Three things scientists want to know after California’s July earthquakes | Science News for Students

Three things scientists want to know after California’s July earthquakes

One of them: Is tectonic activity slowly shifting away from the San Andreas Fault?
Jul 24, 2019 — 6:45 am EST
damaged road

This section of California’s State Route 178 crumbled and closed after a powerful magnitude 7.1 earthquake struck the region on July 5.

USGS

In early July, two large earthquakes rattled southern California. Scientists are now scrambling to understand what led to the temblors and what they might tell us about future quakes.

A magnitude 6.4 quake struck July 4 near the town of Ridgecrest. That’s about 194 kilometers (121 miles) northeast of Los Angeles. The next day, a magnitude 7.1 quake shook the same region. 

Both quakes took place in a high desert area. The crisscrossing faults here are known as the Eastern California Shear Zone. They are quite a distance from California’s infamous San Andreas Fault.

That fault stretches nearly 1,300 kilometers (some 800 miles) and generally takes center stage for California’s earthquake activity. There, the Pacific tectonic plate and the North American tectonic plate slowly grind past each other. This can cause sections of ground to lock together for a while. That brake on their movement allows strain to buildup. Eventually it will suddenly release, producing powerful quakes.

For the last few tens of millions of years, the San Andreas has been the primary origin of massive earthquakes in southern California. It’s also now overdue for a massive earthquake, based on historic trends. Many people fear it’s only a matter of time before another truly “Big One” strikes.

But as shown by the July 4 and July 5 quakes — and their many aftershocks —the San Andreas Fault system isn’t the only area of concern. California is riddled with faults, notes geophysicist Susan Hough. She works for the U.S. Geological Survey in Pasadena, Calif. Almost all of the state is part of the general boundary between the Pacific and North American plates. The Eastern California Shear Zone itself has been the source of several large quakes in the last few decades. These include the magnitude 7.1 Hector Mine quake in 1999. There was also the magnitude 6.7 Northridge quake in 1994 and a magnitude 7.3 Landers quake in 1992.

Here are three questions scientists are trying to answer in the wake of quakes on July 4 and 5.

Which faults ruptured, and how?

The quakes appear to have occurred, here, along previously unmapped faults. These include a section known as the Little Lake Fault Zone. Its broad bunch of cracks are difficult to map, Hough says. “It’s not like the San Andreas, where you can go out and put your hand on a single fault,” she explains. And, she adds, the zone also lies within a U.S. Navy base. Such military sites generally are not open for mapping by geologists.

But preliminary data do offer some clues. They suggest that the first rupture may actually have been a two-fer: Instead of one fault rupturing, two connected faults — or conjugate faults — may have ruptured at almost the same time. They would have produced the July 4 quake.

It’s possible that the first quake didn’t fully release the strain on that fault, but that the larger, second quake did. “My guess is that they will turn out to be complementary,” Hough says. By that, she means they will turn out to be related.

The jury is still out, though, says Wendy Bohon. She’s a geologist at Incorporated Research Institutions for Seismology in Washington, D.C. “What parts of the fault broke, and whether a part of the fault broke twice … I’m waiting to see what the scientific consensus is on that.”

It is not yet clear, she adds, whether a simultaneous rupture of a conjugate fault is surprising. It may turn out to be common, she says. The data simply haven’t amassed to show that yet. “In nature, we see a lot of conjugate-fault pairs,” she says. “I don’t think they normally rupture at the same time.” But if they do, “We haven’t had enough data to see that.”

Is the center of tectonic action moving away from the San Andreas?

Data from Global Positioning System (GPS) satellites have revealed exactly how the ground is shifting in California as the giant tectonic plates slide past one another. The San Andreas Fault system bears most of the strain, those data show — some 70 percent. But the Eastern California Shear Zone bears the other 30 percent. And the large quakes seen there over the last few decades raise an interesting possibility, Hough says: We may be witnessing the birth pangs of a new boundary.

“The plate boundary system has been evolving for a long time already,” Hough says. For the last 30 million years or so, the action has focused along the San Andreas Fault. But just north of Santa Barbara, Calif., lies a “big bend” in the fault. This kink separates the northern and southern portions of the fault. Where the fault bends, the Pacific and North American plates aren’t sliding past one another but colliding into each other.

“The plates are trying to move,” she says. “But the San Andreas is actually not well aligned with that motion.” The Eastern California Shear Zone is. And some geologists are now asking whether this is a new plate boundary in the making. The changeover would take “millions of years,” she adds. “It’s not going to be in anyone’s lifetime.”

Will these quakes trigger the Big One on the San Andreas?

Such large quakes inevitably raise fears of setting off the Big One. Historically, the San Andreas has produced a massive quake about once every 150 years. “It has been pretty quiet in the San Andreas since 1906,” Hough notes. That’s when an estimated magnitude 7.9 quake along the northern portion of the fault devastated San Francisco. The southern portion of the San Andreas is even more overdue for a massive quake. Its last biggie was an estimated magnitude 7.9 quake in 1857, she says. 

The recent quakes aren’t likely to change that situation. Subsurface shifting due to a large earthquake can alter strains on nearby faults. But it’s unlikely that the quakes either relieved stress or will ultimately trigger another quake along the San Andreas system, Hough says. The reason? Basically, the early July quakes were too far away. “The disruption [from one earthquake] of other faults decreases really quickly with distance,” she explains.

Some early assessments do suggest that the 7.1 earthquake on July 5 triggered some slippage, also known as creep, along at least one shallow fault in the southern San Andreas system. But such slow, shallow slips don’t produce earthquakes, Hough points out.

July’s back-to-back quakes could have perturbed much closer faults. One of them, the Garlock Fault, runs roughly west to east along the northern edge of the Mojave Desert. That would be nothing novel: The 1992 Landers quake may have triggered a magnitude 5.7 quake two weeks later along the Garlock Fault.

“Generations of graduate students are going to be studying these events,” notes Bohon. They’ll be looking, she says, into angles of the faults, how the ground moved — even how the visible evidence of a rupture can disappear over time. 

For now, scientists are eagerly trading ideas on social media. “It’s the equivalent of listening in on scientists shouting down the hallway: ‘Here’s my data — what do you have?’” Bohon explains. Those initial ideas and explanations will almost certainly evolve as more information comes in, she adds. “It’s early days yet.”

Power Words

(more about Power Words)

aftershock     One or more smaller earthquakes which often follow a major earthquake.

angle     The space (usually measured in degrees) between two intersecting lines or surfaces at or close to the point where they meet.

conjugated     An adjective that describes a system with two, paired units.

consensus     An opinion or conclusion shared by most, if not all, of a specific group.

data     Facts and/or statistics collected together for analysis but not necessarily organized in a way that gives them meaning. For digital information (the type stored by computers), those data typically are numbers stored in a binary code, portrayed as strings of zeros and ones.

earthquake     A sudden and sometimes violent shaking of the ground, sometimes causing great destruction, as a result of movements within Earth’s crust or of volcanic action.

evolve     (adj. evolving) To change gradually over generations, or a long period of time. Nonliving things may be described as evolving if they change over time. For instance, the miniaturization of computers is sometimes described as these devices evolving to smaller, more complex devices.

fault     In geology, a fracture along which there is movement of part of Earth’s lithosphere.

generation     A group of individuals (in any species) born at about the same time or that are regarded as a single group. The term also is sometimes extended to year classes of other animals or to types of inanimate objects (such as electronics or automobiles).

geological     Adjective to describe things related to Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists.

global positioning system     Best known by its acronym GPS, this system uses a device to calculate the position of individuals or things (in terms of latitude, longitude and elevation — or altitude) from any place on the ground or in the air. The device does this by comparing how long it takes signals from different satellites to reach it.

graduate student     Someone working toward an advanced degree by taking classes and performing research. This work is done after the student has already graduated from college (usually with a four-year degree).

magnitude     (in geology) A number used to describe the relative size of an earthquake. It runs from 1 to more than 8 and is calculated by the peak ground motion as recorded by seismographs. There are several magnitude scales. One of the more commonly used ones today is known as the moment magnitude. It’s based on the size of a fault (crack in Earth’s crust), how much the fault slips (moves) during a quake, and the energy force that was required to permit that movement. For each increase in magnitude, an earthquake produces 10 times more ground motion and releases about 32 times more energy. For perspective, a magnitude 8 quake can release energy equivalent to detonating 6 million tons of TNT. 

novel     Something that is clever or unusual and new, as in never seen before.

plate boundary     (in geology) The edge of a tectonic plate, or the place where two or more tectonic plates meet.

preliminary     An early step or stage that precedes something more important.

primary     An adjective meaning major, first or most important. 

seismology     The science concerned with earthquakes and related phenomena. People who work in this field are known as seismologists.

social media     Internet-based media, such as Facebook, Twitter and Tumblr, that allow people to connect with each other (often anonymously) and to share information.

strain      (in physics) The forces or stresses that seek to twist or otherwise deform a rigid or semi-rigid object.

stress      (in physics) Pressure or tension exerted on a material object.

tectonic     Surface activity on a large rocky body (such as a planet or moon) as liquid rock flows up to the surface where it solidifies, then slowly drifts atop molten rock, carrying surface features with it.

tectonic plates     The gigantic slabs — some spanning thousands of kilometers (or miles) across — that make up Earth’s outer layer.

temblor     Another term for an earthquake or Earth-shaking tremor.

U.S. Geological Survey (or USGS)     This is the largest nonmilitary U.S. agency charged with mapping water, Earth and biological resources. It collects information to help monitor the health of ecosystems, natural resources and natural hazards. It also studies the impacts of climate and land-use changes. A part of the U.S. Department of the Interior, USGS is headquartered in Reston, Va.

Citation

Report:​ ​​U.S. Geological Survey Earthquake Hazards Program. M 7.1 - 18km W of Searles Valley, CA.

Report:​ U.S. Geological Survey Earthquake Hazards Program. M 6.4 - 12km SW of Searles Valley, CA.