Thousands return home after magnitude...

Thousands return home after magnitude 7.7 quake triggered tsunami

There are 1 comment on the Philippines Today story from Sep 3, 2012, titled Thousands return home after magnitude 7.7 quake triggered tsunami. In it, Philippines Today reports that:

Thousands of people on the country's eastern coast returned to their homes on Saturday after fleeing inland when a magnitude 7.7 earthquake off Samar triggered a tsunami warning.

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“Geologist [I'm Climate Change]”

Since: Mar 07

formerly Nuneaton

#1 Sep 5, 2012
Interesting one.

Looked it up on the NEIC website and was unusual in that it was a thrust quake in the descending plate. Normal quakes in that location (descending plate) are rift events caused by bending opening fractures in the top of the plate which then falls into the plate.
This one was deeper then the Tonga 2009 type plate bend rift types and was instead in the interior cumulate layer at ~35Km where the bend and combination of compression versus forearc results in horizontal compression with resulting thrust faulting at ~45 degrees to horizontal and usually in the up direction.

The fact that there was a tsunami indicates that the thrust cut right through the rubble pile to the surface. This one joins the NGDC list of mundane pop up tsunami typical of arcs under compression.

For the really big & nasty one similar to 2011 Tohoku look at the volcanoes in the region. Bulusan is now relatively compressed as also are the volcanoes further south in the S. part adjacent to the Sanghie arc. The northern volcano Mayon is still active and squeezing in the longest fracture. it still has a way to go to drive the fracture flat, so rebound/long duration Mercalli X quake (r/lx) which relaxes the crust here is still a long way off.
The exact length of the Mayon fracture and its tectonic segment is unclear. As the Philippines are a Zygomorph there is a megathrust on BOTH sides of the islands, the major r/lx (probably 1600s) may have also relaxed the crust & opened up a fracture @ Canlaon which is also still squeezing in a long narrow fracture & occasionally going "pop". Looks like r/lx along the main Philippines arc is still a long way off.
Other indicators of relative decompression in the r/lx cycle is the descending plate, which still shows evidence of compression quakes in the descending slab as a result of relatively free descent. In the latest stage with volcanoes hot & dormant, this free descent will slow as a result of a failed attempt to subduct the forearc slab (bigger pop up quakes), and a general slowing of the descending plate compression events. This effect was visible in Tohoku (Japan) which is the only modern case of r/lx in a zygomorph with a megathrust on both sides. The only other historically documented case of a zygomorph undergoing r/lx was the 1755 r/lx of the Cadiz & Marquis de Pombal fault zone & Lisbon area quake & tsunami(big enough to cause seiches in Scandinavia & scotland).

One other item of note is that as a result of a zygomorph containing 2 megathrusts, 2 forearc wedges & a volcano chain in the middle, the runout is usually 2* that of an arc or larger, and that one or BOTH megathrusts may undergo r/lx at the same time to relieve accumulated oblique stress across the segments as occurred in Lisbon 1755; Bulk @ Cadiz and further SW,(r/lx vector ~SSE) and 2 events under Lisbon area NE part which suffered a direct hit from the quake as well as Tsunami (r/lx vector NW).* Note Lisbon zygomorph conains no volcanoes listed as a result of the descending plate being relatively short and has probably not yer (as under Atlas range) reached a depth able to dewater and produce magma.

Will be able to go paddling @ the seaside for a while longer in central philippines.

Have a nice day: Ag

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