Disputes over sovereignty hide quake ...

Disputes over sovereignty hide quake and tsunami danger in Manila Trench

There are 1 comment on the South China Morning Post story from Aug 2, 2014, titled Disputes over sovereignty hide quake and tsunami danger in Manila Trench. In it, South China Morning Post reports that:

As countries clash over sovereignty in the region, scientists are unable to do crucial research on the quake-prone Manila Trench The risk of a major tsunami in the South China Sea has been underestimated, even ignored, by governments in the region, say scientists from the mainland, Taiwan and the Philippines.

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“Geologist [I'm Climate Change]”

Since: Mar 07

formerly Nuneaton

#1 Aug 4, 2014

I am curently looking over this one having viewed Taiwan area last month or so in google searches on a ramble through the journals. For the record, the kaoshiung tsunami of SW taiwan was actually a sediment slump on a clear day down a submarine canyon @ the head of the river delta & shelf. the resulting slump on a clear day down the canyon resulted in a 10m tsunami @ the then Kaoshiung fishing village.

Have found that the taiwan area is still recompressing following its major past megathrust & that it is compressed enough to now produce transpression in a fracture zone in the phillipine sea plate E of the main island which is the compressed forearc. This arc extends down from the elevated forearc of taiwan to the N. Babuyan islands terminating in the indent in the manila trench @ the area between the babuyan islands proper & Iraya which is the S, most volcano of the Taiwan arc.

Now the fun bit. The major tsunami in 1076 was actually from the SOUTH portion of the Luzon trench, extending from the collision zone with the Palawan & Calamian group island arc @ Mindoro extending N. through Taal & Pinatubo & terminating @ the promontory tip region NW. of Dagupan. the result was a major disaster for central Vietnam (tsunami over 10m runup) & an overwash of the paracel islands.

The South Manila trough is part of a Zygomorph,(megathrust on BOTH sides), with the megathrust on the other side being the Philippines trench. the result is that like a range it stores up a huge amount of compressive stress. The tsunami following the megathrust was at least tohoku sized @ source & may actually be bigger. Major tsunami deposits reported from E. Australia are the results of megathrust events @ the New hebrides, Vanuatu, Santa Cruz, Solomon islands & Bougainville zygomorphs. runups @ Australia in some places are over 30m high. Good news is that zygomorphs take a long time to recompress following megathrusts.

Historic tsunami from the Manila bay region of the Luzon arc are actually from outer rise events from the descending plate following the 1076 megathrust event, resulting in Tonga type tsunami in the plain of the manila delta in early Spanish historic time. Taal volcano has been rather busy squeezing in magma from fractures @ the base of the crust from that megathrust. Pinatubo has also recently got in on the act (remember that one?). It is early days for the recompression of the South Luzon trench segment, so lots of busy volcanism to come in the manila region. The descending plate in South Luzon & also Taiwan is busy descending & is as a result compressing with recompression quakes down to approx 200Km. Descent in both areas is relatively free as there is a lot of remaining arc compressibility to enable descending plate movement.

Now the horror story (cue the jaws music).

The North Luzon trench running from approx Dagupan location, to the Babuyan islands proper is locked & the arc (also a zygomorph) is loading. Being locked, the descending plate is being held up, so the descending plate is quiet & is also inhibiting the descent of the plate in the adjacent segments. The volcanoes in the main part of N.Luzon are quiet (albeit hot & fizzy) as a result of the convergence rate being faster to the south. The babuyan islands however are still receiving magma from the latest stage fractures in the zygomorph (thin dribs & drabs), and have been active in the last century. This means that fortunately there is still a few decades to go before the end point. The forearc in the North Luzon segment is still quiet, & before letting go aft the arc should produce regular compression thrusts in the forearc in the 7 to 8 range of Tohoku type with relatively small tsunami.

The minimum size expected for the tsunami is Tohoku size & the max is probably 50m or more in the San Fernando area. The radiant will be the bit in the report heading to Hong Kong, but may travel closer to Hainan.

Have a nice day: Ag

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