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

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... for nuclear installations. Although the work is still in progress, enough scientific data have been gathered to assist the Philippine government in deciding whether or not to activate the BNPP, and to improve the general hazard preparedness of the communities on the volcano slopes. Very near infrared (VNIR) images from the AVA ASTER (Advanced Spaceborne Thermal Emission and Reflection) archive (NASA 2009) were down- loaded and draped over an ASTER digital elevation model (DEM) using ERDAS (Earth Resources Data Analysis System) processing software. River drai- nage patterns, lava ridges, levees, summit calderas and a flank eruptive centre were identified in the three-dimensional (3D) images and aerial photographs. Lineaments were also delineated from the VNIR images, and from shaded relief and slope aspect maps derived from the DEM, to identify target sites for structural mapping. European Space Agency (ESA) Environmental Satellite (ENVISAT) Advanced Synthetic Aperture Radar (SAR) descending radar images were pro- cessed using the Stanford Method for Persistent Scatterers-Multi-Temporal InSAR (STAMPS-MTI: Hooper 2006). Twenty-one time-series images from 19 March 2003 to 8 March 2006 were used in the persistent scatterer interferometry to evaluate the ground movement of the area adjacent to the Lubao Fault. Before the fieldwork, sites of structural outcrops were selected using the lineament map. In the field, the orientation of joint and fault structures encountered at the target sites were measured, and geometric and kinematic fabrics were recorded for microtectonic analysis. Thick vegetation and soil cover developed from moderate to extreme weathering of the deposits limited access to good rock exposures, limiting most fieldwork to outcrops at quarries, coasts and road cuts. Where tephra deposits cropped out, slope faces were scraped cleaned before examining and describing deposit sequences and lithologies. Mapping of outcrops with pyroclastic deposits and faults was carried out at a scale of 1:2500. Two short-lived isotopes of radon gas have found useful application in evaluating active faults (Crenshaw et al. 1982). Radon 222 ( 222 Rn) is generated naturally by the decay of 238 U, and has a half- life of only 3.8235 days; Radon 220, also called thoron ( 220 Rn or 220 Tn), with an even shorter half- life of only 55.6 s, is the natural decay product of 232 Th, the most stable thorium isotope (Holden 2004). Both isotopes decay by emitting alpha radi- ation, detectable by their unique emission energies of 6.3 MeV for Tn and 5.5 MeV for Rn (Sexton 1994; Papastefano 2002). Ajari & Adepelumi (2002) and Burton et al. (2004) attributed the high content of these radon isotopes in soils underlain by faults and fractures to increased surface-to-volume ratios in the fracturing rock, and increased soil permeability, which facili- tate radon release from the solid matrix. The short half-lives of these isotopes require that measurable quantities must be escaping from free surfaces of the rock. Radon gas was measured at flatland sites where lineament traces appear in the remotely sensed images. At discrete points along transect lines perpendicular to the lineaments, a soil probe was driven 0.4 m into the soil and connected to an TM RAD7 Durridge Co. portable radon detector. Two 5 min readings were taken at each point. Con- centrations were reported in Bq m 2 3 units. Radon background values also were measured at a quarry site 4 km north of the perimeter fence of the nuclear power plant facility. Earthquake hypocentres of the Bataan region for 1976 to the present were obtained online from the Advanced National Seismic System (ANSS), and focal mechanism solutions from 1929 to the present from the Global Centroid Moment Tensor archives (Fig. 1). Earthquake plots were created TM using the Generic Mapping Tools (GMT ) software (Wessel & Smith 1991). The remotely sensed images and DEMs show that Natib’s summit, 1233 m above sea level, rises between two calderas. The largest is 7.5 Â 5 km 2 in plan (Fig. 2). East of it is a younger volcanic cone with a smaller summit caldera measuring 2 Â 1.8 km. Large channels occupy the eastern slopes of this younger volcanic cone, forming a prominent curved feature that resembles a landslide scar. The southern half of the concavity has been filled by a circular planform of rugged terrain. Several ridges originate from the western rim of the larger caldera and extend towards the South China Sea (Fig. 2). Along their axes, these ridges are steepest near the Natib summit, their slope angles of about 30 8 –40 8 decreasing to 0 8 –15 8 as they reach a break in slope at approximately the 114 m elevation. Below this break, single ridges splay out towards the coast, with flatlands occupying the spaces between them. At the coast, they terminate as headlands that form cliffs as high as 30 m. The BNPP is located in one of these headlands, named Napot Point. About 4.2 km SSW of the larger caldera rim (Fig. 2), a high point 348 m in elevation protrudes from the lower midslopes of the edifice. From this topographical high, finger-like ridges emanate and reach the coast near Napot Point. A relatively smooth fan-like feature occupies most of the southern portion of the Natib edifice, terminating where it meets the Bagac River at the base of Mariveles Volcano. Closely spaced lineaments trend S30 8 –35 8 W from the southern rim of the large caldera towards the coast, a prominent one defining the SE coast of Napot Point (Fig. 3). An offshore extension is expressed on bathymetric charts as a submarine scarp at least 10 km long (Fig. 3). The processing of persistent scatterers in the 21 descending radar images reveal a sharp linear boundary of ground movement separating the western and eastern blocks of the Lubao Fault (Fig. 4). Persistent scatterers in the western block of the Lubao Fault show a decrease in the line-of-sight (LOS) of the radar signal by as much as 2.5 cm year 2 1 . The eastern block, however, is characterized by an increase in LOS with a rate of 2 2.5 cm year 2 1 . The change in LOS across the Lubao Fault is most pronounced in transect 4 (Fig. 4), 22 km from the base of Mount Natib. Field mapping of the SW sector of the Natib Volcano from 390 m elevations down to the coast revealed siltstone–sandstone beds, deposits of lahars, pyroclastic flows and surges, and columnar jointed and autobrecciated lavas. These lithologies and their stratigraphy are described in this section according to the areas in which they are exposed (Figs 2 & 5). Lingatin quarry. A quarry site adjacent to the Lingatin River south of Morong town proper exposed an 11 –12 m-thick sequence of at least five deposits. The lowermost unit (NQPF1: Fig. 6a) is massive and composed of poorly sorted lithic clasts in a light-brown clayey matrix. Ranging in size from 2 to 40 cm, the clasts are mostly andesitic, normally graded and typically angular, although the larger ones have been rounded by spheroidal weathering and have rotten cores. A network of holes, commonly with charred-grass stalks, distinguishes this deposit, which is a block-and-ash deposit. NQPF1 is overlain by NQPF2, a 4 m-thick deposit that tapers at the edges (Fig. 6a, b). Massive and poorly sorted, it consists of devitrified pumice lenses (fiammes) 5– 10 cm long and 1 –5 cm thick, set in a pinkish-red ash matrix. Fewer welded- pumice fragments occur at the base but increase in abundance upwards. The pinkish-red colour of the matrix and welding features indicate high- temperature emplacement. Angular –subangular polymictic lithic clasts, ranging in size from about 1 to 20 cm, along with mm-size crystals are dis- persed throughout this unit, which is best interpreted as a pyroclastic-flow deposit. Overlying NQPF2 in sharp contact, NQPF3 is a reddish brown, massive, poorly sorted and clast- supported 4 m-thick deposit. The clasts are lithic and angular– subangular, and range in diameter from 5 to 20 cm. This unit is also interpreted as a massive pyroclastic-flow deposit. NQPF4, overlying NQPF3, is an approximately 4 m-thick, massive, poorly sorted deposit composed of lithic clasts and pumice fragments in a light- yellow, ashy matrix (Fig. 6c). Lithic clasts of variable composition range in size from 1 to 5 cm and are angular– subangular. Juvenile clasts are devitrified to white clay. NQPF4 is another distinct pyroclastic-flow deposit. Overlying NQPF4 is NQPF5, a 3– 4 m-thick sequence of reddish-brown parallel –subparallel layers that grade upwards into a more massive deposit (Fig. 6d). The reddish-brown ash layers contain lithic and pumice fragments that range in size from 2 to 5 cm. Minute crystals are present in the matrix. In the massive and poorly sorted portion of this unit are angular–subangular lithic fragments, 8– 10 cm in diameter, and 1 –2 cm-size pumice fragments that exhibit slight welding. A large brown rip-up clast about 6 m long and 2 m thick containing a smaller chunk of soil within the massive portion of this unit indicates en masse transport of eroded fragments (Fig. 6d). NQPF5 is identified as a pyroclastic-surge deposit that grades into a more massive pyroclastic-flow unit. NQPF1 and NQPF2 also crop out in a smaller adjacent quarry, and NQPF4 is exposed in 1.5 m-deep pits along the road between Lingatin River and the BNPP site. Beside the Lingatin River, NQPF5 overlies a 3 m-thick autobrecciated lava deposit. Cabigo and Yala points. Thickly bedded, poorly sorted deposits are exposed in outcrops as high as 4–5 m along the coast of Cabigo Point. Variably weathered, the clasts range in size from pebbles to boulders, are rounded –subrounded, and are generally polymictic but are mostly andesitic –basaltic. Clasts in each bed typically are supported in matrixes of sand, typically very coarse. Discernable stratification is expressed in variable clast-size layer colours. Individual beds display normal grading (Fig. 7a). These are typical lahar deposits. In ...

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... sequence of reddish-brown parallel –subparallel layers that grade upwards into a more massive deposit (Fig. 6d). The reddish-brown ash layers contain lithic and pumice fragments that range in size from 2 to 5 cm. Minute crystals are present in the matrix. In the massive and poorly sorted portion of this unit are angular–subangular lithic fragments, 8– 10 cm in diameter, and 1 –2 cm-size pumice fragments that exhibit slight welding. A large brown rip-up clast about 6 m long and 2 m thick containing a smaller chunk of soil within the massive portion of this unit indicates en masse transport of eroded fragments (Fig. 6d). NQPF5 is identified as a pyroclastic-surge deposit that grades into a more massive pyroclastic-flow unit. NQPF1 and NQPF2 also crop out in a smaller adjacent quarry, and NQPF4 is exposed in 1.5 m-deep pits along the road between Lingatin River and the BNPP site. Beside the Lingatin River, NQPF5 overlies a 3 m-thick autobrecciated lava deposit. Cabigo and Yala points. Thickly bedded, poorly sorted deposits are exposed in outcrops as high as 4–5 m along the coast of Cabigo Point. Variably weathered, the clasts range in size from pebbles to boulders, are rounded –subrounded, and are generally polymictic but are mostly andesitic –basaltic. Clasts in each bed typically are supported in matrixes of sand, typically very coarse. Discernable stratification is expressed in variable clast-size layer colours. Individual beds display normal grading (Fig. 7a). These are typical lahar deposits. In fault contact and interbedded with lahar deposits is a 3 m-thick sequence of undulating and cross-bedded layers of tephra ranging in thickness from 1 to 12 cm. The thicker beds containing larger lithic clasts, which range in size from a few millimetres up to 6 cm (Fig. 7b). Matrixes are generally composed of white ash containing millimetre- size crystals, and subangular clasts of pumice and lithic fragments. Pumice accumulations occur in some cross-bedded layers; other beds have reversely graded lithic clasts. All of these features, along with impact sags, are characteristic of pyroclastic-surge deposits. Similar but more massive white-coloured tephra deposits crop out further south along the coast of Yala Point. These whitish pyroclastic- flow deposits are overlain by a thick sequence of lahar beds. Napot Point. The rocks exposed in cliffs and islets along the coast of Napot Point are indurated sands and silts, and lahar and pyroclastic-flow deposits. Pyroclastic-flow deposits crop out within the BNPP site itself. The sedimentary sequence is composed of several thick beds of brown–light-brown and well-sorted sandstone separated by thin– medium interbeds of sandstone and siltstone (Fig. 8a). This sequence of beds generally thins upwards. Parallel laminations are also preserved within the silty layers. These features indicate that the sediments were most probably deposited in a low-energy, shallow-marine environment. Joints cut perpendicular to the strike of beds, displacing laminations by about 1 cm in some places. Indurated lahar deposits 5–8 m thick are the most dominant rock type along the coast. They are massive to thickly bedded. Individual beds are poorly sorted and composed of cobble- to boulder- sized rounded–subrounded polymictic lithic clasts. Bases are commonly clast-supported but gradually become matrix supported towards their tops. In one outcrop, lahar beds exhibit normal grading. An approximately 15 m-thick tuffaceous outcrop is exposed along a roadcut 200 m west of the BNPP office (Fig. 8b). The base of this outcrop is about 2 m thick, but only the upper part is well exposed. It is composed of clast-supported grey – light grey subrounded pebble- to cobble-size polymictic lithic fragments. Medium– coarse sand comprises the matrix. Overlying the bottom unit is a 5 m-thick, yellowish-brown, poorly sorted, matrix-supported layer. Resembling NQPF4 of the quarry section, its polymictic clasts range in size from 10 to 30 cm. Above this deposit is a 3.5 m stratified sequence of angular– subangular pumice and lithic clasts in an ashy matrix. Pumice sizes ranges from 1 to 2 cm, but the lithic clasts can be as large as 15 cm. Individual strata range in thickness from 10 cm to 1 m and vary in colour from yellowish tan to reddish orange. A whitish pumice-rich layer 10 cm thick occurs in the upper-middle part of the sequence. Pumice clasts, some subwelded, are common in the reddish-orange tuffaceous layers (Fig. 8c, d), similar in appearance to unit NQPF5 of the quarry deposits. The topmost unit is a poorly sorted light-brown ashy layer containing angular–subangular lithic clasts 1–20 cm in size and subangular white clay particles 1 cm or less in diameter. It filled a 0.4 m-wide channel and is about 4 m thick. All units in this outcrop, except for the basal layer, are interpreted as pyroclastic-flow deposits. A 390 m-high volcanic edifice juts out of the SW slope of the volcano about 5 km NE of Napot Point (Fig. 2). Four elongated ridges extend radially from the summit towards the south, SW and SE, forming headlands on the coast. One ridge also extends NNE from the summit, forming a saddle as it joins the slope of Mount Natib. Outcrops on the summit of this satellite cone are indurated, dark grey, massive breccias consisting of dominantly 1 –8 cm-sized porphyritic andesite clasts set in a coarse-grained brecciated andesitic matrix. These massive breccias are exposed on a steep wall on one of the ridges, overlying what appears to be another massive layer composed of poorly sorted brecciated material that was inaccess- ible for closer inspection. Metro Highlands Marucdoc. Columnar lava deposits exposed on a steep slope at the side of one of the tributaries of the Marucdoc River and upstream of the Metro Subic Highlands Resort (Fig. 9) are composed of euhedral pyroxene and plagioclase laths in a fine-grained crystalline groundmass. Phenocryst sizes are 0.5– 0.8 cm. Boulder-sized float of similar petrology are abundant along the Marucdoc River Bayandati and on River. slopes Massive of the resort and autobrecciated up to the gate of lava the BNPP deposits property. up to 5 m high and at least 50 m long crop out along the banks of the Bayandati River. The massive but jointed lava is dark grey in colour, and is composed of euhedral pyroxene and amphibole phenocrysts together with trachytic plagioclase laths in a fine-grained crystalline groundmass. Bayandati River. Massive and autobrecciated lava deposits up to 5 m high and at least 50 m long crop out along the banks of the Bayandati River. The massive but jointed lava is dark grey in colour, and is composed of euhedral pyroxene and amphibole phenocrysts together with trachytic plagioclase laths in a fine-grained crystalline groundmass. A fault that cuts northwestwards across Natib Volcano was delineated by Wolfe & Self in 1983 from aerial images and topographic maps (Wolfe & Self 1983) (Fig. 10). The same fault was described in the environmental management report for the PNOC geothermal exploration of Mount Natib (PNOC 1988) and belongs to a set of subpar- allel faults superimposed on the other structures of the Natib Volcano, including its caldera (Cabato 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 & Dimalanta (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 M w 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 Manila Bay (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 southeastern block by as much as 3.5 m over the past 1.5 ka, based on palaeosea-level reconstructions from a peat layer taken in Lubao. Preliminary results of the persistent scatter interferometry of the Lubao area reveal differential ground movement, with a linear boundary corresponding to the trace of the lineament. The name Lubao Fault is thus more appropriate based on evidence of movement along the structure. US Geological Survey (USGS) epicentre data for M w 3.6 earthquakes from 1973 to 2008 include several shallow events that plot close to the fault (Fig. 1). The lineaments SW of Natib Volcano identified in the remotely sensed images are exposed as faults at Cabigo and Napot points (Fig. 11a). At Cabigo Point, faults striking N20 8 –30 8 E and dipping 60 8 –70 8 SE truncate pyroclastic-surge deposits and bring them into contact with lahar deposits (Fig. 11b). Approximately 500 m NE along the coast, about 20 similarly oriented fractures cut indurated lahar deposits (Fig. 11b). At Napot Point, a cliff exposes indurated lahar deposits transected by faults that strike N13 8 –33 8 E and dip 28 8 –41 8 NW. Fault displacements, drag folding and rhomboid shear lenses along the fracture zones (Fig. 12) document thrust faulting. A scarp extends NE from the faulted outcrop at Napot Point into the fenced BNPP perimeter. This feature may be the morphological expression of the faulted rocks and needs further investigation through palaeoseismology (i.e. ...

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... 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 volcanic hazards that even a preliminary map of the geology of the SW sector of the volcano and its stratigraphy could provide. The work was guided by the IAEA volcanic and seismic guidelines (IAEA 2002, 2003, 2005, 2009) and the recommen- dations of Hill et al. (2009) for evaluating the volcanic hazards at sites for nuclear installations. Although the work is still in progress, enough scientific data have been gathered to assist the Philippine government in deciding whether or not to activate the BNPP, and to improve the general hazard preparedness of the communities on the volcano slopes. Very near infrared (VNIR) images from the AVA ASTER (Advanced Spaceborne Thermal Emission and Reflection) archive (NASA 2009) were down- loaded and draped over an ASTER digital elevation model (DEM) using ERDAS (Earth Resources Data Analysis System) processing software. River drai- nage patterns, lava ridges, levees, summit calderas and a flank eruptive centre were identified in the three-dimensional (3D) images and aerial photographs. Lineaments were also delineated from the VNIR images, and from shaded relief and slope aspect maps derived from the DEM, to identify target sites for structural mapping. European Space Agency (ESA) Environmental Satellite (ENVISAT) Advanced Synthetic Aperture Radar (SAR) descending radar images were pro- cessed using the Stanford Method for Persistent Scatterers-Multi-Temporal InSAR (STAMPS-MTI: Hooper 2006). Twenty-one time-series images from 19 March 2003 to 8 March 2006 were used in the persistent scatterer interferometry to evaluate the ground movement of the area adjacent to the Lubao Fault. Before the fieldwork, sites of structural outcrops were selected using the lineament map. In the field, the orientation of joint and fault structures encountered at the target sites were measured, and geometric and kinematic fabrics were recorded for microtectonic analysis. Thick vegetation and soil cover developed from moderate to extreme weathering of the deposits limited access to good rock exposures, limiting most fieldwork to outcrops at quarries, coasts and road cuts. Where tephra deposits cropped out, slope faces were scraped cleaned before examining and describing deposit sequences and lithologies. Mapping of outcrops with pyroclastic deposits and faults was carried out at a scale of 1:2500. Two short-lived isotopes of radon gas have found useful application in evaluating active faults (Crenshaw et al. 1982). Radon 222 ( 222 Rn) is generated naturally by the decay of 238 U, and has a half- life of only 3.8235 days; Radon 220, also called thoron ( 220 Rn or 220 Tn), with an even shorter half- life of only 55.6 s, is the natural decay product of 232 Th, the most stable thorium isotope (Holden 2004). Both isotopes decay by emitting alpha radi- ation, detectable by their unique emission energies of 6.3 MeV for Tn and 5.5 MeV for Rn (Sexton 1994; Papastefano 2002). Ajari & Adepelumi (2002) and Burton et al. (2004) attributed the high content of these radon isotopes in soils underlain by faults and fractures to increased surface-to-volume ratios in the fracturing rock, and increased soil permeability, which facili- tate radon release from the solid matrix. The short half-lives of these isotopes require that measurable quantities must be escaping from free surfaces of the rock. Radon gas was measured at flatland sites where lineament traces appear in the remotely sensed images. At discrete points along transect lines perpendicular to the lineaments, a soil probe was driven 0.4 m into the soil and connected to an TM RAD7 Durridge Co. portable radon detector. Two 5 min readings were taken at each point. Con- centrations were reported in Bq m 2 3 units. Radon background values also were measured at a quarry site 4 km north of the perimeter fence of the nuclear power plant facility. Earthquake hypocentres of the Bataan region for 1976 to the present were obtained online from the Advanced National Seismic System (ANSS), and focal mechanism solutions from 1929 to the present from the Global Centroid Moment Tensor archives (Fig. 1). Earthquake plots were created TM using the Generic Mapping Tools (GMT ) software (Wessel & Smith 1991). The remotely sensed images and DEMs show that Natib’s summit, 1233 m above sea level, rises between two calderas. The largest is 7.5 Â 5 km 2 in plan (Fig. 2). East of it is a younger volcanic cone with a smaller summit caldera measuring 2 Â 1.8 km. Large channels occupy the eastern slopes of this younger volcanic cone, forming a prominent curved feature that resembles a landslide scar. The southern half of the concavity has been filled by a circular planform of rugged terrain. Several ridges originate from the western rim of the larger caldera and extend towards the South China Sea (Fig. 2). Along their axes, these ridges are steepest near the Natib summit, their slope angles of about 30 8 –40 8 decreasing to 0 8 –15 8 as they reach a break in slope at approximately the 114 m elevation. Below this break, single ridges splay out towards the coast, with flatlands occupying the spaces between them. At the coast, they terminate as headlands that form cliffs as high as 30 m. The BNPP is located in one of these headlands, named Napot Point. About 4.2 km SSW of the larger caldera rim (Fig. 2), a high point 348 m in elevation protrudes from the lower midslopes of the edifice. From this topographical high, finger-like ridges emanate and reach the coast near Napot Point. A relatively smooth fan-like feature occupies most of the southern portion of the Natib edifice, terminating where it meets the Bagac River at the base of Mariveles Volcano. Closely spaced lineaments trend S30 8 –35 8 W from the southern rim of the large caldera towards the coast, a prominent one defining the SE coast of Napot Point (Fig. 3). An offshore extension is expressed on bathymetric charts as a submarine scarp at least 10 km long (Fig. 3). The processing of persistent scatterers in the 21 descending radar images reveal a sharp linear boundary of ground movement separating the western and eastern blocks of the Lubao Fault (Fig. 4). Persistent scatterers in the western block of the Lubao Fault show a decrease in the line-of-sight (LOS) of the radar signal by as much as 2.5 cm year 2 1 . The eastern block, however, is characterized by an increase in LOS with a rate of 2 2.5 cm year 2 1 . The change in LOS across the Lubao Fault is most pronounced in transect 4 (Fig. 4), 22 km from the base of Mount Natib. Field mapping of the SW sector of the Natib Volcano from 390 m elevations down to the coast revealed siltstone–sandstone beds, deposits of lahars, pyroclastic flows and surges, and columnar jointed and autobrecciated lavas. These lithologies and their stratigraphy are described in this section according to the areas in which they are exposed (Figs 2 & 5). Lingatin quarry. A quarry site adjacent to the Lingatin River south of Morong town proper exposed an 11 –12 m-thick sequence of at least five deposits. The lowermost unit (NQPF1: Fig. 6a) is massive and composed of poorly sorted lithic clasts in a light-brown clayey matrix. Ranging in size from 2 to 40 cm, the clasts are mostly andesitic, normally graded and typically angular, although the larger ones have been rounded by spheroidal weathering and have rotten cores. A network of holes, commonly with charred-grass stalks, distinguishes this deposit, which is a block-and-ash deposit. NQPF1 is overlain by NQPF2, a 4 m-thick deposit that tapers at the edges (Fig. 6a, b). Massive and poorly sorted, it consists of devitrified pumice lenses (fiammes) 5– 10 cm long and 1 –5 cm thick, set in a pinkish-red ash matrix. Fewer welded- pumice fragments occur at the base but increase in abundance upwards. The pinkish-red colour of the matrix and welding features indicate high- temperature emplacement. Angular –subangular polymictic lithic clasts, ranging in size from about 1 to 20 cm, along with mm-size crystals are dis- persed throughout this unit, which is best interpreted as a pyroclastic-flow deposit. Overlying NQPF2 in sharp contact, NQPF3 is a reddish brown, massive, poorly sorted and clast- supported 4 m-thick deposit. The clasts are lithic and angular– subangular, and range in diameter from 5 to 20 cm. This unit is also interpreted as a massive pyroclastic-flow deposit. NQPF4, overlying NQPF3, is an approximately 4 m-thick, massive, poorly sorted deposit composed of lithic clasts and pumice fragments in a light- yellow, ashy matrix (Fig. 6c). Lithic clasts of variable composition range in size from 1 to 5 cm and are angular– subangular. Juvenile clasts are devitrified to white clay. NQPF4 is another distinct pyroclastic-flow deposit. Overlying NQPF4 is NQPF5, a 3– 4 m-thick sequence of reddish-brown parallel –subparallel layers that grade upwards into a more massive deposit (Fig. 6d). The reddish-brown ash layers contain lithic and pumice fragments that range in size from 2 to 5 cm. Minute crystals are present in the matrix. In the massive and poorly sorted portion of this unit are angular–subangular lithic fragments, 8– 10 cm in diameter, and 1 –2 cm-size pumice fragments that exhibit slight welding. A large brown rip-up clast about 6 m long and 2 m thick containing a smaller chunk of soil within the massive portion of this unit indicates en masse transport of eroded fragments (Fig. 6d). NQPF5 is identified as a pyroclastic-surge deposit that grades into a more massive pyroclastic-flow unit. NQPF1 and NQPF2 also crop out in a smaller adjacent quarry, and NQPF4 is exposed in 1.5 m-deep pits along the road between Lingatin River and the BNPP site. Beside the Lingatin River, NQPF5 overlies a 3 m-thick autobrecciated lava deposit. Cabigo and Yala points. Thickly bedded, ...

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... cataclysmic eruption of Pinatubo only 60 km away, it is puzzling that Natib remains so poorly understood. Part of the reason is the difficulties posed to geological mappers: the large size of the volcano, its steep slopes, highly weathered exposures and dense vegetation. Thus, our field data were gathered mainly on Natib’s midslopes and footslopes during five field campaigns conducted from May 2009 to January 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 volcanic hazards that even a preliminary map of the geology of the SW sector of the volcano and its stratigraphy could provide. The work was guided by the IAEA volcanic and seismic guidelines (IAEA 2002, 2003, 2005, 2009) and the recommen- dations of Hill et al. (2009) for evaluating the volcanic hazards at sites for nuclear installations. Although the work is still in progress, enough scientific data have been gathered to assist the Philippine government in deciding whether or not to activate the BNPP, and to improve the general hazard preparedness of the communities on the volcano slopes. Very near infrared (VNIR) images from the AVA ASTER (Advanced Spaceborne Thermal Emission and Reflection) archive (NASA 2009) were down- loaded and draped over an ASTER digital elevation model (DEM) using ERDAS (Earth Resources Data Analysis System) processing software. River drai- nage patterns, lava ridges, levees, summit calderas and a flank eruptive centre were identified in the three-dimensional (3D) images and aerial photographs. Lineaments were also delineated from the VNIR images, and from shaded relief and slope aspect maps derived from the DEM, to identify target sites for structural mapping. European Space Agency (ESA) Environmental Satellite (ENVISAT) Advanced Synthetic Aperture Radar (SAR) descending radar images were pro- cessed using the Stanford Method for Persistent Scatterers-Multi-Temporal InSAR (STAMPS-MTI: Hooper 2006). Twenty-one time-series images from 19 March 2003 to 8 March 2006 were used in the persistent scatterer interferometry to evaluate the ground movement of the area adjacent to the Lubao Fault. Before the fieldwork, sites of structural outcrops were selected using the lineament map. In the field, the orientation of joint and fault structures encountered at the target sites were measured, and geometric and kinematic fabrics were recorded for microtectonic analysis. Thick vegetation and soil cover developed from moderate to extreme weathering of the deposits limited access to good rock exposures, limiting most fieldwork to outcrops at quarries, coasts and road cuts. Where tephra deposits cropped out, slope faces were scraped cleaned before examining and describing deposit sequences and lithologies. Mapping of outcrops with pyroclastic deposits and faults was carried out at a scale of 1:2500. Two short-lived isotopes of radon gas have found useful application in evaluating active faults (Crenshaw et al. 1982). Radon 222 ( 222 Rn) is generated naturally by the decay of 238 U, and has a half- life of only 3.8235 days; Radon 220, also called thoron ( 220 Rn or 220 Tn), with an even shorter half- life of only 55.6 s, is the natural decay product of 232 Th, the most stable thorium isotope (Holden 2004). Both isotopes decay by emitting alpha radi- ation, detectable by their unique emission energies of 6.3 MeV for Tn and 5.5 MeV for Rn (Sexton 1994; Papastefano 2002). Ajari & Adepelumi (2002) and Burton et al. (2004) attributed the high content of these radon isotopes in soils underlain by faults and fractures to increased surface-to-volume ratios in the fracturing rock, and increased soil permeability, which facili- tate radon release from the solid matrix. The short half-lives of these isotopes require that measurable quantities must be escaping from free surfaces of the rock. Radon gas was measured at flatland sites where lineament traces appear in the remotely sensed images. At discrete points along transect lines perpendicular to the lineaments, a soil probe was driven 0.4 m into the soil and connected to an TM RAD7 Durridge Co. portable radon detector. Two 5 min readings were taken at each point. Con- centrations were reported in Bq m 2 3 units. Radon background values also were measured at a quarry site 4 km north of the perimeter fence of the nuclear power plant facility. Earthquake hypocentres of the Bataan region for 1976 to the present were obtained online from the Advanced National Seismic System (ANSS), and focal mechanism solutions from 1929 to the present from the Global Centroid Moment Tensor archives (Fig. 1). Earthquake plots were created TM using the Generic Mapping Tools (GMT ) software (Wessel & Smith 1991). The remotely sensed images and DEMs show that Natib’s summit, 1233 m above sea level, rises between two calderas. The largest is 7.5 Â 5 km 2 in plan (Fig. 2). East of it is a younger volcanic cone with a smaller summit caldera measuring 2 Â 1.8 km. Large channels occupy the eastern slopes of this younger volcanic cone, forming a prominent curved feature that resembles a landslide scar. The southern half of the concavity has been filled by a circular planform of rugged terrain. Several ridges originate from the western rim of the larger caldera and extend towards the South China Sea (Fig. 2). Along their axes, these ridges are steepest near the Natib summit, their slope angles of about 30 8 –40 8 decreasing to 0 8 –15 8 as they reach a break in slope at approximately the 114 m elevation. Below this break, single ridges splay out towards the coast, with flatlands occupying the spaces between them. At the coast, they terminate as headlands that form cliffs as high as 30 m. The BNPP is located in one of these headlands, named Napot Point. About 4.2 km SSW of the larger caldera rim (Fig. 2), a high point 348 m in elevation protrudes from the lower midslopes of the edifice. From this topographical high, finger-like ridges emanate and reach the coast near Napot Point. A relatively smooth fan-like feature occupies most of the southern portion of the Natib edifice, terminating where it meets the Bagac River at the base of Mariveles Volcano. Closely spaced lineaments trend S30 8 –35 8 W from the southern rim of the large caldera towards the coast, a prominent one defining the SE coast of Napot Point (Fig. 3). An offshore extension is expressed on bathymetric charts as a submarine scarp at least 10 km long (Fig. 3). The processing of persistent scatterers in the 21 descending radar images reveal a sharp linear boundary of ground movement separating the western and eastern blocks of the Lubao Fault (Fig. 4). Persistent scatterers in the western block of the Lubao Fault show a decrease in the line-of-sight (LOS) of the radar signal by as much as 2.5 cm year 2 1 . The eastern block, however, is characterized by an increase in LOS with a rate of 2 2.5 cm year 2 1 . The change in LOS across the Lubao Fault is most pronounced in transect 4 (Fig. 4), 22 km from the base of Mount Natib. Field mapping of the SW sector of the Natib Volcano from 390 m elevations down to the coast revealed siltstone–sandstone beds, deposits of lahars, pyroclastic flows and surges, and columnar jointed and autobrecciated lavas. These lithologies and their stratigraphy are described in this section according to the areas in which they are exposed (Figs 2 & 5). Lingatin quarry. A quarry site adjacent to the Lingatin River south of Morong town proper exposed an 11 –12 m-thick sequence of at least five deposits. The lowermost unit (NQPF1: Fig. 6a) is massive and composed of poorly sorted lithic clasts in a light-brown clayey matrix. Ranging in size from 2 to 40 cm, the clasts are mostly andesitic, normally graded and typically angular, although the larger ones have been rounded by spheroidal weathering and have rotten cores. A network of holes, commonly with charred-grass stalks, distinguishes this deposit, which is a block-and-ash deposit. NQPF1 is overlain by NQPF2, a 4 m-thick deposit that tapers at the edges (Fig. 6a, b). Massive and poorly sorted, it consists of devitrified pumice lenses (fiammes) 5– 10 cm long and 1 –5 cm thick, set in a pinkish-red ash matrix. Fewer welded- pumice fragments occur at the base but increase in abundance upwards. The pinkish-red colour of the matrix and welding features indicate high- temperature emplacement. Angular –subangular polymictic lithic clasts, ranging in size from about 1 to 20 cm, along with mm-size crystals are dis- persed throughout this unit, which is best interpreted as a pyroclastic-flow deposit. Overlying NQPF2 in sharp contact, NQPF3 is a reddish brown, massive, poorly sorted and clast- supported 4 m-thick deposit. The clasts are lithic and angular– subangular, and range in diameter from 5 to 20 cm. This unit is also interpreted as a massive pyroclastic-flow deposit. NQPF4, overlying NQPF3, is an approximately 4 m-thick, massive, poorly sorted deposit composed of lithic clasts and pumice fragments in a light- yellow, ashy matrix (Fig. 6c). Lithic clasts of variable composition range in size from 1 to 5 cm and are angular– subangular. Juvenile clasts are devitrified to white clay. NQPF4 is another distinct pyroclastic-flow deposit. Overlying NQPF4 is NQPF5, a 3– 4 m-thick sequence of reddish-brown parallel –subparallel layers that grade upwards into a more massive deposit (Fig. 6d). The reddish-brown ash layers contain lithic and pumice fragments that range in size from 2 to 5 cm. Minute crystals are present in the matrix. In the massive and poorly sorted portion of this unit are angular–subangular lithic fragments, 8– 10 cm in diameter, and 1 –2 cm-size pumice fragments that exhibit slight welding. A large brown rip-up clast about 6 m long and 2 m thick containing a smaller chunk of soil within the massive portion of this unit indicates ...