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West Valley Fault very active - Phivolcs

posted Mar 4, 2012, 8:45 PM by Ka Noel

ABS-CBNnews.com
Posted at 03/01/2012 9:18 PM | Updated as of 03/02/2012 2:42 PM

MANILA, Philippines – The Philippine Institute of Volcanology and Seismology (Phivolcs) on Thursday inspected the West Valley Fault in an effort to mitigate the risks that come with an active fault.

The fault spans 90 to 100 kilometers and passes through several areas in Metro Manila as well as Bulacan, Rizal, Cavite and Laguna.

Philvolcs said the fault is still very active.

“Makikita natin na madalas kumilos ang fault. Kailangan paghandaan ang pagkilos ng fault na ito,” said Phivolcs director Renato Solidum.

The agency wants to determine how often the fault moves and how intense the movement is.

“Posibleng magnitude 7.2 dito sa fault manggaling ang lindol at intensity 8 to intensity 9 shaking. Ang intensity 8 destructive earthquake na ‘yon,” Solidum said.

Soil samples from the West Valley Fault will be sent abroad for analysis.

Results are expected within two months and will be used to help government prepare for the effects of another massive earthquake.

Meanwhile, the National Disaster Risk Reduction and Management Council (NDRRMC) also conducted a nationwide earthquake drill Thursday.

NDRRMC executive director Benito Ramos said they have been doing the drills every quarter to raise awareness since no one really knows when the next big earthquake will strike next.

Residents of Negros Oriental are still reeling from the 6.9-magnitude earthquake that hit the province last month.

Aside from the dozens of fatalities, many lost their homes and millions of pesos of infrastructure were destroyed.

The cause of the earthquake was a blind fault—a fault line that had been previously undiscovered. -- Reports from Jasmin Romero, ABS-CBN News; ANC


What Causes a Tsunami?

posted Mar 4, 2012, 8:16 PM by Ka Noel   [ updated Mar 4, 2012, 8:16 PM ]

from Geology.com   

What causes a tsunami?... A tsunami is a large ocean wave that is caused by sudden motion on the ocean floor. This sudden motion could be an earthquake, a powerful volcanic eruption, or an underwater landslide. The impact of a large meteorite could also cause a tsunami. Tsunamis travel across the open ocean at great speeds and build into large deadly waves in the shallow water of a shoreline.

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Benham Plateau

posted Feb 29, 2012, 11:08 PM by Ka Noel   [ updated Feb 29, 2012, 11:09 PM ]

From Wikipedia, the free encyclopedia  

Benham Plateau (coordinates: 17°N 125°ECoordinates: 17°N 125°E), also known as the Benham Rise, is a seismically active undersea region and extinct volcanic ridge east of the Philippines, in the Philippine Sea. Under the Philippine Sea lies a number of Basins including the West Philippine Basin (WPB) of which inside the Basin is located the Central Basin Fault (CBF). The Benham Plateau is located in the CBF and its basement probably represents a micro-continent. Several scientific surveys have been made on the feature to study its nature and its impact on tectonic subduction, including one about its effects on the 1990 Luzon earthquake, which devastated the northern city of Baguio. The area is currently claimed, as part of its continental shelf, by the Republic of the Philippines, which has lodged a claim with the United Nations Commission on the Limits of the Continental Shelf on April 8, 2009.

Geological features

Benham Rise is a submerged extinct volcanic ridge located at 16 degrees 30 minutes N, 124 degrees 45 minutes E off the coast of Luzon, with the size of about 250 km in diameter and rises over 2,000 meters above the sea floor, from below 5,000 meters below sea level to above 3,000 meters below sea level. Its area is close to the Benham Seamount, located at 15 degrees 48 minutes N, 124 degrees 15 minutes E. The precise location is somewhere near the east of the Philippine Trench and near the south of the East Luzon Trench, both of which absorb the subducting force of the Philippine Sea Plate under the Philippine Mobile Belt, a collage of large blocks of that crust that amalgamated prior to the collision of the Philippine Sea Plate with the Eurasian Plate.

The origin of the landform, along with a fellow landform, the Urdaneta Plateau (a remnant of mantle plume), is identified in one study as at least five sequences of propagating rifts, probably triggered by mantle flowing away from the mantle thermal anomaly. Its presence of the landform disrupts the continuity of this region (known as the Philippine-East Luzon Trench) by continuously colliding with the Sierra Madre mountain range of eastern portion of the island of Luzon. Though it is generally thought that the Philippine Sea Plate is being subducted under the Philippine Mobile Belt, under the rules of tectonic subduction, there appears to be a resistance to this because of the presence of the landform, and instead, the plate is being displaced into the northern portion of Luzon to the west.

The geophysical features of the plateau may have been the result of an early Miocene collision event between the Benham Rise and the eastern margin of Luzon, which may have also allowed the inception of the NW striking strand of the Philippine fault. These forces may have impacted the shape of the island of Luzon because of the basaltic sea floor resisting the subduction that may have also cause the bending of the Philippine Fault. The active basins in Central Luzon, which trace an asymmetrical V shape, is the best place to observe recent tectonic evolution of the fault system.

History

The landform is presumably named after Admiral Andrew Ellicot Kennedy Benham (1832–1905) by American surveyors who were the probable discoverers of the geological feature. He was a United States Navy officer, who served with both the South Atlantic and West Gulf Blockading Squadrons during the American Civil WarThere has been speculation in the scientific community about the nature of the landform. Following the major 16 July 1990 Luzon earthquake, scientists reconsidered their fault models and decided it likely that Benham Plateau has similarly displaced the Philippine Fault System to the westAfter analysing older models such as that of Pinet and Stephan (1989), scientists reconsidered their fault models. They thought that it is highly likely that the Benham Plateau is still displacing Central Luzon and the Philippine Fault System to the west, which may have had an impact in causing such a catastrophic earthquake. The 20 second to 50 second wave in the 1990 quake that developed a new east-west sub-fault was so strong that it terminated disastrously at the city of Baguio in Benguet, Cordillera. Several scientific surveys, conducted between 2004 to 2008, collected hydrographic data that determined the morphology of the seabed in the region. Additional data from international bathymetric surveys and an analysis of international research projects were collected to support the findings.

Philippine claim

Despite its proximity to the archipelago, the plateau was previously not included in the territory of the Philippine Islands. On 8 April 2009, the Republic of the Philippines lodged a partial territorial waters claim with the United Nations Commission on the Limits of the Continental Shelf in relation to the continental shelf in the region of Benham RiseIt was submitted as part of petition expanding the archipelago's baselines and exclusive economic zone through a law that also included other claims involving disputed territories of the Kalayaan Islands (Spratly Islands) and Scarborough Shoal. Although the landform, in itself, is not disputed, the petition still received some criticism inside and outside the country because of its controversial natureAccording to the government's claim, based on a set of guidelines by the Commission on the Limits of the Continental Shelf, the area satisfies the 350-mile constraint line since the outer limits of the continental shelf are located landward of the constraint line, which is located 350 miles from the baselines where the measurement of the breadth of the territorial sea begins

The Congress of the Philippines enacted Republic Act No. 9522, also known as the Archipelagic Baselines Law, which is the basis of the claim. According to the document the region is bounded by the Philippine Basin on the north and east, and by Luzon on the west and south. It asserted that, according to scientific data based on seismic, magnetic, other geological features, the Benham Rise is an extension of the Philippines’ continental shelf. In summary, the baselines, the basis used for delineating the maritime territorial and jurisdictional zones (including the continental shelf), conform with the requirements of the UN Convention of the Law of the Sea (UNCLOS). The claim is only a partial claim since the law that allows the Philippines to expand its territorial boundaries also includes islands in the South China Sea.

U.N Decision

According to, Philippine Environment Secretary Ramon Paje,the United Nations is expected to come out with a decision by the middle of 2012 on the Philippines’undisputed claim over the 13- million hectare Benham Rise. Paje told the Senate during a hearing on the Department of Environment and Natural Resources (DENR) proposed P16.990- billion budget for 2012 that the country’s claim over Benham Rise would be recognized by the UN as part of the territory and extension of the continental shelf of the Philippines.  The Philippines filed the claim in 2008 in compliance with the requirements of the UN Convention on the Law of the Sea (UNCLOS).

Petrology and Geochemistry of the 1991 Eruption Products of Mount Pinatubo

posted Feb 28, 2012, 10:56 PM by Ka Noel

By Alain Bernard,1 Ulrich Knittel,2 Bernd Weber,2 Dominique Weis,1 Achim Albrecht,3 Keiko Hattori,4 Jeffrey Klein,5 and Dietmar Oles2

1 Department of Geology, CP 160/02, Université Libre de Bruxelles, 50 Ave. F.D. Roosevelt, B-1050 Brussels, Belgium.

2 Institut für Mineralogie und Lagerstättenlehre, Wüllnerstrasse 2, D-52056 Aachen, Germany. Now at: Institut für Geowissenschaften/Mineralogie Saarstr. 21, D-55099 Mainz, Germany.

3 Department of Chemistry, Rutgers University, Piscataway, NJ 08855; now at: EAWAG/ETH, Umweltphysik, 8600 Duebendorf, Switzerland.

4 Department of Geology, University of Ottawa, Ottawa, K1N 6N5 Canada.

5 Department of Physics, The University of Pennsylvania, Philadelphia, PA 19104.


ABSTRACT

The June 1991 activity of Mount Pinatubo erupted about 5 cubic kilometers of magma, mainly during the paroxysmal June 15 eruption. Most of the erupted magma was dacite with an overall constant composition (64.5 weight percent SiO2) but with variable amounts of phenocrysts. Small volumes of andesite (56 weight percent SiO2) characterized the June 7-12 dome-building phase and the June 12 eruption. This andesite contains inclusions of a primitive basalt (50.6 weight percent SiO2). The dacite is extraordinarily enriched in sulfur (1,474 to 2,211 ppm sulfur), most of which is present as phenocrysts and microphenocrysts of anhydrite. The dacite also contains xenocrystic olivine, which, because of abundant chromite inclusions, is considered to be of juvenile nature. Major and trace element compositions of dacite and andesite display the same geochemical features as the other silicic rocks from the Bataan arc volcanoes of the Philippines. Strontium and neodymium isotopic compositions of all 1991 dacite and andesite and their phenocrysts are identical, with 87Sr/86Sr = 0.7042 to 0.7043 and 143Nd/144Nd = 0.51286 to 0.51298. The basalt contains slightly more radiogenic strontium and slightly less radiogenic neodymium. The isotopic signatures are in agreement with the volcano's location, which is relatively far from the North Palawan continental terrane (NPCT) collision zone as compared with the Macolod corridor, Luzon, or the Mindoro segment. These compositions suggest also that an unusually large contamination by marine sediments is not likely to explain the large sulfur enrichment observed in the Pinatubo magma. Dacite and andesite also have identical lead isotopic compositions characterized by high 208Pb/204Pb and 207Pb/204Pb for relatively low 206Pb/204Pb, while the basalt contains slightly more radiogenic lead. The basalt is likely to have been generated by partial melting of peridotite because of its high MgO content and high forsterite content of olivine pheno-crysts and the abundance of chromite inclusions. It is close to the primitive basalts found in the Macolod corridor.


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The Philippine Fault System

posted Feb 28, 2012, 10:07 PM by Ka Noel   [ updated Feb 29, 2012, 11:14 PM ]


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 tectonic geophysicists call the 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 demarkated 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. The main fault runs most of the length of the Philippines and is called the Philippine Fault (see Gallery). In northern Luzon, the fault has become braided and is no longer single. All faults in the Philippines are inter-related by the tectonic forces causative 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.

The Philippine fault, probably active since the Middle Miocene is a recent example of the north-south strike-slip fault zones of Luzon. The N150E segment, which breaks the meridian continuity of the Philippine Fault system in Luzon, is located to the west of Benham Plateau on the western edge of the Philippine Sea Plate. The N150E fault probably has a recent genesis and could be a mechanism to absorb backwards the convergence between Benham Plateau and the east Luzon arc.

Following the major 1990 Luzon earthquake, scientists reconsidered their fault models and following the lead of Pinet and Stephan (1989), thought it likely that Benham Plateau is still displacing central Luzon and the Philippine Fault System to the west, and so caused the 1990 Luzon earthquake. The 20 second or 50 second wave in the 1990 quake developed a new east-west sub-fault, which terminated disastrously at Baguio.

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