Tuesday, September 22, 2015

Marinduque tremors could have really triggered Bohol quakes after all. Scientists discover that seismic pressure transfers up to 1,000 km!

Marinduquenos will probably still recall that two days before a 7.2-magnitude earthquake struck Bohol and Central Visayas on Oct. 15, 2013, an earthquake swarm took place in Boac,Marinduque epicentered offshore northwest of the island. The said earthquake swarm meant a stunning 25 tremors in a matter of 8 days with 19 of them occuring in 3 days and 15 occuring within 12 hours.

Two days later came the devastating  7.2 magnitude earthquake that struck Bohol and Cebu. 


Intrigued by the possibility that the Marinduque earthquake swarms could have triggered the Bohol tremors, I threw the question then via email to the dedicated administrator of Earthquake-Report, Armand Vervaeck (Belgium). He responded in part as follows:
"Nobody will be able to tell you as science isn't able to make prognoses in strength and time. Swarms are not unusual and 98/100 nothing important happens thereafter. In some cases (like in l'Aquila Italy) swarms are precursors of something bigger to come. So I don't think that something will happen BUT we are not really sure about this :))
"Taken into account the location of the Marinduque quakes, they could also have been the trigger to free the Bohol energy BUT nobody will be able to prove that."
But now comes a discovery made by Italian scientists that seismic pressure transfers up to 1,000 km - causing other earthquakes! Read the story below.

Note that the distance between Marinduque and Bohol is 458.85 km, well within the stated distance. 


Distance between Marinduque and Bohol is 468.85 km. This distance is equal to 291.33 miles, and 252.99 nautical miles.

How earthquakes can trigger copycat quakes 1000 kilometres away
New Scientist, 21 September 2015
“Seismic waves unleashed during Wednesday’s magnitude 8.3 earthquake in Chile could have triggered aftershocks as far as 1000 kilometres away.
That’s because they can shake up grains of rock wedged inside distant faults. According to computer models, even weak waves at the right frequency could be enough to start a new quake by vibrating that grist into a more slippery, liquid-like layer.
Earthquakes often happen when two tectonic plates that have been pressed together suddenly slip. But we’ve seen that major earthquakes like 1992’s Landers earthquake in California can also send out waves that spark copycat quakes 1000 kilometres away, even though the waves get weaker as they travel.
The mysterious remote triggering of quakes may have also played a role in events in Chile in 2014, and Japan in 2011.
“We were wondering: how could it happen that a very tiny wave with a very small amplitude could trigger earthquakes a thousand kilometres away?” says Lucilla de Arcangelis of the Second University of Naples in Italy.
One idea is that sound waves can lather up the grains between the two plates in a way that decreases friction, to make a slip easier. Now a team including de Arcangelis has built a computer model that shows the process as it happens.
They found that seismic waves could trigger an earthquake in the simulated fault only if they came in a narrow range of frequencies. If the fault was just about to slip, it would hasten the process by starting vibrations in that range. Only the frequency really mattered – weak waves, or even waves that would actually push in the direction against a slip, could still induce an earthquake.
“Each fault will have its own acoustic resonance frequency,” de Arcangelis says. “If a signal arrives at this frequency, the fault that without perturbation would be quiet will trigger an earthquake.”
Combined with a 2005 lab experiment that also showed a resonant frequency could jiggle glass beads in a fake fault into slipping, this simulation could suggest that actual faults have specific frequencies they’re susceptible to.

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