It has taken me a while to dig deeper into this. I just haven't got around to it but have been interested in it for a number of reasons. There is more info out there than I thought. Basuanga, Palawan and Zamboanga are all, according to the scientific community, part of the Eurasian continent off to the west. The rest of the Philippines being made up of a number of small platelets or blocks which are collectively known as the Philippine Mobile Belt. The red saw tooth lines are the lines of subduction where plates either side of Luzon are being stuffed down and sinking into the earths "mantle". The Philippine Mobile Belt is bound on all sides by zones of subduction as illustrated in the diagram above. The Philippines is a bit of a complex mash up of platelets that are a manifestation of forces coming from two main directions ie from the west ie the Sundaland Block of Eurasia and from the East South East, from the Philippine Sea Plate. The faults that run through Luzon in the diagram "accommodate large-scale tectonic motions between the Philippine Sea Plate (PHSP) and the Sundaland Block of Eurasia (SUND). The major plate motion is accommodated by deformation along a series of arc-bounding and intra-arc faults that accumulate elastic strain between major earthquakes". (taken from the link below) Luzon itself is made up of 6 or 7 blocks / plates of varying shapes and sizes. You can make out the Marikana fault just on the west side of the orange wedge shaped "fragment" located to the north west of Laguna. The fault lines depicted in the diagram are live and the landmass(es) that make up Luzon are under stress from the combined processes of subduction from both directions. On a day to day basis, these stresses build up with little to no movement until eventually something somewhere has to give within the fault system i.e. an earthquake event occurs. The fact that stress is continually being applied means that an earthquake somewhere in Luzon (and other areas in the Philippines) is always likely to happen at any time. The only aseismic (non seismic) areas of the Philippines are the areas of Basuanga, Palawan and Zamboanga which are said to be part of Eurasia. Having said that it looks like Palawan is on a collision course with the rest of the Philippines. http://geology.indiana.edu/hamburger/
Interesting Mt Pinatubo is right underneath where you would expect the melting zone of the Eurasian plate, just look how close that subduction zone is to the west coast, I wonder what evidence there is of massive earthquakes in the past?
There are people about that look into that kind of question in terms of the geological past. Historically there appears to be a long record from what I have read. http://en.wikipedia.org/wiki/List_of_earthquakes_in_the_Philippines
From wikipedia: The Philippine Sea Plate is subducting under the Philippine Mobile Belt at the rate of about 16 cm (6.3 in) per year.
Do I see a fault line passing roughly N-S straight through Metro Manila? Incidentally I think this thread has great potential for Euro Pinoy
Zamboanga is thought to be a chip off the same block as Palawan ie from the main Eurasian plate. Zamboanga and Eurasia has collided into Mindanao in the past, Mindanao being part of the Philippine Mobile Belt and under similar duress as Luzon. Mindanao Island is being "squeezed" from the west by subduction along the Cotabato Trench and from the east along the southern Philippine Fault and Philippine Trench. The Cotobato Trench featuring prominently in 1976: http://www.british-filipino.com/sho...slands&p=38146&highlight=earthquake#post38146 There is a 1,200-km-long Philippine fault zone that transects the whole Philippine archipelago from northwestern Luzon to southeastern Mindanao and has been the source of large-magnitude earthquakes in recent years, such as the 1973 Ragay Gulf earthquake (M 7.0), 1990 Luzon earthquake (Mw 7.7), and 2003 Masbate earthquake (Ms 6.2).
