“Predictive Forensics for Averting Possible Disasters: A FORIN Template for Tackling Issues Related to the Valley Fault System and the Angat Dam in Luzon, Philippines” FINAL REPORT July 2012 to January 2013

DECIBEL V. FAUSTINO-ESLAVA., Ph.D. Project Leader School of Environmental Science and Management University of the Philippines - Los Baños Laguna, PHILIPPINES 15 January 2013

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TABLE OF CONTENTS

I. Abstract 3 II. Project Information 4 III. Introduction 5 IV. Activities Conducted 9 1. Review of Literature 9 a. Geology Regional Tectonics The Valley Fault System b. Angat Dam c. The study area: Angat, Bustos and Norzagaray 2. Geophysical Assessment 38 a. Geologic mapping b. Geohazards assessment c. Geophysical Surveys 3. Socio-Economic Studies 89 a. Focus group discussions b. Economic valuation V. Outcomes and Products 92 Geohazards maps Geophysical evaluation Socio-economic evaluation Focus group discussion results Watershed management and flooding VI. Conclusions 139

VII. Future Directions 142 VIII. Appendix 143

A. Focus group discussion photos B. Saaty Rating Scale C. List of Acronyms D. References

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I. ABSTRACT

The Philippines is a tectonically active region and is subject to a variety of attendant geohazards. Of particular interest to this research is the Valley Fault System on the island of Luzon that traverses several provinces and a good part of Metro Manila. This fault system consists of two sub-parallel, northeast-trending faults: the West and the East Valley Faults. The West Valley Fault extends northeastwards into the province of Bulacan and passes very close to the >40 year old Angat Dam. One of the dykes of the dam is actually believed to have been built on top fault system splays. This study looks into the surrounding communities‟ level of preparedness in terms of the hazards posed by the presence of the Valley Fault System and how such issues can be incorporated in their Local Government Unit‟s planning considerations. Looking at the geophysical, social and economic issues of this region, the most striking realization is that scientific information available at the national agency level do not filter down to the local communities where they truly matter. Hence, more effective ways of information communication is needed, especially with the recognition that the best tool for coping with the Valley Fault‟s perils is knowledge.

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II. PROJECT INFORMATION TOTAL PROJECT FUNDING Fifteen thousand US Dollars (US$ 15,000) PROJECT DURATION August 15, 2012 to January 15, 2013 PROJECT STAFF Principal Investigator Decibel V. Faustino-Eslava, Ph.D.

Associate Professor School of Environmental Science and Management University of the Philippines, Los Baños, Laguna Telephone: +63 49 5362251 e-mail: dei_faustino@yahoo.com dfeslava@uplb.edu.ph http://www.uplb.edu.ph/index.php/environmental-science-and-management

Project Staff Leonardo M. Florece, Ph.D. and Rico Ancog, Ph.D.

School of Environmental Science and Management University of the Philippines, Los Baños, Laguna Noelynna T. Ramos, Ph.D. Rushurgent Working Group Tectonics and Geodynamics Laboratory Ozzy Boy Nicopior, M.Sc. candidate School of Environmental Science and Management University of the Philippines, Los Baños, Laguna

Research Assistant Teodorico Marquez, M.Sc. candidate

School of Environmental Science and Management University of the Philippines, Los Baños, Laguna

Consultants Carla B. Dimalanta, D.Sc.

Rushurgent Working Group Tectonics and Geodynamics Laboratory

Graciano P. Yumul, Jr., D.Sc. Monte Oro Resources and Energy, Inc. Ortigas, Pasig City

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III. INTRODUCTION

The Philippines is a nation that is naturally predisposed to a number of geohazards and its tectonic nature is at the core of these complexities. The archipelago is a composite product of various tectonic processes that involve arc magmatism, ophiolite accretion, ocean formations and closures and arc-continent collisions. These processes are consequent to the country‟s position at one of the most complex collision boundaries between Mainland Asia and the Philippine Sea Plate (Fig.III-1).

As a direct consequence of the country‟s tectonic setting, numerous extensive fault lines can be found cutting across the islands. One such geologic feature is the Valley Fault System (VFS) on the northern island of Luzon.

The VFS is an active fault system that traverses the provinces of Laguna and Cavite, cuts across eastern Metro Manila and propagates north to the province of Bulacan (Fig. III-2). It consists of two sub-parallel, northeast-trending faults: the West and the East Valley Faults (WFV and EVF, respectively) that bound the Marikina Valley. The EVF extends NE for more than 38 kilometers (Daligdig et al., 1997) from Marikina City. The WVF extends from as far north as the Angat Dam in the province of Bulacan, passes across Metro Manila (affected cities include Quezon, Marikina, Pasig, Makati, Pateros, Taguig and Muntinlupa) to the province of Laguna to the south. The VFS is considered by Philippine Institute of Volcanology and Seismology (PHIVOLCS) as active and it can trigger earthquakes as strong as 7.2 in magnitude. Such movements can potentially result in 1 to 2-meter horizontal and 0.5-meter vertical ruptures (Fig. III-3).

Despite having been recognized as early as 1929, prior to the rapid expansion and development of Metro Manila, the relevance of the VFS has not been seriously considered in many of the development plans of the metropolis and the surrounding provinces. It is only recently that the government grasped the importance of the recommendations from the scientific community to incorporate the VFS in policy and planning considerations. The project: Earthquake Impact Reduction Study for Metropolitan Manila (MMEIRS; Punongbayan, 2011) conducted from 2002 to 2004 by the Japan International Cooperation Agency (JICA), Metropolitan Manila Development Authority (MMDA) and PHIVOLCS is one of the few researches that looked into the possible catastrophic effects of the VFS on Metro Manila. The project primarily aimed to evaluate seismic hazards, damages and the vulnerability level of Metro Manila by considering 18 earthquake scenarios from different earthquake generators near Luzon, such as the Manila Trench and the Philippine Fault zone and the VFS. The project evaluated potential effects to buildings, the metro‟s lifeline and its population and prepared the framework of the earthquake disaster management master plan for the metropolis. However, the study did not cover areas outside of Metro Manila. Our research was proposed to investigate the VFS as it relates to the province of Bulacan to the north of Metro Manila. In particular, it hopes to address issues on the WVF‟s possible effects to the Angat Dam and its downstream communities, most especially because of the recognition that the dam was built very close to the WVF.

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Figure III-1. Map showing some of the tectonic elements of the archipelago. Figure indicated by the blue box, approximates the location of Bulacan. Figure is modified from Perez et al., 2012.

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Source: HIGP, Univ. Hawaii

West Valley Fault

Manila Bay

Subic

West Valley Fault

Angat Dam

La MesaDam

Figure III-2. The West Valley Faultextends from Taal Volcano through Laguna, the western edge of Laguna de Bay up to Angat Dam in the north. Modified from Punongbayan (2011).

Province of

Laguna

Taal Volcano Laguna

De Bay

METRO

MANILA

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Figure III-3. The Philippine Institute of Volcanology and Seismology has traced the extent of the West Valley Fault cuts across several cities of Metro Manila. Figure is from Punongbayan (2011).

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Angat Dam is an earth and rock-filled hydro-electric reservoir that supplies about 90% of the water requirement of Metro Manila, the political and economic center of the Philippines. It also generates ~240,000 kilowatts of power and irrigates 28,000 hectares of farmlands in the nearby provinces of Bulacan and Pampanga. After more than 43 years of use, the Manila Water and Sewerage System (MWSS) conducted a six-month study to evaluate the structural integrity of the aging dam. For this purpose, the MWSS hired consultants Tonkin and Taylor International (TTI) in collaboration with the Engineering and Development Corporation of the Philippines (EDCOP). Based on their review of the dam‟s preconstruction reports and from previous studies of PHILVOLCS (Daligdig et al., 1997), they reaffirmed earlier reports that Angat Dam‟s eastern dyke had been built on a splay of the WVF. Because of the presumed active nature of this fault system, they included earthquake considerations in their recommendations. They also generated flooding models for a total dam break and a total dyke failure that could result from earthquake activity. They found that should a catastrophic engineering failure occur, resulting flood waves will not only affect areas near the Angat River. Flooding can spread and occupy vast areas well beyond the Angat River channel and affect both upstream and downstream areas in the floodplain of the Pampanga River. Thirty (30) cities and towns in Metro Manila, Bulacan and Pampanga are likely to be flooded and damages exceeding PhP200 billion may be incurred. As a result of that study, the Philippine national government released PhP5.7 billion for its rehabilitation.

However, the absence of recent strong fault activity along the VFS has proven to be a major stumbling block in convincing various sectors of society on the VFS‟s potential to cause disasters. Many local government units across the metropolis have not truly internalized the fault system‟s relevance to their development plans. Communities‟ grasp of what hazards loom with the VFS appears to remain poor. In contrast to some disaster stories in the Philippines where affected communities have had strings of previous events that people would have learned from, communities near and downstream of Angat Dam lack a history of earthquake-related events that resulted in major losses. Residents of this region will now have to contend with the new reality (newly made available to most of them) that an active fault cuts across or is very proximal to their towns. A new set of hazards will have to be considered in many of their development plans. Old hazards will have to be reevaluated since its potential could be exacerbated consequent to the presence of the VFS. People will have to adjust their perception on their levels of exposure to these hazards. Hence, the existence of the WVF at the Angat Dam opens up a chance for a multidisciplinary investigation of projective or predictive forensic work that merges geophysical issues with socio-economic concerns in order to prepare the stakeholders for the fault‟s current and future negative consequences. With these recognitions, this research was proposed to:

1. assess the various hazards posed by activity along the VFS; 2. gauge the level of preparedness or unpreparedness of local communities to these hazards; and 3. look into possible policy recommendations that may be forwarded to the local and national government units to boost community resilience to the negative impacts (physical, social and economic) that may arise from the fault‟s destructive movements and from the stigma of its mere presence in their region.

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IV. ACTIVITIES CONDUCTED A. Review of Literature a. Tectonic setting and regional geology

In order to determine how activity along the WVF can affect the surrounding areas, the normal geophysical conditions of the region need to be understood first. The following discusses the regional geologic makeup of the study area.

Tectonics The Philippines is a nation that is naturally predisposed to a number of hazards and its

tectonic nature is often at the core of these complexities. Most of the archipelago forms part of the Philippine Mobile Belt (PMB), an active deformational zone sandwiched between the Philippine Sea Plate to the east and the continental Sundaland-Eurasian Plate to the west. The archipelago is therefore classified into two terranes: the PMB and the Palawan Continental Block. The Palawan Continental Block is suggested to be a fragment of southern China that was translated south- eastwards, following the opening of the South China Sea during the Oligocene (Lewis & Hayes, 1984; Rangin, 1989; Pubellier et al., 1991; Quebral et al., 1996; Yumul et al., 2003, 2008). The western margin of the Philippine Mobile Belt is bounded by the discontinuous east-dipping Manila-Negros-Sulu-Cotabato trench system and at the east by the west-dipping East Luzon Trough-Philippine Trench system (Taylor and Hayes, 1980; Aurelio, 2000; Yumul et al., 2009).

Based on the ages and distribution of volcanoes along the eastern volcanic arc of the Philippines, the Philippine Trench system on the east is generally considered to be young(~8 Ma; Ozawa et al., 2004). Similarly, activity along the East Luzon Trough is thought to be young, given the shallow west-dipping Benioff zone associated with this plate boundary (Bautista et al., 2001). The presence of an eastward verging thrust zone observed in Taiwan (north of Luzon) also suggests incipient convergence between the Benham Plateau (a large igneous province within the West Philippine Basin) and Luzon along the East Luzon Trough (e.g. Cardwell et al., 1980; Stephan et al., 1986; Rangin and Pubellier, 1990).

Because of the stresses applied by these two subduction systems on the archipelago, the Philippine Fault Zone (PFZ) developed along its entire length from the northernmost Luzon to the southernmost Mindanao. It is believed to have formed to decouple the northwestward movement of the Philippine Sea Plate from the southeastward movement of the Eurasian plate/Palawan Continental Block (Barrier et al., 1991; Aurelio, 2000). Another theory is that the PFZ accommodates boundary-parallel forces of the overall plate convergence as a trench-linked strike-slip fault related to the Manila trench and/or the Philippine trench (Fitch, 1972; Karig, 1983; Yeats et al., 1997). The known and predicted slips of this fault zone and most of its splays are consistent with a west-northwest to north-west motion of the Philippine Sea Plate.

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Regional geology The study area is located at the western foothills of the Sierra Madre Range and is the

southeastern-most margin of the Central Luzon Basin. It is underlain by igneous and sedimentary formations that range in age from Mesozoic (Cretaceous) to Cenozoic (Plio-Pleistocene) (Figures IV-1 and IV-2).

Figure IV-2. Geologic map covering Angat. The area is underlain by igneous and sedimentary formations that range from Mesozoic to Cenozoic ages. The boxed area is the approximate location of Angat Dam. Map is from the Geological Survey Division, Bureau of Mines and Geo-sciences – Department of Environment and Natural Resources.

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Figure IV-2. Stratigraphic column of the study area showing the basement to be composed of the Montalban Ophiolitic Complex and overlain by younger sedimentary and igneous formations. Adapted from Peña, 2008. Montalban Ophiolitic Complex The oldest rocks east of the Central Luzon Basin, and considered the basement complex of the Southern Sierra Madre range, is represented by the Montalban Ophiolitic Complex (MGB, 2004). Originally, gabbros exposed at Angat, Bulacan were named the Angat Ophiolite (Karig, 1983) and correlated with various components of the ophiolite exposed from Montalban, Rizal through eastern Bulacan, Nueva Ecija and south of the Laur-Dingalan area. So as not to be confused with the Early-Middle Miocene sedimentary Angat Formation, the name Montalban Ophiolitic Complex was proposed by Mines and geo-Sciences Bureau (MGB) in 2004. This ophiolitic complex is composed of layered massive gabbros, sheeted dike complex, pillow basalts and turbiditic sedimentary rocks (Arcilla, 1983). An early Late Cretaceous age is deduced from the paleontological dating of red siliceious mudstones intercalated with the pillow basalts (Revilla and Malaca, 1987). Radiolarians and pelagic foraminifera from the turbidites overlying the pillow basalts yielded similar ages (Arcilla, 1991). Hence, the Montalban Ophiolite is assumed to be of early Late Cretaceous age. Geochemical investigations into its crustal

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section reveal that the lower basalts have typical mid-oceanic ridge characteristics while the upper andesites and basalts have island-arc tholeiitic characteristics. Barenas-Baito Formation The volcanic-sedimentary sequence overlying the pillow basalts of the Montalban Ophiolite have been collectively referred to as the Barenas-Baito Formation (Revilla and Malaca, 1987). These rocks are considered to be the oldest sedimentary units east of the Central Luzon Basin. It is made up of splitic and basic to intermediate volcanic flows and breccias with intercalations of metasediments. The volcanic unit is thought to include the pillow basalts of the Angat Ophiolite, the volcaniclastic member of the Maybangain Formation in southern Sierra Madre and the Coronel and Dingalan Formations in the Laur-Dingalan fault zone (Rutland, 1968). Alternatively, because this formation is considered to be equivalent to the volcanic and sedimentary carapace of the Angat Ophiolite, it is therefore older than the Maybangain Formation and is equivalent to the Kinabuan Formation (Ringenbach, 1992). Dating studies using radiolarians collected from mudstones along the Tayabasan River indicate an early Late Cretaceous age (Blome, 1985). Kinabuan Formation Considered by some works as equivalent to the Barenas-Baito Formation of Revilla and Malaca (1987), the Kinabuan Formation is thought to also represent the sedimentary rocks directly overlying the volcanic carapace of the Montalban Ophiolitic Complex. It is characterized by thinly bedded silty shale and calcareous sandstone with tuffaceous and siliceous layers capped by thin beds of limestone. The lower part of the sedimentary sequence is composed of fine to medium-grained calcarenite and calcisiltite, pelagic limestone and minor calcareous lithic to feldspathicarenite with black organic calcareous shale (Haeck, 1987). The upper limestone is pelagic (Reyes and Ordoñez, 1970) and contains radiolarians which indicate a bathyal depositional environment (Ringenbach, 1992). This formation has been dated Santonian to Early Maastrichtian based on planktonic foraminifera (Reyes and Ordoñez, 1970; Haeck, 1987). Contrary to these earlier data however, radiolarians yielded an older Turonian age based on radiolarians and pelagic foraminifera (Arcilla, 1992). Nevertheless, these slightly different ages still fall within the Late Cretaceous period. Maybangain Formation Conformable to the Kinabuan Formation is the Maybangain Formation whose best exposures are found along Maybangain Creek in Tanay, Rizal. It consists of the lower Masungi Limestone Member and an overlying or partly intertonguingclastic-volcanic member. The Masungi Limestone is interpreted to have been deposited in a fore-reef setting. It consists mainly of redeposited limestones, debris flow materials and turbiditic materials that are interbedded with calcareous and non-calcareous mudstones and minor volcaniclastic rocks (Haeck, 1987). The volcaniclastic member is typically exposed to the west of the Masungi Limestone. It consists of several sub-members including a lower turbiditic sequence of volcanogenic sandstones and siltstones with minor mudstones. This sub-member also contains olistostromes derived from the Masungi Limestone and gravity slides and olistoliths of the Kinabuan Formation. Above this is the Susongdalaga sub-member which is composed of a thick sequence of sandstones, volcanic breccias and conglomerate. The Kanumay sub-member is composed of turbiditic sandstones and siltstones overlain by the upper-most San Ysidro sub-member which is lithologically similar to

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the lower Susongdalaga. This formation is believed to range from Early/Middle Paleocene to Middle Eocene (MGB, 2004) based numerous foraminiferal studies (Reyes and Ordoñez, 1970; Hashimoto, 1981; Haeck, 1987; Ringenbach, 1992). The Maybangain Formation is correlated by Peña (2008) with the Bayabas Formation of Revilla and Malaca (1987) in the eastern Central Luzon Basin. Bayabas Formation Overlying the Barenas-Baito Formation in the east Central Luzon Basin stratigraphy and correlated with the Maybangain Formation of the Southern Sierra Madre is the Bayabas Formation. It consists of metavolcanics and sedimentary units dated to be Late Eocene to Early Oligocene (Peña, 2008 and references cited therein). The metavolcanic units are composed of andesite flows and pyroclastic rocks that include andesitic tuff-breccias. The sedimentary units comprise bedded siltstones, shaley sandstones and conglomerates. Small lenses of marbleized limestone are also reportedly intercalated with the clastic rocks. Sta. Ines Diorite Diorites exposed in Tanay, Rizal which intruded into Cretaceous-Eocene sedimentary units are collectively referred to as the Sta. Ines Diorite (MGB, 2004). This intrusive body is associated with pyrometasomitic deposits of iron ore where they intruded into limestone. It is dominantly composed of medium to coarse-grained hornblende diorite with local quartz diorite, gabbro and diabase. Radiometric dating (K-Ar) of a diorite yielded an Early Oligocene (36.9 Ma) age (Wolfe, 1981). Binangonan Formation Deposited unconformably on the Maybangain Formation is the Binangonan Formation. It consists of the lower Teresa Siltstone member and the upper limestone member (MGB, 2004). The lower clastic member is characterized by tuffaceouscalcerous siltstones and marl, deposited by turbidity currents in a shallow basin (Peña, 2008 and references cited therein). Based on its overall sedimentological characteristic, this member is believed to represent a proximal shallow water fan deposit. The upper limestone member is typically fossil-rich, massive and typical of shallow-water reefs. A Late Oligocene to Early Miocene age was adopted by MGB (2004) based on the range of ages derived by earlier works using both paleontological (Peña, 2008 and references cited therein) and radiometric methods (Ringenbach, 1992). Angat Formation In the eastern Central Luzon Basin, the Angat Formation rests unconformably over the Sta. Ines Diorite and the Bayabas Formation as observed along the Angat River in Bulacan. Towards the southern Sierra Madre region, this formation was observed to have been deposited on the Sta. Ines Diorite in Camachile, Bulacan and over the Barenas-Baito and Binangonan Formations farther east (Peña, 2008). It consists of a subordinate clastic member at the base and a limestone member at the top. Thin beds of calcareous shale and clayey sandstone characterize the clastic member. Paleo-environment indicators suggest it to have been deposited in a shallow marine environment. Interfingering with this clastic member is the upper limestone facies made up of lower reef-flank and deposit and an upper biohermal mass. A late Early to early Middle Miocene is reported for this formation (Peña, 2008 and references cited therein).

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Madlum Formation Conformably overlying the Angat Formation is the Madlum Formation. It consists of 3 members: a lower clastic member, the AlagaoVolcanics and the Buenacop Limestone. The lower clastic member is a thick sequence of bedded sandstones and siltyshales with minor basal conglomerate and limy sandstone interbeds. The AlagaoVolcanics is composed of a sequence of pyroclastic breccia, tuffs, argillites, indurated greywacke and andesite flows exposed in Alagao, San Ildefonso, Bulacan. Also included in this member are metavolcanics previously assigned to the Sibul Formation of Corby et al. (1951) and the andesite-basal sequence in the Rodriguez-Teresa, Rizal areas. The Buenacop Limestone, best exposed along Ganlang River in San Ildefonso, Bulacan, occurs as narrow discontinuous strips of north-south aligned ridges and small patches between Sta. Maria and Sumacbao rivers. The basal sections of this member is composed of crystalline, slightly tuffaceous, porous limestone with numerous fragments of volcanic rocks, chert nodules and mafic minerals. The upper section is massive, cavernous and contains occasional andesite fragments, volcanic debris and fragments of reefal organisms. Paleontological examinations indicate a Middle Miocene age of deposition in shelfal conditions. Lambak and Makapilapil Formations In Bulacan, the Madlum Formation is unconformably overlain by the Lambak Formation. This unit is correlative to the Makapilapil Formation exposed in eastern Nueva Ecija. These formations are composed of sequences of massive to poorly bedded, well indurated, tuffaceous sandy shale and massive, well-indurated, poorly sorted, medium to coarse-grained arkosic sandstone. Coarse components in the sandstones are mainly crystals of quartz and feldspar set in a clayey, tuffaceous and calcareous matrix. The minor congomeratic beds comprise cobbles and pebbles of volcanic rocks and diorite set in coarse tuffaceous matrix. Paleontological studies indicate a Late Miocene age of deposition of these formations in an open sea condition (Peña, 2008). Tartaro Formation A sequence of clayey mudstones and sandstones found along Madlum, Baliculing and Salapungan Rivers in Bulacan is assigned to the Tartaro Formation. It is typically massive or poorly bedded, poorly indurated and contains abundant mollusk shells. Based on nannofossils, a probable Late Miocene to Early Pliocene age is assigned to this formation (Villanueva et al., 1995). The assumed paleo-environment for this formation is a shallow, lagoonal, near-shore setting. Antipolo Basalt Basalts exposed around Antipolo in Rizal, Binangonan, Morong, Angat-Novaliches and Talim Island comprise the Antipolo Basalt. Exposures of this formation are typically brecciated and amygdaloidal. Some units in Antipolo exhibit columnar jointing which indicates that the basalts were deposited as thick lava flows which developed columnar jointing at its fringes. The Antipolo Basalt is believed to be Miocene, but could be as young as Pleistocene (Peña, 2008). Guadalupe Formation Most of Metro Manila is underlain by the tuff sequence belonging to the Guadalupe Formation. Two members comprise this formation: the lower Alat Conglomerate and the upper Diliman Tuff. The Alat Conglomerate is a sequence of a conglomerate, sandstone and

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mudstones. The predominant rock type is the conglomerate which usually occurs as massive, poorly sorted units composed of pebbles and small boulders cemented by coarse grained, calcareous matrix. The interbedded sandstone is typically massive to poorly bedded, tuffaceous, fine to medium grained, friable and exhibits cross bedding. The mudstones are medium to thin bedded, soft and tuffaceous. The Diliman Tuff is composed of pyroclastic rocks which cover a large portion of the Metro Manila, southern Rizal and southern Bulacan. It is composed of fine-grained vitric tuffs and welded pyroclastic breccias with minor tuffaceous sandstones. The Guadalupe Formation unconformably overlies the Tartaro and on the basis of the presence of Stegodon fossils and other vertebrate remains, leaf imprints and artifacts, it is assigned a Pleistocene age. Manila Formation

Unconsolidated fluvial, deltaic and marine deposits of the Manila Formation overlie the Diliman Tuff (Purser and Diomampo, 1995). Deposition of this formation is believed to have been during the Holocene. It is composed of unconsolidated deposits consist of clay, silt, gravelly sand and tuffaceous silt.

The Valley Fault System The Valley Fault System is a NE-SW trending 135-km long fault system transecting

eastern Metro Manila, Cavite, Laguna and Bulacan (PHIVOLCS, 1999). It is part of the Philippine Fault Zone system that accommodates much of the relative movement of the surrounding subduction systems. However, unlike the sinistral or left-lateral strikeslip PFZ, the VFS is predominantly dextral or right-lateral strikeslip fault system, except for its northeastern segments. Features of the VFS were first identified in the city of Marikina, hence, it was initially called the Marikina Valley Fault System (Daligdig et al., 1997). However, because later studies showed that the fault in fact traverses other cities and provinces and in order to avoid imparting stigma on Marikina City, the word Marikina was dropped and the VFS term was adapted.

The VFS consists of two sub-parallel, northeast-trending faults: the Western and Eastern Valley Faults hat bound the Marikina Valley. Alvir (1929) first recognized the Marikina Valley as a fault-bounded graben that developed from repeated vertical movements along the VFS. Irving (1947) suggested a more complex uplift process that resulted in the greater uplift of the eastern block relative to the western block. Later studies (e.g., Gervasio, 1968; Arcilla et al., 1983; Daligdig et al., 1997) have unraveled additional intricacies to the nature of the VFS, including its recently active nature (Daligdig et al., 1997). The EVF extends NE for more than 38 kilometers (Daligdig et al., 1997) from Marikina City. The WVF extends from as far north as the Angat Dam in the province of Bulacan, passes across Metro Manila (affected cities include Quezon, Marikina, Pasig, Makati, Pateros, Taguig and Muntinlupa) to the province of Laguna to the south.

The VFS branches southward from the PFZ and bounds the Marikina Valley where it is interpreted as a pull-apart basin. The East VFS traverses the western side of the Southern Sierra Madre and the West VFS bounds the west side of the Marikina Valley. The VFS appears to end to the south against another tectonic feature, the rift-related Macolod Corridor (Fig. IV-3) which is a zone of volcanoes related to northeast- trending extensional structures. Whether it extends farther south of this region, traces of the VFS appear to have already been buried by young

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eruptive materials of the Taal Volcano.

Geologic mapping and morphotectonic analysis consistently indicate a dominantly dextral strikeslip motion along the VFS during the recent geologic past (Rimando and Kneupfer, 2006). Researches that look into the more recent kinematics of the VFS using GPS data have been unable to resolve details of its movement because of the huge uncertainty involved. As such, Thibault (1999) suggested that the resolvable movement along the VFS can be considered as being minor. However, large-scale differences in the vertical to horizontal displacements of the WVFS and the EVFS indicate slightly different styles of fault movements between the two. The WVFS is dominantly dextral whereas the EVFS is oblique dextral. There is an observed larger cumulative slip along the EVF but similar range in single-event scarp heights between the two segments. These characteristics are taken to indicate differences in the age of faulting, slip rate and/or recurrence interval between the two. Present-day movements along some segment of the VFS suggest a pull-apart basin bounded by the East and West lineaments. The normal component of displacement is most evident along the EVF. Earlier studies on the EVFS suggested that these vertical movement features reflect a distinct phase in the VFS‟ development (Alvir, 1929; Irving, 1947; Gervasio, 1968; Arcilla et al., 1989). However, more recent studies suggest that the vertical component of slip during the contemporary phase of deformation that has generated strike-slip faulting actually accounts for all of the vertical deformation.

Analysis by Rimando and Kneupfer (2006) also suggest that rupturing along the EVFS involved multiple segments and likely occurred separately from rupturing events along the WVFS. Their offset-based magnitude estimates for both faults fall within the range M 7.3 and 7.7, hence, the VFS has the potential to generate magnitude 7 or greater earthquakes.

Recent tectonics strongly control the kinematics of the VFS. Retardation of the subduction along the East Luzon Trough because of the supposed impingement of the Benham Rise and compression because of the collision of the PMB with the Palawan Continental Block have resulted in the development thrust and strike-slip faults with large thrust components in the northern Philippines. Part of the system that accommodates these compressions include movement along the sinistral PFZ and the dextral VFS by lateral extrusion and rotation (Fig. IV-4).

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Figure IV-3. The VFS appears to terminate to the south against the rift-related Macolod Corridor. Faults are represented by hachured lines (hachures indicate the downthrown area), open arrows point to the inferred direction of minimum horizontal stress and ongoing extension of the Macolod Corridor. Figure is modified from Vogel et al., 2006.

West

Valley

Fault

East

Valley

Fault

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Figure IV-4. The kinematics of the VFS is attributed to the lateral extrusion of the block between VFS and the PFZ and the complementary clockwise rotation of the same block that is being extruded laterally. Figure is from Rimando and Knuepfer, 2006.Both the EVF and the WVF were estimated to have moved at similar slip rates consequent to the extrusion process. Based on the combination of magnitude, recurrence and slip rate studies, maximum slip rates of 7–10 mm/yr and 6–8 mm/yr were estimated for the WVF and the EVF, respectively (Rimando and Knuepfer, 2006). The recurrence information of 400-600 years was derived from paleoseismic investigations by Nelson et al. (2000).

b. The Angat Dam

Angat Dam is an earth and rock-filled dam that is one of the closest large-water reservoir to Metro Manila. It is operated by the National Power Corporation (NPC) to provide power, irrigation and domestic water supply. The major structures of the dam include a 131 m high main

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rockfill dam, a 55 m high rockfill dyke and a 3-gate radial concrete spillway. Its water-holding capacity is estimated at 850 m3. Electricity generated at this hydro-electric power plant is supplied to the Luzon power grid and water is supplied to both the Metropolis and surrounding provinces for agricultural, domestic and industrial purposes.

LOCATION

Angat Dam is located in eastern Central Luzon, Philippines. It is approximately 60 kilometers northeast of Metro Manila and lies on the southwestern reaches of the Sierra Madre Range. Its watershed, the Angat Watershed reservation, is situated largely in the Province of Bulacan within the municipalities of Doña Remedios Trinidad, Norzagaray and San Jose Del Monte. Its northeastern boundaries stretch into the Municipalities of General Tiñio, Province of Nueva Ecija and Infanta, Province of Quezon (Fig. IV-5). Angat Dam‟s hydroelectric power plant is located in the community of San Lorenzo, Norzagaray, Bulacan.

Figure IV-5. Location of Angat Dam relative to the various provinces of Central Luzon.

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HYDROLOGY

The Angat Dam watershed reservation is approximately 600 km2 (Briones and Castro, 1986) and is characterized by moderate to intensive forest cover (Fig. IV-6). It is one of the few remaining well-forested and well-managed watersheds in the country. The NPC, a state-owned corporation which is the largest provider and generator of electricity in the Philippines, has the mandate to manage this Reservation.

Figure IV-6. The Angat Watershed, From Briones and Castro, 1986

The effective drainage area of the Reservation is 568 km2 (NPC, 2012). It has two sub-catchments areas: the larger Angat Watershed Metropolitan Water District with an area of 562 km2 (which drains into the Angat Dam itself); and the Angat Watershed Pilot Reserve which

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covers an area of 66 km2, is downstream of the main dam and the water from which drains into Ipo Dam (Fig. IV-7). The Sierra Madre Ranges feed the Angat River‟s water source from three major tributaries: Talaguio, Catmon and Matulid Rivers, which stretch downstream into various municipalities of Bulacan, drains into the Pampanga River in Calumpit, Bulacan before eventually exiting into the Manila Bay. Additional water supply is fed into the Angat Dam through the Angat-Umiraytransbasin tunnel. This tunnel runs for about 13 km from Umiray River on the eastern side of the Sierra Madre and conveys between 7 to 9 m3/sec of water into the dam.

Figure IV-7. Angat Dam‟s reservoir. (Photo taken by the TLM,Jr.)

Construction of the Angat River multipurpose water project began in 1961 and was completed in 1967. It has an installed hydroelectric capacity of 218,000 kwh and has been generating an approximated 530 Gwh annually. The MWSS receives 500 million gallons per day of water from this reservoir which they supply to Metro Manila for domestic and industrial use. Through the National Irrigation Administration (NIA) in the province of Bulacan, water from the dam is also used to irrigate about 315 km3 of farmlands, mostly rice fields.

Based on the Modified Coronas Classification map by the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA; Fig. IV-28), the Angat Watershed is characterized by Type III climate. This means that wet and dry periods are not very distinct in this region. However, the area is relatively dry between December to February or March to May and relatively wet during the rest of the year. The average annual rainfall in the area is 4,200 mm with the maximum annual rainfall recorded at 4,919 mm in 1972 and the least annual rainfall at 1,430 mm in 1982 (EDCOP T&TI, 2012).

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ANGAT DAM STRUCTURES

Figure IV-8. Major facilities of the Angat Hydroelectric Power Plant (main dam, powerhouse, spillway and dyke).

Figure IV-8 shows the major facilities of Angat Hydroelectric Power Plant. Angat Dam has a 131-m high inclined core rockfill dam, as the major water retaining structure (Fig. IV-9 and IV-10), and a 55-m high rockfill dyke (Fig. IV-11) with a rolled earth core to the east of the main dam. In between these two structures is a concrete spillway (Fig. IV-12 and IV-13). The spillway has an ogee crest and three radial gates where water passes through the chute, out the flip bucket and through a sharp bend that directs the flow downstream. The intake of the power station is located to the (east) of the main dam where the main powerhouse and auxiliary units are also located (Fig. IV-14).

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Main Dam

Figure IV-9. Angat Main Dam taken from the Guesthouse. Photo is looking northeast. (Photo taken by the TLM,Jr.)

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Figure IV-10. Downstream view of Angat Main Dam. (Photo taken by the TLM,Jr.)

Figure IV-11. Angat Dyke. (Photo taken by the TLM,Jr.)

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Figure III-12. Spillway. (Photo taken by the TLM,Jr.)

Figure III-13. Spillway‟s radial gate. (Photo taken by the TLM,Jr.)

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Figure IV-14. Angat Hydroelectric Power Plant. (Photo taken by the TLM,Jr.)

CONFLUENCE

South of the powerhouse, Angat Dam converges with a minor tributary some 7.5 kilometers downstream to pool into the smaller Ipo Dam. The Angat River then turns westwards to flow through the towns of Norzagaray, Angat, Bustos, etc. Another small reservoir, about 45 km from Angat Dam, has also been built in Bustos. Each confluence has smaller reservoirs vital for water distribution within their respective localities (Fig. IV-15). The Ipo Dam reservoir gets its supply from the auxiliary units of the powerhouse and the spillway. It is then supplied to MWSS for distribution to Metro Manila. On the other hand, water from the main units of the power station are transferred through a tail-race tunnel that empties into the Angat River downstream of Ipo and Bustos Dams. Water from this channel is generally used for irrigation. There are five power generation auxiliary units that operate 24/7 at a combined rated capacity of 46 MW. This produces the required 46 m3/sec outflow needed for the water supply service of the facility.

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Figure IV-15. Angat Dam‟s confluence. (ADB, 2004)

ANGAT DAM AND DYKE SAFETY PROJECT

Philippine Institute of Volcanology and Seismology in 2004 published the Metro Manila Earthquake and Impact Reduction Study to assess the levels of possible damages that movement along various earthquake generators may inflict on the metropolis. One of the geologic structures that the study modeled is a 7.2 magnitude earthquake along the West Valley Fault (Scenario 8). From PHIVOLCS‟ estimate, a Scenario 8 event will destroy > 40 % of residential buildings in Metro Manila and kill >100,000 people (MMIERS, 2003). Because the WVF does not terminate in Metro Manila, and in fact it is believed to traverse the province of Bulacan near Angat Dam, concerns about its effect on other areas has been considered in the Dam‟s safety evaluation. In particular, the Power Sector Assets and Liabilities Management Corp (PSALM) and MWSS,

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owner of the dam and dyke, commissioned a study in 2011 to evaluate the structural integrity of the dam. Included in the study are flood models should there be a dam or dyke collapse resulting from structural destruction caused by WVF Recommendations for remediation works through retrofitting and dam and dyke reinforcement were put forward to bring the Angat Dam up to standards with respect to seismic resiliency performance. The six-month study was conducted by the Engineering and Development Corporation of the Philippines (EDCOP) through a joint venture with Tonkin & Taylor International (Philippines), Inc.

The EDCOP and T&TI initiated the study of Angat dam safety particularly the dam and dyke seismic loadings as well as probable maximum flood of the spillway. Also, the consultancies offered a detailed rehabilitation work arrangements and construction cost estimates for Angat Dam and dyke embankments.

Results of their investigation highlight the following:

During the construction of the dam, fault traces were identified in the foundation of the dyke (Fig. IV-16). These faults were traced for 15 km upstream of the dyke. These faults trend north-northeast at about 20º and generally dip at 75-90º SE

Figure IV-16. Fault traces in dyke foundation during construction, (EDCOP T&TI, 2012)

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As the usual practice at that time, these fractures were treated with consolidation grouting to reduce the seepage potential.

Based on a study conducted by the PHIVOLCS it was suggested that the WVF runs

northeast a few hundred meters east of the dyke. Faults mapped beneath the dyke are splays of this main fault trace. Movement along the main WVF can potentially trigger movement along these splays as well (Fig. IV-17).

Figure IV-17. Traces of WVF traversing Angat Dam‟s reservoir. (Modified from PHIVOLCS Map)

No rigorous seismic hazard assessment has been done on the nature of the WVF, much less on its splays. Rough estimates by PHIVOLCS suggest that the structure can generate earthquakes with possible magnitudes of 6 to 7 at intervals of 200 to 400 years with a right-lateral strikeslip movement.

Structural analyses of the dam, dyke and spillway revealed that in terms of stability, their

downstream slopes are extremely steep and do not pass a factor of safety of 1.5 for static conditions.

The presence of a potentially active fault beneath the dyke raises the risks even higher.

Should there be movement of the fault, dislocation of the embankment and internal drainage systems can occur. This will likely result in either loss of freeboard or damage to the internal core, filter and drainage zones of these structures. Piping failure (Fig.IV-18) may develop that could eventually lead to a catastrophic collapse.

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Figure IV-18. Approximate pattern of piping failure, (EDCOP T&TI, 2012)

The suggested rehabilitation design for the main dam is the construction of a rockfill

buttress in the downstream slope with a finished slope angle of 1V:1.65H (Fig. IV-19). For the dyke rehabilitation, a finished slope angle of 1V:1.54H (Fig. IV-20) is recommended for the rockfill buttress in the downstream slope of the dyke. The rehabilitation project cost estimates has been set at 134.75 million US dollars which had already been released in September 2013. Rehabilitation work is scheduled to commence in 2013.

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Figure IV-19. Slope buttress for suggested rehabilitation design for the main dam (EDCOP T&TI, 2012)

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Figure IV-20. Slope buttress for suggested rehabilitation design for the dyke (EDCOP T&TI, 2012)

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C. DESCRIPTION OF THE STUDY AREAS

I. MUNICIPALITY OF ANGAT Angat is bordered by the towns of San Rafael and Bustos in the north; Sierra Madre Range in

the east; municipalities of Santa Maria and Norzagaray in the south; and municipality of Pandi in the west. With a total land area of 6,526 hectares, Angat is comprised of 16 barangays, namely: Banaban, Baybay, Binagbag, Donacion, Encanto, Laog, Marungko, Niugan, Paltok, Pulong Yantok, San Roque, Sta Cruz, Sta. Lucia, Sto. Cristo, Sulucan and Taboc (Fig. IV-21).

The topography in Angat ranges from flat or level land, level and nearly level, very gently sloping or gently undulating to gently sloping land. The panoramic view of Angat River that flows in the area serves as a local attraction.

Type I climates prevails in the area with pronounced wet and dry season. Angat experiences wet season from the months of June to November while dry from December until May. The annual average rainfall and temperature is 2,500 mm and 26.60C, respectively (Angat CLUP, 2001-2005).

The 2010 Census of Population and Housing report of National Statistics Office (NSO) revealed a total population of 55,332. The heavily-dense barangay in Angat are the following: Sta. Cruz (5,633); Sulucan (5,557); Sto. Cristo (4.859); San Roque (4,642); and Binagbag (4,641).

II. MUNICIPALITY OF BUSTOS

The Municipality of Bustos is located about 50 kilometers northeast of Metro Manila and 19 kilometers away from Malolos City (Provincial Capital of Bulacan). Bustos is centrally located among some municipalities of Bulacan such as Municipality of San Rafael in the north; Municipality of Angat in the east; Municipalities of Pandi and Pladirel in the south; and Municipality of Baliuag in the west. Bustos is a 2nd-class semi-urban and the smallest municipality in the province of Bulacan with a total land area of 4,250 hectares (MPDO, 2009). It is consists of 14 barangays namely: Bonga Mayor, Bonga Menor, Buisan, Camachilihan, Cambaog, Catacte, Liciada, Malamig, Malawak, Poblacion, San Pedro, Talampas, Tanawan, and Tibagan (Fig. IV-22).

The terrain in the area is generally flat with Angat River acting as a natural boundary between the northern and western side of Bustos. The eastern side of the town locates the Bustos Dam which is an eight-kilometre multi-purpose dam.

Bustos has two distinct climatic seasons. The month of April marks the start of rainy season and lasts in November. The highest amount of rainfall was observed during February while least during March. Occasional rainfall is widely dispersed in the town while dry season is during the remaining months of the year. The average annual rainfall and temperature is 100.58 inches and 280C, respectively (Bustos CLUP, 2001-2005).

NSO reported that Bustos has a total population of 62,415 with 13,503 households based on 2010 Census of Population and Housing. Among the populated barangays in Bustos are Poblacion (9,641); San Pedro (6,506); Tibagan (6,084); Cambaog (5,592); and Malamig (5,313).

III. MUNICIPALITY OF NORZAGARAY Norzagaray is 46 kilometers north of Manila and about 36 kilometers west of Malolos City

(Provincial Capital of Bulacan). With its location in the southeastern side of Bulacan, Norzagaray is bordering the Municipality of Montalban under the province of Rizal. The town of

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Norzagaray is bounded by Municipality of Doña Remedios Trinidad in the north; General Nakar, Quezon in the east; and City of San Jose Del Monte in the southwest. The total land area of Norzagaray is 30,819 hectares. It is consists of 13 barangays namely: Bangkal, Baraka, Bigte, Bitungol, Friendship Village Resources (FVR), Matictic, Minuyan, Partida, Pinagtulayan, Poblacion, San Lorenzo, San Mateo and Tigbe (Fig. IV-23).

The topography in the area is varied ranging from flat to gently sloping lands and hilly lands to mountainous terrains. The area falls under Type I climate which is dry during November to April and wet in the remaining months of the year. The average annual rainfall is 3,000 mm while average monthly temperature is 26.60C (Norzagaray CLUP, 2011-2020).

As of 2010, the total number of population in Norzagaray is 103,095 according to Census of Population and Housing published by NSO. The barangays with the most numbered of population are the following: Poblacion (15,642); Tigbe (14,846); Bigte (11,032); Matictic (10,395); and San Mateo (9,089).

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Figure IV-21. Location and boundary map of Angat, Bulacan.

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Figure IV-22. Location and boundary map of Bustos, Bulacan.

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Figure IV-23. Location and boundary map of Norzagaray, Bulacan.