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Geological Society, London, Special Publications

Geological hazards of SW Natib , site of the Nuclear Power Plant, the

A. M. F. Lagmay, R. Rodolfo, H. Cabria, J. Soria, P. Zamora, C. Abon, C. Lit, M. R. T. Lapus, E. Paguican, M. G. Bato, G. Tiu, E. Obille, N. E. Pellejera, P. C. Francisco, R. N. Eco and J. Aviso

Geological Society, London, Special Publications 2012, v.361; p151-169. doi: 10.1144/SP361.13

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Geological hazards of SW Natib Volcano, site of the Bataan Nuclear Power Plant, the Philippines

A. M. F. LAGMAY1*, R. RODOLFO1, H. CABRIA1, J. SORIA2, P. ZAMORA2, C. ABON1, C. LIT1, M. R. T. LAPUS1, E. PAGUICAN1,3, M. G. BATO1,3, G. TIU1,3, E. OBILLE4, N. E. PELLEJERA1, P. C. FRANCISCO1, R. N. ECO1 & J. AVISO1 1National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City 1101, the Philippines 2Marine Science Institute, College of Science, University of the Philippines, Diliman, Quezon City 1101, the Philippines 3Clermont Universite´, Universite´ Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, France 4National Institute for Science and Mathematics Education Development, University of the Philippines, Diliman, Quezon City 1101, the Philippines *Corresponding author (e-mail: [email protected])

Abstract: The SW sector of , a potentially active volcano in the Bataan volcanic arc in western Luzon, is the site of a mothballed nuclear power plant that members of the national legislature have proposed to activate. Detailed geological fieldwork was conducted to assess the capability of the volcano and to identify any volcanic hazards it might pose to the nuclear plant. The nearest eruptive centre is 5.5 km away from the plant. SW Natib Volcano is underlain by lava flows, lahar deposits and at least six pyroclastic density current (PDC) deposits, three directly underlying the nuclear reactor facility. A fault trending N308E is aligned with the Lubao Fault, a capable fault NE of the volcanic edifice. Radon emissions at the traces of these faults are high and comparable to those at known active faults. An associated thrust fault at the nuclear site cuts through lahars up to the ground surface. The results presented here can be used for general hazard preparedness of local communities, and may assist the government to decide whether or not to recommission the nuclear power plant.

Natib Volcano is one of several calderagenic volca- Plant (BNPP). Construction began in 1976 and was noes comprising the Bataan volcanic arc (Fig. 1) in temporarily suspended in 1979, following the Three west Luzon, in the Philippines (Defant et al. 1988). Mile Island nuclear accident and a subsequent safety The most famous of these volcanoes is Pinatubo, inquiry into the plant. Construction was resumed which erupted in 1991 after 540 years of dormancy later but, before it was activated, the nuclear plant (Newhall & Punongbayan 1996). Reaching plume was mothballed in 1986. In 2008, 26 years later, heights of up to 35 km (Koyaguchi & Tokuno a Congressional Bill ‘mandating the immediate 1993), the Plinian eruption left 11 109 + recommissioning and commercial operation of the 0.5 109 m3 of tephra (Siebert & Simkin 2002) Bataan Nuclear Power Plant’ was filed (Cojuanco and a 2.5 km wide. To the south, separated 2008). from Pinatubo by about 17 km of intervening Ceno- When the BNPP was built in the late 1970s and zoic volcanic terrain, are the mountains of Natib early 1980s (Volentik et al. 2009), the planning of and , which together comprise the entire nuclear power plant facilities did not involve well- Bataan Peninsula. Natib and Mariveles are less established, internationally accepted guidelines to famous, but nonetheless equally impressive in set criteria and procedures for assessing potential their edifices, and have even larger . The volcanic hazards (McBirney et al. 2003). Permits largest of Natib’s two summit calderas has three for constructing the BNPP were granted based on times the diameter of Pinatubo’s, and that of Mari- investigations carried out according to local prac- veles is nearly 1.5 times larger. tices (EBASCO 1977, 1979; Newhall 1979) and Mount Natib is also well known locally because based on science that necessarily could not take its SW slope is the site of the Bataan Nuclear Power into account many relevant aspects of volcanology

From:Terry,J.P.&Goff, J. (eds) 2012. Natural Hazards in the Asia–Pacific Region: Recent Advances and Emerging Concepts. Geological Society, London, Special Publications, 361, 151–169, http://dx.doi.org/10.1144/SP361.13 # The Geological Society of London 2012. Publishing disclaimer: www.geolsoc.org.uk/pub_ethics Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

152 A. M. F. LAGMAY ET AL.

Fig. 1. The Bataan Arc, composed in part of Pinatubo, Natib and Mariveles volcanoes, and plots of seismic epicentres, focal mechanism solutions of earthquakes and lineaments. Data sources: Advanced National Seismic System Catalogue, Global Centroid Moment Tensor (Dziewonski & Gilbert 1976) and USGS Global Land Cover Facility (USGS 2004). that have rapidly developed only over the past Metro , and in view of the recent cataclysmic 30 years (Schmincke & Sumita 2008). eruption of Pinatubo only 60 km away, it is puzzling Even today, adequate geological maps of Natib that Natib remains so poorly understood. Part of Volcano do not exist, the same criticism of the the reason is the difficulties posed to geological original hazard assessment for the BNPP given mappers: the large size of the volcano, its steep by experts of the International Atomic Energy slopes, highly weathered exposures and dense veg- Agency (1978), volcanologist C. G. Newhall etation. Thus, our field data were gathered mainly (1979) and oversight panels in the Philippines. on Natib’s midslopes and footslopes during five Prior to the present report, the best available geo- field campaigns conducted from May 2009 to logical assessments of Natib Volcano were those January 2010, three of which were severely ham- of Almero (1989), Ruaya & Panem (1991) and pered by continuous heavy rain. Payot et al. (2008). However, Ruaya & Panem’s Nevertheless, the rapidity with which the legis- work, which delineates the summit caldera deposits, lation to activate the BNPP is proceeding necessi- springs and faults, was narrowly focused on the tated the improved understanding of the volcanic geothermal prospects of the Bataan volcanic arc. hazards that even a preliminary map of the For the purposes of the present report, the most sig- geology of the SW sector of the volcano and its nificant result of that work was to determine geo- stratigraphy could provide. The work was guided chemically the relationship to an active volcanic by the IAEA volcanic and seismic guidelines hydrothermal system of the water in some of the (IAEA 2002, 2003, 2005, 2009) and the recommen- 16 hot springs. The maps of Almero and Payot dations of Hill et al. (2009) for evaluating the vol- et al. are either too generalized or do not assess canic hazards at sites for nuclear installations. the volcanic hazards. Without detailed geological Although the work is still in progress, enough maps that identify the stratigraphy and distribution scientific data have been gathered to assist the of Natib’s eruptive products, the volcanic hazards Philippine government in deciding whether or not at the site cannot be assessed properly. to activate the BNPP, and to improve the general Considering the importance of this controversial hazard preparedness of the communities on the site for a nuclear power plant only 80 km away from volcano slopes. Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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Methodology of 6.3 MeV for 220Tn and 5.5 MeV for 222Rn (Sexton 1994; Papastefano 2002). Remote sensing and lineament analysis Ajari & Adepelumi (2002) and Burton et al. (2004) attributed the high content of these radon iso- Very near infrared (VNIR) images from the AVA topes in soils underlain by faults and fractures to ASTER (Advanced Spaceborne Thermal Emission increased surface-to-volume ratios in the fracturing and Reflection) archive (NASA 2009) were down- rock, and increased soil permeability, which facili- loaded and draped over an ASTER digital elevation tate radon release from the solid matrix. The short model (DEM) using ERDAS (Earth Resources Data half-lives of these isotopes require that measurable Analysis System) processing software. River drai- quantities must be escaping from free surfaces of nage patterns, lava ridges, levees, summit calderas the rock. and a flank eruptive centre were identified in the Radon gas was measured at flatland sites where three-dimensional (3D) images and aerial photo- lineament traces appear in the remotely sensed graphs. Lineaments were also delineated from the images. At discrete points along transect lines VNIR images, and from shaded relief and slope perpendicular to the lineaments, a soil probe was aspect maps derived from the DEM, to identify driven 0.4 m into the soil and connected to an target sites for structural mapping. TM RAD7 Durridge Co. portable radon detector. European Space Agency (ESA) Environmental Two 5 min readings were taken at each point. Con- Satellite (ENVISAT) Advanced Synthetic Aperture centrations were reported in Bq m23 units. Radon Radar (SAR) descending radar images were pro- background values also were measured at a quarry cessed using the Stanford Method for Persistent site 4 km north of the perimeter fence of the Scatterers-Multi-Temporal InSAR (STAMPS-MTI: nuclear power plant facility. Hooper 2006). Twenty-one time-series images from 19 March 2003 to 8 March 2006 were used in the persistent scatterer interferometry to evaluate Seismicity the ground movement of the area adjacent to the Lubao Fault. Earthquake hypocentres of the Bataan region for 1976 to the present were obtained online from the Advanced National Seismic System (ANSS), and Structural and lithological field mapping focal mechanism solutions from 1929 to the Before the fieldwork, sites of structural outcrops present from the Global Centroid Moment Tensor archives (Fig. 1). Earthquake plots were created were selected using the lineament map. In the TM field, the orientation of joint and fault structures using the Generic Mapping Tools (GMT ) soft- encountered at the target sites were measured, and ware (Wessel & Smith 1991). geometric and kinematic fabrics were recorded for microtectonic analysis. Thick vegetation and soil cover developed from moderate to extreme weather- Results ing of the deposits limited access to good rock Remote sensing/geomorphological and exposures, limiting most fieldwork to outcrops at quarries, coasts and road cuts. Where tephra depos- morphotectonic analyses its cropped out, slope faces were scraped cleaned The remotely sensed images and DEMs show that before examining and describing deposit sequences Natib’s summit, 1233 m above sea level, rises and lithologies. Mapping of outcrops with pyro- between two calderas. The largest is 7.5 5km2 clastic deposits and faults was carried out at a in plan (Fig. 2). East of it is a younger volcanic scale of 1:2500. cone with a smaller summit caldera measuring 2 1.8 km. Large channels occupy the eastern Radon measurements slopes of this younger volcanic cone, forming a pro- minent curved feature that resembles a landslide Two short-lived isotopes of radon gas have found scar. The southern half of the concavity has been useful application in evaluating active faults filled by a circular planform of rugged terrain. (Crenshaw et al. 1982). Radon 222 (222Rn) is gener- Several ridges originate from the western rim of ated naturally by the decay of 238U, and has a half- the larger caldera and extend towards the South life of only 3.8235 days; Radon 220, also called China Sea (Fig. 2). Along their axes, these ridges thoron (220Rn or 220Tn), with an even shorter half- are steepest near the Natib summit, their slope life of only 55.6 s, is the natural decay product of angles of about 308–408 decreasing to 08–158 232Th, the most stable thorium isotope (Holden as they reach a break in slope at approximately 2004). Both isotopes decay by emitting alpha radi- the 114 m elevation. Below this break, single ation, detectable by their unique emission energies ridges splay out towards the coast, with flatlands Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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Fig. 2. Morphological interpretation of the summit area and SW flanks of Mount Natib. Napot Point is the location of the Bataan Nuclear Power Plant (BNPP). occupying the spaces between them. At the coast, line-of-sight (LOS) of the radar signal by as much they terminate as headlands that form cliffs as as 2.5 cm year21. The eastern block, however, is high as 30 m. The BNPP is located in one of these characterized by an increase in LOS with a rate of headlands, named Napot Point. 22.5 cm year21. The change in LOS across the About 4.2 km SSW of the larger caldera rim Lubao Fault is most pronounced in transect 4 (Fig. 2), a high point 348 m in elevation protrudes (Fig. 4), 22 km from the base of Mount Natib. from the lower midslopes of the edifice. From this topographical high, finger-like ridges emanate and Geology reach the coast near Napot Point. A relatively smooth fan-like feature occupies most of the Field mapping of the SW sector of the Natib southern portion of the Natib edifice, terminating Volcano from 390 m elevations down to the coast where it meets the River at the base of revealed siltstone–sandstone beds, deposits of Mariveles Volcano. lahars, pyroclastic flows and surges, and columnar Closely spaced lineaments trend S308–358W jointed and autobrecciated lavas. These lithologies from the southern rim of the large caldera towards and their stratigraphy are described in this section the coast, a prominent one defining the SE coast of according to the areas in which they are exposed Napot Point (Fig. 3). An offshore extension is (Figs 2 & 5). expressed on bathymetric charts as a submarine scarp at least 10 km long (Fig. 3). Lingatin quarry. A quarry site adjacent to the Linga- The processing of persistent scatterers in the tin River south of Morong town proper exposed an 21 descending radar images reveal a sharp linear 11–12 m-thick sequence of at least five deposits. boundary of ground movement separating the The lowermost unit (NQPF1: Fig. 6a) is massive western and eastern blocks of the Lubao Fault and composed of poorly sorted lithic clasts in a (Fig. 4). Persistent scatterers in the western block light-brown clayey matrix. Ranging in size from 2 of the Lubao Fault show a decrease in the to 40 cm, the clasts are mostly andesitic, normally Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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Fig. 3. Lineament interpretation of Mount Natib based on satellite images and a bathymetric chart. The map shows the Lubao Fault, interpreted lineaments on the surface of Mount Natib’s edifice and offshore extension of the lineaments based on bathymetry. The two calderas of Mount Natib and the caldera of Mount Mariveles are outlined. graded and typically angular, although the larger the matrix and welding features indicate high- ones have been rounded by spheroidal weathering temperature emplacement. Angular–subangular and have rotten cores. A network of holes, com- polymictic lithic clasts, ranging in size from about monly with charred-grass stalks, distinguishes this 1 to 20 cm, along with mm-size crystals are dis- deposit, which is a block-and-ash deposit. persed throughout this unit, which is best interpreted NQPF1 is overlain by NQPF2, a 4 m-thick as a pyroclastic-flow deposit. deposit that tapers at the edges (Fig. 6a, b). Overlying NQPF2 in sharp contact, NQPF3 is a Massive and poorly sorted, it consists of devitrified reddish brown, massive, poorly sorted and clast- pumice lenses (fiammes) 5–10 cm long and 1–5 cm supported 4 m-thick deposit. The clasts are lithic thick, set in a pinkish-red ash matrix. Fewer welded- and angular–subangular, and range in diameter pumice fragments occur at the base but increase in from 5 to 20 cm. This unit is also interpreted as a abundance upwards. The pinkish-red colour of massive pyroclastic-flow deposit. Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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Fig. 4. Persistent scatterer interferometery of the NW flank of Mount Natib and the Lubao Fault. A notable change in line-of-sight (LOS) of the radar signal occurs at the boundary of the fault indicating differential movement of the western block relative to the eastern block. Transect A–A0 shows the most abrupt change in LOS across the Lubao Fault. The Y-axis corresponds to the change in LOS from March 2003 to March 2006. The centre of the X-axis in the cross-section is the approximate location of the Lubao Fault trace.

NQPF4, overlying NQPF3, is an approximately identified as a pyroclastic-surge deposit that grades 4 m-thick, massive, poorly sorted deposit composed into a more massive pyroclastic-flow unit. of lithic clasts and pumice fragments in a light- NQPF1 and NQPF2 also crop out in a smaller yellow, ashy matrix (Fig. 6c). Lithic clasts of vari- adjacent quarry, and NQPF4 is exposed in able composition range in size from 1 to 5 cm and 1.5 m-deep pits along the road between Lingatin are angular–subangular. Juvenile clasts are devitri- River and the BNPP site. Beside the Lingatin fied to white clay. NQPF4 is another distinct River, NQPF5 overlies a 3 m-thick autobrecciated pyroclastic-flow deposit. lava deposit. Overlying NQPF4 is NQPF5, a 3–4 m-thick sequence of reddish-brown parallel–subparallel Cabigo and Yala points. Thickly bedded, poorly layers that grade upwards into a more massive sorted deposits are exposed in outcrops as high as deposit (Fig. 6d). The reddish-brown ash layers 4–5 m along the coast of Cabigo Point. Variably contain lithic and pumice fragments that range in weathered, the clasts range in size from pebbles to size from 2 to 5 cm. Minute crystals are present in boulders, are rounded–subrounded, and are gener- the matrix. In the massive and poorly sorted ally polymictic but are mostly andesitic–basaltic. portion of this unit are angular–subangular lithic Clasts in each bed typically are supported in fragments, 8–10 cm in diameter, and 1–2 cm-size matrixes of sand, typically very coarse. Discernable pumice fragments that exhibit slight welding. A stratification is expressed in variable clast-size layer large brown rip-up clast about 6 m long and 2 m colours. Individual beds display normal grading thick containing a smaller chunk of soil within (Fig. 7a). These are typical lahar deposits. the massive portion of this unit indicates en masse In fault contact and interbedded with lahar transport of eroded fragments (Fig. 6d). NQPF5 is deposits is a 3 m-thick sequence of undulating and Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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Fig. 5. Stratigraphy of SW Natib based on detailed interpretations of outcrops at 1:2500. cross-bedded layers of tephra ranging in thickness the sediments were most probably deposited in a from 1 to 12 cm. The thicker beds containing low-energy, shallow-marine environment. Joints larger lithic clasts, which range in size from a few cut perpendicular to the strike of beds, displacing millimetres up to 6 cm (Fig. 7b). Matrixes are gen- laminations by about 1 cm in some places. erally composed of white ash containing millimetre- Indurated lahar deposits 5–8 m thick are the size crystals, and subangular clasts of pumice and most dominant rock type along the coast. They are lithic fragments. Pumice accumulations occur in massive to thickly bedded. Individual beds are some cross-bedded layers; other beds have reversely poorly sorted and composed of cobble- to boulder- graded lithic clasts. All of these features, along with sized rounded–subrounded polymictic lithic clasts. impact sags, are characteristic of pyroclastic-surge Bases are commonly clast-supported but gradually deposits. Similar but more massive white-coloured become matrix supported towards their tops. In one tephra deposits crop out further south along the outcrop, lahar beds exhibit normal grading. coast of Yala Point. These whitish pyroclastic- An approximately 15 m-thick tuffaceous out- flow deposits are overlain by a thick sequence of crop is exposed along a roadcut 200 m west of the lahar beds. BNPP office (Fig. 8b). The base of this outcrop is about 2 m thick, but only the upper part is well Napot Point. The rocks exposed in cliffs and islets exposed. It is composed of clast-supported grey– along the coast of Napot Point are indurated sands light grey subrounded pebble- to cobble-size poly- and silts, and lahar and pyroclastic-flow deposits. mictic lithic fragments. Medium–coarse sand Pyroclastic-flow deposits crop out within the comprises the matrix. Overlying the bottom unit BNPP site itself. The sedimentary sequence is com- is a 5 m-thick, yellowish-brown, poorly sorted, posed of several thick beds of brown–light-brown matrix-supported layer. Resembling NQPF4 of the and well-sorted sandstone separated by thin– quarry section, its polymictic clasts range in size medium interbeds of sandstone and siltstone from 10 to 30 cm. Above this deposit is a 3.5 m stra- (Fig. 8a). This sequence of beds generally thins tified sequence of angular–subangular pumice and upwards. Parallel laminations are also preserved lithic clasts in an ashy matrix. Pumice sizes ranges within the silty layers. These features indicate that from 1 to 2 cm, but the lithic clasts can be as large Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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Fig. 6. Deposits in the quarry near the Lingatin River. (a) The oldest pyroclastic-flow deposit in the Lingatin Quarry (NQPF1) is overlain by an NQPF2 lens and massive NQPF3 layer. (b) Welded pumice fragments in NQPF2. (c) Yellowish pyroclastic-flow deposit. (d) NQPF5 deposit with a rip-up pyroclastic-flow deposit megaclast that in turn contains ripped-up soil. as 15 cm. Individual strata range in thickness from forming headlands on the coast. One ridge also 10 cm to 1 m and vary in colour from yellowish extends NNE from the summit, forming a saddle tan to reddish orange. A whitish pumice-rich layer as it joins the slope of Mount Natib. 10 cm thick occurs in the upper-middle part of the Outcrops on the summit of this satellite cone are sequence. Pumice clasts, some subwelded, are indurated, dark grey, massive breccias consisting common in the reddish-orange tuffaceous layers of dominantly 1–8 cm-sized porphyritic (Fig. 8c, d), similar in appearance to unit NQPF5 clasts set in a coarse-grained brecciated andesitic of the quarry deposits. matrix. These massive breccias are exposed on a The topmost unit is a poorly sorted light-brown steep wall on one of the ridges, overlying what ashy layer containing angular–subangular lithic appears to be another massive layer composed of clasts 1–20 cm in size and subangular white clay poorly sorted brecciated material that was inaccess- particles 1 cm or less in diameter. It filled a ible for closer inspection. 0.4 m-wide channel and is about 4 m thick. All units in this outcrop, except for the basal layer, are Metro Highlands/Marucdoc. Columnar lava depos- interpreted as pyroclastic-flow deposits. its exposed on a steep slope at the side of one of the tributaries of the Marucdoc River and upstream of Peak to the south of Metro Subic Highlands Resort. the Metro Subic Highlands Resort (Fig. 9) are com- A 390 m-high volcanic edifice juts out of the SW posed of euhedral pyroxene and plagioclase laths in slope of the volcano about 5 km NE of Napot a fine-grained crystalline groundmass. Phenocryst Point (Fig. 2). Four elongated ridges extend radially sizes are 0.5–0.8 cm. Boulder-sized float of from the summit towards the south, SW and SE, similar petrology are abundant along the Marucdoc Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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et al. 2005). This NW-oriented fault follows the same trend as the Subic Bay Fault Zone interpreted from gravity and magnetic data by Yumul & Dima- lanta (1997), and appears to control the northern coast of Subic Bay. A marine seismic reflection survey in the bay (Cabato et al. 2005) identified the feature as a fault cutting across 18–8 ka marine sediments, from the inconsistent thicknesses of the packages they disrupt. The focal mechanism solution for a 5.5 Mw earthquake that occurred along this trend NW of Natib on 29 December 1982 is best interpreted as that of an oblique strike- slip fault (Fig. 1). A lineament NE of Natib Volcano separates the dry alluvial fans of the mountains between Natib and Pinatubo from the low-lying coastal wetlands NW of (Fig. 3). First described by Siringan & Rodolfo (2003), localized ground subsi- dence was attributed to vertical movements across this lineament. Soria (2009) formally named it the Lubao Lineament after the municipality where it is best expressed and argued that despite high sedi- mentation due to the Holocene eruptions of Mt Pinatubo, the wetland–dryland boundary has been maintained because it is an active fault. Soria (2009) estimated that vertical components of motion at the lineament have dropped the south- eastern block by as much as 3.5 m over the past 1.5 ka, based on palaeosea-level reconstructions Fig. 7. Deposits along the coast of Cabigo Point. (a) from a peat layer taken in Lubao. Preliminary Fractured lahar beds. (b) Pyroclastic-surge deposit results of the persistent scatter interferometry of showing undulating cross-bed structures, impact marks the Lubao area reveal differential ground move- and reverse grading. ment, with a linear boundary corresponding to the trace of the lineament. The name Lubao Fault is thus more appropriate based on evidence of move- River and on slopes of the resort up to the gate of the ment along the structure. US Geological Survey BNPP property. (USGS) epicentre data for Mw 3.6 earthquakes from 1973 to 2008 include several shallow events Bayandati River. Massive and autobrecciated lava that plot close to the fault (Fig. 1). deposits up to 5 m high and at least 50 m long The lineaments SW of Natib Volcano identified crop out along the banks of the Bayandati River. in the remotely sensed images are exposed as The massive but jointed lava is dark grey in faults at Cabigo and Napot points (Fig. 11a). At colour, and is composed of euhedral pyroxene and Cabigo Point, faults striking N208–308E and amphibole phenocrysts together with trachytic dipping 608–708SE truncate pyroclastic-surge plagioclase laths in a fine-grained crystalline deposits and bring them into contact with lahar groundmass. deposits (Fig. 11b). Approximately 500 m NE along the coast, about 20 similarly oriented fractures Structures cut indurated lahar deposits (Fig. 11b). At Napot Point, a cliff exposes indurated lahar deposits A fault that cuts northwestwards across Natib transected by faults that strike N138–338E and dip Volcano was delineated by Wolfe & Self in 1983 288–418NW. Fault displacements, drag folding from aerial images and topographic maps (Wolfe and rhomboid shear lenses along the fracture & Self 1983) (Fig. 10). The same fault was zones (Fig. 12) document thrust faulting. A scarp described in the environmental management report extends NE from the faulted outcrop at Napot for the PNOC geothermal exploration of Mount Point into the fenced BNPP perimeter. This Natib (PNOC 1988) and belongs to a set of subpar- feature may be the morphological expression of allel faults superimposed on the other structures of the faulted rocks and needs further investigation the Natib Volcano, including its caldera (Cabato through palaeoseismology (i.e. trenching studies). Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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Fig. 8. Deposits in the Napot Point area along the coast and within the BNPP site. (a) Sandstone–siltstone beds. (b) Alternating reddish-brown and yellowish-brown pyroclastic-flow deposits. (c) Baked contact of the upper layers of the pyroclastic-flow deposit. (d) Close-up view of the baked contact showing subwelded pumice clasts.

At SW Natib Point, values of 222Rn and 220Tn (2003) and Soria (2009). Both structures are colli- emitted from identified lineaments ranged from near and have the same orientation. 4000 to 23 000 and from 25 to 4000 Bq m23, respectively (Fig. 13). Values peaked at sharp changes in topography. For comparison, 222Rn Discussion emitted from unfractured outcrops in the Lingatin Volcanic hazard evaluation Quarry were only 3000–3200 Bq m23. The Radon gas emitted from the lineaments are comparable to The general procedure followed for the evaluation values of up to 30 000 Bq m23 that have varied of volcanic hazards for the BNPP was the methodo- only slightly (+1000 Bq m23) during repeated logical approach (Fig. 14) outlined in the draft measurements of the Western Marikina Valley guidelines for volcanic hazards in site evaluation Fault, a known active fault that displaces paved for nuclear installations (IAEA 2009). The approach roads in Pasig City, Metro Manila. involves four stages. Stage 1 is the initial assessment Following recent work describing faults that tra- of volcanism of less than 10 Ma in the region of the verse volcanoes (Lagmay et al. 2000; Wooler 2003; BNPP. As volcanism less than this age was ident- Palomo et al. 2004; Norini et al. 2008; Watt et al. ified for Pinatubo, Natib and Mariveles we pro- 2009; Tibaldi et al. 2010), the faults identified in ceeded to stage 2, which characterizes sources of the SW edifice of Natib are interpreted as the exten- volcanic activity as initiating events. Current volca- sion of the Lubao Fault of Siringan & Rodolfo nic activity is identified for Pinatubo Volcano, Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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Fig. 9. Columnar jointed lava deposit near Marucdoc River. (a) View of the outcrop in the field. (b) Close-up view of the lava deposit. which last erupted in 1990. Available age dates necessary and was determined using screening dis- for Natib Volcano are 0.069–1.6 Ma (EBASCO tance values (SDV), the maximum distance from 1977), 0.54–3.0 Ma (Wolfe 1983), 20–59 ka the source to the site at which each phenomenon (EBASCO 1977, 1979), 27 + 0.63 ka (Volentik could be a hazard (McBirney et al. 2003). Numeri- et al. 2009) and 11.3–18 ka (Cabato et al. 2005). cal simulations by Volentik et al. in 2009 and the Deposits from Mariveles Volcano have dates of tephra fall experienced at the site in 1991 demon- 0.19–4.1 Ma (Wolfe 1983) and as young as 5 ka strate that Pinatubo, Natib and Mariveles are volca- (Siebert & Simkin 2002). Because the potential for noes that produce hazards that are within screening future volcanic activity in the site region cannot distance values and, by definition, are capable vol- be ruled out, hazards screening in stage 3 was canoes (Volentik et al. 2009). A capable volcano Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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Fig. 10. Map showing the locus of volcanism along the Bataan Lineament (BL) and the trace of the Manila Fault (MF) according to Wolfe & Self (1983). or volcanic field is one that: (1) may experience vol- PDC deposits, and their content of welded pumice canic activity during the performance period of the fragments 5 cm in size and lithic clasts of up to nuclear installation; and (2) such an event has the 20 cm, indicate discrete large explosive events that potential to produce phenomena that may affect originated from a nearby source or sources. The the site of the nuclear installation (IAEA 2009). more likely candidates are Natib’s two calderas The classification of all three volcanoes as capable and Mariveles Volcano. Pinatubo is an unlikely prompted an evaluation of hazards at the BNPP source, being separated from the deposits by site outlined in stage 4 of the guidelines. 60 km of topographical barriers. In this work, the evaluation of hazards, develop- Lahar deposits atop, below and in fault contact ment of site-specific models and assessment of site with other volcanic deposits are widespread along suitability is based on the geology of SW Natib. the coastline of Napot Point. Frequent heavy rains The stage 4 assessment of volcanic hazards at the in this humid tropical region can easily remobilize BNPP site began with the identification of volcanic eruptive deposits on Natib’s edifice. Several lava deposits in the field area. As early as the late 1970s, flow ridges were identified, including an eruptive Newhall (1979) collectively described the deposits centre located 5.5 km away from the nuclear site. of lahars and at least six pyroclastic density currents After recognizing the deposits, it is necessary to (PDC) that underlie the SW sector of Natib as the determine probabilistically the potential impacts of ‘Napot Point tephra’. Four are massive; the other the volcanic processes that formed them should two are stratified. Erosional contacts between the future eruptions occur at the Natib or Mariveles Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

GEOLOGICAL HAZARDS OF SW NATIB VOLCANO 163

Fig. 11. Fractures measured along the Cabigo Point coast. (a) Stereoplot of fractures superimposed on the lineaments (red lines) identified from remotely sensed images. The black line refers to the radon survey transect. (b) Fault truncating pyroclastic-surge deposits and bringing them into contact with lahar deposits. volcanoes. This can be achieved with good control probabilities, together with Natib’s active volcanic on the age dates of eruptions, and stratigraphy to hydrothermal system (Ruaya & Panem 1991), determine the frequency and rate of volcanic means that Natib has credible potential for future activity. The probability of a future Natib eruption eruption. Volentik et al. (2009) estimated an even was calculated by Ebasco (1977) at 3 1025 higher probability for a VEI (Volcanic Explosivity year21 and to be an order of magnitude greater by Index) 6–7 eruption of Mariveles Volcano: Volentik et al. (2009) at 1 1024 –2 1024 3.5 1024 –6 1024 year21, with a 95% confi- year21, with a confidence level of 95%. These dence level. In some States a value for the annual

Fig. 12. Faults at Napot Point. (a) A 25 m-high outcrop of faulted indurated lahar deposits. (b) Truncated clasts with drag folding. (c) Rhomboid lenses along the shear plane. (d) Scarp extending in the NNE direction from the faulted outcrop into the BNPP fenced perimeter. Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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Fig. 13. Radon (222Rn) and thoron (220Tn) measurements traversing across identified lineaments (see Fig. 10a). Values of 222Rn peak at points near the lineaments and at sharp changes in relief.

Fig. 14. Methodological approach in determining site suitability of a nuclear power plant site (IAEA 2009). This approach was followed in this study. Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

GEOLOGICAL HAZARDS OF SW NATIB VOLCANO 165 probability of 1027 is used in the hazard assessment According to these guidelines, a ‘Yes’ in the site for external events as a reasonable basis to evaluate selection and evaluation column indicates that a sig- whether a volcano in the region could produce any nificant hazard from this phenomenon generally type of activity in the future that could lead to constitutes an exclusion criterion, and a ‘No’ in serious radiological consequences (IAEA 2009). the design column indicates that it is impractical Although the probability assessment in this study to mitigate a potential hazard by either facility can be improved with more age dates that would design or operational planning (IAEA 2009). better constrain recurrence rates, the study already In the case of BNPP, the area is underlain by provides a conservative probability estimate that deposits of pyroclastic flows and surges, and falls within the stated annual probability limit lahars. Lava deposits and an eruptive centre are observed in some States (IAEA 2009), which also proximal to the BNPP site. Of all these volcanic assumes that future eruptions at Mount Natib and phenomena, the potentials for pyroclastic density Mount Mariveles are possible. currents and lava flows cannot be mitigated by After establishing the probabilities of Natib and engineering solutions. Lahar hazards, however, Mariveles eruptions, the type of hazard responsible can be addressed by engineering design. for each deposit present is assigned a screening dis- tance value. Pyroclastic density currents generated Evaluation of seismic and tectonic hazards by VEI 6–7 eruptions can affect the BNPP site, based on numerical models (Volentik et al. 2009). The IAEA Safety Standard Series (IAEA 2003) The presence of such deposits at Napot Point vali- suggest relevant coverage areas for different levels dates the runout of pyroclastic flows predicted by of investigation for seismic hazard evaluation. the energy cone model of Sheridan (1979) and Typical radial extents are 150 km for regional inves- thereby puts the BNPP site well within the screening tigation, 25 km for near-regional investigation, distance value for this hazard (Volentik et al. 2009). 5 km for the site vicinity and 1 km for the site Lahar deposits are widespread along the coast- area. Any geological structure within these coverage lines of Napot, Cabigo and Emman points, but it is areas will have a corresponding impact on the unclear whether the BNPP site is vulnerable to nuclear power plant. The size, however, may vary future lahars. Napot Point and adjacent headlands depending on the geological and tectonic setting, may have been raised high enough by the volcanic and its shape may be asymmetric to include deposits to isolate them from lahar paths. At Pina- distant significant sources of earthquakes. tubo Volcano, however, lahars remobilize freshly One of the most important elements for evaluat- deposited tephra to completely inundate channels ing a nuclear plant site is surface faulting. Capable (Rodolfo et al. 1996), and lahars erode and form faults are structures that are most relevant when new channels at Volcano (Paguican et al. evaluating the geological features of the site. The 2009). Similarly, the Natib landscape could easily IAEA provides criteria in identifying whether a be altered, rendering obsolete any SDV analysis fault is capable or not (IAEA 2002). The first cri- for lahars based on present topography. With terion is that the fault shows evidence of significant regard to the hazard posed by lava flows, the pres- past deformations or movements of a recurring ence of an effusive eruptive centre 5.5 km away nature during a period that is recent enough to from the BNPP, and lava deposits only a few infer reasonably that further movements at or near hundred metres away, place the installation also the surface could occur. In tectonically active within the screening distance value for lava flows. areas like the Philippines, where both earthquake The foregoing probability analyses will bear data and geological data consistently reveal short uncertainties until the frequency and timing of the earthquake recurrence intervals, periods of the past Natib and Mariveles volcanoes are established order of tens of thousands of years may be appropri- more precisely. There is more certainty about the ate. The second criterion for a capable fault is a physical characteristics of those past events, such structural relationship with another known capable as their volumes and spatial extents. Thus, the volca- fault, such that movement at one may cause move- nic risk assessment for the BNPP leans more ment of the other at or near the surface. towards a deterministic analysis, focused on the Active faults determined by the Philippine Insti- geological characteristics of volcanic phenomena tute of Volcanology and Seismology (Phivolcs) and their spatial extent, rather than an estimation within the region of the BNPP are the Manila of the likelihood of the occurrence of such hazards Trench, East Zambales, Marikina Valley, Iba and (IAEA 2009). Lubang faults, with distances from the site of 140, To determine whether a site should be excluded 90, 82, 75 and 66 km, respectively. In Subic Bay, in the selection for a nuclear facility, the IAEA draft active faults were also identified by Cabato et al. guidelines present the different volcanic phenomena (2005) within 20 km of the BNPP site. The closest that may pose potential hazards to a site (Table 1). faults that have been identified are within 1 km of Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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Table 1. Volcanic phenomena and associated characteristics that could affect nuclear installations, with implications for site selection and evaluation, and design (IAEA 2009)

Phenomena Potentially adverse characteristics for nuclear Site Design/ installations selection operation

Tephra fall Static physical loads, abrasive and corrosive particles No Yes in air and water Pyroclastic density currents: Dynamic physical loads, atmospheric overpressures, Yes No Pyroclastic flows, surges projectile impacts, temperatures .300 8C, abrasive and blasts particles, toxic gases Lava flows and lava domes Dynamic physical loads, water impoundments and Yes No floods, temperatures .700 8C Debris avalanches, landslides Dynamic physical loads, atmospheric overpressures, Yes No and slope failures projectile impacts, water impoundments and floods Debris flows and lahars, floods Dynamic physical loads, water impoundments and Yes Yes floods, suspended particulates in water Opening of new vents Dynamic physical loads, ground deformation, Yes No volcanic earthquakes Ballistic projectiles Projectile impacts, static physical loads, abrasive No Yes particles in water Volcanic gases and aerosols Toxic and corrosive gases, water contamination, No Yes gas-charged lakes Tsunamis, seiches, crater lake Water inundation Yes Yes failure, glacial burst Atmospheric phenomena Dynamic overpressures, lightning strikes, downburst No Yes winds Ground deformation Ground displacements .1 m, landslides Yes No Volcanic earthquakes and Continuous tremor, multiple shocks usually ,M 5NoYes seismic events Hydrothermal systems and Thermal water .50 8C, corrosive water, water Yes No groundwater anomalies contamination, water inundation or upwelling, alteration, landslides

Italicized entries are the hazards pertinent to BNPP without design solutions. the BNPP, with one thrust fault cutting an outcrop at Capable faults are associated with earthquakes. the tip of Napot Point just outside the fenced per- A fault should be considered capable if the imeter of the installation, with its trace only 200 m maximum potential earthquake associated with it away from the nuclear reactor. Aside from very is sufficiently large and at a depth where it is reason- high radon measurements (Fig. 13), there is as yet able to infer that movement at or near the surface no direct evidence of active fault movement could occur (IAEA 2002). The length of the within 1 km of the BNPP because the faulted Lubao Fault, which according to the delineation of rocks have not been dated. The second criterion, Soria (2009) terminates NE of the footslopes of however, gives reason to believe that the faults Natib Volcano, is approximately 42 km. When within 1 km of the BNPP are active. The Lubao extended to the SW part of Natib’s edifice near Fault NE of Natib (Soria 2009) is active, and has Napot Point, the total length is 73 km. According the same orientation and is collinear with the to Wells & Coppersmith (1994), a 73 km fault faults in the SW sector of Natib (Fig. 3). length would be able to generate a magnitude 7.2 Many faults that traverse volcanoes have been (Mw) earthquake. The IAEA suggests that where reported in the literature (Wooler 2003; Palomo capable faults exist within 1 km of the nuclear facil- et al. 2004; Norini et al. 2008; Watt et al. 2009; ity, another site must be considered (IAEA 2005). Tibaldi et al. 2010). A classic example is Mayon Such is the case for the BNPP, where a capable Volcano, which is traversed by the northern bound- fault based on the second criterion of the guidelines ing fault of the Oas Graben (Lagmay et al. 2005; was identified within 1 km of the installation. Lagmay & Zebker 2009). Named the Northern Oas Fault, the surface trace of this structure ends abruptly at the western margin of Mayon and Conclusions re-emerges east of it, hidden by the active deposition of primary and reworked material on the conical Lavas, and the deposits of lahars, pyroclastic flows edifice (Lagmay et al. 2000). and pyroclastic surges, were mapped in the SW Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

GEOLOGICAL HAZARDS OF SW NATIB VOLCANO 167 sector of Natib Volcano. At least six pyroclastic However, the study already provides conserva- density current (PDC) deposits were mapped, tive probability estimates to the hazard’s assess- three directly underlying the nuclear reactor facility. ment. Assuming such probabilities are sufficient to Deposits of at least six pyroclastic density currents consider future eruptions as credible events, the were identified, with three of the units directly presence of at least three PDC deposits clearly underlying the site of the BNPP. A previously uni- show that pyroclastic flows are well within screen- dentified eruptive centre is located 5.5 km from ing distance and can affect the site. According to the main building of the plant. the IAEA draft guidelines, there is no engineering Faults oriented N208E–N308E along the coast of design that can address this type of hazard for a Cabigo Point extend towards the SSW offshore in nuclear power plant. bathymetric charts as a linear change in relief. The Enough data have been gathered to use as one of continuation of these faults into SW Natib can be the scientific bases for the decision of the Philippine traced from lineaments oriented N308E in ASTER government whether or not to activate the moth- images and aerial photographs. Radon emissions balled BNPP. These data will also be useful for at these lineaments are as high as 22 000 Bq m23, general hazard preparedness of communities on against background values of 2000–4000 Bq m23. the slopes of the volcano. A thrust fault at the tip of Napot Point cuts up to the ground surface through lahars. Natib is considered a capable volcano, based on References its active volcanic hydrothermal system and a calcu- Ajari,T.&Adepelumi, A. 2002. Reconnaissance soil– 24 24 21 lated probability of 1 10 –2 10 year , gas Radon survey over faulted crystalline area of with a 95% confidence level of a future VEI 6–7 ile-Ife, Nigeria. Environmental Geology, 41, 608–613. volcanic eruption. The volcanic hazards posed to Almero, R. 1989. Engineering Geology Aspects of the site were assessed based on IAEA draft guide- Characterization: Study for a Radioactive Waste Dis- lines. Among the hazards identified, lava flows posal Site in PNPP-1, Morong, Bataan Province. and pyroclastic density currents are within the Master’s thesis, University of the Philippines. Burton Neri Condarelli screening distance value, the maximum distance , M., ,M.& , D. 2004. High from the source to the site at which the volcanic spatial resolution radon measurements reveal hidden active faults on Mt. Etna. Geophysical Research phenomenon could be a hazard. Of all the volcanic Letters, 31, L07618–L07627. hazards, PDCs and lava flows do not have any Cabato, J. A., Rodolfo,K.S.&Siringan, F. 2005. engineering solutions. Lahar hazards, however, History of sedimentary infilling and faulting in Subic can be addressed by engineering design. Bay, Philippines revealed in high-resolution seismic Faults were mapped in the SW sector of Natib. reflection profiles. Journal of Asian Earth Sciences, One at Napot Point cuts up to the ground surface 25, 849–858. through an indurated lahar deposit. These tectonic Cojuanco, M. O. 2008. House Bill No. 4631: An Act Man- structures are evaluated as capable faults because dating the Immediate Re-commissioning and Commer- they show a structural relationship with the Lubao cial Operation of the Bataan Nuclear Power Plant, Appropriating Funds Therefore, and for Other Pur- Fault, which is considered active based on truncated poses. House of Representatives, Republic of the recent fluviodeltaic sediments and palaeosea-level Philippines, Quezon City, Metro Manila. reconstructions recording as much as 3.5 m move- Crenshaw, W. B., Williams,S.N.&Stoiber,R.E. ment over the past 1.5 ka. When evidence shows 1982. Fault location by radon and mercury detection the existence of capable faults within 1 km of the at an active volcano in Nicaragua. Nature, 300, nuclear facility, another site must be considered. 345–346. Such is the case for the BNPP, where capable faults Defant, M. J., Boer,J.Z.D.&Dietmar, O. 1988. The associated with the Lubao Fault were identified western volcanic arc, the Philippines: within 1 km of the nuclear power plant. two arcs divided by rifting? Tectonophysics, 145, 305–317. The work on Natib Volcano is still in progress Dziewonski,A.M.&Gilbert, F. 1976. The effect of and further characterization of the volcanic deposits small aspherical perturbations on travel times and a and faults is desired. The stratigraphy with corre- re-examination of the corrections for ellipticity. Geo- sponding age dates for each deposit will improve physical Journal of the Royal Astronomical Society, the understanding of eruption recurrence rates and 44, 7–17. probability estimates for future volcanic eruptions EBASCO 1977. Preliminary Safety Analysis Report, at Natib and Mariveles. Subsurface studies from Philippine Nuclear Power Plant #1. Technical report, numerous borehole data and geophysical surveys Philippine Atomic Energy Commission Open-File are also recommended to reduce uncertainties Report and response to questions. Philippine Atomic Energy Commission, Manila. in surface geological mapping of vegetated and EBASCO 1979. Evidence Substantiating the Incredibility highly weathered terrain. With regard to seismic of Volcanism on the West Flank of Mt. Natib, and the hazards, trenching studies of the faults within Assessment of Volcanic Hazards at Napot Point. 1 km of the BNPP is the next logical step. Response to Philippine Atomic Energy Commission Downloaded from http://sp.lyellcollection.org/ at Nanyang Technological University on June 13, 2012

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