Science
A back-to-back sequence of four 6.3 magnitude earthquakes in just over a week has stunned scientists. “There is no way to know … if the latest ‘domino’ will cause another one to fall.”
ByRobin George Andrews
Published October 18, 2023
• 9 min read
At 11:11 a.m. local time on October 7, Afghanistan’s Herat province was struck by a magnitude 6.3 temblor. A fault at the far western end of the Hindu Kush mountain range ruptured, damaging the populous Herat city and leaving many rural villages in rubble. Then, just 23 minutes later, there was another earthquake, also registering as magnitude 6.3.
Just four days later, on October 11, while bodies were still being recovered from the wreckage, yet another magnitude 6.3 earthquake hit the same region, further traumatizing a devastated populace. By October 15, the death toll had reached almost 3,000—and that’s when a fourth magnitude 6.3 quake rocked the area.
This back-to-back sequence of four equally strong earthquakes in just over a week stunned many scientists. “I was startled and dismayed to see the initial doublet,” says Judith Hubbard, an earthquake scientist at Cornell University. “The third earthquake was surprising; the fourth even more so.”
Scientists have been vigorously debating what may have generated this unlikely seismic cascade. A dearth of local seismometers makes the events difficult to study, but one idea is already gaining support. The cause of the quakes was “most likely a domino effect,” says Harold Tobin, the director of the Pacific Northwest Seismic Network at the University of Washington.
When the first quake struck on October 7, some of the stress from the geologic fault that slipped was transferred to another, already stressed fault. That caused it to rupture as well soon after—and this process happened twice more. This sort of transferal of stresses is seen all over the world, but in the case of these earthquakes, “what’s unusual is that they have all been around the same magnitude and occurred in a very short period of time,” says Tobin.
Earthquake sequences like this are scarce, which makes understanding them tricky. And while scientists continue to puzzle over this latest sequence, they warn that it may not yet be complete—additional quakes in the coming days, weeks, or months remain a concerning possibility.
“There is no way to know, at this point, if the latest ‘domino’ will cause another one to fall,” says Hubbard.
An unlikely occurrence
The network of faults crisscrossing much of Afghanistan is a multi-layered web of possible rupture sites crafted by the inexorable movements of the Arabian, Eurasian, and Indian tectonic plates. Practically, this means that the country hosts plenty of earthquakes, some of them lethal; last summer, a magnitude 6.1 quake on the border with Pakistan killed over 1,000 people.
Normally a pair of strong quakes in Afghanistan wouldn’t seem unusual. But from the start, this pair proved peculiar. “This part of Afghanistan is far less seismically active than the eastern part, so the location of the first two earthquakes was somewhat surprising,” says Wendy Bohon, an earthquake geologist.
But it was the equivalent magnitudes that particularly piqued the interests of scientists. “When the first two very similar magnitude 6.3 earthquakes occurred, I already thought that this was a fairly unusual sequence,” says Jascha Polet, a seismologist and professor emeritus at California State Polytechnic University Pomona. “When the sequence then produced a quadruplet of these events, I was very surprised.”
This is not how earthquakes normally work. Typically you get a mainshock—a relatively large fault rupture—that is followed by many smaller aftershocks that decrease in frequency over time. You can get another quake in the area of a similar magnitude, but the odds of that happening within a week of the mainshock are about 5 percent.
The quake pairing on October 7 defied those odds. It was what is known as a doublet: two distinct quakes of similar or equal magnitudes happening in a short timespan on the same or on closely spaced fault zones. Each have their own aftershock pattern, and due to their proximity, they are thought to be linked by an underlying tectonic or seismic process.
Doublets are not especially uncommon. Coincidentally, there was a doublet in Papua New Guinea the same day. The quakes that struck Turkey and Syria in February were also a doublet. Triplets, although rarer, also happen. “There was a triplet sequence in central Iran that occurred in the space of about two weeks in December 2017,” says Edwin Nissen, an earthquake scientist at the University of Victoria in Canada—three magnitude 6 quakes in succession.
But “a quadruplet, especially one of four that are very close to identical magnitude, is certainly a very rare and unlikely pattern,” says Tobin.
Hubbard notes that there was a quadruplet in the Philippines in 2019, where quakes of magnitude 6.4, 6.6, 6.5, and 6.8 struck Mindanao Island between October and December. But Afghanistan’s equal-magnitude and fast-paced quadruplet appears particularly perplexing.
So what caused it?
A quartet of quakes
That there were four modestly powerful quakes, rather than one extremely strong temblor, is probably due to the piecemeal nature of the region’s mountainous faults, “broken up by gaps, step-overs, or bends,” says Nissen. “These segment boundaries can stop earthquakes from rupturing over a wide fault plane, which is needed to generate a very large magnitude.”
Seismic signals reveal that all four quakes were caused by thrust faulting, where one crustal wedge jolts over another. But limited data means that the specific fault (or faults) responsible have not yet been identified.
Regardless of this uncertainty, the migration of stress through the crust is likely the root cause of the four quakes. “After an earthquake, some nearby areas are put under more stress, making earthquakes on faults in those areas more likely,” says Bohon.
The epicenters of each successive quake are slightly to the east of one another, possibly involving lots of closely spaced faults, or several sections of one long fault broken up into pieces. “It may be that these are faults that are stacked on top of each other, kind of like blocks, and when one moves it makes the others more likely to move,” says Bohon.
But this explanation alone still doesn’t explain why these earthquakes had the same magnitudes, says Polet.
There is, however, no hard-and-fast rule that says quakes in a doublet, triplet, or quadruplet must be a certain size—that depends on the unique dimensions and behaviors of each fault. That these four quakes were the same magnitude, then, “is probably coincidence,” says Hubbard.
A rigged game of dominos
Not everyone has bought into this preliminary analysis. “It doesn’t need to be a domino effect,” says Zachary Ross, a geophysicist at the California Institute of Technology. It could instead be what seismologists broadly refer to as a swarm: a series of quakes of comparable sizes all happening around the same time and in the same region, with none being a distinct mainshock.
Ross considers Afghanistan’s seismic sequence to be “fairly normal for earthquake swarms, in which we often see many earthquakes with similar magnitudes.” But he notes that swarms with magnitudes above 6 are uncommon. Another issue is that, when swarms occur, molten rock or superheated fluids moving through the crust is often implicated as the cause—but this part of Afghanistan is not associated with this type of geologic activity.
Most scientists are leaning toward the domino effect hypothesis, and as they gather more data, they will start to get a clearer picture of the underlying geologic cause. But one thing is already crystal clear: This region was entirely unprepared for any one of the quadruplet’s quakes.
The remote villages most severely affected are collections of unreinforced mud, brick, and timber buildings, the sort that immediately fold during a strong earthquake. Tragically, an estimated 90 percent of the victims were women and children, who were more likely to be inside when the quakes struck. The Taliban has restricted the rights of women to move freely in public spaces, exacerbating the disparity of casualties.
Earthquakes strike when the crust of the planet shifts, but disasters usually have human as well as natural causes. This one is no different.
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