Sunday, June 2, 2013

The Philippine Sea Plate Is Moving

M6.2 - 22km SSE of Buli, Taiwan2013-06-02 05:43:04 UTC

M5.6 - 5km ENE of Carmen, Philippines2013-06-01 14:10:07 UTC

Tectonic Summary

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.

Earthquakes kill 2 in Taiwan, injure 33 in the Philippines

TAIPEI, Taiwan (AP) — A strong earthquake jolted Taiwan on Sunday, killing two people and injuring at least 21 others and causing panicked shoppers to rush out of a shaking multistory department store, officials said.
Another earthquake jolted the southern Philippines late Saturday, injuring at least 33 people and damaging more than 140 houses.

The tremor that hit Taiwan on Sunday afternoon was felt all over the island, but most severely in the central and southern regions. The magnitude-6.3 quake’s epicenter was near Jenai township in Nantou County in central Taiwan, about 155 miles south of Taipei, the Central Weather Bureau said.

In Mountain Ali in the southern part of the island, a man was killed by a rockslide while driving a car on a mountain road, the TaiwanFire Agency said in a statement. Another man was killed by a falling rock when he was working at a farm in Chushan, near the epicenter.

Rockslides at a scenic mountainous area near the epicenter injured several people, the agency said. In all, 21 people were injured by the earthquake, many by fallen objects.

Workers removed fallen rocks and repaired a damaged mountain road in Nantou, allowing more than 100 stranded tourists to pass.

Shoppers screamed and ran out of a 12-story department store that shook violently for nearly a minute, TV stations reported from the central city of Taichung. Households elsewhere in central Taiwan reported cracks on the walls or ceilings falling, the reports said.

The Central Weather Bureau said the tremor had a relatively shallow depth of 6 miles. The U.S. Geological Survey put the magnitude at 6.5.

Earthquakes frequently rattle Taiwan, but most are minor and cause little or no damage. Nantou is near the epicenter of a magnitude-7.6 earthquake that struck Taiwan in 1999 and killed more than 2,300 people.

In the southern Philippines, a magnitude-5.7 earthquake rattled North Cotabato province and nearby areas late Saturday as people slept. The temblor damaged more than 140 houses and several school buildings and set off a landslide that partially blocked a road with boulders, officials said.

At least 33 people, including children, were injured by collapsed walls and falling debris in the hard-hit North Cotabato villages of Kimadzil and Kibugtongan, said Hermes Daquipa, a Philippine Institute of Volcanology and Seismology official who joined a government team that surveyed the hilly villages.

The quake, which was set off by the movement of a nearby fault, damaged the approaches to two bridges and concrete pipes that cut off water supply to the two villages. Some of the damaged school buildings will not be able to be used for Monday’s resumption of classes after a summer break for safety reasons, North Cotabato Gov. Emmylou Tolentino-Mendoza said.

Many residents remained jittery Sunday because of continuing aftershocks, said Ms. Tolentino-Mendoza, who added that she scrambled out of her home like other villagers when the ground started to shake and objects fell from shelves.
“It’s a big relief that no motorist was passing through our highway when boulders rolled down from the mountainside,” she said.

The Philippine archipelago is located in the Pacific “Ring of Fire,” where earthquakes and volcanic activity are common. A magnitude-7.7 quake killed nearly 2,000 people on the northern island of Luzon in 1990.

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A Filipino science research specialist views electronic graphs of seismic activities on a computer used to monitor earthquakes, at the Philippine Institute of Volcanology and Seismology-Department of Science and Technology (Phivolcs-DOST) office in Quezon City, east of Manila, Philippines, 02 June 2013. Eight children were injured and structures damaged by a magnitude-5.7 earthquake in the southern Philippines on 01 June, officials said on 02 June. The children from two villages in Carmen town of North Cotabato province were hurt by falling debris and landslides, according to a Cotabato police officer. The Phivolcs-DOST said the epicenter was 10 kilometers west of Carmen town. Photo: EPA/ROLEX DELA PENA, epa03727373


Quakes jolt Philippines, Taiwan

Earthquakes have jolted Taiwan, the southern Philippines and ocean depths off the Solomon Islands. At least 33 Filipinos were hurt as they slept. In Taiwan falling rocks killed a Taiwanese mountain climber.
A magnitude 5.7 earthquake rattled the southern Philippines on Sunday, injuring at least 33 residents as they slept. A stronger 6.3 magnitude quake was felt across Taiwan on Sunday, especially in central and southern regions. It killed one person and injured at least 18 others.
Hours earlier, a 5.5-magnitude quake rattled the Pacific seabed off the Solomon Islands but triggered no reports of damage or Tsunami alert.

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