Deadly M5.6 earthquake strikes western Nepal
Shaking felt across much of northern India
Citation: Hubbard, J., 2023. Deadly M5.6 earthquake strikes western Nepal. Earthquake Insights, https://doi.org/10.62481/8347a8d3
A magnitude 5.6 earthquake struck Jajarkot district of western Nepal on November 3, 2023 just before midnight local time. The earthquake occurred one month after a M5.7 earthquake ~110 km to the northwest. The October 3 earthquake caused building damage but few deaths. Sadly, the more recent earthquake reportedly killed at least 69 people, and the death toll is likely to increase as areas near the epicenter are contacted. Images of the region show major building collapse.
Given the distance between the two earthquakes on October 3 and November 3, and their relatively small magnitudes, it does not appear that the two events were linked, although people living in the region between the two events likely felt them both.
Nepal sits on an active plate boundary, and so it is expected that earthquakes will occur periodically. I described this setting after the October 3 2023 earthquake. I have temporarily removed the paywall on that post to allow access (although Substack seems to periodically reapply the paywall - leave a comment below if you need me to remove it again!).
Unfortunately, many of the people in this region live in buildings that cannot withstand earthquake shaking, so even these more moderate magnitude earthquakes can have tragic consequences.
Why was the November 3 earthquake so much more damaging than the October 3 earthquake?
At this point, I can only guess. However, a few points stand out:
The October 3 earthquake was preceded by a foreshock (M4.9 half an hour before the mainshock). This foreshock likely acted as a natural warning, causing people to evacuate buildings. One of the biggest risks in earthquakes is falling objects and collapsing buildings, so this foreshock may have prevented significant injury and deaths.
Foreshocks occur before a minority of earthquakes. They are simply smaller earthquakes that trigger later earthquakes that are larger than themselves. There is no way to know if an earthquake was a foreshock until after the mainshock happens.The October 3 earthquake occurred in the afternoon, while the November 3 earthquake occurred in the middle of the night. On October 3, people were more likely to be out and about, while on November 3, they were more likely to be in bed, asleep. Collapsing buildings on November 3 could have caught a lot of people unawares.
Even though the reported magnitude for the October 3 earthquake was slightly larger according to the USGS, and the reported magnitude of the October 3 earthquake was shallower, the USGS PAGER estimates that the shaking was significantly stronger on November 3.
It is not immediately obvious to me what would cause this stronger shaking, but it is important to remember that the earthquake magnitude does not directly correlate to surface shaking. This stronger shaking on November 3 means that the peak shaking was likely stronger on November 3, and about twice as many people experienced “strong” shaking (intensity VI).
I note that the IPGP network (France) reports the November 3 earthquake as larger (M5.8 vs. M5.7) and shallower (14 km vs. 26 km). Differences in network detection may mean that the USGS catalog is not properly identifying relatively subtle differences in the earthquakes that turned into major differences in outcome.
The area where the November 3 earthquake occurred has had fewer earthquakes in the last decades than the area to the northwest, where the October 3 earthquake occurred. The region around the November 3 earthquake may thus contain more buildings untested by shaking, and more slopes that are close to failure and able to collapse in landslides.
Figure 4
Why do different organizations report different magnitudes?
Twitter user @this_is_tckb noticed that the magnitudes being reported by different organizations were significantly different for the recent earthquake, and asked why. It is fairly common to see discrepancies in earthquake magnitudes. This can be due to several reasons:
Different organizations calculate and report magnitudes on different magnitude scales. While many people think that the Richter scale is the only way that earthquake magnitudes are measured, there are actually several different scales, which each rely on different types of measurements. Detailed descriptions of the different scales can be found here.
The recent earthquake was reported by the USGS as Mw5.6. This is the moment magnitude, and it is the most common modern type of measurement of an earthquake. The moment magnitude is intended to capture the total moment of the earthquake, which is sort of like the total energy (but not exactly).
In contrast, Nepal’s National Earthquake Monitoring and Research Center reports the earthquake as ML6.4. ML stands for Richter magnitude. This scale was originally designed to reflect the intensity of shaking as measured by a specific type of seismometer located near the epicenter; modern measurements of ML have to be derived and converted since those types of seismometers are no longer in use.Even when organizations report magnitude using the same scale, they may come up with different results if they have different networks of seismometers. For instance, while the USGS reported Mw5.6, GFZ reported Mw5.7, and IPGP reported Mw5.8.
Ultimately, earthquake scientists don’t tend to pay too much attention to small differences in magnitude, because they know that they come with uncertainty, and also that the impact of the earthquake is based on much more than the magnitude.
What else controls earthquake shaking, other than magnitude?
Earthquake shaking is affected by a variety of factors, many of them quite complicated. Here are a few:
Depth of the earthquake: The deeper the earthquake, the further it is from the Earth’s surface - so the maximum shaking intensity will be lower. However, deeper earthquakes can cause shaking over very wide regions, in contrast to shallow earthquakes, which will typically have more targeted effects.
Amplification: Seismic waves slow down and amplify in softer sediments, so areas like valleys will tend to experience higher intensity shaking than areas on strong bedrock.
Shaking frequencies: Different kinds of earthquakes can produce different kinds of shaking. High frequency shaking will have different effects that low frequency shaking, with impacts on landslides and buildings. Shaking frequencies can be affected not just by the earthquake itself, but by the materials that the earthquake waves travel through to get to the surface.
Earthquake rupture speed and direction: Although small earthquakes are usually visualized as point sources, large earthquakes occur over large faults - sometimes hundreds of kilometers long. The earthquake will start at one point on the fault and then move along the fault, typically at a couple kilometers per second. Different areas may experience different intensities of shaking depending on whether they are near the beginning of the rupture or the end, and how fast the rupture occurs.
What to expect
This part of Nepal is mountainous, and travel often depends on a few key roads. Reports indicate that landslides blocked some of these roads, making it difficult to access and assist areas near the epicenter. It is likely that the death toll will climb as more areas are reached. Rapid access is critical to providing assistance to impacted areas.
We can expect that this earthquake will be followed by aftershocks. It is also possible, although not likely, that the earthquake will trigger an event of the same size or larger. Thus, it is important for residents to be cautious of buildings, especially any buildings that were visibly damaged in the earthquake.
However, it is fall, and weather reports for the region indicate that temperatures are dipping below freezing at night. Thus, it will be critical for residents whose homes were damaged in the earthquake to secure safe housing in the coming days and weeks.
References:
Hubbard, J., Bradley, K., 2023. Buildings collapse in Nepal due to M5.7 earthquake with foreshock. Earthquake Insights, https://doi.org/10.62481/8a43340a
I see that the USGS magnitude is Mww, so they used the W-phase moment as they usually do. Because the W-phase has an extremely long wavelength around 20 km, it can be a little off for shallow thrust earthquakes like this one.