The magnitude 7.8 earthquake that struck Nepal on 25 April killed more than 8800 people, injured another 23,000+, and destroyed at least 500,000 buildings.
But weak and limited ground shaking for a quake of such large magnitude helped keep its impact from being a lot worse, said scientists at a special symposium about the earthquake at the International Union of Geodesy and Geophysics (IUGG) general assembly in Prague, Czech Republic.
The earthquake “wasn’t generating a lot of ground acceleration,” symposium speaker Thorne Lay told Eos. “It just had very concentrated slip that was localized [and] that limited the areas that had the most shaking.”
Lay, who directs the Center for the Study of Imaging and Dynamics of the Earth at the University of California, Santa Cruz, explained that the temblor “wasn’t really jerky” because it was “a very narrow, smooth propagating rupture.”
The shaking from the earthquake better matched what would have been expected from a magnitude 6 or 6.5 quake rather than from almost an 8, symposium convener Domenico Giardini told Eos. He serves as chair of seismology and geodynamics at the Institute of Geophysics at ETH Zurich, Switzerland, and is president of the International Association of Seismology and Physics of the Earth’s Interior (IASPEI), a semiautonomous association of IUGG.
Seismic Risk Remains High
Giardini, Lay, and other speakers in the 27 June session noted that the seismic risk remains very high in the Kathmandu area, which lies along the earthquake-prone boundary where the Indian tectonic plate slowly pushes beneath the Eurasian plate.
The Nepal earthquake “actually ruptured an incredibly small segment of this plate boundary, anywhere along it we know has had major earthquakes in the past,” said Kevin Furlong, a geosciences professor at Pennsylvania State University in University Park. The short length of the rupture, which he noted ended just north of Kathmandu, leaves many other locations along the plate boundary unrelieved of their stress and still capable of generating even stronger quakes. “There are no safe spots,” he said.
Training May Have Saved Lives
Other session speakers suggested that to a limited degree, improvements in earthquake preparedness and building practices in Nepal helped lessen the number of deaths and amount of destruction from the quake.
Harsh Gupta, president of the Geological Society of India and past president of IUGG, told Eos that although the structural damage to Nepal was what one would expect from a magnitude 7.8 earthquake, the loss of human lives was much less than from similar earthquakes. He partially attributed this decrease to earthquake safety training.
An earthquake awareness campaign takes place in the city every year, so “people knew how to conduct themselves,” he said. “That must have been another factor in few lives lost.”
Training of engineers and masons in Nepal over the past several decades to design and build more earthquake-resistant structures “definitely had an effect,” added Mohsen Ghafory-Ashtiany, president of the Iranian Earthquake Engineering Association. “But it was not as effective as we would expect.”
Engineers and scientists have not been successful in Nepal at translating their knowledge into “simple, doable, affordable, culturally acceptable solutions,” said Ghafory-Ashtiany, who chairs IASPEI’s Commission on Earthquake Hazard, Risk and Strong Ground Motion. Although Nepal has building codes and a master plan for dealing with major disasters, he told Eos, there is a big gap “between implementation and theory.”
—Randy Showstack, Staff Writer
Citation: Showstack, R. (2015), Weak shaking lessened Nepal earthquake impact, Eos, 96, doi:10.1029/2015EO032443. Published on 7 July 2015.