It’s not the size of the earthquake so much as its rate of oscillation that determines the extent of injuries and damage, suggest two scientific papers published recently. Following the 7.8 magnitude earthquake near Kathmandu in April, experts expected the number of deaths to be comparable to a 2005 quake that hit a less-populated area of Kashmir. The actual death toll was close to 9,000 in Nepal, nowhere near the 85,000 people who died in the earlier quake. That “is actually a small number given the density of the population in the Kathmandu area and the vulnerability of the buildings,” Jean-Philippe Avouac, the author of the papers, told The New York Times.
The papers suggest that if an earthquake results in quick oscillations which shake the ground back and forth approximately every second, small buildings are more at risk. During the Nepal quake, the ground oscillated only every 4 or 5 seconds, putting taller buildings, such as towers and skyscrapers at greater risk. This could explain why many low, relatively unstable buildings survived the quake while a 200-foot-tall watchtower was destroyed.
Dr. Avouac, a professor of geology at the California Institute of Technology and the University of Cambridge in England, explained in one paper that during the April quake, an 87-mile stretch of the seismic fault between India and Asia broke relatively smoothly. This smooth break generated the long-period waves and limited the faster oscillations that would have resulted from a more uneven break.
Scientists don’t know if this type of long-wavelength earthquake is common in the Himalaya region or whether faults in other parts of the world could produce similar shaking.
“There’s one in the Los Angeles area, for example. Five-second energy like this earthquake released would be especially damaging to 50-story buildings,” seismologist Susan E. Hough told The New York Times.
Image via Hilmi Hacaloğlu, video via Jean-Philippe Avouac, Caltech Tectonics Observatory and Tim Pyle, Caltech IPAC