The faults bounding the Himalayan range front could produce great earthquakes, similar to those which struck the Aleutian arc during the twentieth century.
By Akash Kharita, Indian Institute of Technology, Roorkee
Citation: Kharita, A., 2020, Himalayan range awaits large quakes, Temblor, http://doi.org/10.32858/temblor.120
On 25 April, 2015 — a typical Saturday morning in Kathmandu — shops, restaurants and bazaars bustled with their usual weekend crowds. At 11:56 a.m. local time, a magnitude-7.8 earthquake rocked the city, killing approximately 9,000 people throughout Nepal. Collapsed buildings and triggered landslides resulted in more than 22,000 injuries and numerous reports of missing persons. The quake destroyed several important architectural sites and leveled villages in nearby districts. Although the jolt shocked the city and surrounding countryside, geoscientists who study the region expressed little surprise.
The Himalayan range marks the collision zone between two tectonic plates, where India thrusts itself beneath Eurasia along the plate boundary called the Main Himalayan Thrust. The Main Frontal Thrust is the southernmost surface expression of the Main Himalayan Thrust. Large earthquakes can occur on these faults when they get stuck, causing stress to build up.
In a new study published in Seismological Research Letters, author Steven Wesnousky, a paleoseismologist at the University of Nevada, Reno, synthesized past earthquake data at 30 sites along the Himalayan arc as a way to forecast both the timing and magnitude of future great quakes. “Sufficient stress,” Wesnousky says, “has been accumulating in the Himalaya along the Main Himalayan Thrust for years.” He says this pent-up stress could produce a sequence of megathrust earthquakes, with multiple events greater than magnitude-8.5.
Quantifying past large earthquakes
Wesnousky and his colleagues located and trenched faults in the Himalayan foothills. They measured the height of fault scarps in the field to figure out how much these faults moved, and dated sediment samples in the lab to determine when the faults ruptured. In the current study, he used this information to calculate how fast the fault moved vertically — the vertical slip rate. By combining vertical slip rates with the rate of convergence between India and Eurasia, gleaned from geodetic methods that use GPS to monitor relative plate motion, and the dip of India underthrusting Eurasia — approximately 30° — obtained via seismic reflection imaging and borehole analysis, Wesnousky calculated that the next big quakes should happen approximately 500 to 1000 years after the most recent event at each location. For some locations along the Himalayan range, that time has already passed, for others it is 1,000 years from now.
The sources for uncertainties in each component of Wesnousky’s calculations include imprecise age determinations of sediment and sedimentary rocks, large ranges for the vertical slip rate at some sites, and a poorly constrained angle of underthrusting between India and Eurasia along the Main Frontal Thrust. These uncertainties are significant, resulting in huge ranges for when the next quakes may occur — on the order of the length of a human life. Refining the geometry of the Main Frontal Thrust directly below each study site, Wesnousky says, can help reduce uncertainty in the forecasting of future great earthquakes.
Expecting great earthquakes
“As horrible as it sounds, we should expect great magnitude-8.5 [or larger] megathrust earthquakes to occur again in the Himalaya,” says Roland Bürgmann, professor at the University of California, Berkeley, who was not part of this study.
A weakness in studies like Wesnousky’s is that available data only record one large earthquake, says Roger Bilham, a research scientist at the University of Colorado, Boulder, who was not part of this study. He notes that researchers working on the problem want to see the paleoseismic record extended back 10,000 years — multiple earthquake cycles — at each location along the arc to determine the intervals between great quakes.
Nevertheless, if Wesnousky is right, the arc may be ready to rupture in multiple locations, which could result in a series of great earthquakes similar to a recent earthquake sequence in Alaska, where damaging earthquakes occurred in 1906, 1946, 1957, 1964 and 1965. The 1950 magnitude-8.7 earthquake in Assam may have begun a similar sequence in the Himalayas, according to Wesnousky.
“When will [these earthquakes] occur? No one can tell exactly,” says Wesnousky. For communities along the Himalayan range, they can minimize damage and loss of life that will certainly accompany a large earthquake by encouraging state and local governments to develop earthquake resistant infrastructure and emergency response plans. Check the National Disaster Management Authority’s website for how you can be ready. Remember that during an earthquake if you are outdoors, get into the open and get away from anything that might fall on you. If you are indoors, drop, cover and hold on.
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Bilham, R. (2019), Himalayan earthquakes: a review of historical seismicity and early 21st century slip potential. Geological Society, London, Special Publications, v. 483, 423-482. https://doi.org/10.1144/SP483.16
Freymueller, J., Bilham, R., Burgmann, R., Larson, K. M., Paul, J., Jade, S., and Gaur, V. (1996), Global Positioning System measurements of Indian plate motion and convergence across the Lesser Himalaya. Geophysical Research Letters, v. 23, 3107-3110. https://doi.org/10.1029/96GL02518
Wesnousky, S.G. (2020), Great pending Himalaya earthquakes: Seismological Research Letters, https://doi.org/10.1785/0220200200