MARIKINA VALLEY FAULT SYSTEM: UNDERSTANDING METRO MANILA’S VULNERABILITY TO EARTHQUAKES



MARIKINA VALLEY FAULT SYSTEM:

UNDERSTANDING METRO MANILA’S VULNERABILITY
TO EARTHQUAKES

        When the earth shook, the Algonquian Indians used to say the Great Tortoise, which supported the world, was shifting its weight. Japanese legends, on the other hand, blamed the movements to a giant spider. Native folklore attributed earthquakes to the anger of the earth goddess. The great Aristotle had an equally mistaken notion that prevailed for 2,500 years. He thought earthquakes were caused by powerful subterranean winds.

        An earthquake, in the most basic definition, is a naturally induced shaking of the ground, caused by the fracture and sliding of rocks within the earth’s crust. Because the Philippines is within the Circumpacific Belt, an area characterized by a concentration of earthquake epicenters and active volcanoes, it is an absolute necessity for the Filipinos to recognize, learn and understand the predicament they have.

        Major seismic events in the last three decades, July 16, 1990 (Baguio and Cabanatuan), November 15, 1994 (Mindoro), April 21, 1995 (Samar), January 1, 2001 (Davao Region), July 23, 2010 (Moro Gulf), August 31, 2012 (Samar), October 15, 2013 (Bohol), December 29, 2018 (Davao Region), and the successive strong earthquakes (Magnitude 6 and higher) that hit Mindanao since October 2019, makes it imperative for Filipinos to have the necessary knowledge about earthquakes. 

The Philippine Geologic Past

        As an archipelago, the Philippines had no definitive existence since about 30 million years ago. The country was never a part of the Asian Mainland or the Australian Continent even during the days of Pangaea. What are called “landbridges” that connected the archipelago with Formosa, Mainland Asia, Indonesia and Malaysia, were more recent products of the recession of the sea in the Pleistocene Epoch. One theory is that like the Philippine Archipelago, the landbridges were submerged under the ocean during the early times.

        About 16 million years ago, during the Miocene Epoch of the Tertiary Period, the Indo-Australian Plate moved and crashed with both the Pacific Plate in the northeast and the China Plate in the northwest generating massive shocks and diastrophisms along a line from the Indonesian Archipelago to the Island of Japan, lifting upward the lands in which more or less where we stand today. Thus began the birth of the Philippine landmass from the bosom of the ocean.

        In the next five to 20 million years, the Philippine Plate suffered intense compression from two sides, one from the southeast, generating the great Philippine Fault and uplifting islands in the eastern coasts; and in the south creating mountain foldings, raising the island of Mindanao, and thrusting up previously submerged lands like the island of Jolo.

        During the Second Glacial Age, approximately 700,000 years ago, the greater part of what is known today as the Cavite Province and the submarine slope of the Taal Volcano were tilted up and raised by about 400 meters in the vicinity of Tagaytay, assisted by the southern projection of the Marikina Valley Fault lines. The lifted ridge from Parañaque to Las Piñas provided a natural dam separating the Laguna Lake from the Manila Bay.

        The geomorphology of the Philippines underwent radical changes during the glacial period, causing the rise and submersion of lands. When the glaciers began to thaw about 80,000 years ago, the landbridges connecting the Philippine landmass with the rest of Asia and Australia and much of the land went underwater by 35 to 120 meters.

        By the year 10,000 B.C., the Philippines, more or less, became what it is today – a conglomeration of roughly 7,100 islands enveloped by eight shoreline and offshore troughs and trenches. 

A Glimpse of Philippine Seismic History



On the left is the Map of Seismicity History (1600-2006) and Active Faults in the Philippines (Wong, Dawson, and Dober), and on the right in the Seismicity Map of the Philippines (Historical and Instrumental) as provided by PHIVOLCS in the Philippine Earthquake Model (2017) Atlas.

        The Philippine territory is within the Circumpacific Belt, a seismically active region better known as the “Ring of Fire.” The region covers the length of the Philippine and Japan Archipelagos, extending through the Aleutians, Alaska, and the western coasts of the Americas, westward north of the Antarctic, east of Australia and back to the Philippines through the Indonesian Archipelago. It is here where 77 percent of major earthquake epicenters and 82 percent of the active volcanoes in the world are located.

        According to the Philippine Institute of Volcanology and Seismology (PHIVOLCS), the Philippine Archipelago is one of the world’s most tectonically and, therefore, seismically active areas. Statistically speaking, the Philippines host at least five imperceptible to perceptible earthquakes per day.

        The strongest earthquakes observed in Manila had been of Intensity X, with an average return period of about 130 years. According to an investigation conducted by the National Society for Seismology and Earthquake Engineering of the Philippines (NASSEP) in 1979-1980, a very strong earthquake of about Intensity IX or X hit Manila on November 30, 1645, destroying all existing buildings especially along the Pasig River.

        On June 3, 1803, another earthquake probably of the same intensity, hit Manila destroying the Manila Cathedral and 527 other buildings, killing about 400 and injuring more than 2,000 persons.

        The earthquake of August 2, 1968, is always remembered in our history because of the collapse of the Ruby Tower that killed hundreds of people in downtown Manila. The 7.3-magnitude earthquake hit at exactly 4:19 a.m., while most of the people were still sleeping, followed by a 5.9-magnitude aftershock just after 20 minutes. Ruby Tower collapsed, allegedly because of the poor design and the substandard construction materials used. Since then, our interest on the subject of earthquakes has been enhanced.

        By far, the most destructive earthquake to hit the Philippines was the Magnitude 8 temblor that hit Mindanao on August 17, 1976. The earthquake triggered a tidal wave that left more than 3,000 persons dead, another 3,000 missing and rendered about 20,000 families homeless. Another is that of July 16, 1990, that hit Baguio and Cananatuan, a killer quake comparable to the 1906 California earthquake or the 1964 Alaskan earthquake. 

Connected Calamities

Illustration shows the sequence of events that scientists believed

to have led to the awakening and eruption of Mt. Pinatubo (1991).

At right, map shows the location of the Philippines

in relation to four tectonic plates.

        The re-awakening of long-dormant volcano, Mt. Pinatubo, was believed to have been induced by the July 16, 1990 earthquake. Many scientists believe that a strong earthquake gives rise to identifiable volcanic eruptions.

        An example is the destruction of the cities of Pompeii and Herculaneum. Mt. Vesuvius, 210 kilometers southeast of Rome, had been a peaceful mountain for several thousand years. Then on February 5, 63 A.D., a severe earthquake jolted its vicinity. This event started a series of intermittent earthquakes that lasted for 10 years. Consequently on August 24, 79 A.D., the dormant Mt. Vesuvius volcano erupted, burying the cities of Pompeii and Herculaneum.

        On the western edge of the Apennines, severe faulting over the last two million years gave birth to a line of volcanoes that runs from Mt. Amiata in Tuscany to as far as Mt. Etna in Sicily, passing through the volcanic lakes of Bolsena, Vico, Bracciano, Albano and Nemi and continuing through the volcanoes of Roccamonfina, Vesuvius and the Isole Eolie.

        Powerful Chilean earthquakes from May 21 to 29, 1960, also triggered the eruption of nearby dormant Puyuehue Volcano. The Mexican earthquake of September 19, 1985, was believed to reawaken the Nevada del Ruiz Volcano which has been dormant for 140 years. Adding to the list are Mt Unzen in Southwestern Japan and Mt. Pinatubo in the Philippines.

        For our part of the puzzle, the Philippine Archipelago is situated between the Philippine Plate and the China Plate. The Philippine Plate is moving westward at the rate of approximately seven centimeters per year, colliding with the China Plate. The upper surface of the Philippine Plate bends at the East Luzon Trench and the Philippine Trench, and slides beneath the lower surface of the China Plate in the subduction zone. This created several fissures that cut through Lingayen Gulf down to Central, Eastern and Southern Luzon, and then Leyte, going to Mindanao. This is known as the Philippine Fault System.

        As the Philippine Plate continues to slide downward, stresses accumulate in the Philippine Fault System and are released from time to time resulting in earthquakes.

        Friction at the subduction zone exerted tremendous pressure on the subterranean rock formations like a grinding machine. Rocks are forced deeper, gigantic erosions occur and the rocks begin to melt creating magma.

        Long dormant volcanoes like Mt. Pinatubo are characterized by seeming disappearance of visible craters. This is because the magma in the vent leading down its crater has solidified forming a dome or volcanic plug just like a cork stopper plugging a bottle’s mouth. But the powerful earthquake of July 16, 1990, cracked the interior of the dome creating vent fissures that eventually became the exit point of massive and tremendous pressure built-up for more than 600 years. Thus Mt. Pinatubo began erupting on June 12, 1991. 

The Marikina Valley Fault System

Two comparative MVFS mapping: On the left, by UNDRO (1977),

and on the right, 40 years later, by PHIVOLCS (1017).

        The adobe formation, running from the north in the foot of the Sierra Madre mountains to the south in the slopes of Taal Volcano cutting through Metro Manila Area (MMA), a thick sequence of well-bedded volcanic tuff and tuffaceous clastics, which dated back from the early Pleistocene Ice Age, roughly about 1 to 3 million years ago, is generally associated with the possible development of faulting activity the experts called the Marikina Valley Fault System (MVFS).

        It is speculated that the Marikina Valley Fault System had moved at least three times in the last 15 centuries, the last verifiable earthquake emanating from it was that of August 20, 1658.
        The existence of the MVFS, however, was recognized by various workers only as early as 1923. But according to PHIVOLCS, “the MVFS’s activity has yet to be fully evaluated. Field mapping augmented by topographic map and air photo interpretations conducted in April and May (1991) of the Marikina Valley and surrounding areas revealed previously unrecognized geologic and geomorphic evidences for the recent activity of the Marikina Fault System.” In 2017, PHIVOLCS released the PhilippineEarthquake Model (PEM) atlas, an extensive seismicity map of the Philippines which includes the “Distribution of Active Faults in (the) National Capital Region” showing the mapped location of the MVFS, and spectral acceleration maps of Metro Manila, a valuable data source for seismic-structural engineers.

        Much of the early research about the MVFS was obtained through the 1977 United Nations Disaster Relief Coordinator (UNDRO) Report. In October 1976, the then Human Settlements Commission (HSC), headed by First Lady Imelda R. Marcos, requested the assistance of the office of the UNDRO to conduct a systematic vulnerability analysis in the Metro Manila Area and, on this basis, to prepare a composite risk map for inclusion in the urban development master plan of the metropolis.

        The mission was carried out by UNDRO consultants, Michel Couillaud and Jacques Didon, from October 13, 1976 to March 5, 1977, under the umbrella of the HSC.

        During the course of its comprehensive research and geologic and aerial investigations, the UNDRO discovered the following subsoil conditions:

        (a) The Guadalupe Formation (adobe): ……. Westward towards Manila the formation extends underneath the delta sediments where the beds inter-tongue with compacted marine sand, gravel and silt along the coastal area. They thin out towards the west and are wedged in with marine sediments… East and North of Manila and in Parañaque these tuffs are overlain by brown clay loam passing to light gray or brownish compact clay. The thickness ranges from 0.5 meter near Quezon City to two meters near Novaliches to the North.

        (b) Marikina Alluvial Plain: This graben valley, well-delimited by the tuff escarpment and the fault-truncated ridges, was almost completely filled with alluvial sediments transported by the Marikina River… The alluvium is made up of an unconsolidated mixture of sand, some gravel and considerable silt and clay derived chiefly from weathering of pyroclastic and volcanic rocks. Sand layers with considerable amounts of marine shell fragments were found at depths between 6.5 and 18 meters from the surface of the ground in Sucat and Napindan… The thickness of alluvium varies from zero at contact with the bedrock to at least 75 meters at the valley in Pinagbuhatan and Napindan. From Bambang, Pasig, thins out gradually eastward across the Marikina Valley through Pinagbuhatan and Anzano…

        (c) Manila Deltaic Plain: After the raising of Guadalupe ridge, the Pasig River received the impounded lake water and, at the same time, provided a large volume of fluvial materials that, mixed with marine sediments, rapidly expanded into a large deltaic plain… This plain…, encompasses the Manila area and extends southward near Pasay City… Based on actual drilling data and core analyses, it can be stated that generally the commercial district of Sta. Cruz, Sampaloc, Quiapo, Escolta, Intramuros, Port Area, Ermita, Paco and Malate, all in Manila, are underlain by plastic clays, silts, sands and gravels with an intricate admixture of marine shells, corals and decayed plants… Lateral persistency among individual beds is so poorly developed that even a thick bed may terminate abruptly in as short a distance as three meters. A maximum thickness of 61 meters to 90 meters is indicated, the thickest being along the banks of Pasig River in Quiapo, Avenida, Escolta and Port Area.

        It can also be noted that in the intensity map prepared by the former Weather Bureau for August, 1968 Luzon earthquakes, an isolated higher intensity was observed in downtown Manila. This was caused by the soft soil layers underlying the area.

        Furthermore, the UNDRO Mission Report noted the following:

        ● Certain parts of Intramuros (Binondo and Sta. Cruz) have sustained ground subsidence and tilting, which in principle may have been (at least partly) caused by the liquefaction of loose sand layers under the deltaic plain of Manila. A certain degree of liquefaction may have occurred toward the end of the 16th century when Manila was rocked by particularly violent earthquakes.

        ● As far as the MMA is concerned, there is no historical evidence of fault displacement, even in the case of violent tremors. The evidence of last displacement (and associated deformation) dates back to the second Glacial Age, i.e., well beyond an arbitrary, though usual limit of, say, 15,000 years, up to which time one may assume a fault to be active. Nevertheless, taking into account the importance of past displacements (more than 80 meters in Pasig) and the fact that MMA forms a “fragile zone” liable to be affected by strong shaking, this factor should be considered in the total seismic risk estimation.

        ● It is conceivable that fault traces in the MMA may experience movements in the future. Earthquakes occurring in a fault may be the source of severe local shaking. Surface fault displacement and an associated deformation should be localized along the faults… The judgment of whether or not a fault is likely to move in the near future is based on its behavior in the recent geologic past. It is prudent to consider that a fault, which has moved within the past 15,000 years, is still active and is a factor to be weighed carefully in physical planning.

        It can also be noted that in the UNDRO mapping, several branches of the MVFS are plotted including two presumed fault lines traversing parallel the Pasig River. Because this area in question is highly urbanized, there is much difficulty in making geological and geomorphical investigations. However, the UNDRO map indicated faults emanating near the North Harbor (crossing the northernmost pier) and South Harbor (crossing the U.S. Embassy area), as well as their presumed counterparts emanating from the junction of the Pasig and Marikina Rivers. The UNDRO map also indicated three main fault lines. The UNDRO-plotted faults extend farther cutting through Sucat in Parañaque, and Alabang, Muntinlupa.

        In 1980, NASSEP suggested the possible existence of a Manila Fault line cutting along the Pasig River. Investigation of building ruins dating back to the middle of the 17th century indicated that the buildings were destroyed not by liquefaction alone but by the surge of very powerful vertical and lateral forces, theoretically suggesting that the epicenter of the earthquake is very near the vicinity. The Manila Faults are said to be branches of the more extensive MVFS. 

The PHIVOLCS Report

        During the time of PHIVOLCS Director Raymundo Punongbayan, investigation, trenching and mapping of the MVFS began. Here is the summary of the preliminary results on that mapping activity:

        ● The Marikina Valley Fault System (MVFS) consists of two main northeast-trending faults – the East Valley Fault (EVF) and the West Marikina Fault – that bound the Marikina Valley and adjoining towns of Montalban, San Mateo, Antipolo and parts of Eastern Metro Manila… Repeated movements along the MVFS greatly influenced the present morphology of the area wherein the Marikina Valley was downthrown relative to the Diliman-Pasig and Montalban-San Mateo-Antipolo areas on the west and east, respectively.

        ● The WVF has been mapped for a distance of at least 23 kilometers from Lower Macabod, Rodriguez, in the north down to the vicinity of the Ultra Sports Complex in Pasig, Metro Manila. Mapping of the northernmost and southernmost extensions of the WMF has been constrained by similar conditions as in the EVF. The areas directly lying along the fault trace are the following: Macabod, Rodriguez and the vicinity north of Amityville, eastern part of Amityville, western part of Christineville, eastern Quezon City/western Marikina area, downslope area east of Violago and BF Homes; eastern Payatas, Bagong Silangan, Fil-Invest Homes III; eastern Capitol Park Homes; Loyola Grand Villa Subdivision; western Loyola Subdivision’s Barangka, Cinco Hermanos, eastern parts of Don Juan, Industrial Valley and White Plains Subdivisions, and St. Ignatius Village; western parts of Green Meadows and Valle Verde Subdivisions and the Golf and Country Club.

        ● The EVF was mapped as far north as San Rafael, Rodriguez and down south just north of Marvi Hills subdivision and Modesta Village for a distance of at least eight kilometers. The northern terminus of the EVF has not been fully mapped while its southern extent is poorly-defined as a large part of the area has been greatly modified by present-day development. Among the areas transected by the EVF are the following: San Rafael north of Wawa River, eastern San Rafael, Gloria Vista Subdivision, eastern San Mateo and northwestern Antipolo.

        As of 2018, the main trunk of WVF have been mapped and labeled up to a length of 129.47 kilometers (80.45 mi) from Doña Remedios Trinidad to Calamba, in Laguna, and runs through Metro Manila traversing the cities of Marikina, Quezon City, Mandaluyong, Pasig, Makati, Taguig and Muntinlupa and passing the west bank of Laguna Bay, and moving in a dominantly dextral strike-slip motion, in the general direction towards Taal Lake. The eastern segment, EVF, moves in an oblique dextral motion. It extends to about 17.24 kilometers (10.71 miles) from Rodriguez to San Mateo in the province of Rizal.

The Vulnerability of Metro Manila

         Most of Luzon, particularly Metro Manila, where the MVFS cuts through, because of its peculiar geologic condition, is said to be prone to natural hazards like earthquakes. On an average, destructive earthquakes have shaken the city of Manila once in every 14 and a half years. The former Weather Bureau published a paper on Significant Philippine Earthquakes, which give the dates, times, and locations of epicenters and the reported intensities of all the earthquakes that hit the country since 1949. Since that year it was estimated that Manila is “liable to be affected by an earthquake of Intensity IV (based on the Rossi-Forel Scale) every year.” This average magnitude is relatively high because of the given predominant geological conditions underlying the city.

        During the July 16, 1990 earthquake, Manila was rocked by an Intensity VII tremor although the epicenter of the earthquake was about 200 kilometers away. The isolated higher earthquake intensity experienced in Manila was caused by the soft, unconsolidated soil layers underlying the city. Manila’s buildings suffered slight structural damage. But what if the epicenter is near, say within a 10-kilometer radius? What potential effect can be expected in the area?

        According to PHIVOLCS, any moderate to strong earthquakes from the MVFS is “expected to have considerable impact on the present population and building density within Metro Manila and adjoining areas.”

        The metropolis is especially prone to the fault-related hazard called liquefaction. The process occurs when water-soaked sediments, such as the case in the many places within the Marikina Valley and in the western part of Metro Manila especially those lying along the coastal and reclaimed areas, river deltas and similar settings, are subjected to strong ground shaking. During the process, the sediments acquire a more compacted state resulting in an increase in hydrostatic or pore water pressure thus causing the solid particles to behave like liquid and seek areas of least stress, more likely along the ground surface. The transfer of underlying materials to the surface is compensated in adjoining areas by subsidence. That means while one area is lifted upward, others sink down. This process was responsible for the extent and magnitude of damage sustained by the commercial district of Dagupan City, in Pangasinan, during the July 16, 1990 earthquake.

        Based on existing land laws, the Civil Code, administrative orders of the Bureau of Lands and Bureau of Forest Development, several executive orders, presidential decrees and zoning ordinances, the following summary can be noted on building easement along riverbanks:

        No building shall be erected within three (3) meters in urban areas, twenty (20) meters in agricultural areas and forty (40) meters in forest areas of the original width of esteros, streams and rivers, whereby the margins are allotted for open spaces, parks, recreation areas, navigation and permanent forest cover.

        It can also be noted that Quezon City (although already urbanized), and the Marikina valley area are still classified as forest areas, and Caloocan City, Malabon and Navotas are still classified as agricultural lands. But zoning and easement requirements were never followed, even in the urban areas. In fact, encroachments of waterways are prevalent in Metro Manila, which is also the number one cause of flooding in the city. Titles issued to these pieces of land according to all existing laws are, per se, illegal and null and void from the beginning and must be cancelled.

        Buildings atop river deposit areas such as these are in high risk during earthquakes because of the danger of ground collapse and liquefaction.

        A HSC research team reported in 1984 that a portion of Metro Manila, west of the Marikina River and atop the Pasig River delta, and the Tondo foreshore area, have sunk about 18 inches in two and a half decades. It was also observed that the towns along the Laguna lakeside have also subsided an average depth of about a foot.

        According to PHIVOLCS, another hazard that is expected along active faults is ground rupturing or the generation of cracks on the ground surface accompanied by either horizontal or vertical movements or a combination of both. This hazard usually affects the areas directly along and immediately astride the fault traces. Areas along the boundaries of Quezon City and Marikina going parallel the Marikina River on both sides, passing through Pasig, Mandaluyong, Pateros, Taguig, continuing along the lakeside of Laguna de Bay along the Sucat and Alabang area (UNDRO mapping), are quite prone to this disaster. A combination of liquefaction and ground rupturing is also expected along these areas and also parallel the banks of the Pasig River on both sides.

        The UNDRO recommended a vulnerability index map to redefine land-use and building constraints which, if applied, will result in mitigating the impact of natural phenomena, and avert disaster. These constraints are applicable both to zones which are already built up and to areas planned for new development. Thus, in the former case, the constraints indicated might lead to the removal of extremely vulnerable buildings or activities, to programs of urban renewal in which the risk factor has been taken into account, or to temporary-adjusted land-uses. In the latter case, they will simply indicate restrictions on land-use and building.

        In relation to the MVFS, in 2004, the Japan International Cooperation Agency (JICA) in cooperation with PHIVOLCS and the Metropolitan Manila Development Authority (MMDA) to put together the 2004 Metro Manila Earthquake Impact Reduction Study (MMEIRS). The comprehensive study covered damage scenarios and estimation results if the metropolis is hit with a Magnitude 7.2 earthquake. It outlined which areas are most vulnerable, the extent of worst-case-scenario damage, and recommended action plans to mitigate the disaster.

        According to the MMEIRS, residential buildings will be most at risk, with at least 25.6 percent or 339,800 damaged and another half of this figure heavily damaged. On  fire-related incidents (as a consequence of the earthquake) alone, there will be a potential 18,000 casualties, and 113,600 possible injured. The direct human impact is estimated at 33,500 deaths, and 3.1 million people homeless.

        In another study, Evaluating the Seismic Hazard of Metro Manila, Philippines, by Ivan Wong, Timothy Dawson and Mike Dober, the MVFS is the closest active fault to the Manila metropolitan region and represents the most likely near-field source of large, damaging earthquakes to Manila.” The same study calculated that the full rupture of the Manila Trench, around 1,000 kilometers in length, would generate a megatrust earthquake of Magnitude 8.8.

        In 2011, a survey after the release of the 2010 National Structural Code of the Philippines (NSCP), revealed that 35 percent of low-rise residential buildings and more than 25 percent of mid-rise buildings in Metro Manila are not up to standards as they adhere to older construction codes and standards.

        As of July 2019, there are 104 subdivisions on the direct path of the MVFS. With it, 6,415 structures are inside or within 10 meters of the faults, mostly residential buildings. There are also 18 school campuses, of which around 30 buildings, constructed on top of the faults. These are extremely susceptible to collapse and destruction, most probably the perceived source of fatalities and casualties in a major earthquakes epicentered near or within the MVFS.

        As studies and investigations of the MVFS continue, the threat of a major earthquake emanating from it exists. 

Comparative Scenarios

        So far, comparing all records from various institutions, government and academic entities, the earthquakes of November 30, 1645 and August 20, 1658, probably, are the only two earthquakes in recorded history believed to may have originated near or within the Metro Manila area. According to NASSEP, these two earthquakes, estimated to be around Magnitude 7 or higher, are linked to the MVFS, the former to the WVF and the latter to the EVF.

        If the MVFS would indeed move and cause an earthquake of staggering magnitude, Metro Manila would be subjected to a catastrophe also of staggering magnitude.

        Most of the structures in Metro Manila are designed to resist a Magnitude 6 earthquake as per the code requirement. Others can resist up to Magnitude 8.5 such as the Light Railway Transit (LRT) system.

        There are many scenarios that can be simulated if an earthquake epicentered in the MVFS within the Metro Manila area. To have a better-simulated view of forthcoming events, let us study earthquakes of similar nature and circumstances.

        With this in mind, two earthquakes can be recalled: The April 18, 1906 California quake that totally destroyed the San Francisco Bay area, and the infamous Magnitude 8.1 Good Friday quake in Anchorage, Alaska, in 1964.

        In these two cases the terrains were similar in that the land areas in question were bordered by the sea and sliced by the fault lines.

        Furthermore, with the case of the Anchorage quake, the epicenter occurred at the junction of four known fault lines: Lake Clark, Cook Inlet, Seldovia and Fairweather Faults. A similar nature is observed at the confluence of the Marikina and Pasig Rivers, where, according to the UNDRO mapping, several continuous and discontinuous fault traces are in junction.

        In both the California and Alaska temblors, all buildings including residential houses near the faults were totally destroyed.

        In California, many experts feared that if a very strong earthquake emanates from the San Andreas Fault, the entire California coastline could disappear and sink into the sea. Similarly, an earthquake of Magnitude 8 or greater could trigger the same catastrophe in Metro Manila along the Pasig River delta fronting the Manila Bay. Buildings on reclaimed areas and soft silt and clay foundation in the vicinity would be almost if not totally destroyed.

        On the economic point of view, the existence of the MVFS and its eventual seismic activities may bring down real estate investments in Metro Manila. Prices of lands in the metropolis would also go down. However, there are also some good effects. Real estate investors will look for alternative sites in the country, thereby widening the potentials of other places in the Philippines like Palawan, an area the least visited by earthquakes. Commerce and industry would be decentralized benefiting underdeveloped districts and municipalities. Population density would be lessened, and consequently, other relative factors like traffic congestion, unemployment problems, dispersal of commercial establishments, etc. Initially, the same were observed in California, Alaska and in Japan. Areas within at least a kilometer from potential faults were abandoned and declared open spaces or parks. Establishments and settlements were relocated.

        Having discussed all these scenarios, the designated authorities in government should do their parts. Stricter building and construction laws would have to be followed. Zoning and easement ordinances must be implemented to the letter.

MAJOR REFERENCES:

Couillard, M., and Didon, J., “Comprehensive Vulnerability Investigation and Analysis of Metro Manila With Regards to the Hazards of Earthquakes and Floods” (1978), United Nation Disaster Relief Coordinator (UNDRO) Report to the Human Settlement Commission (HSC).

Limeta, F. B., “Seismicity History of the Philippines and Its Application in Engineering Design” (1983), National Society for Seismology and Earthquake Engineering in the Philippines (NASSEP) Compilation of Research Studies and Porposition Paper. 

Malicdem, E. B., “Barangays and Buildings Traversed by the Valley Fault System” (2017)

Miyamoto, H., and Amir, G., “Comprehensive Seismic Risk Reduction Program for Public Buildings in Metro Manila, Philippines” (2015).

Philippine Institute of Volcanology and Seismology (PHIVOLCS), Japan International Cooperation Agency (JICA), Metropolitan Manila Development Authority (MMDA), “Metro Manila Earthquake Impact Reduction Study” (MMEIRS) (2004).

Philippine Institute of Volcanology and Seismology (PHIVOLCS), The Philippine Earthquake Model (PEM): A Probabilistic Seismic Hazard Assessment (PSHA) of the Philippines and Metro Manila (2017).

Punongbayan, R. S., Rasdas, A., Rimando, R. E., Tungol, N., Mirabueno, H., Nelson, A. R., Personius, S. F., “Multiple Large Earthquakes in the Past 1500 Years on a Fault in Metropolitan Manila, Philippines” (2000), Bulletin of Seismological Society of America.

Rimando, R. E., and Knuepter, P. L. K., “Neotectonics of the Marikina Valley Fault System (MVFS) and Tectonic Framework of Structures in Northern and Central Luzon, Philippines” (2006), Tectonophysics. 

Wong, I., Dawson, T., Didon, J., “Evaluating the Seismic Hazards in Metro Manila, Philippines” (2009).