地学雑誌 Journal of Geography（Chigaku Zasshi） 123（1）133–142 2014 doi:10.5026/jgeography.123.133

Caldera Structure Inferred from Gravity Basement around Volcano, Southern Luzon,

Masao KOMAZAWA＊, Jose PANTIG＊＊ and Eddie L. LISTANCO＊＊＊

［Received 9 June, 2011; Accepted 23 April, 2013］

Abstract A gravity survey was carried out in and around Bulusan volcano in February 1996. The gravity stations totaled to 225, which were restricted to roads. The value of density used for both terrain and Bouguer corrections was 2,300 kg / m3（ 2.3 g / cm3）, the value commonly used in volcanic terrains and the surface layer density is considered to be geologically low. A semi-circular feature with steep gravity gradient was recognized in the Bouguer anomalies from the east through the south and west of Bulusan volcano. This semi-circular feature corresponds clearly to the southern rim of the Irosin caldera. However, the northern caldera rim is not clear from gravity anomalies. In contrast, the result of three-dimensional analysis of residual gravity anomalies indicates that, the gravity basement has a circular structure, with its diameter significantly smaller than that of topographic depression. It is important to note that this circular depression is similar to funnel-shaped（ or inverted cone） caldera rather than to a piston-cylinder type of caldera. The mass deficiency of the Irosin caldera was estimated to be 1.1×1010 tons by applying the Gaussian theorem gravity anomalies.

Key words： gravity anomalies, Irosin caldera, Bulusan volcano, gravity basement, gravity residuals, funnel-shaped structure, mass deficiency

the caldera structure can still be inferred and I．Introduction traced at its southern wall. The caldera structure Bulusan volcano, one of the active volcanoes of is known as the“ Irosin caldera”. Subsequent the Bicol volcanic arc, is located in the south end activities include growth of Jormajan and Bulusan of Luzon Island. According to radiometric dating volcanoes on the caldera floor. Acting as a dam, and geologic data, a calderagenic eruption oc- Jormajan volcano helped to form a lake on the curred about 41 cal kBP（ Mirabueno et al., 2007）. caldera floor. The lake became a place where Voluminous rhyolitic pumice and ash covered the present Irosin-Juban valley was generated. almost the entire province, leaving only Breaching at the western side of Jormajan volcano the prominent peaks exposed. The eruption cre- led to the draining of the lake（ Jesse Umbal, ated a depression about 200 m deep and 10 km in written communication）. diameter on the basis of topography and geology. Recent eruptions of Bulusan volcano occurred Although erosion must have breached the caldera from July to October 2007, November 2010 and at its northwestern side and succeeding eruptions February 2011. Eruption of pyroclastic flows and masked its northern and northeastern limits, ash demonstrates its explosivity and the associ-

＊ Geological Survey of Japan, AIST, Tsukuba, 305-8567, Japan / Oyo Corporation, Tsukuba, 305-0841, Japan ＊＊ Philippine Institute of Volcanology and Seismology, Quezon City, Philippines ＊＊＊ University of the Philippines, Quezon City, Philippines

— —133 ated volcanic hazards. The Philippine National former Bulusan Volcano Observatory in Irosin Oil Company（ PNOC） and Energy Development town, which is located almost in the central area Corporation（ EDC） conducted geologic mapping, of gravity survey. The coordinate of the base hydrogeochemical studies, and resistivity survey station was 12°43.80' N and 124°01.60' E. The in the Irosin caldera some years ago, and some published 1 / 50,000 topographic maps for the of the results were published in the geothermal Irosin caldera and vicinity were drawn with the literature. However, the underground structure geodetic system named“ Luzon local” and were of the caldera within which Irosin town is located useful in survey planning. However, details of has not been confirmed by gravity survey. Thus, parameters of the ellipsoid adopted for maps were a gravity survey was carried out in February uncertain and their accuracy was not enough for 1996 around Bulusan volcano to delineate and reading the latitude and longitude. For these confirm the caldera rim limits and to determine reasons, the geodetic system“ WGS-84” was the underground structure. The work was a part adopted for this survey. An accuracy of 20–30 m of the 1995 financial term of the ITIT（ Institute was easily attained by this method which satisfied for Transfer of Industrial Technology） project, the minimum requirements for a regional survey. a joint research of Geological Survey of Japan Two hundred twenty five total gravity stations and the Philippine Institute of Volcanology and were set up along the roads at intervals of 1–2 km Seismology（ PHIVOLCS）. （Fig. 1）. There was no station in the elevated and forested areas of the volcano. II．Gravity survey at Bulusan volcano III．Processing of measured gravity data The gravity meter, LaCoste & Romberg G-type gravity meter（ G-277） of PHIVOLCS, used for All measured gravity data were referred to the this survey is the same instrument used for the Cagsawa Ruins gravity base station（ in , monitoring of volcano（ Jentzsch et al., , Philippines）, which was used for the 2001）. The gravity meter calibration was carried gravity monitoring of Mayon volcano, but not to out along the calibration route of Hannover, the International Gravity Standardization Net Germany. It was shown that the instrument 1971（ IGSN71）. The offset of the local gravity factor was suitable for the gravity survey. values against IGSN71 was estimated to be less To fix the position and altitude of gravity than 50 μN / kg（ 5 mGal） by comparing them stations, differential GPS and leveling are prefer- with the global gravity field（ e.g., Sandwell and able, but such methods were not cost effective Smith, 2009; Satellite Geodesy1））. Normal gravity for economic and practical reasons, we used values corrected for the latitude of gravity station a set consisting of one Magellan normal GPS were obtained using the gravity formula, Geodetic device（ Magellan DX5000） and two barometric Reference System 1980. Bouguer corrections altimeters of TOMMEN. The altitude was within a range of 60 km in arc-distances were determined by averaging observed values of performed with a spherical cap crust formula. the two barometric altimeters. As the reference Terrain corrections were done within a range of observation of atmospheric pressure at the base 60 km, the same range for Bouguer correction, by station was not carried out, the loop-closing approximating real topography to an assemblage time between two stations with known altitude, of annular prisms which were produced by such as coasts or spot heights, was shortened as interpolating mesh terrain data and random ter- much as possible to within two hours. Using this rain data of gravity stations（ Komazawa, 1988）. method, the accuracy of position and altitude was The effect of the earth's curvature on terrain estimated to be 20 m and 5 m, respectively. This correction was also taken into consideration. The makes it possible to draw a gravity map with detailed terrain data of the world, Shuttle Radar contour interval of 10μN / kg（ 1 mGal）. Topography Mission（ SRTM）, was produced by The gravity base station was established at the NASA2）. Mesh size of SRTM is 3" arc-distance

— —134 Fig. 1 Distribution of Gravity Stations. Gravity stations are shown with dot marks. Relief is topography and made from the 3" （about 90 m） mesh data of Shuttle Radar Topography Mission produced by NASA. Line A-B is the 2-D modeling line. A circular dotted line denotes a caldera estimated from gravity basement shown in Fig. 4.

（about 90 m）, which makes it possible to perform 2,300 kg / m3 and the central cone of Aso caldera is terrain correction for topography in a close 2,278 kg / m3（ Komazawa, 1995）. Since the entire zone（ 0–500 m）. In order to perform terrain measurement area of this survey is covered with corrections for the topography in the near zone volcanic sediment or pyroclastic flows related （500 m–4 km）, the middle zone（ 4 m–16 km） to Quaternary volcano, it can be assumed from and the far zone（ 16 m–60 km）, three types of geological consideration that, the density of digital elevation model, 7.5" DEM, 30" DEM and surface is significantly low, similar to that of above 2' DEM, respectively, were newly produced from mentioned examples. SRTM and“ etopo2”（ 2' arc distance mesh data of Bouguer anomaly map with assumed density bathymetry）. of 2,300 kg / m3（ Fig. 2） shows that the area of the The value of density used for both terrain Irosin caldera has a low gravity anomaly. correction and Bouguer corrections was chosen IV． Characteristics of Bouguer gravity to be 2,300 kg / m3 according to the result of anomalies surface density analysis of some volcanoes. For example, the bulk density of Mt. Fuji, the shape Bouguer anomalies with assumed rock density of which is similar to Mayon volcano, is about of 2,300 kg / m3 are considered to show the real

— —135 Fig. 2 Bouguer Anomalies with an assumed density of 2,300 kg/m3（ 2.3 g/cm3）. Contour interval is 10 μN/kg（ 1 mGal）. Gravity stations are shown with dot marks. Others are same as Fig. 1. structural features of central caldera and / or that the Irosin caldera might not be classified as central cone of Bulusan volcano, because the ter- a piston-cylinder type of caldera（ Williams, 1941; rain effects may be reduced correctly if the density Smith and Bailey, 1968）. It resembles a funnel- of mountain（ volcano） is chosen as an assumed shaped caldera（ Yokoyama, 1963）. Of course, all density suitably. calderas are collapsed features. Funnel shaped As seen in Fig. 2, a semi-circular feature with and piston shaped are end members, and there steep gravity-gradient is recognized in the are many variants between the two shaped. If a Bouguer anomalies from the east, through south caldera is of multiple type with nested concentric and west of Bulusan volcano. Unlike other cal de- pistons, it can look like a funnel. ras, no broad flat low anomaly is found inside the North of Bulusan volcano, there is no clear topographical caldera wall. It is clear that the evidence of a caldera structure either in Bouguer semi-circular feature in the gravity-anomalies anomalies or in the topography. This can be corresponds to a caldera structure. It should be interpreted that the broad low gravity anomalies noted, however, that there is a difference between extending to the north and west of the volcano, this caldera and the Aso caldera of Japan; the lat- including the southern shore of Sorsogon bay, cor- ter has steep cliff-like relief near along the inner respond to an old structure which was generated wall of topographic caldera and flat ground in its as a pre-Bulusan tectonic depression. On the central part（ Komazawa, 1995）. This implies other hand, the semi-circular feature with steep

— —136 Fig. 3 Gravity residuals inferred from a shallow structure. Regional trends and noise components are removed with the 2 km and 50 m upward-continuation, respectively. Contour interval is 10 μN/kg（ 1 mGal）. Others are same as Fig. 1.

gravity-gradient mentioned above is considered to shaded area representing negative anomalies. reflect a comparatively young structure, generated In general, the zero value lines on the residual in relation with volcanic eruption about 41,000 anomaly map indicate the places of discontinuous years ago（ Mirabueno et al., 2007）. density structure, i.e. faults or gaps of basement The low gravity anomaly feature of the cald era rocks. It is clear from Fig. 3 that, there exists becomes clearer by mapping residual gravity a localized area of negative residual anomalies anomalies obtained by using upward-continuation about 5 km in diameter. This suggests that the filter to remove regional trends of gravity field. caldera structure inferred from gravity anomalies The residual anomalies related to the underground might be smaller than that which is estimated structure shallower than several kilometers can from topography and geology by previous works be calculated using a band-pass filer designed such as Mirabueno et al（. 2007）. with two upward-continuation filters（ Komazawa, V．Basement structure inferred from 3-D 1995）. In this study, the band-pass filter consist- analysis and 2-D modeling ing of 2 km and 50 m upward-continuation filters was designed to remove the regional trends and Three-dimensional analysis and two-dimen- noise components, respectively. Fig. 3 shows sional multi-layered modeling are based on the a map of the residual gravity anomalies, with method developed by Komazawa（ 1995）. A 3-D

— —137 Fig. 4 Gravity basement in meter above sea level with density contrast of 300 kg/m3（ 0.3 g/cm3）. Contour interval is 100 m. gravity basement map was derived from the The northeast region of the dashed line shows the residual gravity anomalies. In this case, the infor- gravity basement is depressed several hundred mation on the geological setting was referred from meters. The southern semi-circular structure the PHIVOLCS geologic maps and the geologic corresponds to the caldera rim, but the northern map by Delfin et al（. 1993）. However, the undula- caldera rim was masked with the gravity-high tion of basement was not clear from these maps. of post-caldera central cone around Mt. Bulusan. Thus there are several analytical constraints: Being located on the dashed line, Bulusan volcano the basement was adjusted to coincide with the is similar in its structural situation to Sakurajima ground surface in the areas of local maximum volcano and Satsuma-Iojima of Japan, which are in the residual gravity anomalies; the density located on the marginal area of the Aira caldera contrast between the surface layer and the base- and the Kikai caldera, respectively（ Kawanabe et ment was assumed to be 300 kg / m3（ 0.3 g / cm3）, al., 2004; Komazawa et al., 2008）. A hollow-like i.e. the densities of surface layer and basement structure with a steep-gradient wall exists inside are 2,000 kg / m3 and 2,300 kg / m3, respectively. the circular dashed lines（ caldera region）（ Fig. The 3 D gravity basement map was obtained 4）, about 5 km northwest from Irosin town. This using this method（ Fig. 4）. It is evident in the hollow, about 5 km in diameter and more than obtained 3 D map that there is a circular wall 1.5 km in depth, is considered to be filled with structure with a diameter of about 15 km（ Fig. 4） voluminous rhyolitic pumice and ash. which corresponds to the inferred caldera region. Two-dimensional multi-layered modeling was

— —138 Fig. 5 Two-dimensional multi-layered modeling. Gravity residuals profiles（ top） and crustal cross section（ bottom） along line A-B are shown. The values of model denote density in 1,000 kg/m3（ 1.0 g/cm3）. The location of the profile A-B is shown in Fig. 1. Exaggeration of vertical direction horizontal is 10 times. made along one profile（ line A-B in Fig. 1）, which caldera region（ dashed line） shown in Fig. 3. passes near Bulusan volcano and 5 km north The 2.5 mGal of the residual gravity anomaly is of Irosin town. Fig. 5 shows the resultant three considered as an average value of the margin of layered structural model. In this model, the base- the caldera region（ dashed line）. However, this ment density of 2,300 kg / m3 was adopted from the result is considered to be underestimated because result of 3-D analysis shown in Fig. 4, while thin the gravity-high effect of post-caldera central cone surface layer with the density of 1,800 kg / m3 was shall be limited only around Mt. Bulusan. At estimated around Irosin town in order for residual present there is difficulty in estimating the effect gravity anomalies to be explained. The depression of post-caldera central cone because gravity sur- structure around Irosin town, shown in Fig. 5, vey has not been carried out in that area. In fact, might resemble a funnel-shaped（ or inverted gravity low area of Japanese caldera is broader cone） caldera（ Yokoyama, 1963）. due to denser station distribution. The mass deficiency can also be estimated from the gravity VI．Mass deficiency basement structure shown in Fig. 4. The volume It is very important to estimate the mass of the sedimentary layer within the caldera region deficiency, which might correspond to the between the gravity basement and an elevation of total mass of the materials ejected outside the 0-meter above sea level amounts to about 40 km3. caldera. The mass deficiency of the Irosin caldera The basement rocks of this volume is considered amounts to 1.1×1010 tons, which was estimated to be replaced with the low density surface by using the Gaussian theorem from the residual layer. The 0-meter elevation above sea level is gravity anomalies less than 2.5 mGal within the considered as an average height of the surround-

— —139 ing basement rocks outside the caldera（ Fig. 4）. estimated to be 1.1×1010 tons by the Gaussian If the density contrast between the sedimentary theorem, and almost the same result was obtained （fills） and the basement is assumed to be 0.3 g / cm3 from the gravity basement structure. or less, the mass deficiency of the caldera will be In order to investigate the detailed subsurface 1.2×1010 tons or less. The two results obtained structure, it is necessary to carry out more dense above nearly coincide with each other, and the gravity measurement with use of differential mass deficiency and the diameter（ about 12 km） GPS system around Bulusan volcano. was plotted on a diagram of calderas proposed by Yokoyama（ 1963）. Acknowledgements In order to get the actual volume of DRE（ dense The authors thank two reviewers, Dr. C. Newhall rock equivalent）, the mass deficiency would need and Dr. K. Nishimura for valuable comments. The authors express their deepest appreciation to Dr. S. to be calculated from the difference between the Suto of Geological Survey of Japan, National Institute pre-eruption basement structure and the present of Advanced Industrial Science and Technology for his basement structure, but this is hard to know assistance and useful information on the ITIT project. exactly. In this study, we assumed that the pre- The authors also thank Professor M. Okuno of Fukuoka eruption basement structure had a flat-topped University and Professor T. Kobayashi of Kagoshima surface of 0-meter elevation above sea level. So, University for their assistance to this publication. The the volume of DRE is estimated to be about authors are indebted to all staff members of Mayon 40 km3 which is the same as the volume of the Volcano Observatory and Bulusan Volcano Observatory sedimentary layer within the caldera region. The of Philippine Institute of Volcanology and Seismology for total mass of DRE is about 1.0×1011 tons, when their assistance and information about gravity survey. 3 density is 2,500 kg / m . This result is almost Notes of the same order of magnitude of the probable 1） http.://topex.ucsd.edu/WWW_html/mar_grav.html volume of magma erupted based on a correlation ［ Cited 2011/06/09］. of volume of eruption versus caldera diameter 2）http://www.jpl.nasa.gov/srtm/［ Cited 2011/06/09］. （Lipman, 2000）. References VII．Conclusions Delfin, F.G., Panem, C.C. and Defant, M.J.（ 1993）: Although measured stations were few, a low Eruptive history and petrochemistry of the Bulusan gravity anomaly corresponding to a caldera volcanic complex: Implications for the hydrothermal system and volcanic hazards of Mt. Bulusan, Philip- structure and a semi-circular with steep gravity- pines. Geothermics, 22, 417-434. gradient near along the southern caldera rim were Jentzsch, G., Punongbayan, R.S., Schreiber, U., Seeber, found in the vicinity of Irosin town. However, G., Völksen, C. and Weise, A.（ 2001）: Mayon volcano, the northern rim was not clear in the gravity Philippines: Change of monitoring strategy after anomalies. By means of three-dimensional microgravity and GPS measurements. Journal of Volcanology and Geothermal Research, 109, 219-234. analysis, it was found that the gravity basement Kawanabe, Y., Sakaguchi, K., Saito, M., Komazawa, M. has a circular structure about 15 km in diameter and Yamazaki, T.（ 2004）: Geological Map of Japan corresponding to the caldera region. Inside 1:200,000, Kaimon Dake and a Part of Kuro Shima. the circular structure, there is a funnel-shaped Geological Survey of Japan, AIST.（in Japanese with English abstract depression structure 5 km in diameter and similar ） Komazawa, M.（ 1988）: A gravimetric terrain correction to an inverted cone（ instead of a piston-cylinder method by assuming annular prism model. Journal of structure）. Even if the caldera is multiple, this the Geodetic Society of Japan, 34, 11-23.（in Japanese implies that the extrusion sites of pyroclastic with English abstract） flows, which caused the basement rock collapses Komazawa, M.（ 1995）: Gravimetric analysis of Aso volcano and its interpretation. Journal of the Geodetic and the funnel-shaped depression, were restricted Society of Japan, 41, 17-45. to the narrow region. Komazawa, M., Nakamura, K., Yamoto, K., Iguchi, The mass deficiency of the Irosin caldera was M., Akamatsu, J., Ichikawa, N., Takayama, T. and

— —140 Yamazaki, T.（ 2008）: Gravity anomalies at Sakura- ma rine gravity from retracked Geosat and ERS- jima volcano, southwest Japan. Annuals of Disaster 1 altim etry: Ridge segmentation versus spreading Prevention Research Institute Kyoto University, 51B, rate. Journal of Geophysical Research, 114, B01411, 261-266（. in Japanese with English abstract） doi:10.1029/2008JB006008. Lipman, P.W.（ 2000）: Calderas. Encyclopedia of Vol­ Smith, R.L. and Bailey, R.A.（ 1968）: Resurgent caul- canoes, 643-662. drons. Memoir of the Geological Society of America, Mirabueno, M.H.T., Okuno, M., Nakamura, T., Laguerta, 116, 613-662. E.P. and Newhall, C.G.（ 2007）: AMS radiation dating Williams, H（. 1941）: Calderas and their origin. Univer­ sity­ of a charcoal fragment from the Irosin ignimbrite, of California Publications. Bulletin of the Department Sorsogon province, southern Luzon, Philippines. Bul­ of Geological Sciences, 25, 239-346. letin of the Volcanological Society of Japan, 52, 241- Yokoyama, I.（ 1963）: Structure of caldera and gravity 244. anomaly. Bulletin of Volcanology, 26, 67-73. Sandwell, D.T. and Smith, W.H.F. （2009）: Global

— —141 ブルサン火山の重力基盤からみえるカルデラ構造

駒 澤 正 夫＊ ホセ パンティーク＊＊ エディー リスタンコ＊＊＊

イロシンカルデラの地下構造を把握するため デラ壁を含むカルデラ領域を示す直径 15 km ほ 1996 年 2 月に Bulusan 火山およびその周辺で重 どの円形構造があることがわかった。さらに，そ 力調査を実施した。測点は標高の低い山麓の道沿 の円形構造の内側には直径 5 km ほどの急勾配の いに限られ，測定数は 225 点となった。火山の 壁で仕切られた漏斗状（上下逆さまの円錐）の構 山体に近い密度である 2,300 kg/m3（2.3 g/cm3） 造が存在し，深さは 1.5 km に達することがわ の仮定密度のブーゲー異常図は，山体部に測点が かった。つまり，イロシンカルデラは，陥没構造 なくても実際の重力異常を表すと考えられる。 が一カ所だけ確認でき，大量の火山砕屑物の噴出 重力異常にはカルデラ壁に対応する急勾配が を伴った大規模噴火（複数回の場合も含む）は， Bulusan 火山の東から南を経て西に存在するこ ごく狭い領域に限られることを示している。ま とがわかった。しかし，カルデラの北縁について た，重力異常による質量欠損の計算から約 40 は勾配構造が明瞭ではなかった。Irosin town は km3 の領域から 1.1×1010 トンの火山砕屑物を噴 急勾配構造の内側にある低重力異常域にある。重 出したと推定され，既存のカルデラの直径と質量 力の 3 次元解析から得られた重力基盤にはカル 欠損の関係と整合的である。

キーワード：重 力異常，イロシンカルデラ，ブルサン火山，重力基盤，残差重力，漏斗型構造，質量欠損

＊ 産業技術総合研究所地質情報部門 / 応用地質株式会社 ＊＊ フィリピン火山地震研究所 ＊＊＊ フィリピン大学

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