EXPERTS ADVISORY

Though today’s magnitude-7.3 Nepal earthquake was powerful, the devastating April 25 event released 5.6 times more energy and exposed about 1.3 million more people to violent shaking, according to University of Michigan earthquake geophysicist Eric Hetland.

He is one of several experts from the U-M Department of Earth and Environmental Sciences who can discuss today’s earthquake and how it compares to last month’s magnitude-7.8 Nepal quake.

Hetland calculated the number of people exposed to violent shaking during the two earthquakes and concluded that about 1.4 million people were exposed to “violent” shaking during the April 25 earthquake. He said his estimate for the April quake is a bit lower than the number generated by the U.S. Geological Survey shortly after that event, which killed more than 8,000 people.

In today’s aftershock, only about 65,000 people were exposed to “violent” shaking, or Level IX shaking on the Modified Mercalli Intensity Scale, he said. That level of shaking is capable of causing great damage to structures, including partial collapse.

“Fewer people were exposed to severe or violent shaking this time, and most of the shaking at those levels occurred in non-urban areas,” said Hetland, an assistant professor in the department. Hetland is trying to understand the stresses that lead to earthquakes, the regions on a fault that are most likely to fail in future earthquakes, and the loss of life and property that will result.

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Larry Ruff is a seismologist who studies large earthquakes around the world. He said today’s magnitude-7.3 earthquake qualifies as an aftershock of the April 25 event.

“A strict definition of an aftershock is that the event should be smaller than the main shock, located within the rupture zone of the main shock, have the same fault geometry as the main shock, and occur soon enough after the main shock to be part of the aftershock sequence,” Ruff said. “Today’s earthquake satisfies all of these conditions and thus should be viewed as an aftershock.”

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Ben van der Pluijm, an earthquake geologist, said he does not consider today’s Nepal earthquake an aftershock of the April 25 event.

“It is the next in a predicted progression of fault unlocking along the active Himalayan front,” van der Pluijm said. “The style is very similar to April’s earthquake, but it’s a little bit smaller.”

Van der Pluijm said large earthquakes in this area are relatively rare but are ultimately responsible, over millions of years, for the uplift of the Himalayas.
“The setting of these earthquakes exactly fits the predicted scenario of the Indian plate diving beneath the Asian plate,” van der Pluijm said.

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Marin Clark is a geomorphologist and geophysicist who studies tectonic movements in the Himalayan region and who is an expert on landslides triggered by earthquakes there. Clark can discuss the general plate tectonics of the region, where the recent earthquakes occurred on the plate boundary between India and Eurasia. She can also comment on the history of past earthquakes in the Himalaya and the potential side effects of such large events, such as landslides and avalanches.

The threat of landslides and mudslides remains high across much of Nepal’s high country, and the risk is likely to increase when the monsoon rains arrive this summer, Clark said.

Within hours of the April 25 magnitude-7.8 earthquake, Clark and two colleagues assessed the landslide hazard in Nepal. They looked for locations where landslides likely occurred during the earthquake, as well as places that are at high risk in the coming weeks and months.

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