Seismotectonics of the Philippine Sea and Vicinity
The Philippine Sea plate is bordered by the larger Pacific and Eurasia
plates and the smaller Sunda plate. The Philippine Sea plate is unusual
in that its borders are nearly all zones of plate convergence. The Pacific
plate is subducted into the mantle, south of Japan, beneath the
Izu-Bonin and Mariana island arcs, which extend more than 3,000 km along
the eastern margin of the Philippine Sea plate. This subduction zone is
characterized by rapid plate convergence and high-level seismicity extending
to depths of over 600 km. In spite of this extensive zone of plate
convergence, the plate interface has been associated with few great
(M>8.0) ‘megathrust’ earthquakes. This low seismic energy release is thought
to result from weak coupling along the plate interface
(Scholz and Campos, 1995). These convergent plate margins are also
associated with unusual zones of back-arc extension (along with resulting
seismic activity) that decouple the volcanic island arcs from the remainder of
the Philippine Sea Plate (Karig et al., 1978; Klaus et al., 1992).
South of the Mariana arc, the Pacific plate is subducted beneath the Yap
Islands along the Yap trench. The long zone of Pacific plate subduction at
the eastern margin of the Philippine Sea Plate is responsible for the generation
of the deep Izu-Bonin, Mariana, and Yap trenches as well as parallel chains of
islands and volcanoes, typical of circum-pacific island arcs. Similarly, the
northwestern margin of the Philippine Sea plate is subducting beneath the
Eurasia plate along a convergent zone, extending from southern Honshu to
the northeastern coast of Taiwan, manifested by the Ryukyu Islands and the
Nansei-Shoto (Ryukyu) trench. The Ryukyu Subduction Zone is associated with
a similar zone of back-arc extension, the Okinawa Trough. At Taiwan, the plate
boundary is characterized by a zone of arc-continent collision, whereby the
northern end of the Luzon island arc is colliding with the buoyant crust of the
Eurasia continental margin offshore China.
Along its western margin, the Philippine Sea plate is associated with a zone of
oblique convergence with the Sunda Plate. This highly active convergent plate
boundary extends along both sides the Philippine Islands, from Luzon in the
north to the Celebes Islands in the south. The tectonic setting of the Philippines
is unusual in several respects: it is characterized by opposite-facing subduction
systems on its east and west sides; the archipelago is cut by a major transform
fault, the Philippine Fault; and the arc complex itself is marked by active volcanism,
faulting, and high seismic activity. Subduction of the Philippine Sea Plate occurs at
the eastern margin of the archipelago along the Philippine Trench and its northern
extension, the East Luzon Trough. The East Luzon Trough is thought to be an
unusual example of a subduction zone in the process of formation, as the Philippine
Trench system gradually extends northward (Hamburger et al., 1983). On the west
side of Luzon, the Sunda Plate subducts eastward along a series of trenches,
including the Manila Trench in the north, the smaller less well-developed Negros
Trench in the central Philippines, and the Sulu and Cotabato trenches in the south
(Cardwell et al., 1980). At its northern and southern terminations, subduction at
the Manila Trench is interrupted by arc-continent collision, between the northern
Philippine arc and the Eurasian continental margin at Taiwan and between the
Sulu-Borneo Block and Luzon at the island of Mindoro. The Philippine fault, which
extends over 1,200 km within the Philippine arc, is seismically active. The fault has
been associated with major historical earthquakes, including the destructive M7.6
Luzon earthquake of 1990 (Yoshida and Abe, 1992). A number of other active
intra-arc fault systems are associated with high seismic activity, including the
Cotabato Fault and the Verde Passage-Sibuyan Sea Fault (Galgana et al., 2007).
Relative plate motion vectors near the Philippines (about 80 mm/yr) is oblique to the
plate boundary along the two plate margins of central Luzon, where it is partitioned
into orthogonal plate convergence along the trenches and nearly pure translational
motion along the Philippine Fault (Barrier et al., 1991). Profiles B and C reveal
evidence of opposing inclined seismic zones at intermediate depths (roughly 70-300 km)
and complex tectonics at the surface along the Philippine Fault.
Several relevant tectonic elements, plate boundaries and active volcanoes, provide a
context for the seismicity presented on the main map. The plate boundaries are most
accurate along the axis of the trenches and more diffuse or speculative in the South
China Sea and Lesser Sunda Islands. The active volcanic arcs (Siebert and Simkin, 2002)
follow the Izu, Volcano, Mariana, and Ryukyu island chains and the main Philippine islands
parallel to the Manila, Negros, Cotabato, and Philippine trenches.
