PREFACE
In 2009, after more than a week of continuous rainfall and the onslaught of Typhoon Ondoy (Ketsana), most of Luzon including Metro Manila were inundated by rushing flood waters. Thousands of homes were flooded up to the rooftops. The more alarming news is about the cracks on some parts of the Angat Dam in Bulacan. We also heard in the news that a large portion of the neighboring province of Pampanga is sinking. Then there is the perennial observation in some subdivisions in Metro Manila, particularly in parts of Mandaluyong, Pasig, eastern Quezon City, western Marikina, northwestern Antipolo and parts of the Rizal Province (which were also heavily flooded during Ondoy’s wrath) of cracks and displacements on concrete structures and pavements. What is happening to our geologic terrain? Do these events have a connection or a common cause?
In the middle of 1991, just after Mt. Pinatubo’s eruption, a very good friend of mine, broadcast journalist Lito Villarosa, began a series of exposé regarding the controversial Marikina Fault System. According to Villarosa, he saw a bunch of documents in the defunct Ministry of Human Settlement on its way to the junk bin. When he tried to read the contents, it turn out to be a 1976 report of the United Nations Disaster Relief Coordinator (UNDRO) to the former Human Settlement Commission, detailing a systematic vulnerability analysis of Metro Manila area with regards to disasters like earthquake and flood. There was also a comprehensive study regarding the Marikina Fault System. To cut the story short, he gave me a copy of this report and, together, we conducted extensive research on the subject and simultaneously revealed to the public what we have uncovered, Villarosa through the broadcast media and me through the pages of MOD Magazine as an exclusive three-part special report, "
The Philippines: Land of Earthquakes and Volcanoes." Exactly a year after, that article won for me the grand prize in the Second Annual Science and Technology Journalism Award sponsored by the Philippine Press Institute and the Philippine Geothermal Inc. That same article is now the banner story of this blog that would also answer the two aforementioned questions.
Part I
A Study of Our Restless Terrain
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.
What really does happen when the ground trembles violently as it did in Baguio and Cabanatuan on July 16, 1990, one of the most unforgettable geologic disasters in Philippine history? 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 P
hilippines is within the Circumpacific Belt, an area characterized by the concentration of earthquake epicenters and active volcanoes, it is an absolute necessity for the Filipinos to recognize, learn and understand the predicament they have.
Events like the 1990 killer quake, the Mt. Pinatubo eruptions, the series of tectonic tremors in Luzon, and the focus of attention to the highly controversial but otherwise proverbial Marikina Valley Fault System, prompted me to re-publish an updated version of this article.
The Philippine’s 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 is 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 times. It is believed 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 Archipelago of Japan, lifting upward the lands in which more or less where we stand today. Thus began the birth of the Philippine Archipelago from the bosom of the ocean.
In the next five to 15 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 islands in 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 Fault lines. The lifted ridge from Parañaque to Las Piñas provided a natural dam separating the Laguna Lake from the Manila Bay.
The face of the Philippine Archipelago 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 archipelago with the rest of Asia and Australia went underwater by 35 to 120 meters.
By the year 10,000 B.C., our archipelago 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
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 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.
In 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 had been enhanced.
By far, the most destructive earthquake to hit the Philippines other than that of July 16, 1990, was the Intensity VIII temblor that hit Mindanao on August 17, 1976. The earthquake epicentered in the Moro Gulf, triggered a tidal wave that left more than 3,000 persons dead, another 3,000 missing and rendered about 20,000 families homeless.
Then the July 16, 1990 killer quake – an agonizing event that is now recorded in the annals of history as one of the most destructive inland-epicentered earthquakes that the world has ever experienced and comparable only or perhaps greater than the 1906 California earthquake or the 1964 Alaskan earthquake.
On November 15, 1994, an earthquake occurred near Verde Island, off the coast of Mindoro. The 7.1-magnitude earthquake generated a tsunami that hit approximately 40 kilometers of the northern and eastern shoreline of Mindoro Island from Puerto Galera up to Pinamalayan. Areas hardest hit by the tsunami are in Barangays Malaylay, Old Baco, Wawa, and Baco Islands where six to ten meters of vertical run-up was believed to have smashed the shoreline up to more than a quarter of a kilometer inland, destroying completely the houses nearshore and leaving at least 41 persons dead, mostly children and old people.
Mindoro was again hit by a magnitude 6 earthquake last September 18, 2009.
Interrelated Calamities
During the 1990s and early 2000s, Central Luzon has been devastated by lahar flood. Lahar is being induced by monsoon rains, which according to meteorology experts were induced by the Mt. Pinatubo volcanic ash that remained afloat high above our atmosphere.
On the other hand, the Mt. Pinatubo eruptions were 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 21-29, 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. And in recent times, Mt Unzen in Southwestern Japan and Mt. Pinatubo in the Philippines.
Mt. Unzen had been dormant for 199 years and Mt. Pinatubo’s last eruption was placed by carbon dating method between 1342-1380. But why did these long dormant volcanoes suddenly heat-up and blow their tops? Why, for that matter, several volcanoes in this part of the world suddenly acted up one after the other?
And earthquakes seem to be scheduled in a daily basis in this part of the world?
Last September 28, 2009, the Samoas, a group of islands in the Pacific, was hit by a magnitude 8 earthquake and a tsunami and, the following day, Sumatra, Indonesia, was struck by a 7.6-magnitude earthquake followed by another 6.8-magnitude temblors near the Padang town. And most recently, the Vanuatu archipelago in the South Pacific was hit my several earthquakes. Leading geologists in Japan, a center for volcanic research, agree that tectonic plate movements under the Pacific Ocean caused the violent eruptions of Unzen and Pinatubo some 2,000 kilometers apart.
For neighboring country, Indonesia, on the other hand, Sumatra, or northwestern Indonesia, is right on the India-Burma (on the Indo-Australian) tectonic plate boundary. Part of the Indo-Australian plate that has the Indian Ocean on it is moving roughly northeastward at a rate of six centimeters per year relative to the Burma plate and colliding with Sumatra. This results in oblique convergence that is accommodated on the right-lateral transform faults and rifts along the Sunda trench.
The devastating megathrust earthquake of December 26, 2004 that killed more than 230,000 people occurred on the interface of the India and Burma plates, off the west coast of Northern Sumatra some 225 kilometers south-southeast of Banda Aceh) at a depth of 10 kilometers (6.2 miles) and was cause by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate at the Sunda trench, which lies to the west of the earthquake's epicenter. It is the fourth strongest earthquake in recorded history. According to geologist, the magnitude 9.0 earthquake was so powerful, that the energy it released made the Earth wobble on its axis and permanently altered the regional map.
For our part of the puzzle, at least four major tectonic plates are directly affecting the Philippine archipelago. Aside from these, marine geologists at the Mines and Geosciences Bureau (MGB) of the Department of Environment and Natural Resources (DENR) say the Philippines is at the center of five undersea trenches that are earthquake-prone and could trigger tsunamis.
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 like that of July 16, 1990.
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 “twin” earthquakes 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.
Mt. Pinatubo has spewed over 12 cubic kilometers of volcanic debris throughout the provinces of Pampanga, Tarlac and Zambales. This is worsened by the fact that the areas are catchbasins during continuous downpour of monsoon rains. Tectonic tremors have been continuously recorded around the aforementioned areas. Could it be possible that the large volume of subterranean mass spewed by the volcano had created some hollow formation deep underground that is now causing geologic displacements in these areas? Could it be that these tectonic movements are due to the fact that the Philippine Plate inside the subduction zone is greatly eroding?
Notice that the southern part of central Luzon is checkered by several faultlines. Considering that because of the previous earthquakes and the subsequent eruptions of Mt. Pinatubo depositing more than 40 billion tons of volcanic matter, have weaken the geologic slab in the vicinity, there is the possibility of a worst scenario of massive geologic
caving-in in that part of Central Luzon. If this is so, then the devastation we have so far experienced is not even a minute comparison to the immensity of what is to come. Much of Pampanga could, in the near future, become a huge lake!
It is also highly probable that earthquakes emanating from active faultlines could reactivate long inactive faults such as that of the Marikina Fault System. Considering the awesome effect that these cataclysms could inflict to human lives and properties, we have to know and study all the possibilities. We need to be prepared
http://erneelawagan.blogspot.com/2009/10/things-to-do-before-during-and-after.html.
Part II
The Marikina Valley Fault System: The Truth
In the 1990s. the media revealed a startling discovery that alarmed the people of Metro Manila particularly those living in Marikina. This was the announcement made by PHIVOLCS regarding the existence and possible reactivity of the Marikina Valley Fault System (so named because it traverses an area parallel to the Marikina River, but later renamed simply Valley Fault System). Thus, we must be inclined to know the truth and possible consequences, and the program and countermeasures fundamental to our safety.
The Valley Faults System
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 the provinces of Pampanga and Bulacan, 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 initially termed as the Marikina Fault System (MFS).
The existence of the Marikina Fault System, however, was recognized by various workers only as early as 1923. But according to PHIVOLCS, “the MFS’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.”
The UNDRO Report
In October 1976, the Human Settlements Commission (HSC) requested the assistance of the office of the United Nations Disaster Relief Coordinator (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 MFS 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, Parañaque, and Alabang, Muntinlupa.
In 1980, the National Society for Seismology and Earthquake Engineering of the Philippines (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 Marikina Fault System.
The PHIVOLCS Report
Here is the summary of the preliminary results on the MFS mapping activity as so far compiled by PHIVOLCS:
● The Marikina Fault System (MFS) consists of two main northeast-trending faults – the East Marikina Fault (EMF) and the West Marikina Fault (WMF) – that bound the Marikina Valley and adjoining towns of Montalban, San Mateo, Antipolo and parts of Eastern Metro Manila… Repeated movements along the MFS 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 EMF 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 EMF 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 subdivision development. Among the areas transected by the EMF are the following: San Rafael north of Wawa River, eastern San Rafael, Gloria Vista Subdivision, eastern San Mateo and northwestern Antipolo.
● The WMF has been mapped for a distance of around 30 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 EMF. 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’ Brangka, 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.It was during the time of PHIVOLCS director Raymundo Punongbayan when the July 16, 1990 Baguio killer quake happened, followed less than a year after by the Mount Pinatubo eruption. This prompted PHILVOCS to publicly disseminate information on the activity of the MFS, to make the public know of its existence and the potential danger it posts so that the proper authorities can make long-range preparations to meet the probable consequences of such an impending event. However, subdivision developers earnestly tried to stop the dissemination of information by lobbying in Congress and Malacañang to stop Director Punongbayan, until such time that a full and comprehensive study have been produced. These somehow slowdown the facilitation of vital information regarding the MPS.
Part III
The Marikina Fault System: The Consequences
Most of Luzon, particularly Metro Manila, because of its peculiar geologic condition, is said to be prone to natural hazards like earthquakes. On an average, the city of Manila has been shaken by destructive earthquakes once in every 14 and a half years. The former Weather Bureau published a paper on Significant Philippine Earthquakes, which gives the date, time, location of epicenter 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.
The Vulnerability of Metro Manila
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 Marikina Valley Fault System 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.
Another potential threat to the aforementioned areas is large-scale geologic displacement, which although may occur slowly, say a few centimeters per decade, but these areas will be proned to heavy floodings during high tides, storm surges, and typhoon or monsoon-induced rains. These is now quite apparent, especially with global warming added to the scenario.
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 highly urbanized), and the Marikina valley area are still classified as forest areas, and Caloocan City, Malabon City 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 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 and Taguig, continuing along the lakeside of Laguna de Bay along the Sucat and Alabang area (UNDRO mapping), are quite prone to this disaster. This is evident in many subdivisions in the area where buildings and civil work facilities experienced structural, semi-structural and masonry cracking in the last two decades or so. A combination of liquefaction and ground rupturing are 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 (Restrictions in land usage were implemented when the Ministry of Human Settlement was still existing, but it was completely forgotten after the agency was closed).
As studies and investigations of the MFS continue, the threat of a major earthquake emanating from it exists.
Possible Scenarios
So far, comparing all records from various institutions, government and academic entities, the earthquake of November, 1645, probably, is the only earthquake in recorded history believed to may have originated near or within the Metro Manila area. The intensity of that earthquake was hypothetically placed between XI and X.
If the MFS 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 at a minimum and a magnitude 8 at the maximum.
There are many scenarios that can be simulated if an earthquake epicentered in the MFS 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 8.1-magnitude (Richter Scale) 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.
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.
On the economic point of view, the existence of the MFS 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.
Most if not all the scenarios regarding earthquake aftermath may seem grim, but let us not get into panic. Rather, let us compose ourselves and take all the necessary precautions. It is, however, inevitable and no man can challenge the might of Mother Nature nor can anyone predict exactly, at present, the time of her outburst. Prudence and prayers would certainly help a lot. Let us take these as signs that God is reminding us of His presence.
