Caltech team, in collaboration with the Department of Mines and Geology of Nepal studied the Himalayas mountain for over 20 years, to learn about mountain-building and earthquake triggering processes. For the purpose of monitoring the rhythm of Earth's crust, the research team led by geologist Jean-Philippe Avouac has installed a network of GPS stations across Nepal. Thanks to the years long effort of studying the quakes cycles, the scientist have been able to interpret the severe Gorkha earthquake in a new light.
The earthquake happened in Gorkha near Kathmandu, Nepal, on April 25. It's magnitude was estimated at 7.8 by USGS, causing a severe devastation across the affected area and deaths of thousands of people. The epicenter of the quake was located 75 kilometers (46.6 miles) to the west-northwest of Kathmandu, and propagated eastward with the speed of 2.8 km/s.
"At first when I saw the news trickling in from Kathmandu, I thought there was a problem of communication, that we weren't hearing the full extent of the damage," said Avouac. "As it turns out, there was little damage to the regular dwellings, and thankfully, as a result, there were far fewer deaths than I originally anticipated."
The research team has gathered data from the GPS stations and combined them with the data from accelerometer which measured ground motion in Kathmandu, data from international seismological stations and radar images from orbiting satellites. The goal was to investigate physical processes involved in the Gorkha earthquake and to explain the unusual quake consequences – how an earthquake of this magnitude left most of the low-story buildings intact, while completely devastating the taller structures.
Build up and release of stress on MHT. Video credit: Caltech
The researchers showed the earthquake struck on the main megathrust fault (Main Himalayan Thrust – MHT), where northern India is thrusting underneath Eurasia, pushing the Himalayas upward. This thrusting is ongoing, at a rate of around 2 cm (0.7 inches) annually. GPS measurements have showed that a big part of this fault is locked. A huge stress on a fault like this causes a large earthquake, such as the Gorkha earthquake.
There are several areas like this along the fault in western Nepal, which have not experienced a major earthquake since the great the big quake in 1505. The April earthquake, has only ruptured a small part of the locked fault, so the potential for the major earthquake has still remained substantial.
Propagation of the Gorkha earthquake. Video credit: Caltech
"The Gorkha earthquake didn't do the job of transferring deformation all the way to the front of the Himalaya," explained Avouac. "So the Himalaya could certainly generate larger earthquakes in the future, but we have no idea when."
"With the geological context in Nepal, this is a place where we expect big earthquakes. We also knew, based on GPS measurements of the way the plates have moved over the last two decades, how 'stuck' this particular fault was, so this earthquake was not a surprise," says Jean Paul Ampuero, assistant professor of seismology at Caltech and coauthor on the Nature Geoscience paper. "But with every earthquake there are always surprises."
Unlike the previous earthquakes on this fault, where the ruptures have reached the surface, this earthquake remained contained at a depth of about 15 km (9.32 miles). "That was good news for Kathmandu," noted Ampuero. "If the earthquake had broken all the way to the surface, it could have been much, much worse."
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However, the Gorkha earthquake enhanced the amount of stress on the neighboring part of the fault, closer to Kathmandu, that remained locked. The accumulated stress might, in time, trigger another earthquake or the fault could "creep", allowing the Indian and Eurasian plate to slowly pass one another, dissipating the stress in that way. To further investigate the possibilities the team is building models and monitoring post earthquake deformation of the Earth's crust.
Another key finding from this research explained why some buildings were spared while other were destroyed by the earthquake. The answer lies in the frequency of shaking. The high frequency waves, the vibrations of which have periods of the order of 1 second, are responsible for affecting the low-story buildings. These waves were pretty mild, as the are produced by the deeper edges of the fault ruptures, located on the northern end, away from Kathmandu.
Also, the GPS records showed the onset of slip during an earthquake was relatively gradual, which is why the radiated high-frequency seismic waves were mild, as well.
"It would be good news if the smooth onset of slip, and hence the limited induced shaking, were a systematic property of the Himalayan megathrust fault, or of megathrust faults in general." said Avouac. "Based on observations from this and other megathrust earthquakes, this is a possibility."
On the other hand, the research showed the unexpected amount of low-frequency waves, with period longer than 5 seconds, was produced by the quake. This type of shaking caused the taller buildings in Kathmandu to collapse. The example of this is the 60 meter high tower which was utterly devastated in the quake, although it previously survived the stronger earthquakes in 1833 and 1934.
The reason for that is a natural phenomenon called resonance. Buildings and all the infrastructure have their own natural vibration frequency – the resonance frequency. The taller the building is, the longer is the period of it's resonance. If an earthquake causes the grounds to shake with a frequency equal to the building's resonance frequency, the shaking within the building structure will be amplified, causing the structure to collapse.
In the case of Gorkha earthquake, the scientist discovered the effect of low frequency waves was amplified by the geology of the Kathmandu basin. It's a natural lakebed filled with soft sediment, which caused the seismic quake waves to remain trapped within the basin and continue to reverberate in a 5 second period. "That's just the right frequency to damage tall buildings like the Dharahara Tower because it's close to their natural period," Avouac explained.
Domniki Asimaki, the professor of mechanical and civil engineering at Caltech, noted that the city of Los Angeles is also lying on top of the sedimentary deposit, and is highly likely to be prone to this type of increased shaking during a strong magnitude earthquake. "In fact," she said, "the buildings in downtown Los Angeles are much taller than those in Kathmandu and therefore resonate with a much lower frequency. So if the same shaking had happened in L.A., a lot of the really tall buildings would have been challenged."
This is a good reason to put the efforts into understanding the reasons and consequences of the Gorkha earthquake, according to the team leader: "Such studies of the site effects in Nepal provide an important opportunity to validate the codes and methods we use to predict the kind of shaking and damage that would be expected as a result of earthquakes elsewhere, such as in the Los Angeles Basin," Avouac explained.
- "Lower edge of locked Main Himalayan Thrust unzipped by the 2015 Gorkha earthquake" – Jean Phillippe Avouac, Lingsen Meng, Shengji Wei, Teng Wang, Jean-Paul Ampuero – Nature Geoscience (2015) – doi:10.1038/ngeo2518
- "Slip pulse and resonance of Kathmandu basin during the 2015 Mw 7.8 Gorkha earthquake, Nepal imaged with geodesy" – J. Galetzka, D. Melgar, J.F. Genrich, J. Geng, S. Owen, E.O. Lindsey, X.Xu, Y. Bock, J.P. Avouac, L.B. Adhikari, B.N. Upreti, B. Pratt-Situala, T.N. Bhattarai, B.P. Situala, A. Moore, K.W. Hudnut, W. Szeliga, J. Normandeau, M. Fend, M. Flouzat, L. Bollinger, P.Shrestha, B. Koirala, U. Gautam, M. Bhatterai, R. Gupta, T. Kandel, C. Timsina, S.N. Sapkota, S. Rajaure, N. Maharjan – Science (2015) – doi: 10.1126/science.aac6383
Featured image: Three months after the earthquake, Nepal, July 29, 2015. Image credit: @orderofmalta
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