Gorkha aftershocks reveal seismic hazard in Nepal

by Jennifer Schmidt, Ph.D. (@DrJenGEO)

Researchers used seismic signals from 8,000 earthquakes after the Gorkha quake in 2015 to image the fault structure beneath Nepal. They found a system of stacked-up faults previously only hypothesized and modeled.

CITATION: Schmidt, Jennifer (2019), Gorkha aftershocks reveal seismic hazard in Nepal, Temblor, http://doi.org/10.32858/temblor.058

We don’t often consider the benefits of a natural disaster in its immediate aftermath.
But this is precisely what a team of researchers did following the magnitude-7.8 earthquake that rocked Gorkha, Nepal, on April 25, 2015. In a recently published study in Nature Geoscience, the team reports how aftershocks from the Gorkha earthquake illuminated the fault structure beneath Nepal, revealing how such large earthquakes occur in this region.

Big mountains make big earthquakes

The active collision of India with the Eurasian continent is what caused the Himalayas to become the tallest mountain range on Earth. When continents collide, stresses build and large volumes of rock are pushed together. Those rocks have to go somewhere, and in this case, rocks from Asia were stacked on top of one another along thrust faults. This is what built the Himalayan mountain range. When these faults slip, they can generate very large earthquakes.

Himalayas near Gorkha
Snowcapped peaks of the high Himalayas, below which the Gorkha earthquake occurred. In the relatively low-lying foreground is the surface above the crustal duplex imaged by Mendoza and colleagues. Credit: Peter DeCelles

The location and geometry of these faults are not straightforward however, making it difficult to predict where these large earthquakes may occur. In particular, the geometry of the Main Himalayan Thrust (MHT), the major fault that separates Indian and Eurasian crust, is debated.

Much of what we know about the faults below the surface in the Himalayas comes from what we can see at the surface, according to Peter DeCelles, a professor of geosciences at the University of Arizona, who was not involved in the new study. Geologists project what they see beneath their feet and make informed interpretations about the geometry of faults at depth. Surface mapping throughout the Himalayas tells us that there must be thrust faults down there, but geologists have competing theories about the geometry, with no way of imaging the crust to sufficient depth and resolution. Until now, that is.

If we know something about the fault geometry, we can identify areas of high stress that are primed for future earthquakes, says Matt Mendoza, a graduate student at the University of California, Riverside and the study’s lead author.

Evidence for a longstanding theory

The team traveled to Nepal weeks after the Gorkha quake to deploy a network of seismometers to record aftershocks. Using the locations of 8,000 earthquakes, smaller aftershocks of the main event, the team pinpointed the location of the MHT and illuminated a crustal duplex: a series of thrust faults stacked on top of one another.

Fault Duplexes
Duplexes form when multiple thrust faults that slip sequentially overlap. Credit: Mike Norton, CC BY-SA 3.0

This finding, according to Mendoza, affirms what some geologists have been saying about this area for more than two decades. But, he says, this is the first time we have been able to image this duplex structure. The study’s result, DeCelles says, “is very informative about how this particular thrust belt behaves”; but he says this work also has global implications.

Seismometer deployment in Nepal
Team members deploy a seismometer in Nepal as locals look on. Credit: Marianne Karplus

A natural laboratory

What we know about how duplexes form has generally come from analog sandbox models, DeCelles says. But the new study shows us a “natural example of something that is just cranking away right under our feet.” Such a region could be used as a natural laboratory to test theories in an active duplex, which can be applied to similar regions around the world. “We can get at some of these burning questions we all have,” DeCelles says.

Breaking the rules

The area where the Gorkha earthquake occurred generally experiences much smaller earthquakes, those that are only detected by seismometers [Bilham, 2019]. “Gorkha kind of broke some rules,” DeCelles says. Indeed, the large historic earthquakes that have occurred in the Himalayas were centered much farther south.

This work shows that the duplex is active, he says, and the Gorkha event shows that large magnitude earthquakes can occur in the region.

References

Mendoza, M.M., et al. (2019). Duplex in the Main Himalayan Thrust illuminated by aftershocks of the 2015 Mw 7.8 Gorkha earthquake. Nature Geoscience. 12, 1018–1022 (2019) DOI:10.1038/s41561-019-0474-8

Bilham, R. (2019). Himalayan earthquakes: a review of historical seismicity and early 21st-century slip potential. Geological Society, London, Special Publications, 483, SP483-16. DOI: 10.1144/SP483.16