The Philippine Fault System is an inter-related system of faults
throughout the whole of the Philippine Archipelago, primarily caused by
tectonic forces compressing the Philippines into what geophysicists call
the Philippine Mobile Belt. Philippine Mobile Belt
The Philippine Mobile Belt is composed of a large number of accretionary blocks
and strips. Most strips are long and narrow like the Zambales ophiolites
which is at least 400 km long and 50 km wide. The strips generally run
north-south and the zones of convergence are usually demarcated by fault lines.
The Philippine Mobile Belt is compressed on the west by the Eurasian Plate and
two arms of the Sunda Plate, and on the east by the Philippine Sea Plate. These
tectonic plates have compressed and lifted parts of the Philippines causing
extensive faulting, primarily on a north-south axis. All faults in the
Philippines are inter-related by the tectonic forces of the Philippine Mobile
Belt, or its tectonic induced volcanism. A more complete understanding can be
gained by viewing the faults in the Philippines as an inter-related
Philippine Fault System. Philippine Fault Zone
The Philippine Fault Zone extends 1200 km across the Philippine archipelago
behind the convergent boundary of the Philippine Trench and the subduction of
the Philippine Sea Plate. This left-lateral strike-slip fault extends
NW-SE accommodating the lateral oblique motion of the subducting Philippine Sea
Plate with respect to the Philippine Trench. It extends from Davao Gulf in
the south, bisects the Caraga region at the Agusan River basin, crosses to Leyte
and Masbate islands, and traverses Quezon province in eastern Luzon before
passing through Nueva Ecija up to the Ilocos region in northwest Luzon. The
northern and southern extensions of the PFZ are characterized by branching splay
faults due to brittle terminations. These horsetail faults are indicative of
the lateral propagation and further development of the PFZ. The fault’s
current activity can be observed in Holocene sandstone outcrops on the Mati
and Davao Oriental islands. The fault experiences a slip rate of approximately
2-2.5 cm/year. Formation
It is proposed that the Philippine Trench and PFZ represent a ‘shear
partitioning’ mechanism, where the oblique physical motions of subduction
at the convergent zone resulted in the development of the major strike-slip
fault. In the Philippine Sea, the oblique motion of the subducting
Philippine Sea Plate resulted in the formation of the Philippine trench and
the PFZ back arc fault system. The oblique motion is accommodated by two
vector components; one vector perpendicular to the converging
Philippine Trench and one vector parallel to the PFZ. Approximately 30%
of the oblique motion is accommodated by the PFZ while the remaining proportions
are displaced along other regional tectonic features as the Philippine Sea
Plate currently subducts below the Philippine archipelago at a rate of 6–8
cm/year. These two tectonic features thus correlate to a similar time of
development. The formation of the PFZ was a result of two stages. The first
stage began at ~10 Ma, when the northern segments of the PFZ developed due to the
convergence of the China Sea Crust underneath the nearby Manila Trench. The
lack of accretionary prism at the Philippine Trench is suggestive of young
origin correlating to an early second stage of development with the central
PFZ proposed to have developed between 2.7 and 3.8 Ma.
Earthquakes The central Philippine Fault Zone
consisting of the Guinyangan, Masbate, and Central Leyte faults are the most
seismically active regions transecting the islands of Bondoc to Leyte. The
northern and southern extensions of the Philippine Fault Zone experience
infrequent earthquakes. The largest and most destructive earthquakes are
generated along the Guinyangan fault every 30 to 100 years with slip rates of
20–33 mm/year as determined by GPS and historical records. Moderate earthquakes
are observed along the Masbate fault with frequent aftershocks indicative of
continued displacement and regional slip of 5–35 mm/year. The northern and
southern segments of the Central Leyte fault experiences different seismic
activity dependent on regional geology. While the Southern Central Leyte fault
experiences moderate seismic events, the Northern Central Leyte fault creeps at
approximately 25 mm/year. Historical data on the PFZ is limited due to the
faults geographical location predominantly offshore and lack of
permanent Global Positioning System. See also
Molucca Sea Collision Zone Marikina Valley Fault System
References Gallery
External links Latest Philippine Earthquakes
Phivolcs Earthquake Information webpage