geological hazards of sw natib volcano, site of the bataan … · 2015. 1. 8. · and mariveles,...

Geological Society, London, Special Publications

doi: 10.1144/SP361.13p151-169.

2012, v.361;Geological Society, London, Special Publications Obille, N. E. Pellejera, P. C. Francisco, R. N. Eco and J. AvisoAbon, C. Lit, M. R. T. Lapus, E. Paguican, M. G. Bato, G. Tiu, E. A. M. F. Lagmay, R. Rodolfo, H. Cabria, J. Soria, P. Zamora, C. Bataan Nuclear Power Plant, the PhilippinesGeological hazards of SW Natib Volcano, site of the

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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 Philippines2Marine Science Institute, College of Science, University of the Philippines, Diliman,

Quezon City 1101, the Philippines3Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448,

F-63000 Clermont-Ferrand, France4National 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 Mount Natib, a potentially active volcano in the Bataan volcanic arcin western Luzon, is the site of a mothballed nuclear power plant that members of the nationallegislature have proposed to activate. Detailed geological fieldwork was conducted to assess thecapability 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 bylava flows, lahar deposits and at least six pyroclastic density current (PDC) deposits, three directlyunderlying the nuclear reactor facility. A fault trending N308E is aligned with the Lubao Fault, acapable fault NE of the volcanic edifice. Radon emissions at the traces of these faults are high andcomparable to those at known active faults. An associated thrust fault at the nuclear site cutsthrough lahars up to the ground surface. The results presented here can be used for generalhazard preparedness of local communities, and may assist the government to decide whether ornot to recommission the nuclear power plant.

Natib Volcano is one of several calderagenic volca-noes comprising the Bataan volcanic arc (Fig. 1) inwest Luzon, in the Philippines (Defant et al. 1988).The most famous of these volcanoes is Pinatubo,which erupted in 1991 after 540 years of dormancy(Newhall & Punongbayan 1996). Reaching plumeheights of up to 35 km (Koyaguchi & Tokuno1993), the Plinian eruption left 11 � 109 +0.5 � 109 m3 of tephra (Siebert & Simkin 2002)and a caldera 2.5 km wide. To the south, separatedfrom Pinatubo by about 17 km of intervening Ceno-zoic volcanic terrain, are the mountains of Natiband Mariveles, which together comprise the entireBataan Peninsula. Natib and Mariveles are lessfamous, but nonetheless equally impressive intheir edifices, and have even larger calderas. Thelargest of Natib’s two summit calderas has threetimes the diameter of Pinatubo’s, and that of Mari-veles is nearly 1.5 times larger.

Mount Natib is also well known locally becauseits SW slope is the site of the Bataan Nuclear Power

Plant (BNPP). Construction began in 1976 and wastemporarily suspended in 1979, following the ThreeMile Island nuclear accident and a subsequent safetyinquiry into the plant. Construction was resumedlater but, before it was activated, the nuclear plantwas mothballed in 1986. In 2008, 26 years later,a Congressional Bill ‘mandating the immediaterecommissioning and commercial operation of theBataan Nuclear Power Plant’ was filed (Cojuanco2008).

When the BNPP was built in the late 1970s andearly 1980s (Volentik et al. 2009), the planning ofnuclear power plant facilities did not involve well-established, internationally accepted guidelines toset criteria and procedures for assessing potentialvolcanic hazards (McBirney et al. 2003). Permitsfor constructing the BNPP were granted based oninvestigations carried out according to local prac-tices (EBASCO 1977, 1979; Newhall 1979) andbased on science that necessarily could not takeinto account many relevant aspects of volcanology

From: Terry, J. P. & Goff, J. (eds) 2012. Natural Hazards in the Asia–Pacific Region: Recent Advances and EmergingConcepts. 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


http://www.geolsoc.org.uk/pub_ethicshttp://sp.lyellcollection.org/

that have rapidly developed only over the past30 years (Schmincke & Sumita 2008).

Even today, adequate geological maps of NatibVolcano do not exist, the same criticism of theoriginal hazard assessment for the BNPP givenby experts of the International Atomic EnergyAgency (1978), volcanologist C. G. Newhall(1979) and oversight panels in the Philippines.Prior to the present report, the best available geo-logical assessments of Natib Volcano were thoseof Almero (1989), Ruaya & Panem (1991) andPayot et al. (2008). However, Ruaya & Panem’swork, which delineates the summit caldera deposits,springs and faults, was narrowly focused on thegeothermal prospects of the Bataan volcanic arc.For the purposes of the present report, the most sig-nificant result of that work was to determine geo-chemically the relationship to an active volcanichydrothermal system of the water in some of the16 hot springs. The maps of Almero and Payotet al. are either too generalized or do not assessthe volcanic hazards. Without detailed geologicalmaps that identify the stratigraphy and distributionof Natib’s eruptive products, the volcanic hazardsat the site cannot be assessed properly.

Considering the importance of this controversialsite for a nuclear power plant only 80 km away from

Metro Manila, and in view of the recent cataclysmiceruption of Pinatubo only 60 km away, it is puzzlingthat Natib remains so poorly understood. Part ofthe reason is the difficulties posed to geologicalmappers: the large size of the volcano, its steepslopes, highly weathered exposures and dense veg-etation. Thus, our field data were gathered mainlyon Natib’s midslopes and footslopes during fivefield campaigns conducted from May 2009 toJanuary 2010, three of which were severely ham-pered by continuous heavy rain.

Nevertheless, the rapidity with which the legis-lation to activate the BNPP is proceeding necessi-tated the improved understanding of the volcanichazards that even a preliminary map of thegeology of the SW sector of the volcano and itsstratigraphy could provide. The work was guidedby the IAEA volcanic and seismic guidelines(IAEA 2002, 2003, 2005, 2009) and the recommen-dations of Hill et al. (2009) for evaluating the vol-canic hazards at sites for nuclear installations.Although the work is still in progress, enoughscientific data have been gathered to assist thePhilippine government in deciding whether or notto activate the BNPP, and to improve the generalhazard preparedness of the communities on thevolcano slopes.

Fig. 1. The Bataan Arc, composed in part of Pinatubo, Natib and Mariveles volcanoes, and plots of seismicepicentres, focal mechanism solutions of earthquakes and lineaments. Data sources: Advanced National Seismic SystemCatalogue, Global Centroid Moment Tensor (Dziewonski & Gilbert 1976) and USGS Global Land Cover Facility(USGS 2004).

A. M. F. LAGMAY ET AL.152


http://sp.lyellcollection.org/

Methodology

Remote sensing and lineament analysis

Very near infrared (VNIR) images from the AVAASTER (Advanced Spaceborne Thermal Emissionand Reflection) archive (NASA 2009) were down-loaded and draped over an ASTER digital elevationmodel (DEM) using ERDAS (Earth Resources DataAnalysis System) processing software. River drai-nage patterns, lava ridges, levees, summit calderasand a flank eruptive centre were identified in thethree-dimensional (3D) images and aerial photo-graphs. Lineaments were also delineated from theVNIR images, and from shaded relief and slopeaspect maps derived from the DEM, to identifytarget sites for structural mapping.

European Space Agency (ESA) EnvironmentalSatellite (ENVISAT) Advanced Synthetic ApertureRadar (SAR) descending radar images were pro-cessed using the Stanford Method for PersistentScatterers-Multi-Temporal InSAR (STAMPS-MTI:Hooper 2006). Twenty-one time-series images from19 March 2003 to 8 March 2006 were used inthe persistent scatterer interferometry to evaluatethe ground movement of the area adjacent to theLubao Fault.

Structural and lithological field mapping

Before the fieldwork, sites of structural outcropswere selected using the lineament map. In thefield, the orientation of joint and fault structuresencountered at the target sites were measured, andgeometric and kinematic fabrics were recorded formicrotectonic analysis. Thick vegetation and soilcover developed from moderate to extreme weather-ing of the deposits limited access to good rockexposures, limiting most fieldwork to outcrops atquarries, coasts and road cuts. Where tephra depos-its cropped out, slope faces were scraped cleanedbefore examining and describing deposit sequencesand lithologies. Mapping of outcrops with pyro-clastic deposits and faults was carried out at ascale of 1:2500.

Radon measurements

Two short-lived isotopes of radon gas have founduseful application in evaluating active faults(Crenshaw et al. 1982). Radon 222 (222Rn) is gener-ated naturally by the decay of 238U, and has a half-life of only 3.8235 days; Radon 220, also calledthoron (220Rn or 220Tn), with an even shorter half-life of only 55.6 s, is the natural decay product of232Th, the most stable thorium isotope (Holden2004). Both isotopes decay by emitting alpha radi-ation, detectable by their unique emission energies

of 6.3 MeV for 220Tn and 5.5 MeV for 222Rn(Sexton 1994; Papastefano 2002).

Ajari & Adepelumi (2002) and Burton et al.(2004) attributed the high content of these radon iso-topes in soils underlain by faults and fractures toincreased surface-to-volume ratios in the fracturingrock, and increased soil permeability, which facili-tate radon release from the solid matrix. The shorthalf-lives of these isotopes require that measurablequantities must be escaping from free surfaces ofthe rock.

Radon gas was measured at flatland sites wherelineament traces appear in the remotely sensedimages. At discrete points along transect linesperpendicular to the lineaments, a soil probe wasdriven 0.4 m into the soil and connected to anRAD7

TM

Durridge Co. portable radon detector.Two 5 min readings were taken at each point. Con-centrations were reported in Bq m23 units. Radonbackground values also were measured at a quarrysite 4 km north of the perimeter fence of thenuclear power plant facility.

Seismicity

Earthquake hypocentres of the Bataan region for1976 to the present were obtained online from theAdvanced National Seismic System (ANSS), andfocal mechanism solutions from 1929 to thepresent from the Global Centroid Moment Tensorarchives (Fig. 1). Earthquake plots were createdusing the Generic Mapping Tools (GMT

TM

) soft-ware (Wessel & Smith 1991).

Results

Remote sensing/geomorphological and

morphotectonic analyses

The remotely sensed images and DEMs show thatNatib’s summit, 1233 m above sea level, risesbetween two calderas. The largest is 7.5 � 5 km2in plan (Fig. 2). East of it is a younger volcaniccone with a smaller summit caldera measuring2 � 1.8 km. Large channels occupy the easternslopes of this younger volcanic cone, forming a pro-minent curved feature that resembles a landslidescar. The southern half of the concavity has beenfilled by a circular planform of rugged terrain.

Several ridges originate from the western rim ofthe larger caldera and extend towards the SouthChina Sea (Fig. 2). Along their axes, these ridgesare steepest near the Natib summit, their slopeangles of about 308–408 decreasing to 08–158as they reach a break in slope at approximatelythe 114 m elevation. Below this break, singleridges splay out towards the coast, with flatlands

GEOLOGICAL HAZARDS OF SW NATIB VOLCANO 153



occupying the spaces between them. At the coast,they terminate as headlands that form cliffs ashigh as 30 m. The BNPP is located in one of theseheadlands, named Napot Point.

About 4.2 km SSW of the larger caldera rim(Fig. 2), a high point 348 m in elevation protrudesfrom the lower midslopes of the edifice. From thistopographical high, finger-like ridges emanate andreach the coast near Napot Point. A relativelysmooth fan-like feature occupies most of thesouthern portion of the Natib edifice, terminatingwhere it meets the Bagac River at the base ofMariveles Volcano.

Closely spaced lineaments trend S308–358Wfrom the southern rim of the large caldera towardsthe coast, a prominent one defining the SE coast ofNapot Point (Fig. 3). An offshore extension isexpressed on bathymetric charts as a submarinescarp at least 10 km long (Fig. 3).

The processing of persistent scatterers in the21 descending radar images reveal a sharp linearboundary of ground movement separating thewestern and eastern blocks of the Lubao Fault(Fig. 4). Persistent scatterers in the western blockof the Lubao Fault show a decrease in the

line-of-sight (LOS) of the radar signal by as muchas 2.5 cm year21. The eastern block, however, ischaracterized by an increase in LOS with a rate of22.5 cm year21. The change in LOS across theLubao Fault is most pronounced in transect 4(Fig. 4), 22 km from the base of Mount Natib.

Geology

Field mapping of the SW sector of the NatibVolcano from 390 m elevations down to the coastrevealed siltstone–sandstone beds, deposits oflahars, pyroclastic flows and surges, and columnarjointed and autobrecciated lavas. These lithologiesand their stratigraphy are described in this sectionaccording to the areas in which they are exposed(Figs 2 & 5).

Lingatin quarry. A quarry site adjacent to the Linga-tin River south of Morong town proper exposed an11–12 m-thick sequence of at least five deposits.The lowermost unit (NQPF1: Fig. 6a) is massiveand composed of poorly sorted lithic clasts in alight-brown clayey matrix. Ranging in size from 2to 40 cm, the clasts are mostly andesitic, normally

Fig. 2. Morphological interpretation of the summit area and SW flanks of Mount Natib. Napot Point is the locationof the Bataan Nuclear Power Plant (BNPP).




graded and typically angular, although the largerones have been rounded by spheroidal weatheringand have rotten cores. A network of holes, com-monly with charred-grass stalks, distinguishes thisdeposit, which is a block-and-ash deposit.

NQPF1 is overlain by NQPF2, a 4 m-thickdeposit that tapers at the edges (Fig. 6a, b).Massive and poorly sorted, it consists of devitrifiedpumice lenses (fiammes) 5–10 cm long and 1–5 cmthick, set in a pinkish-red ash matrix. Fewer welded-pumice fragments occur at the base but increase inabundance upwards. The pinkish-red colour of

the matrix and welding features indicate high-temperature emplacement. Angular–subangularpolymictic lithic clasts, ranging in size from about1 to 20 cm, along with mm-size crystals are dis-persed throughout this unit, which is best interpretedas a pyroclastic-flow deposit.

Overlying NQPF2 in sharp contact, NQPF3 is areddish brown, massive, poorly sorted and clast-supported 4 m-thick deposit. The clasts are lithicand angular–subangular, and range in diameterfrom 5 to 20 cm. This unit is also interpreted as amassive pyroclastic-flow deposit.

Fig. 3. Lineament interpretation of Mount Natib based on satellite images and a bathymetric chart. The map shows theLubao Fault, interpreted lineaments on the surface of Mount Natib’s edifice and offshore extension of the lineamentsbased on bathymetry. The two calderas of Mount Natib and the caldera of Mount Mariveles are outlined.




NQPF4, overlying NQPF3, is an approximately4 m-thick, massive, poorly sorted deposit composedof lithic clasts and pumice fragments in a light-yellow, ashy matrix (Fig. 6c). Lithic clasts of vari-able composition range in size from 1 to 5 cm andare angular–subangular. Juvenile clasts are devitri-fied to white clay. NQPF4 is another distinctpyroclastic-flow deposit.

Overlying NQPF4 is NQPF5, a 3–4 m-thicksequence of reddish-brown parallel–subparallellayers that grade upwards into a more massivedeposit (Fig. 6d). The reddish-brown ash layerscontain lithic and pumice fragments that range insize from 2 to 5 cm. Minute crystals are present inthe matrix. In the massive and poorly sortedportion of this unit are angular–subangular lithicfragments, 8–10 cm in diameter, and 1–2 cm-sizepumice fragments that exhibit slight welding. Alarge brown rip-up clast about 6 m long and 2 mthick containing a smaller chunk of soil withinthe massive portion of this unit indicates en massetransport of eroded fragments (Fig. 6d). NQPF5 is

identified as a pyroclastic-surge deposit that gradesinto a more massive pyroclastic-flow unit.

NQPF1 and NQPF2 also crop out in a smalleradjacent quarry, and NQPF4 is exposed in1.5 m-deep pits along the road between LingatinRiver and the BNPP site. Beside the LingatinRiver, NQPF5 overlies a 3 m-thick autobrecciatedlava deposit.

Cabigo and Yala points. Thickly bedded, poorlysorted deposits are exposed in outcrops as high as4–5 m along the coast of Cabigo Point. Variablyweathered, the clasts range in size from pebbles toboulders, are rounded–subrounded, and are gener-ally polymictic but are mostly andesitic–basaltic.Clasts in each bed typically are supported inmatrixes of sand, typically very coarse. Discernablestratification is expressed in variable clast-size layercolours. Individual beds display normal grading(Fig. 7a). These are typical lahar deposits.

In fault contact and interbedded with lahardeposits is a 3 m-thick sequence of undulating and

Fig. 4. Persistent scatterer interferometery of the NW flank of Mount Natib and the Lubao Fault. A notable change inline-of-sight (LOS) of the radar signal occurs at the boundary of the fault indicating differential movement of thewestern 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 thecross-section is the approximate location of the Lubao Fault trace.




cross-bedded layers of tephra ranging in thicknessfrom 1 to 12 cm. The thicker beds containinglarger lithic clasts, which range in size from a fewmillimetres up to 6 cm (Fig. 7b). Matrixes are gen-erally composed of white ash containing millimetre-size crystals, and subangular clasts of pumice andlithic fragments. Pumice accumulations occur insome cross-bedded layers; other beds have reverselygraded lithic clasts. All of these features, along withimpact sags, are characteristic of pyroclastic-surgedeposits. Similar but more massive white-colouredtephra deposits crop out further south along thecoast of Yala Point. These whitish pyroclastic-flow deposits are overlain by a thick sequence oflahar beds.

Napot Point. The rocks exposed in cliffs and isletsalong the coast of Napot Point are indurated sandsand silts, and lahar and pyroclastic-flow deposits.Pyroclastic-flow deposits crop out within theBNPP site itself. The sedimentary sequence is com-posed of several thick beds of brown–light-brownand well-sorted sandstone separated by thin–medium interbeds of sandstone and siltstone(Fig. 8a). This sequence of beds generally thinsupwards. Parallel laminations are also preservedwithin the silty layers. These features indicate that

the sediments were most probably deposited in alow-energy, shallow-marine environment. Jointscut perpendicular to the strike of beds, displacinglaminations by about 1 cm in some places.

Indurated lahar deposits 5–8 m thick are themost dominant rock type along the coast. They aremassive to thickly bedded. Individual beds arepoorly sorted and composed of cobble- to boulder-sized rounded–subrounded polymictic lithic clasts.Bases are commonly clast-supported but graduallybecome matrix supported towards their tops. In oneoutcrop, lahar beds exhibit normal grading.

An approximately 15 m-thick tuffaceous out-crop is exposed along a roadcut 200 m west of theBNPP office (Fig. 8b). The base of this outcrop isabout 2 m thick, but only the upper part is wellexposed. It is composed of clast-supported grey–light grey subrounded pebble- to cobble-size poly-mictic lithic fragments. Medium–coarse sandcomprises the matrix. Overlying the bottom unitis a 5 m-thick, yellowish-brown, poorly sorted,matrix-supported layer. Resembling NQPF4 of thequarry section, its polymictic clasts range in sizefrom 10 to 30 cm. Above this deposit is a 3.5 m stra-tified sequence of angular–subangular pumice andlithic clasts in an ashy matrix. Pumice sizes rangesfrom 1 to 2 cm, but the lithic clasts can be as large

Fig. 5. Stratigraphy of SW Natib based on detailed interpretations of outcrops at 1:2500.




as 15 cm. Individual strata range in thickness from10 cm to 1 m and vary in colour from yellowishtan to reddish orange. A whitish pumice-rich layer10 cm thick occurs in the upper-middle part of thesequence. Pumice clasts, some subwelded, arecommon in the reddish-orange tuffaceous layers(Fig. 8c, d), similar in appearance to unit NQPF5of the quarry deposits.

The topmost unit is a poorly sorted light-brownashy layer containing angular–subangular lithicclasts 1–20 cm in size and subangular white clayparticles 1 cm or less in diameter. It filled a0.4 m-wide channel and is about 4 m thick. Allunits in this outcrop, except for the basal layer, areinterpreted as pyroclastic-flow deposits.

Peak to the south of Metro Subic Highlands Resort.A 390 m-high volcanic edifice juts out of the SWslope of the volcano about 5 km NE of NapotPoint (Fig. 2). Four elongated ridges extend radiallyfrom the summit towards the south, SW and SE,

forming headlands on the coast. One ridge alsoextends NNE from the summit, forming a saddleas it joins the slope of Mount Natib.

Outcrops on the summit of this satellite cone areindurated, dark grey, massive breccias consistingof dominantly 1–8 cm-sized porphyritic andesiteclasts set in a coarse-grained brecciated andesiticmatrix. These massive breccias are exposed on asteep wall on one of the ridges, overlying whatappears to be another massive layer composed ofpoorly sorted brecciated material that was inaccess-ible for closer inspection.

Metro Highlands/Marucdoc. Columnar lava depos-its exposed on a steep slope at the side of one of thetributaries of the Marucdoc River and upstream ofthe Metro Subic Highlands Resort (Fig. 9) are com-posed of euhedral pyroxene and plagioclase laths ina fine-grained crystalline groundmass. Phenocrystsizes are 0.5–0.8 cm. Boulder-sized float ofsimilar petrology are abundant along the Marucdoc

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 turncontains ripped-up soil.




River and on slopes of the resort up to the gate of theBNPP property.

Bayandati River. Massive and autobrecciated lavadeposits up to 5 m high and at least 50 m longcrop out along the banks of the Bayandati River.The massive but jointed lava is dark grey incolour, and is composed of euhedral pyroxene andamphibole phenocrysts together with trachyticplagioclase laths in a fine-grained crystallinegroundmass.

Structures

A fault that cuts northwestwards across NatibVolcano was delineated by Wolfe & Self in 1983from aerial images and topographic maps (Wolfe& Self 1983) (Fig. 10). The same fault wasdescribed in the environmental management reportfor the PNOC geothermal exploration of MountNatib (PNOC 1988) and belongs to a set of subpar-allel faults superimposed on the other structures ofthe Natib Volcano, including its caldera (Cabato

et al. 2005). This NW-oriented fault follows thesame trend as the Subic Bay Fault Zone interpretedfrom gravity and magnetic data by Yumul & Dima-lanta (1997), and appears to control the northerncoast of Subic Bay. A marine seismic reflectionsurvey in the bay (Cabato et al. 2005) identifiedthe feature as a fault cutting across 18–8 kamarine sediments, from the inconsistent thicknessesof the packages they disrupt. The focal mechanismsolution for a 5.5 Mw earthquake that occurredalong this trend NW of Natib on 29 December1982 is best interpreted as that of an oblique strike-slip fault (Fig. 1).

A lineament NE of Natib Volcano separates thedry alluvial fans of the mountains between Natiband Pinatubo from the low-lying coastal wetlandsNW of Manila Bay (Fig. 3). First described bySiringan & Rodolfo (2003), localized ground subsi-dence was attributed to vertical movements acrossthis lineament. Soria (2009) formally named it theLubao Lineament after the municipality where it isbest expressed and argued that despite high sedi-mentation due to the Holocene eruptions of MtPinatubo, the wetland–dryland boundary has beenmaintained because it is an active fault. Soria(2009) estimated that vertical components ofmotion at the lineament have dropped the south-eastern block by as much as 3.5 m over the past1.5 ka, based on palaeosea-level reconstructionsfrom a peat layer taken in Lubao. Preliminaryresults of the persistent scatter interferometry ofthe Lubao area reveal differential ground move-ment, with a linear boundary corresponding to thetrace of the lineament. The name Lubao Fault isthus more appropriate based on evidence of move-ment along the structure. US Geological Survey(USGS) epicentre data for Mw 3.6 earthquakesfrom 1973 to 2008 include several shallow eventsthat plot close to the fault (Fig. 1).

The lineaments SW of Natib Volcano identifiedin the remotely sensed images are exposed asfaults at Cabigo and Napot points (Fig. 11a). AtCabigo Point, faults striking N208–308E anddipping 608–708SE truncate pyroclastic-surgedeposits and bring them into contact with lahardeposits (Fig. 11b). Approximately 500 m NEalong the coast, about 20 similarly oriented fracturescut indurated lahar deposits (Fig. 11b). At NapotPoint, a cliff exposes indurated lahar depositstransected by faults that strike N138–338E and dip288–418NW. Fault displacements, drag foldingand rhomboid shear lenses along the fracturezones (Fig. 12) document thrust faulting. A scarpextends NE from the faulted outcrop at NapotPoint into the fenced BNPP perimeter. Thisfeature may be the morphological expression ofthe faulted rocks and needs further investigationthrough palaeoseismology (i.e. trenching studies).

Fig. 7. Deposits along the coast of Cabigo Point. (a)Fractured lahar beds. (b) Pyroclastic-surge depositshowing undulating cross-bed structures, impact marksand reverse grading.




At SW Natib Point, values of 222Rn and 220Tnemitted from identified lineaments ranged from4000 to 23 000 and from 25 to 4000 Bq m23,respectively (Fig. 13). Values peaked at sharpchanges in topography. For comparison, 222Rnemitted from unfractured outcrops in the LingatinQuarry were only 3000–3200 Bq m23. The Radongas emitted from the lineaments are comparable tovalues of up to 30 000 Bq m23 that have variedonly slightly (+1000 Bq m23) during repeatedmeasurements of the Western Marikina ValleyFault, a known active fault that displaces pavedroads in Pasig City, Metro Manila.

Following recent work describing faults that tra-verse volcanoes (Lagmay et al. 2000; Wooler 2003;Palomo et al. 2004; Norini et al. 2008; Watt et al.2009; Tibaldi et al. 2010), the faults identified inthe SW edifice of Natib are interpreted as the exten-sion of the Lubao Fault of Siringan & Rodolfo

(2003) and Soria (2009). Both structures are colli-near and have the same orientation.

Discussion

Volcanic hazard evaluation

The general procedure followed for the evaluationof volcanic hazards for the BNPP was the methodo-logical approach (Fig. 14) outlined in the draftguidelines for volcanic hazards in site evaluationfor nuclear installations (IAEA 2009). The approachinvolves four stages. Stage 1 is the initial assessmentof volcanism of less than 10 Ma in the region of theBNPP. As volcanism less than this age was ident-ified for Pinatubo, Natib and Mariveles we pro-ceeded to stage 2, which characterizes sources ofvolcanic activity as initiating events. Current volca-nic activity is identified for Pinatubo Volcano,

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 thepyroclastic-flow deposit. (d) Close-up view of the baked contact showing subwelded pumice clasts.




which last erupted in 1990. Available age datesfor Natib Volcano are 0.069–1.6 Ma (EBASCO1977), 0.54–3.0 Ma (Wolfe 1983), 20–59 ka(EBASCO 1977, 1979), 27 + 0.63 ka (Volentiket al. 2009) and 11.3–18 ka (Cabato et al. 2005).Deposits from Mariveles Volcano have dates of0.19–4.1 Ma (Wolfe 1983) and as young as 5 ka(Siebert & Simkin 2002). Because the potential forfuture volcanic activity in the site region cannotbe ruled out, hazards screening in stage 3 was

necessary and was determined using screening dis-tance values (SDV), the maximum distance fromthe source to the site at which each phenomenoncould be a hazard (McBirney et al. 2003). Numeri-cal simulations by Volentik et al. in 2009 and thetephra fall experienced at the site in 1991 demon-strate that Pinatubo, Natib and Mariveles are volca-noes that produce hazards that are within screeningdistance values and, by definition, are capable vol-canoes (Volentik et al. 2009). A capable volcano

Fig. 9. Columnar jointed lava deposit near Marucdoc River. (a) View of the outcrop in the field. (b) Close-up viewof the lava deposit.




or volcanic field is one that: (1) may experience vol-canic activity during the performance period of thenuclear installation; and (2) such an event has thepotential to produce phenomena that may affectthe site of the nuclear installation (IAEA 2009).The classification of all three volcanoes as capableprompted an evaluation of hazards at the BNPPsite outlined in stage 4 of the guidelines.

In this work, the evaluation of hazards, develop-ment of site-specific models and assessment of sitesuitability is based on the geology of SW Natib.The stage 4 assessment of volcanic hazards at theBNPP site began with the identification of volcanicdeposits in the field area. As early as the late 1970s,Newhall (1979) collectively described the depositsof lahars and at least six pyroclastic density currents(PDC) that underlie the SW sector of Natib as the‘Napot Point tephra’. Four are massive; the othertwo are stratified. Erosional contacts between the

PDC deposits, and their content of welded pumicefragments 5 cm in size and lithic clasts of up to20 cm, indicate discrete large explosive events thatoriginated from a nearby source or sources. Themore likely candidates are Natib’s two calderasand Mariveles Volcano. Pinatubo is an unlikelysource, being separated from the deposits by60 km of topographical barriers.

Lahar deposits atop, below and in fault contactwith other volcanic deposits are widespread alongthe coastline of Napot Point. Frequent heavy rainsin this humid tropical region can easily remobilizeeruptive deposits on Natib’s edifice. Several lavaflow ridges were identified, including an eruptivecentre located 5.5 km away from the nuclear site.

After recognizing the deposits, it is necessary todetermine probabilistically the potential impacts ofthe volcanic processes that formed them shouldfuture eruptions occur at the Natib or Mariveles

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).




volcanoes. This can be achieved with good controlon the age dates of eruptions, and stratigraphy todetermine the frequency and rate of volcanicactivity. The probability of a future Natib eruptionwas calculated by Ebasco (1977) at 3 � 1025year21 and to be an order of magnitude greater byVolentik et al. (2009) at 1 � 1024–2 � 1024year21, with a confidence level of 95%. These

probabilities, together with Natib’s active volcanichydrothermal system (Ruaya & Panem 1991),means that Natib has credible potential for futureeruption. Volentik et al. (2009) estimated an evenhigher probability for a VEI (Volcanic ExplosivityIndex) 6–7 eruption of Mariveles Volcano:3.5 � 1024–6 � 1024 year21, with a 95% confi-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 withdrag folding. (c) Rhomboid lenses along the shear plane. (d) Scarp extending in the NNE direction from the faultedoutcrop into the BNPP fenced perimeter.

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 truncatingpyroclastic-surge deposits and bringing them into contact with lahar deposits.




Fig. 13. Radon (222Rn) and thoron (220Tn) measurements traversing across identified lineaments (see Fig. 10a). Valuesof 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). Thisapproach was followed in this study.




probability of 1027 is used in the hazard assessmentfor external events as a reasonable basis to evaluatewhether a volcano in the region could produce anytype of activity in the future that could lead toserious radiological consequences (IAEA 2009).Although the probability assessment in this studycan be improved with more age dates that wouldbetter constrain recurrence rates, the study alreadyprovides a conservative probability estimate thatfalls within the stated annual probability limitobserved in some States (IAEA 2009), whichassumes that future eruptions at Mount Natib andMount Mariveles are possible.

After establishing the probabilities of Natib andMariveles eruptions, the type of hazard responsiblefor each deposit present is assigned a screening dis-tance value. Pyroclastic density currents generatedby VEI 6–7 eruptions can affect the BNPP site,based on numerical models (Volentik et al. 2009).The presence of such deposits at Napot Point vali-dates the runout of pyroclastic flows predicted bythe energy cone model of Sheridan (1979) andthereby puts the BNPP site well within the screeningdistance value for this hazard (Volentik et al. 2009).

Lahar deposits are widespread along the coast-lines of Napot, Cabigo and Emman points, but it isunclear whether the BNPP site is vulnerable tofuture lahars. Napot Point and adjacent headlandsmay have been raised high enough by the volcanicdeposits to isolate them from lahar paths. At Pina-tubo Volcano, however, lahars remobilize freshlydeposited tephra to completely inundate channels(Rodolfo et al. 1996), and lahars erode and formnew channels at Mayon Volcano (Paguican et al.2009). Similarly, the Natib landscape could easilybe altered, rendering obsolete any SDV analysisfor lahars based on present topography. Withregard to the hazard posed by lava flows, the pres-ence of an effusive eruptive centre 5.5 km awayfrom the BNPP, and lava deposits only a fewhundred metres away, place the installation alsowithin the screening distance value for lava flows.

The foregoing probability analyses will bearuncertainties until the frequency and timing of thepast Natib and Mariveles volcanoes are establishedmore precisely. There is more certainty about thephysical characteristics of those past events, suchas their volumes and spatial extents. Thus, the volca-nic risk assessment for the BNPP leans moretowards a deterministic analysis, focused on thegeological characteristics of volcanic phenomenaand their spatial extent, rather than an estimationof the likelihood of the occurrence of such hazards(IAEA 2009).

To determine whether a site should be excludedin the selection for a nuclear facility, the IAEA draftguidelines present the different volcanic phenomenathat may pose potential hazards to a site (Table 1).

According to these guidelines, a ‘Yes’ in the siteselection and evaluation column indicates that a sig-nificant hazard from this phenomenon generallyconstitutes an exclusion criterion, and a ‘No’ inthe design column indicates that it is impracticalto mitigate a potential hazard by either facilitydesign or operational planning (IAEA 2009).

In the case of BNPP, the area is underlain bydeposits of pyroclastic flows and surges, andlahars. Lava deposits and an eruptive centre arealso proximal to the BNPP site. Of all these volcanicphenomena, the potentials for pyroclastic densitycurrents and lava flows cannot be mitigated byengineering solutions. Lahar hazards, however,can be addressed by engineering design.

Evaluation of seismic and tectonic hazards

The IAEA Safety Standard Series (IAEA 2003)suggest relevant coverage areas for different levelsof investigation for seismic hazard evaluation.Typical radial extents are 150 km for regional inves-tigation, 25 km for near-regional investigation,5 km for the site vicinity and 1 km for the sitearea. Any geological structure within these coverageareas will have a corresponding impact on thenuclear power plant. The size, however, may varydepending on the geological and tectonic setting,and its shape may be asymmetric to includedistant significant sources of earthquakes.

One of the most important elements for evaluat-ing a nuclear plant site is surface faulting. Capablefaults are structures that are most relevant whenevaluating the geological features of the site. TheIAEA provides criteria in identifying whether afault is capable or not (IAEA 2002). The first cri-terion is that the fault shows evidence of significantpast deformations or movements of a recurringnature during a period that is recent enough toinfer reasonably that further movements at or nearthe surface could occur. In tectonically activeareas like the Philippines, where both earthquakedata and geological data consistently reveal shortearthquake recurrence intervals, periods of theorder of tens of thousands of years may be appropri-ate. The second criterion for a capable fault is astructural relationship with another known capablefault, such that movement at one may cause move-ment of the other at or near the surface.

Active faults determined by the Philippine Insti-tute of Volcanology and Seismology (Phivolcs)within the region of the BNPP are the ManilaTrench, East Zambales, Marikina Valley, Iba andLubang faults, with distances from the site of 140,90, 82, 75 and 66 km, respectively. In Subic Bay,active faults were also identified by Cabato et al.(2005) within 20 km of the BNPP site. The closestfaults that have been identified are within 1 km of




the BNPP, with one thrust fault cutting an outcrop atthe tip of Napot Point just outside the fenced per-imeter of the installation, with its trace only 200 maway from the nuclear reactor. Aside from veryhigh radon measurements (Fig. 13), there is as yetno direct evidence of active fault movementwithin 1 km of the BNPP because the faultedrocks have not been dated. The second criterion,however, gives reason to believe that the faultswithin 1 km of the BNPP are active. The LubaoFault NE of Natib (Soria 2009) is active, and hasthe same orientation and is collinear with thefaults in the SW sector of Natib (Fig. 3).

Many faults that traverse volcanoes have beenreported in the literature (Wooler 2003; Palomoet al. 2004; Norini et al. 2008; Watt et al. 2009;Tibaldi et al. 2010). A classic example is MayonVolcano, which is traversed by the northern bound-ing fault of the Oas Graben (Lagmay et al. 2005;Lagmay & Zebker 2009). Named the NorthernOas Fault, the surface trace of this structure endsabruptly at the western margin of Mayon andre-emerges east of it, hidden by the active depositionof primary and reworked material on the conicaledifice (Lagmay et al. 2000).

Capable faults are associated with earthquakes.A fault should be considered capable if themaximum potential earthquake associated with itis sufficiently large and at a depth where it is reason-able to infer that movement at or near the surfacecould occur (IAEA 2002). The length of theLubao Fault, which according to the delineation ofSoria (2009) terminates NE of the footslopes ofNatib Volcano, is approximately 42 km. Whenextended to the SW part of Natib’s edifice nearNapot Point, the total length is 73 km. Accordingto Wells & Coppersmith (1994), a 73 km faultlength would be able to generate a magnitude 7.2(Mw) earthquake. The IAEA suggests that wherecapable faults exist within 1 km of the nuclear facil-ity, another site must be considered (IAEA 2005).Such is the case for the BNPP, where a capablefault based on the second criterion of the guidelineswas identified within 1 km of the installation.

Conclusions

Lavas, and the deposits of lahars, pyroclastic flowsand pyroclastic surges, were mapped in the SW

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

Phenomena Potentially adverse characteristics for nuclearinstallations

Siteselection

Design/operation

Tephra fall Static physical loads, abrasive and corrosive particlesin air and water

No Yes

Pyroclastic density currents:Pyroclastic flows, surgesand blasts

Dynamic physical loads, atmospheric overpressures,projectile impacts, temperatures .300 8C, abrasiveparticles, toxic gases

Yes No

Lava flows and lava domes Dynamic physical loads, water impoundments andfloods, temperatures .700 8C

Yes No

Debris avalanches, landslidesand slope failures

Dynamic physical loads, atmospheric overpressures,projectile impacts, water impoundments and floods

Yes No

Debris flows and lahars, floods Dynamic physical loads, water impoundments andfloods, suspended particulates in water

Yes Yes

Opening of new vents Dynamic physical loads, ground deformation,volcanic earthquakes

Yes No

Ballistic projectiles Projectile impacts, static physical loads, abrasiveparticles in water

No Yes

Volcanic gases and aerosols Toxic and corrosive gases, water contamination,gas-charged lakes

No Yes

Tsunamis, seiches, crater lakefailure, glacial burst

Water inundation Yes Yes

Atmospheric phenomena Dynamic overpressures, lightning strikes, downburstwinds

No Yes

Ground deformation Ground displacements .1 m, landslides Yes NoVolcanic earthquakes and

seismic eventsContinuous tremor, multiple shocks usually ,M 5 No Yes

Hydrothermal systems andgroundwater anomalies

Thermal water .50 8C, corrosive water, watercontamination, water inundation or upwelling,alteration, landslides

Yes No

Italicized entries are the hazards pertinent to BNPP without design solutions.




sector of Natib Volcano. At least six pyroclasticdensity current (PDC) deposits were mapped,three directly underlying the nuclear reactor facility.Deposits of at least six pyroclastic density currentswere identified, with three of the units directlyunderlying the site of the BNPP. A previously uni-dentified eruptive centre is located 5.5 km fromthe main building of the plant.

Faults oriented N208E–N308E along the coast ofCabigo Point extend towards the SSW offshore inbathymetric charts as a linear change in relief. Thecontinuation of these faults into SW Natib can betraced from lineaments oriented N308E in ASTERimages and aerial photographs. Radon emissionsat these lineaments are as high as 22 000 Bq m23,against background values of 2000–4000 Bq m23.A thrust fault at the tip of Napot Point cuts up tothe ground surface through lahars.

Natib is considered a capable volcano, based onits active volcanic hydrothermal system and a calcu-lated probability of 1 � 1024–2 � 1024 year21,with a 95% confidence level of a future VEI 6–7volcanic eruption. The volcanic hazards posed tothe site were assessed based on IAEA draft guide-lines. Among the hazards identified, lava flowsand pyroclastic density currents are within thescreening distance value, the maximum distancefrom the source to the site at which the volcanicphenomenon could be a hazard. Of all the volcanichazards, PDCs and lava flows do not have anyengineering solutions. Lahar hazards, however,can be addressed by engineering design.

Faults were mapped in the SW sector of Natib.One at Napot Point cuts up to the ground surfacethrough an indurated lahar deposit. These tectonicstructures are evaluated as capable faults becausethey show a structural relationship with the LubaoFault, which is considered active based on truncatedrecent fluviodeltaic sediments and palaeosea-levelreconstructions recording as much as 3.5 m move-ment over the past 1.5 ka. When evidence showsthe existence of capable faults within 1 km of thenuclear facility, another site must be considered.Such is the case for the BNPP, where capable faultsassociated with the Lubao Fault were identifiedwithin 1 km of the nuclear power plant.

The work on Natib Volcano is still in progressand further characterization of the volcanic depositsand faults is desired. The stratigraphy with corre-sponding age dates for each deposit will improvethe understanding of eruption recurrence rates andprobability estimates for future volcanic eruptionsat Natib and Mariveles. Subsurface studies fromnumerous borehole data and geophysical surveysare also recommended to reduce uncertaintiesin surface geological mapping of vegetated andhighly weathered terrain. With regard to seismichazards, trenching studies of the faults within1 km of the BNPP is the next logical step.

However, the study already provides conserva-tive probability estimates to the hazard’s assess-ment. Assuming such probabilities are sufficient toconsider future eruptions as credible events, thepresence of at least three PDC deposits clearlyshow that pyroclastic flows are well within screen-ing distance and can affect the site. According tothe IAEA draft guidelines, there is no engineeringdesign that can address this type of hazard for anuclear power plant.

Enough data have been gathered to use as one ofthe scientific bases for the decision of the Philippinegovernment whether or not to activate the moth-balled BNPP. These data will also be useful forgeneral hazard preparedness of communities onthe slopes of the volcano.

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