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Sunday, May 17

Strange Doings in Nepal? Third Earthquake Strikes

Reporting on the mag 7.3 that struck on May 12 (Nepal’s Recent Quakes Don’t Mean a Bigger One Isn’t Coming), Neel Patel at Wired adamantly maintained it wasn't an aftershock;
Hundreds of aftershocks [in the wake of the mag 7.8 earthquake on April 25] -- some as strong as 6.7 magnitude -- have continued to hamper relief efforts and keep residents in a panicked state.
This latest quake, however, is not an aftershock, but a brand new seismic event.
According to the United States Geological Survey, today’s earthquake occurred 9.3 miles deep in the earth’s crust -- the same depth as the April event. Cities and villages in the area have already felt six aftershocks, and the new quake created a whole new wave of landslides further north in the Himalayan mountains.
Is this just horrible luck, or are we seeing Nepal turn into a haven for earthquakes? The USGS previously estimated a 1-in-200 chance of another event similar to the April earthquake occurring.
“It wasn’t a high probability, but it wasn’t unexpected,” says Rich Briggs, a USGS research geologist.

Unfortunately, it might not even be the last quake to strike region in the near future -- and it certainly won’t be the last seismic event that’s observed there. [...]
Despite this, some news outlets referred to the May 12 event as an aftershock.  Yet Patel's observation was prophetic. From a May 16 report at ABC News, which I see isn't bothering with calling them aftershocks:
A 6.3-magnitude earthquake hit Nepal Saturday as the country was still recovering from previous earthquakes in recent weeks.
The tremor occurred 15 miles north of Ramechhap, about 50 miles east of the capital of Kathmandu and south of Mount Everest, according to the United States Geological Survey. There was no immediate word on damages.
The earthquake follows the deadly 7.8-magnitude earthquake from April 25 that killed over 8,200 people. Further activity, including a 7.3-magnitude earthquake, happened Tuesday [May 12], killing 117 people. [...]
The U.S. Geological Survey tagged Saturday's quake as 5.7 mag, so I don't know where ABC got 6.3 from. But the question is whether geologists are a little more surprised by the strength of this third quake within less than a month.  From a Press Trust of India report on the third quake:
The tremor was felt at 5.04 pm [in India's Bihar state], Director State Meteorological department A K Sen told PTI, adding it had its epicenter in Nepal at a depth of 10 km.
Referring to the evening quake, Sen said an aftershock of this magnitude is generally not seen three days after a high intensity one like the one that took place on May 12.
“An aftershock of 5.7 intensity after three days of high intensity earthquake on May 12 measuring 7.3 on the Richter Scale is rare in weather science,” Sen said.
Generally, aftershocks after a high intensity tremor like that of May 12 are not more than 4.0.
As Patel reported, there were 6+ mag aftershocks after the April 25 quake, but maybe those happened within the first 48 hours of the quake; I don't remember. And I didn't know earthquakes were in the purview of "weather" science.

So I don't know how unusual yesterday's quake, its magnitude (whether 6.3 or 5.7), is considered by geologists.  I suppose this will come clear as more reporters interview more experts about the quake. However, if Sen's observations hold up, things are getting seismically strange in Nepal. Stranger than usual.


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