On the night of June 22, an earthquake shook the towns and villages of the remote Sulaiman Mountains on the Afghanistan-Pakistan border. The collapse of numerous apartments claimed more than a thousand victims. It’s been dubbed the “Khost earthquake,” named after the nearby town of about 150,000 people.
In a region at the interface between the Indian and Eurasian tectonic plates – which also form our planet’s highest mountain range, the Himalayas, when they collide – this rare event is a poignant reminder of the big questions associated with a better understanding of faults and tectonic forces at work.
Early scientific observations of the Khost earthquake challenge what is known (or believed to be understood) about nearby faults.
An almost inconspicuous leaf-slip plate boundary
The Indian Plate is moving north at a rate of about 3 centimeters per year, roughly the rate at which our fingernails grow. On its border with Eurasia in Afghanistan and Pakistan, it slides along strike faults; That is, the plates move in opposite directions across faults, like crossing trains, without bumping into each other.
If the faults were straight, smooth, and vertical, everything would run like rails, but the fact is, millions of years of tectonic history and more complicated movements than expected have twisted and thickened this plate boundary.
Considerable damage for the extent
The June 22 earthquake has an estimated moment magnitude of 6.0. This figure indicates a moderate energy release, more than a thousand times less than that of the 2011 Japanese earthquake associated with the Fukushima disaster, with a moment magnitude of 9.1 (moment magnitude scale is said to be “logarithmic”). On average, there are about 100 earthquakes of similar magnitude around the world each year, but few prove so devastating.
Two smaller earthquakes (magnitude 4.5 and 4.3) in the immediate vicinity were detected one hour and three days later. These are the largest “aftershocks” of this earthquake, which likely has dozens of others that are smaller and undetectable due to the lack of sensors in that region.
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In fact, risk can only be estimated by comparing the physical characteristics of an earthquake with a region’s susceptibility factors, including the proximity of populations and the resilience of structures. In the case of the Khost earthquake, for example, it was pointed out that the extraordinary extent of the damage caused by such an earthquake is largely related to the vulnerability of the unreinforced buildings. However, other explanatory factors can be cited, such as, on the one hand, the shallow depth of the earthquake (it started at 10 kilometers) and, on the other hand, local effects of amplification of seismic waves related to the nature of the subsoil different.
Why is it difficult to estimate seismic risk in the region?
In general, it is the analysis of current and historic earthquakes in a region that provides the primary source of information when attempting to predict the location and magnitude of future earthquakes.
However, compared to other plate boundaries, such as California, which are the best studied in the world, very few earthquakes have been documented in this region. The last major earthquake recorded near Khost was in 1956 and was of magnitude estimated from 6.7.
It is also known that Kabul, the Afghan capital, was devastated by an earthquake in 1505, and according to reports, “the ground rose in some places as high as an elephant and collapsed so badly elsewhere”. Other reports, combined with geographic information, led to estimates that it was a magnitude 7.3 earthquake – 90 times stronger than that in 2022.
Taken together, these observations are insufficient to have an accurate idea of the active seismic faults along this more than 100-kilometer-wide plate boundary. The current scientific knowledge about faults in this region is therefore based to a large extent on the traces that tectonic movements have left in the landscape and geology.
Given the region’s geopolitical context, seismologists have worked primarily with satellite imagery, i.e. “remote sensing,” for several decades. The sparse vegetation associated with the dry climate invites you to observe the soil and its tectonic movements. Viewed from the air, however, it is not easy to distinguish today’s active and fossilized faults.
Thus, we know that the June 22, 2022 earthquake occurred in close proximity to many known short faults, in an area where the topography shows characteristic bulges of a compression zone (the blocks on either side of the faults collide and continue to ride on top of each other), but the seismic activity of these faults is undetermined. Can they accommodate slip? How much and in which direction?
A sobering first look at seismic movement
Part of the answer is provided by the “focus mechanism” of this earthquake, which was obtained in the hours after the event from signals recorded on the global network of seismic stations.
Seismologists are used to depicting the three-dimensional sliding of the earthquake on a “beachball” seen from the sky. The ball, with four red and white quarters, is precisely aligned so that the movement is from white to red. Plans between neighborhoods define two probable failures and other data such as satellite imagery, need to know what actually represents the bug that was being played. In our case, the planes are close to vertical and the motion is “stripping,” meaning that the two sides of the fault are sliding with no associated net contraction or extension.
This observation contrasts with our geological and satellite imagery-based view of faults, which tell us that strike-slip movement between India and Eurasia is localized into two parts. The large strike-slip structures that are the Chaman and Garde fault zones and that could have hosted such an earthquake are further east, while the region where the Khost earthquake occurred is associated with structures known to be “compressive ” have been identified.
Consequently, this earthquake surprised the scientific community by the significant damage it caused, but also by the direction of strike-slip movement in an area believed to be in tectonic compression. It is a new element (among others) that helps clarify the current geometry of the plate boundary and the patterns of its past and future evolution.
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