Fourth M6.3 earthquake strikes Afghanistan
Limited data suggest eastward propagation in unusual cascade
See all of our posts about this earthquake sequence here.
Citation: Hubbard, J. and Bradley, K., 2023. Fourth M6.3 earthquake strikes Afghanistan. Earthquake Insights, https://doi.org/10.62481/2209acfd
On the morning of October 15th, a magnitude 6.3 earthquake struck northwestern Afghanistan, near the city of Herat. Shaking locally exceeded intensity VII (very strong).
This is the fourth M6.2-6.3 earthquake to strike the region in a little over a week. Two M6.3 earthquakes occurred on October 7, and a third on October 10. Today’s earthquake marks the fourth in an unusual cascade - one that raises questions about what people in the region should expect.
After the third earthquake, we wrote:
Doublets are rare, triplets are rarer. There is no reason to think that there couldn’t be a quaduplet. In some regions, we have seen faults rupture piecemeal over the course of decades - we call these rupture cascades - where it seems clear that the first rupture influenced the second one, the second one led to the third, and so on.
We could update this statement today - “triplets are rare, quadruplets are rare. There is no reason to think that there couldn’t be a quintuplet.” It also remains possible that a larger earthquake could be triggered, either on the same fault system that we have already seen rupture, or on one of the larger fault systems nearby.
Limited scientific data so far
After the first two earthquakes, we were able to pinpoint the region of the crust that went up or down due to fault slip using satellite-based radar measurements. These measurements indicated that slip had occurred on a poorly known fault system with subtle geomorphic signals, rather than the larger, more continuous fault systems to the north and south.
As we noted in our previous posts, the earthquake epicenters are poorly located, due to the sparse seismic networks in the region. However, although the absolute locations are poor, the relative locations of the earthquakes suggest that we are seeing eastward propagation of seismicity.
It would be helpful to be able to confirm that pattern using satellite measurements. However, although COMET highlighted another flyover on October 14, it appears that the images collected in that pass just barely missed the earthquakes - they were too far to the east, and show no distinguishable signal of deformation. Another flyover should have occurred about two hours before the most recent earthquake, on October 15. The location of that image - which should include deformation from the first three mainshocks - appears more promising. Unfortunately, the processed image is not yet online, indicating that either the data were not collected as expected, or processing is ongoing.
The fourth earthquake will take more time to map. According to the COMET website for the most recent earthquake, the next relevant satellite flyover will occur on October 19.
Hazard in triggered earthquakes
Earthquakes are most dangerous when they trigger other hazards: building collapses, landslides, liquefaction, tsunamis. Reports from the first two earthquakes indicated severe and widespread collapse of mud-brick buildings, with a death toll approaching 3,000, associated largely with these structural failures. Less information was forthcoming following the third earthquake, and even less after the more recent fourth. However, reports indicate that many people are aware of the risk of aftershocks, and sleeping outside. It is also likely that many of the weakest structures have already collapsed. These factors should have reduced the fatalities in these later events.
However, we also note that the more recent events have been located to the east of the previous ones. While the regions of shaking mostly overlap, some new areas will have experienced higher intensity shaking than previously, potentially leading to new structural failures. Some buildings that accrued progressive damage through repeated shaking may also have become more dangerous.
A rare cascade
Sequences like this are very unusual. We are aware of only a few. Most recently, in 2019, a quadruplet struck Mindanao island in the Philippines, with a sequence of earthquakes M6.4, 6.6, 6.5, and 6.8 between October and December. Those earthquakes, which were strike-slip, occurred on a set of poorly known orthogonal faults. The triggered cascade is thought to be related to a combination of the geometry and variations in friction on the faults.
Given that the earthquakes in the Philippines were strike-slip, while those in Afghanistan were thrusts, these two sequences are not very similar.
Following the third earthquake, we noted a triplet in Iran that occurred in 2017. Those earthquakes were thrusts, more similar to the recent Afghanistan sequence. Like in the Philippines, those events occurred on different faults - faults with similar strikes, but opposing dips.
We do not yet know if the earthquakes in Afghanistan occurred on different faults, like the sequences in the Philippines and Iran. Given the limited data available, it is possible. However, there is no direct evidence for such behavior.
These sorts of cascades pose a challenge for earthquake communication: residents in the area naturally question why earthquake science is unable to provide clear answers about why these events keep happening, or useful guidance on what to expect. Understanding the conditions that lead to cascades, as opposed to more common mainshock-aftershock sequences, should be a priority for future earthquake research.
References:
Hubbard, J. and Bradley, K., 2023. Third M6.3 earthquake strikes western Afghanistan. Earthquake Insights, https://doi.org/10.62481/0c8f9a02
Hubbard, J. and Bradley, K., 2023. Two magnitude 6.3 earthquakes strike northwestern Afghanistan. Earthquake Insights, https://doi.org/10.62481/5776c220
Jiang, Y., Wei, S., Hubbard, J., Hu, W.L. and Salman, R., 2023. Modelling cascading ruptures on near-orthogonal strike-slip fault system: the 2019 Cotabato (the Philippines) earthquake sequence (No. EGU23-16210). Copernicus Meetings. https://doi.org/10.5194/egusphere-egu23-16210
The interferograms that I saw on LinkedIn had a noticeable phase discontinuity on the south side, so at least one of the first set of quakes ruptured to the surface on a north-dipping fault.
Thanks for another update on this earthquake sequence.