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Caldera Structure Inferred from Gravity Basement around

Bulusan Volcano, Southern Luzon, Philippines

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

I．Introduction

　Bulusan volcano, one of the active volcanoes of the Bicol volcanic arc, is located in the south end of Luzon Island. According to radiometric dating and geologic data, a calderagenic eruption oc-curred about 41 cal kBP （Mirabueno et al., 2007）. Voluminous rhyolitic pumice and ash covered almost the entire Sorsogon province, leaving only the prominent peaks exposed. The eruption cre-ated a depression about 200 m deep and 10 km in diameter on the basis of topography and geology. Although erosion must have breached the caldera at its northwestern side and succeeding eruptions masked its northern and northeastern limits,

the caldera structure can still be inferred and traced at its southern wall. The caldera structure is known as the “Irosin caldera”. Subsequent activities include growth of Jormajan and Bulusan volcanoes on the caldera floor. Acting as a dam, Jormajan volcano helped to form a lake on the caldera floor. The lake became a place where the present Irosin-Juban valley was generated. Breaching at the western side of Jormajan volcano led to the draining of the lake （Jesse Umbal, written communication）.　Recent eruptions of Bulusan volcano occurred from July to October 2007, November 2010 and February 2011. Eruption of pyroclastic flows and 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

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

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ated volcanic hazards. The Philippine National Oil Company （PNOC） and Energy Development Corporation （EDC） conducted geologic mapping, hydrogeochemical studies, and resistivity survey in the Irosin caldera some years ago, and some of the results were published in the geothermal literature. However, the underground structure of the caldera within which Irosin town is located has not been confirmed by gravity survey. Thus, a gravity survey was carried out in February 1996 around Bulusan volcano to delineate and confirm the caldera rim limits and to determine the underground structure. The work was a part of the 1995 financial term of the ITIT （Institute for Transfer of Industrial Technology） project, a joint research of Geological Survey of Japan and the Philippine Institute of Volcanology and Seismology （PHIVOLCS）.

II．Gravity survey at Bulusan volcano

　The gravity meter, LaCoste & Romberg G-type gravity meter （G-277） of PHIVOLCS, used for this survey is the same instrument used for the monitoring of Mayon volcano （Jentzsch et al., 2001）. The gravity meter calibration was carried out along the calibration route of Hannover, Germany. It was shown that the instrument factor was suitable for the gravity survey. 　To fix the position and altitude of gravity stations, differential GPS and leveling are prefer-able, but such methods were not cost effective for economic and practical reasons, we used a set consisting of one Magellan normal GPS device （Magellan DX5000） and two barometric altimeters of TOMMEN. The altitude was determined by averaging observed values of the two barometric altimeters. As the reference observation of atmospheric pressure at the base station was not carried out, the loop-closing time between two stations with known altitude, such as coasts or spot heights, was shortened as much as possible to within two hours. Using this method, the accuracy of position and altitude was estimated to be 20 m and 5 m, respectively. This makes it possible to draw a gravity map with contour interval of 10μN / kg （1 mGal）. 　The gravity base station was established at the

former Bulusan Volcano Observatory in Irosin town, which is located almost in the central area of gravity survey. The coordinate of the base station was 12°43.80' N and 124°01.60' E. The published 1 / 50,000 topographic maps for the Irosin caldera and vicinity were drawn with the geodetic system named “Luzon local” and were useful in survey planning. However, details of parameters of the ellipsoid adopted for maps were uncertain and their accuracy was not enough for reading the latitude and longitude. For these reasons, the geodetic system “WGS-84” was adopted for this survey. An accuracy of 20–30 m was easily attained by this method which satisfied the minimum requirements for a regional survey. Two hundred twenty five total gravity stations were set up along the roads at intervals of 1–2 km （Fig. 1）. There was no station in the elevated and forested areas of the volcano.

III．Processing of measured gravity data

　All measured gravity data were referred to the Cagsawa Ruins gravity base station （in Daraga, Albay, Philippines）, which was used for the gravity monitoring of Mayon volcano, but not to the International Gravity Standardization Net 1971 （IGSN71）. The offset of the local gravity values against IGSN71 was estimated to be less than 50μN / kg （5 mGal） by comparing them with the global gravity field （e.g., Sandwell and Smith, 2009; Satellite Geodesy1））. Normal gravity values corrected for the latitude of gravity station were obtained using the gravity formula, Geodetic Reference System 1980. Bouguer corrections within a range of 60 km in arc-distances were performed with a spherical cap crust formula. Terrain corrections were done within a range of 60 km, the same range for Bouguer correction, by approximating real topography to an assemblage of annular prisms which were produced by interpolating mesh terrain data and random ter-rain data of gravity stations （Komazawa, 1988）. The effect of the earth's curvature on terrain correction was also taken into consideration. The detailed terrain data of the world, Shuttle Radar Topography Mission （SRTM）, was produced by NASA2）. Mesh size of SRTM is 3" arc-distance

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（about 90 m）, which makes it possible to perform terrain correction for topography in a close zone （0–500 m）. In order to perform terrain corrections for the topography in the near zone （500 m–4 km）, the middle zone （4 m–16 km） and the far zone （16 m–60 km）, three types of digital elevation model, 7.5" DEM, 30" DEM and 2' DEM, respectively, were newly produced from SRTM and “etopo2” （2' arc distance mesh data of bathymetry）.　The value of density used for both terrain correction and Bouguer corrections was chosen to be 2,300 kg / m3 according to the result of surface density analysis of some volcanoes. For example, the bulk density of Mt. Fuji, the shape of which is similar to Mayon volcano, is about

2,300 kg / m3 and the central cone of Aso caldera is 2,278 kg / m3 （Komazawa, 1995）. Since the entire measurement area of this survey is covered with volcanic sediment or pyroclastic flows related to Quaternary volcano, it can be assumed from geological consideration that, the density of surface is significantly low, similar to that of above mentioned examples.　Bouguer anomaly map with assumed density of 2,300 kg / m3 （Fig. 2） shows that the area of the Irosin caldera has a low gravity anomaly.

IV． Characteristics of Bouguer gravity anomalies　　　　　　　　　　

　Bouguer anomalies with assumed rock density of 2,300 kg / m3 are considered to show the real

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.

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structural features of central caldera and / or central cone of Bulusan volcano, because the ter-rain effects may be reduced correctly if the density of mountain （volcano） is chosen as an assumed density suitably.　As seen in Fig. 2, a semi-circular feature with steep gravity-gradient is recognized in the Bouguer anomalies from the east, through south and west of Bulusan volcano. Unlike other cal de-ras, no broad flat low anomaly is found inside the topographical caldera wall. It is clear that the semi-circular feature in the gravity-anomalies corresponds to a caldera structure. It should be noted, however, that there is a difference between this caldera and the Aso caldera of Japan; the lat-ter has steep cliff-like relief near along the inner wall of topographic caldera and flat ground in its central part （Komazawa, 1995）. This implies

that the Irosin caldera might not be classified as a piston-cylinder type of caldera （Williams, 1941; Smith and Bailey, 1968）. It resembles a funnel-shaped caldera （Yokoyama, 1963）. Of course, all calderas are collapsed features. Funnel shaped and piston shaped are end members, and there are many variants between the two shaped. If a caldera is of multiple type with nested concentric pistons, it can look like a funnel.　North of Bulusan volcano, there is no clear evidence of a caldera structure either in Bouguer anomalies or in the topography. This can be interpreted that the broad low gravity anomalies extending to the north and west of the volcano, including the southern shore of Sorsogon bay, cor-respond to an old structure which was generated as a pre-Bulusan tectonic depression. On the other hand, the semi-circular feature with steep

Fig. 2　 Bouguer Anomalies with an assumed density of 2,300 kg/m3 （2.3 g/cm3）. Contour interval is 10μN/kg （1 mGal）. Gravity stations are shown with dot marks. Others are same as Fig. 1.

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gravity-gradient mentioned above is considered to reflect a comparatively young structure, generated in relation with volcanic eruption about 41,000 years ago （Mirabueno et al., 2007）.　The low gravity anomaly feature of the cal dera becomes clearer by mapping residual gravity anomalies obtained by using upward-continuation filter to remove regional trends of gravity field. The residual anomalies related to the underground structure shallower than several kilometers can be calculated using a band-pass filer designed with two upward-continuation filters （Komazawa, 1995）. In this study, the band-pass filter consist-ing of 2 km and 50 m upward-continuation filters was designed to remove the regional trends and noise components, respectively. Fig. 3 shows a map of the residual gravity anomalies, with

shaded area representing negative anomalies. In general, the zero value lines on the residual anomaly map indicate the places of discontinuous density structure, i.e. faults or gaps of basement rocks. It is clear from Fig. 3 that, there exists a localized area of negative residual anomalies about 5 km in diameter. This suggests that the caldera structure inferred from gravity anomalies might be smaller than that which is estimated from topography and geology by previous works such as Mirabueno et al. （2007）.

V．Basement structure inferred from 3-D analysis and 2-D modeling　　　　

　Three-dimensional analysis and two-dimen-sional multi-layered modeling are based on the method developed by Komazawa （1995）. A 3-D

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.

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gravity basement map was derived from the residual gravity anomalies. In this case, the infor-mation on the geological setting was referred from the PHIVOLCS geologic maps and the geologic map by Delfin et al. （1993）. However, the undula-tion of basement was not clear from these maps. Thus there are several analytical constraints: the basement was adjusted to coincide with the ground surface in the areas of local maximum in the residual gravity anomalies; the density contrast between the surface layer and the base-ment was assumed to be 300 kg / m3 （0.3 g / cm3）, i.e. the densities of surface layer and basement are 2,000 kg / m3 and 2,300 kg / m3, respectively. The 3 D gravity basement map was obtained using this method （Fig. 4）. It is evident in the obtained 3 D map that there is a circular wall structure with a diameter of about 15 km （Fig. 4） which corresponds to the inferred caldera region.

The northeast region of the dashed line shows the gravity basement is depressed several hundred meters. The southern semi-circular structure corresponds to the caldera rim, but the northern caldera rim was masked with the gravity-high of post-caldera central cone around Mt. Bulusan. Being located on the dashed line, Bulusan volcano is similar in its structural situation to Sakurajima volcano and Satsuma-Iojima of Japan, which are located on the marginal area of the Aira caldera and the Kikai caldera, respectively （Kawanabe et al., 2004; Komazawa et al., 2008）. A hollow-like structure with a steep-gradient wall exists inside the circular dashed lines （caldera region） （Fig. 4）, about 5 km northwest from Irosin town. This hollow, about 5 km in diameter and more than 1.5 km in depth, is considered to be filled with voluminous rhyolitic pumice and ash.　Two-dimensional multi-layered modeling was

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.

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made along one profile （line A-B in Fig. 1）, which passes near Bulusan volcano and 5 km north of Irosin town. Fig. 5 shows the resultant three layered structural model. In this model, the base-ment density of 2,300 kg / m3 was adopted from the result of 3-D analysis shown in Fig. 4, while thin surface layer with the density of 1,800 kg / m3 was estimated around Irosin town in order for residual gravity anomalies to be explained. The depression structure around Irosin town, shown in Fig. 5, might resemble a funnel-shaped （or inverted cone） caldera （Yokoyama, 1963）.

VI．Mass deficiency

　It is very important to estimate the mass deficiency, which might correspond to the total mass of the materials ejected outside the caldera. The mass deficiency of the Irosin caldera amounts to 1.1×1010 tons, which was estimated by using the Gaussian theorem from the residual gravity anomalies less than 2.5 mGal within the

caldera region （dashed line） shown in Fig. 3. The 2.5 mGal of the residual gravity anomaly is considered as an average value of the margin of the caldera region （dashed line）. However, this result is considered to be underestimated because the gravity-high effect of post-caldera central cone shall be limited only around Mt. Bulusan. At present there is difficulty in estimating the effect of post-caldera central cone because gravity sur-vey has not been carried out in that area. In fact, gravity low area of Japanese caldera is broader due to denser station distribution. The mass deficiency can also be estimated from the gravity basement structure shown in Fig. 4. The volume of the sedimentary layer within the caldera region between the gravity basement and an elevation of 0-meter above sea level amounts to about 40 km3. The basement rocks of this volume is considered to be replaced with the low density surface layer. The 0-meter elevation above sea level is considered as an average height of the surround-

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.

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ing basement rocks outside the caldera （Fig. 4）. If the density contrast between the sedimentary （fills） and the basement is assumed to be 0.3 g / cm3 or less, the mass deficiency of the caldera will be 1.2×1010 tons or less. The two results obtained above nearly coincide with each other, and the mass deficiency and the diameter （about 12 km） was plotted on a diagram of calderas proposed by Yokoyama （1963）.　In order to get the actual volume of DRE （dense rock equivalent）, the mass deficiency would need to be calculated from the difference between the pre-eruption basement structure and the present basement structure, but this is hard to know exactly. In this study, we assumed that the pre-eruption basement structure had a flat-topped surface of 0-meter elevation above sea level. So, the volume of DRE is estimated to be about 40 km3 which is the same as the volume of the sedimentary layer within the caldera region. The total mass of DRE is about 1.0×1011 tons, when density is 2,500 kg / m3. This result is almost of the same order of magnitude of the probable volume of magma erupted based on a correlation of volume of eruption versus caldera diameter （Lipman, 2000）.

VII．Conclusions

　Although measured stations were few, a low gravity anomaly corresponding to a caldera structure and a semi-circular with steep gravity-gradient near along the southern caldera rim were found in the vicinity of Irosin town. However, the northern rim was not clear in the gravity anomalies. By means of three-dimensional analysis, it was found that the gravity basement has a circular structure about 15 km in diameter corresponding to the caldera region. Inside the circular structure, there is a funnel-shaped depression structure 5 km in diameter and similar to an inverted cone （instead of a piston-cylinder structure）. Even if the caldera is multiple, this implies that the extrusion sites of pyroclastic flows, which caused the basement rock collapses and the funnel-shaped depression, were restricted to the narrow region.　The mass deficiency of the Irosin caldera was

estimated to be 1.1×1010 tons by the Gaussian theorem, and almost the same result was obtained from the gravity basement structure.　In order to investigate the detailed subsurface structure, it is necessary to carry out more dense gravity measurement with use of differential GPS system around Bulusan volcano.

Acknowledgements　The authors thank two reviewers, Dr. C. Newhall and Dr. K. Nishimura for valuable comments. The authors express their deepest appreciation to Dr. S. Suto of Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology for his assistance and useful information on the ITIT project. The authors also thank Professor M. Okuno of Fukuoka University and Professor T. Kobayashi of Kagoshima University for their assistance to this publication. The authors are indebted to all staff members of Mayon Volcano Observatory and Bulusan Volcano Observatory of Philippine Institute of Volcanology and Seismology for their assistance and information about gravity survey.

Notes

1） http.://topex.ucsd.edu/WWW_html/mar_grav.html ［Cited 2011/06/09］.2）http://www.jpl.nasa.gov/srtm/ ［Cited 2011/06/09］.

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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1996年 2月に Bulusan火山およびその周辺で重力調査を実施した。測点は標高の低い山麓の道沿いに限られ，測定数は 225点となった。火山の山体に近い密度である 2,300 kg/m3（2.3 g/cm3）の仮定密度のブーゲー異常図は，山体部に測点がなくても実際の重力異常を表すと考えられる。 重力異常にはカルデラ壁に対応する急勾配がBulusan火山の東から南を経て西に存在することがわかった。しかし，カルデラの北縁については勾配構造が明瞭ではなかった。Irosin townは急勾配構造の内側にある低重力異常域にある。重力の 3次元解析から得られた重力基盤にはカル

デラ壁を含むカルデラ領域を示す直径 15 kmほどの円形構造があることがわかった。さらに，その円形構造の内側には直径 5 kmほどの急勾配の壁で仕切られた漏斗状（上下逆さまの円錐）の構造が存在し，深さは 1.5 kmに達することがわかった。つまり，イロシンカルデラは，陥没構造が一カ所だけ確認でき，大量の火山砕屑物の噴出を伴った大規模噴火（複数回の場合も含む）は，ごく狭い領域に限られることを示している。また，重力異常による質量欠損の計算から約 40

km3の領域から 1.1×1010トンの火山砕屑物を噴出したと推定され，既存のカルデラの直径と質量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