sure I would've been a scientist, perhaps even a geologist or seismologist. But the reality is, as much as I find science fascinating, math has never come easy to me beyond basic arithmetic. I blew the lid off the SATs with my English scores, but my math scores were lurking somewhere near the basement, and I never received much academic encouragement to more fully pursue earth science studies.
Nevertheless, a friend recently asked about a story in the Los Angeles Times detailing how the Garlock Fault in Southern California is experiencing, according to the headline, "unprecedented" movement as the result of the magnitude 6.4 and 7.1 earthquakes near Ridgecrest over the July 4th holiday. My friend also asked whether the Garlock Fault is capable of producing a magnitude 8.0 earthquake, as the newspaper headline indicates. It is, and the L.A. Times story is in fact based on a new study released by Caltech scientists last week.
The Garlock Fault runs east-west across Southern California, essentially from Death Valley National Park at its east end to the Grapevine along I-5 at its west end. The Garlock is unusual because it's an east-west trending fault, and most faults in California tend to run northwest to southeast along larger fault systems like the San Andreas and San Jacinto. Most earthquake faults in San Diego County also follow this alignment.
Geologists have been aware of the Garlock Fault for decades, in part because they see the trace of the fault and the impact it appears to have had on the landscape over time. But there haven't been earthquakes that can be attributed to the Garlock in about 500 years of recorded history, and that includes about 130 years' worth of modern seismology that indicates the fault has simply not been an active geological factor in recent centuries. While the Garlock may have played a role in a magnitude 5.7 earthquake near the town of Mojave in 1992, about two weeks after the big Landers Earthquake to the south near Joshua Tree, no evidence of surface displacement was ever found.
That changed with the magnitude 6.4 quake on July 4th, and the bigger 7.1 quake on July 5th near Ridgecrest. The July 5th quake was the largest to hit California in 20 years, and large earthquakes like those in the Ridgecrest area often have a corresponding effect on other faults. Sometimes nearby faults "loosen up" and move as a result of a big quake, sometimes nearby faults "freeze" and stay locked as the result of a nearby earthquake. Most of the time, nothing happens.
A good way to think of California fault geology is to imagine the state as a big, broken dinner plate that's assembled back together, piece by piece, with a layer of topsoil on it. Some pieces of the broken plate are big chunks while other pieces are small, but they all move relative to one another even when one little piece of the broken plate moves – even a tiny one.
The Garlock Fault runs about 15 miles to the south of Ridgecrest, cutting across U.S. 395 north of Mojave, and when the quakes happened near Ridgecrest this summer many observers initially thought they were on the Garlock. They weren't. Instead, the Ridgecrest quakes were centered in a seismically active area called the Little Lake Fault Zone on a relatively unknown "strike slip" fault, which is a fault that moves horizontally and parallel to a surface fault trace. There are perhaps hundreds of these unmapped faults around the state, some of which may not be visible from the surface. In contrast, it was an undetected "blind thrust" fault, which is a fault at an angle within the earth's crust below the surface, that was the cause of the magnitude 6.4 Northridge Earthquake in L.A. in 1994.
While the Garlock Fault has historically been unaffected by nearby quakes with either an increase or decrease in stress, the Garlock is now moving, albeit very slowly, for the first time in centuries as a result of the Ridgecrest quakes. That movement, as detailed by the Los Angeles Times story, is a big deal in the Earth Sciences world. In seismology, this kind of slow, barely-detectable movement is called "fault creep," or "creeping."
In 1952, a large portion of Kern County and the southern San Joaquin Valley northeast of the Grapevine was hammered by one of the state's most violent earthquakes. It was a big jolt, bigger than the biggest Ridgecrest quake, with a magnitude of 7.3. The quake and its aftershocks killed a dozen people, but that big earthquake happened on an otherwise little fault called the White Wolf Fault. The White Wolf had been identified and mapped prior to the 1952 earthquake, but no one was particularly familiar with it. The White Wolf is also an east-west trending fault, running parallel to the Garlock to the north, and it's located near the western end of the Garlock, where the Garlock meets the San Andreas Fault at the Grapevine near Tejon Pass. Despite the violent shaking and proximity of the White Wolf Fault to the Garlock and San Andreas, the 1952 Kern County Earthquake didn't seem to have any affect on either of those nearby faults.
Meanwhile, this summer's Ridgecrest Earthquake Sequence certainly did affect the Garlock. The depth of the respective quakes may have had something to do with it, another part of the reason the reactivation of the Garlock has become such a compelling story. While a bigger quake 65 years ago did nothing to affect the Garlock, the ones this summer near Ridgecrest rattled it loose. Maybe the Ridgecrest quakes were simply closer to a geologic "sweet spot" for the Garlock where it was "hung up" on rock, perhaps for centuries, and unable to move. Perhaps the Ridgecrest sequences avoided a more destructive quake. Perhaps some other geologic event has now been set in motion.
If you're interested in further layperson reading, there's a seismologist named Susan Hough (a very nice person but no relation) who works at the U.S. Geological Survey. She published a book in 2004 called Finding Fault in California: An Earthquake Tourist's Guide, and it's a handy book to have in the car on road trips or even if you're just driving around L.A., as it points out all kinds of interesting seismic features in the state, some of which you can hike or drive right up to and see for yourself.
Tejon Pass / Grapevine photo by Tommy Hough
Ridgecrest Earthquake photos courtesy of the U.S. Geological Survey
A San Diego County planning commissioner and former radio host at 91X and FM 94/9, Tommy Hough works as an environmental consultant with the ReWild Mission Bay campaign, and is a California Democratic Party delegate and the co-founder and former president of San Diego County Democrats for Environmental Action.