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M7.6 - 96km E of
Sulangan, Philippines 2012-08-31 12:47:34 UTC
Tectonic Summary
The August 31, 2012 M
7.6 earthquake off the east coast of the Philippines occurred as a result of
reverse faulting within the oceanic lithosphere of the Philippines Sea plate.
The preliminary location of the earthquake indicates this is an intraplate event,
50 or more kilometers to the east of the subduction zone plate boundary between
the Philippine Sea and Sunda plates. At the latitude of the earthquake, the
Philippine Sea plate moves west-northwest at a velocity of approximately 100
mm/yr.
While this region of the
Philippines experiences moderate-to-large earthquakes fairly frequently - there
have been approximately 40 events of M6 and above over the past 40 years,
within 250 km of the August 31 2012 earthquake - large events outboard of the
subduction zone are unusual. While several moderate-sized events have occurred
in this intraplate region, most have been the result of normal faulting within
the shallower oceanic lithosphere, rather than deeper reverse faulting like
August 31 2012 event. The largest nearby event regardless of mechanism was the
October 1975 M 7.6 earthquake, approximately 200 km to the north of the August
31 2012 event.
For information on
aftershocks within the region of this earthquake, see this map.
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.
Seismic activity along
the boundaries of the Philippine Sea Plate (Allen et al., 2009) has produced 7
great (M>8.0) earthquakes and 250 large (M>7) events. Among the most
destructive events were the 1923 Kanto, the 1948 Fukui and the 1995 Kobe
(Japan) earthquakes (99,000, 5,100, and 6,400 casualties, respectively), the
1935 and the 1999 Chi-Chi (Taiwan) earthquakes (3,300 and 2,500 casualties,
respectively), and the 1976 M7.6 Moro Gulf and 1990 M7.6 Luzon (Philippines)
earthquakes (7,100 and 2,400 casualties, respectively). There have also been a
number of tsunami-generating events in the region, including the Moro Gulf
earthquake, whose tsunami resulted in more than 5000 deaths.
10.839°N, 126.704°E
Depth: 34.9km (21.7mi)
Event Time
2012-08-31 12:47:34 UTC
2012-08-31 20:47:34 UTC+08:00 at epicenter
2012-08-31 08:47:34 UTC-04:00 system time
Nearby Cities
96km (60mi) E of Sulangan, Philippines
109km (68mi) ESE of Guiuan, Philippines
162km (101mi) ESE of Borongan,
Philippines
176km (109mi) NE of Surigao, Philippines
747km (464mi) ESE of Manila, Philippines
--------------------------------
Earthquakes
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Summary
Updated 2012-08-31
20:05:48 UTC
7523 earthquakes
All events in the past 30 days
7522 meet criteria
located in map area
300 displayed
based on sort order
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