Four magnitude-6.3 earthquakes strike in succession near Herat, Afghanistan
On Oct. 7, 2023, a magnitude-6.3 earthquake struck close to the city of Herat in northwestern Afghanistan.
Less than half an hour later, a second earthquake occurred with the same magnitude. The pair of earthquakes caused widespread building collapse, particularly in the affected rural communities surrounding Herat. At least2,400 people were killed and over 2,000 were injuredin the earthquakes. Three days later, while the population reeled from the events, another magnitude-6.3 earthquake jolted the same region. Finally, on Oct. 15, a fourth magnitude-6.3 earthquake hit.
Earthquakes are sometimes twins, occasionally triplets, but rarely quadruplets. This uncommon set of four events raises questions about the tectonic stresses in the region, if communities should expect more shaking, and how to move forward with rebuilding efforts.
This map of Northern Afghanistan shows the location of the Herat earthquake sequence, as well as its proximity to the Hari-Rud fault system. The background colors show the shaking likely in your lifetime, which comes from transforming the Global Earthquake Activity Rate (GEAR) model into a shaking model called Probabilistic Uniform Seismic Hazard (PUSH). PUSH is globally consistent and corrected for soil amplification, and was trained on the USGS shaking model. Credit: Temblor, CC BY-NC-ND 4.0
Distant collisions
Tectonically, there’s a lot going on in Afghanistan. “You have earthquake hazard across the large parts of Afghanistan, most of it in the east,” says Richard Walker, a professor at the University of Oxford. East of Afghanistan lies the India-Eurasia collisional plate boundary that gives rise to the Himalayan orogen and other ranges, including the Karakoram, Pamir and Hindu Kush mountains. To the west, just over the Iranian border, is the edge of the Arabian-Eurasian collision where the Arabian tectonic plate is crashing into the Eurasian plate, Walker says.
Northern Afghanistan features diffuse deformation because it sits between these two massive collision zones. Moreover, in the region around Herat, which is in the foothills of the Hindu Kush, deformation is “slower.” But, just because the deformation is slow doesn’t mean there’s no earthquake risk, Walker says.
The Herat Fault, also called the Hari-Rud fault system, stretches across nearly the entirety of Northern Afghanistan. This major right-lateral fault is probably still accommodating those two big continental collisions from afar, Walker says, though there’s also shortening that needs to be accounted for. But this fault can’t accommodate all the motions, he explains. The shortening is likely accommodated on reverse faults, which are the prime suspects for the four recent earthquakes.
Past Earthquakes
There’s very little historic seismicity on and around the Herat fault. Since 1970, there have only been 40 events recorded in the 80 kilometers surrounding the epicenters of the 2023 earthquakes. Temblor’s simulated earthquakes estimate a similarly low number of events in proximity to the 2023 epicenter. “It’s not a very active place, which suggests to me that this fault probably ruptured prehistorically. It’s in its quiet phase — it’s not inactive,” says Ross Stein, CEO of Temblor.
“The last big earthquake in northern Afghanistan was probably decades, centuries, or in some places even millennia ago,” says Walker, but their effects can be seen in the landscape. For example, satellite imagery shows offset rivers and streams that result from past events.
More evidence of a lack of historical earthquakes comes from city of Herat; many ancient buildings are still standing, and show little evidence of earthquake damage, says Zakeria Shnizai, an earthquake researcher at the University of Oxford. The ancient buildings in Herat include the Citadel of Herat built in 330 BCE, and the Great Mosque of Herat, whose construction began in 1200 CE.
Temblor’s simulation of 50,000 years of earthquakes greater than or equal to magnitude 5 in Afghanistan identifies that the October earthquakes struck an area of relatively low seismicity. While this could be a weakness of the simulation, it more likely reflects few quakes having occurred there in the past century, as well as lack of strain rate data for the region. Credit: Gabriel Lotto, Temblor CC BY-NC-ND 4.0
Triggered events
It’s unlikely that these four events happened so close together just by chance. It’s far more likely that the earthquakes are triggering each other, like a chain of dominoes being pushed over. When an earthquake occurs, the stress released by the event can be transferred to surrounding faults via coulomb stress transfer.
And reverse faults that don’t reach the surface, like the ones researchers think might be involved in the 2023 Herat sequence, are particularly effective at transferring their stress, Stein says.
Why isn’t this built up stress released at once? If the four magnitude-6.3 events had ruptured together in one earthquake, it would have been equivalent to a single magnitude-6.7 event. It’s likely that some complexity, like a bend in the fault, prevented the stress from being released in a single event, says Walker. This geometric quirk would lead to that domino-style cascade of earthquakes. “It’s not clear right now where the complexity lies,” he says. More observations over the coming weeks will help understand such subtleties.
To understand this unusual sequence, researchers may be able to look to previous earthquake sequences that behaved similarly. They need not look far. In 1994, three shallow earthquakes of about magnitude 6 occurred close together on blind thrusts near Sefidabeh, in eastern Iran (Parsons et al., 2006). The faults were stacked close together in what geologists call en-echelon faults. Slip on one segment then triggered slip on its neighbor. “What we might find out is that the four events in Afghanistan are doing a similar thing,” Walker says.
Are there more earthquakes to come?
It’s hard to know if this earthquake cascade has come to an end, or if another event is on the way. Even if another magnitude-6.3 earthquake were to strike in the same place, it is unlikely to significantly increase the death toll. “Since that first earthquake, most of the people are living outside, not in houses and buildings,” Shnizai says.
A more worrying prospect is if the earthquake sequence triggered a larger magnitude event on one of the major nearby faults, Stein says. For example, an event triggered on the Herat Fault could result in a magnitude-8 earthquake, should the entire fault rupture. That scenario would cause widespread devastation beyond what has already been wrought by the moderately sized events of the ongoing sequence, extending damage to Kabul. Earthquake sequences that begin on a minor fault, as did the Afghanistan quakes, but then cascade into a large strike-slip fault, have been observed before. For instance, the 2002 magnitude-7.9 Denali earth-quake in Alaska, and the 2023 magnitude-7.8 Kahramanmaras earthquake in Turkey followed that pattern. Nevertheless, the chances of this concerning scenario happening are low, simply because earthquakes greater than or equal to magnitude 7.8 are — fortunately — rare, he explains.
A destroyed village in Herat Province after the 7 Oct. 2023 earthquake. Credit: Voice of America News via Wikimedia Commons, Public Domain
Traditional adobe construction
Many of those impacted by the quake lived in rural communities, where people tend to reside in adobe buildings. Those structures are good at keeping warm in the harsh winters and cool in the hot summers, Shnizai says. But, such buildings are also prone to collapse in earthquakes.
Adobe is a construction technique that utilizes “molded earth blocks” to build structures, and is one of many methods of earthen construction. In traditional earthen construction methods, “you use what you have around yourself, either stone, timber, or earth,” says Philippe Garnier, an architect and researcher at the International Centre for Earth Construction, National School of Architecture of Grenoble, France.
Earthen architecture is common across the world. “Conservatively, about 15 to 20% of the world population [lives in earthen constructed buildings],” Garnier says, and many of these places are seismically active.
The citadel of Bam, Iran, or Arg-é Bam, in Kerman Province, Iran is the world’s largest adobe structure, dating to at least 500 BCE. Credit: Ales.kocourek via Wikimedia Commons CC BY-SA 3.0
In certain regions, traditional building techniques have evolved to incorporate features that help prevent deadly collapse during an earthquake. Buildings in Bulgaria, Nepal and China, for example, include one such feature called “seismic bands,” Garnier says. A seismic band is a horizontal timber or brick layer within the walls. Its presence improves the behavior of the walls by increasing its ability to dissipate seismic energy, therefore preventing collapse during an earthquake.
The adobe buildings in the Herat region did not seem to have seismic modifications, like the above-mentioned seismic bands. Although lack of resources play a role in the dearth of seismic modification, that isn’t the only contributing factor. Damaging earthquakes must occur frequently enough to remain in societal memory. Only then do seismic safety modifications tend to be incorporated into traditional building techniques, Garnier says. In other words, earthquakes don’t happen often enough in and around Herat to be factored into building practices.
Other factors can also facilitate building collapse, Garnier says. For example, amplification of shaking may have occurred because of local geological conditions. “The same buildings will not behave the same [in all locations],” he says.
Additionally, conflict in the region may play a role. In 2012, Afghanistan introduced seismic building codes, but aside from not applying to traditional earthen architecture, war and unrest makes enforcement nearly impossible, Shnizai says.
Conflict also results in uncertainty. “People don’t invest in the construction because [they do] not know if they can stay [because of uncertainty and war],” Garnier says.
With winter not far off, rural communities around Herat are considering how to re-build. Constructing modern buildings with seismic safety features may seem like a logical choice for tackling that. However, “I do not think that there will be a lot of money available for reconstruction,” Garnier says. “For me, the objective is really to think about simple things to improve safety.” But it’s also important to consider that importing new building materials and methods may not necessarily suit the needs of the community, he explains. “Reconstruction is a local process that needs to be supported also by local economy…We have to rethink about maximizing the use of local resources and knowledge.”
