Fourth M6.3 earthquake strikes Afghanistan

Limited data suggest eastward propagation in unusual cascade

Citation: Hubbard, J. and Bradley, K., 2023. Fourth M6.3 earthquake strikes Afghanistan. Earthquake Insights, https://doi.org/10.62481/2209acfd

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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.

Figure 1: October 2023 earthquake sequence in northwestern Afghanistan, with events colored by time. Contours show shaking estimated by the USGS.

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.

Figure 2: Map showing uplift in the first two earthquakes (red shades), mapped active faults (black lines), and earthquake locations reported by the NEIC, colored by time. Arrow highlights eastward propagation of seismicity. Note that while the absolute earthquake locations are different on this map than the previous one, due to the different catalog, the pattern of relative seismic locations remains the same.

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.

Figure 3: Screenshot from the COMET data portal for the second earthquake. Red dot shows earthquake epicenter. Boxes show areas of coverage for different satellite image captures, with annotations added to show dates of data collection. Source: https://comet.nerc.ac.uk/earthquakes/us6000ldpm.html

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

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Read our previous posts about this earthquake sequence:

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