Pair of deadly Mexico quakes puzzles scientists Nature ^ | 20 September 201

Pair of deadly Mexico quakes puzzles scientists
Nature ^ | 20 September 2017 | Alexandra Witze

Posted on 9/20/2017, 11:02:34 AM by BenLurkin

Big earthquakes can increase the long-term risk of seismic activity nearby by transferring stress within the Earth’s crust to adjacent geological faults. But that sort of ‘static stress’ transfer usually happens only within a radius equal to about three to four times the length of the original fault’s rupture, says Gavin Hayes, a seismologist at the US Geological Survey…

The 7 September earthquake ruptured about 100 kilometres of the crust, which would imply its stress transfer reached no more than about 300 to 400 kilometres away, Hayes says. That puts the 19 September quake, whose epicentre was 650 kilometres away, outside of the zone of influence. “But the time coincidence makes it pretty suspicious,” Hayes says. “A lot of people will think that they are related, and there’s going to be a lot of work on that.”

Another possibility is that the 19 September quake is an example of ‘dynamic triggering’, in which seismic waves rippling outward from one quake affect faults much more quickly — and at much larger distances — than in static stress transfer. But dynamic triggering usually happens within hours or days after the initial quake, making the 12-day gap between the 7 September event and the latest big tremor hard to explain, says Eric Fielding, a geophysicist at the Jet Propulsion Laboratory in Pasadena, California, who studies dynamic triggering.

His team has been analyzing satellite radar images of the landscape around the 7 September quake, looking for changes in ground level that indicate which parts of the landscape have uplifted and which have dropped down as a result of that event…. Fielding’s team will be looking for similar information in the coming days from the 19 September quake. Radar images can help reveal where geological stress is transferred within the ground after an earthquake.

(Excerpt)


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