日本地熱学会誌 J. Geotherm. 第 31 巻 第 2 号（2009） Res.Soc.Japan 107 頁～116 頁 論 文 Vol.31. No.2(2009) P.107～P.116

The Gravity Anomaly of Ungaran Volcano, :

Analysis and Interpretation

, Agus SETYAWAN* **, Sachio EHARA***, Yasuhiro FUJIMITSU***

Jun NISHIJIMA***, Hakim SAIBI*** and Essam ABOUD****

(Received 18 June 2008, Accepted 2 October 2008)

Abstract Ungaran Volcano is located in the province of , Indonesia, and is a Quaternary volcano that consists of older and younger volcanoes. The older Ungaran volcano formed over 500,000 years ago, and the younger volcano was active until 300,000 years ago. The younger volcano seems to have been constructed inside a caldera formed by the older Ungaran activity. In this study, gravity data was used in an attempt to determine the exact location of the younger and older Ungaran volcanoes, and to investigate the relationship between fault structure and geothermal manifestations. A positive Bouguer anomaly was observed over the volcanic body. From detailed analysis of the gravity data, high

anomalies were located over the northern part of the summit that correlate with the older Ungaran volcano. Various interpretation methods, such as horizontal gradient analysis, spectral analysis, and 2-D forward modeling, were applied to the gravity data. The younger Quaternary volcanic rocks, which consist of hornblende-augite-andesite (andesite lava) of the Gajahmungkur volcanics, have an average density of 2,390 ± 120 kg/m3, while the older Quaternary volcanic rocks, consisting 3 of augite-olivine basalt flows (basaltic lava) from the Kaligesik formation, have an average density of 2,640 ± 100 kg/m . The structural setting of the Ungaran volcano has characterized by circular structure where most geothermal manifestations are

located. The result of gravity analysis shows that Ungaran volcano seems to have occurred in tectonic depression, and prominent caldera depression has not formed within Ungaran volcano without surficial caldera rim. The horizontal gradient analysis indicates that geothermal features at Ungaran volcano are structurally controlled and are located within the younger volcano.

Keywords: Ungaran volcano, gravity anomaly, fault, geothermal manifestations, Indonesia.

1. Introduction is a still an undeveloped geothermal prospect (Fig.1). Ungaran Ungaran Volcano, located about 30 km southwest of formed in a volcanic arc with three other volcanoes, namely , the capital city of Central Java province, Indonesia, Merapi, Merbabu, and Telomoyo, and is situated in the northern

✽✽✽✽ Department of Earth Resources Engineering, Graduate School of Engineering, Kyushu University, 744, Motooka, Nishi-ku, Fukuoka 819-0395, Japan. ✽✽✽✽ Also Department of Physics, Faculty of Mathematics and Natural Science

Diponegoro University, Jl. Prof. Soedarto SH, Tembalang, Indonesia. ✽✽✽✽ Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, 744, Motooka, Nishi-ku, Fukuoka 819-0395, Japan. ✽✽✽✽ National Research Institute of Astronomy and Geophysics, P.O.Box227, 11722 Helwan, Cairo, Egypt. Ⓒ The Geothermal Research Society of Japan, 2009

― 107 ― of the younger and older volcanoes based on analyses of the Bouguer gravity anomaly.

2. General Geological Setting 2-(1) Tectonic Setting The active volcanoes of the Indonesian archipelago are related to several distinct subduction-zone systems, namely the Sunda, Banda, Sangihe, and Halmahera arc systems (Gertisser and Keller, 2003). The tectonic evolution of the Indonesian archipelago from the Late Paleozoic until the Pliocene occurred with subduction and accompanying volcanism spreading systematically in an ever-widening area from the continent towards the ocean. The Java arc-trench system was formed by subduction of an oceanic plate beneath the continental crust. The crust is thin and relatively young, as it consists mostly of a Tertiary age volcano-plutonic arc (Hamilton, 1979). The Quaternary Fig. 1 Location of the study area. Ungaran is situated in volcanism of the Sunda arc is related to the northward Central Java, Indonesia, about 30 km southwest of Semarang city and located in the northern part of the subduction of the Indo-Australian plate beneath the Eurasian volcanic chain that combines with three other plate at a rate of 6 cm/year near Sumatra and 7 cm/year near volcanoes, namely Merapi, Merbabu and Telomoyo. Java (Hamilton, 1979; Jarrad, 1986; Widiyantoro and Van der Physiographically, the line of volcanoes from Merapi to Hilst, 1997; Kohno et al. , 2006). Ungaran occupies the eastern part of the Solo Zone. Physiographically, Bemmelen (1949) mentioned that the (modified from Bemmelen, 1949, based from Landsat TM satellite image 1995). row of volcanoes from Merapi to Ungaran occupies the western part of the Solo Zone. This zone is a depression that is part of this volcanic chain. The chain is believed to have been filled with the Quartenary products of much of the volcanism formed by back-arc magmatism associated with subduction in Central to East Java. (Kohno et al., 2006). Ungaran is a Quaternary volcano 2-(2) Structural Geology comprised of younger and older volcanoes. The younger Gertisser and Keller (2003) summarized the geology of Ungaran body seems to have formed inside a caldera within Central Java as being divided into several structural units: the the older volcano. Gedongsongo, located in the southern part southern coast including the Karangbolong Mountains, the of Ungaran, is the volcano’s main geothermal manifestation, southern Serayu chains, and the western Progo Mountains, the and includes fumaroles, hot springs, hot acid pools, and an southern mountains, the western foothills of the Solo Zone, the acidic surface of hydrothermally altered rocks. Unfortunately, northern Serayu, and the northern coastline. there is a lack of published studies on Ungaran volcano. A Geothermal areas in Central Java, including Ungaran geochemical model developed by Phuong et al. (2005) showed volcano, are located in the Quaternary Volcanic Belt (Solo that the thermal water in the Ungaran volcanic area is made up Zone). This belt is located between the North Serayu of two distinct types: sulphate water (Ca-(Na)-Mg-SO4-HCO3) Mountains and the Kendeng Zone, and contains young in Gedongsongo and bicarbonate waters (Ca-Mg-HCO3 and Quaternary centers of eruption, including Dieng, Sindoro,

Na-(Ca)-HCO3-Cl) in the surrounding areas of Banaran, Sumbing, Ungaran, Soropati, Telomoyo, Merapi, Muria, and Kendalisodo, Diwak, and Kaliulo. Widarto et al. (2005) Lawu (Bemmelen, 1949). suggested that the subsurface temperature of the reservoir Ungaran is a complex volcano consisting of a younger system ranges from 120oC to 290oC, and that a hot water body, which was formed by the most recent volcanic activity, dominated hydrothermal system has developed in the study and an older body formed by prior volcanic activity. The area. However, the deep structure was not clarified. Therefore, Young Ungaran body seems to have been constructed inside a a gravity study was carried out to determine the subsurface caldera formed during the older Ungaran activity. According structural geology of Ungaran and to deduce the exact location to Kohno et al. (2006), the Old Ungaran body formed prior to

― 108 ― volcanic rocks from old to young, as shown in Fig. 2. Ungaran volcanic area is composed of andesitic lava, perlitic lava, and volcanic breccia from the post Ungaran caldera stages, as shown in Fig. 3 (Thanden et al., 1996). There are geothermal manifestations at the piedmont of Ungaran, namely Gedongsongo, Banaran, Kendalisodo, Diwak, Kaliulo, and Nglimut. Gedongsongo is the main geothermal feature associated with the Quaternary Ungaran andesitic volcanic complex. Budiardjo et al. (1997) presented a structural analysis of

Fig.2 SiO2-total alkali elements (Na2O+K2O) in Ungaran this area and revealed that the Ungaran volcanic system is (modified from Kohno et al., 2006). From Old to Young controlled primarily by the occurrence of the Ungaran collapse Ungaran, the silica content increases in volcanic rocks. structure running from the northwest to the southeast. Fault systems trending northwest to southeast and northeast to 500,000 years ago, and the Young Ungaran volcano did not southwest control the old volcanic rocks of the pre-caldera form until 300,000 years ago. The volcanic rocks are rich in formation. The post-caldera volcanic rocks appear to be less alkali elements and are classified as trachyandesite to structurally controlled by the regional fault systems. trachybasaltic andesite. Moreover, the abundance of alkali elements affecting the mineralogy suggests the magma genesis 3. Gravity study as back-arc side volcanism. The silica content increases in 3-(1) Data

Fig. 3 Geologic map of the Ungaran area (modified from Thanden et al., 1996). The stratigraphy of the Ungaran volcanic area was composed of andesitic lava, perlitic lava, and volcanic breccia during the post Ungaran caldera stages.

― 109 ― where gobs is the observed gravity (mGal), γis the normal gravity (mGal), βis the vertical gradient of the normal gravity, h is the observation height (m), G is Newton’s gravitational constant (kg-1m3s-2), ρis the Bouguer density (mGal), T is the terrain correction per unit density, and F is the free-air anomaly expressed by:

F = gobs – γ+βh. (5) H is a coefficient of the Bouguer density, such that: H=2G h - T. (6) There are many techniques for interpreting potential field data, and they can be divided into two categories: forward methods and inverse methods. In our study, forward methods Fig. 4 Ungaran can be divided into two bodies: Old were employed, including spectral analysis, filtering, the Ungaran and Young Ungaran (Kohno et al., 2006). The white circles show the locations of rock Horizontal Gradient Method (HGM), and 2-D forward samples. modeling using Talwani’s algorithm (Talwani et al.,1959) for gravity data. The gravity data for this study is comprised of public The power spectral analysis yields the depths of domain data provided by Gadjah Mada University. The data significant density contrasts in the crust. Spector and were taken for two periods: February 14-22, 2001 and March Bhattacharya (1966) studied the energy spectrum calculated 19-25, 2001, and come from 163 gravity stations covering from different 3-D model configurations. Spector and Grant 143.85 km2. (1970) studied the statistical ensemble of 3-D maps. They 3-(2) Methodology concluded the general form of the spectrum displaying 3-(2)-1) Rock density measurement in laboratory contributions from different factors can be expressed as: Additionally, to obtain the density contrast between the E(rθ) = 〈H(h,r)〉 〈S(a,b,r,θ)〉 〈C(t,Φ,r)〉, (7) younger and older Ungaran rocks, samples were collected where E is the total energy (frequency Hz), r is the radial wave from both areas. The sampling locations are shown in Fig. 4. number (cycles/m),θis the azimuth of the radial wave number The density of each rock, in wet and dry conditions, was (degree), 〈〉 expresses the ensemble average, h is the depth (m), measured by applying the following equations. H is the depth factor, S is the horizontal size (width) factor, C is

FDD = WD / (Ww-Wwat), (1) the vertical size (thickness or depth extent) factor, a and b are

FWD = Ww / (Ww-Wwat), (2) parameters related to the horizontal dimensions of the sources, where FDD is the forced dry density (kg m-3), FWD is the and t and Φ are parameters related to the vertical depth extent -3 forced wet density (kg m ), WD is the rock weight in dry of the source. Only three factors (H, S, and C) are functions of conditions (kg), Ww is the rock weight in wet conditions (kg), the radial frequency r, thus Equation (7) can be written in the and Wwat is the rock weight measured in water (kg). profile form as: 3-(2)-2) Gravity data processing lnEq ( )=++ ln Hhq ( , ) ln Saq ( , ) ln Ct ( ,Φ , q ) +Constant, The gravity method is used to search for changes in geological conditions based on variations in g (gravitational (8) acceleration) (Grant and West, 1965). On the other hand, Olhoeft and Johnson (1989) defined density as a physical where hat,,are the average depth, half width, and thickness of property that changes significantly among various rock types the source ensemble, respectively. This equation demonstrates owing to differences in mineralogy and porosity. If the that contributions from the depths, widths, and thicknesses of distribution of subsurface rock densities is known, much the source ensemble can affect the shape of the energy spectral information about the subsurface structure can be gained. decay curve. According to the result of the power spectral The Bouguer density of 2,470 kg/m3 for the Ungaran analysis using a bandpass filter, anomalies were separated into regional and residual gravity anomalies. volcanic rocks is given by Murata’s (1993) method using: The HGM, which involves mapping local maxima of the g = gobs -γ+βh - 2πGρh + ρT, (3) gravity or pseudogravity (Baranov, 1957), has commonly been g = F - ρH, (4)

― 110 ― used to locate the steepest gradients associated with Table 1 Density of rock samples of Ungaran volcano. near-vertical physical-property boundaries such as faults (Cordell, 1979; Cordell and Grauch, 1985; Blakely and Simpson, 1986). Phillips (1998) suggested that the greatest advantage of the HGM is that it is least susceptible to noise in the data, because it only requires the calculations of the two first-order horizontal derivatives of the field. The method is also robust in delineating both shallow and deep sources, in comparison with the vertical gradient method, which is useful only in identifying shallower structures. The amplitude of the horizontal gradient (Cordell and Grauch, 1985) is expressed as: HGM(x,y) =[(∂g/∂x)2 +(∂g/∂y)2]1/2, (9) where(∂g/∂x) and (∂g/∂y) are the horizontal derivatives of the gravity field in the x and y directions, respectively. In the 2-D forward method, an initial model for the source body is constructed based on geologic and geophysical information. The model’s anomaly is calculated and compared with the observed anomaly, and model parameters are adjusted to improve the fit between the two anomalies. This three-step process of body adjustment, anomaly calculation, and anomaly comparison is repeated until the calculated and observed anomalies are sufficiently alike.

4. Results and Discussions The rock samples from the older Ungaran unit had a slightly more basaltic composition than those from the Young Ungaran volcanic unit, which is composed of andesitic volcanic rocks. Table 1 shows the results of the rock density measurements. The average densities in the forced dry state of the Young and Old Ungaran rocks are 2,300 ± 150 kg/m3 and 2,570 ± 120 kg/m3, respectively. In the wet state, the densities of the young and old rocks are 2,390 ± 120 kg/m3 and 2,640 ± 100 kg/m3, respectively. The density of the rock in the wet ✲ condition was used to determine the density contrast because ( ) All rock samples were divided into several parts for density measurement except rock sample No.8. the rock in the natural state is generally wet. Finally, the (✲✲) The geologic units are based on the geologic map by Kohno et al. ( 2006). density contrast (Δρ) between the Young and Old Ungaran rocks is taken as 250 kg/m3. Reynolds (1997, page 40), the density increases with The Bouguer gravity map is shown in Fig. 5, and shows a decreasing silica content. The Bouguer anomalies in the region positive Bouguer ranging from 20.5 mGals to 56 mGals. A of Ungaran volcano have a good correlation with the silica high Bouguer anomaly was found in the northern part of contents of the older and younger volcanic rocks, as depicted Ungaran volcano. Considering the geologic data, the high in Fig. 2; thus, high silica content indicates low density and a Bouguer anomaly correlates with the older Ungaran volcano. low Bouguer anomaly. Moreover, based on the results of a geochronology The spectral analysis of the Bouguer gravity data is investigation by Kohno et al. (2006). the growth of the volcano shown in Fig. 6-A. The distinguishing feature of the was from north to south. This correlates well with the locations logarithmic decay of the energy curve is the rapid decrease in of the younger and older Ungaran bodies. According to the curve at low wavenumbers, which is indicative of the

― 111 ― response to deeper sources. The gentler decline of the remainder of the curve is related to near-surface sources. The spectrum consists essentially of three components: a very steep gradient at low wavenumbers (0 km-1 ≤ wavenumber ≤ 0.18 km-1), moderate gradient (0.18 km-1 ≤ wavenumber ≤ 0.54 km-1), and a less steep gradient at high wavenumbers (0.54 km-1 ≤ wavenumber ≤ 1.79 km-1). The negative asymptotic character shows that the gravity data has two components: a regional component from deep-seated sources and a local component caused by sources at shallow depths. Between 0 km-1 and 0.5 km-1, the contribution of the regional component decreases as the wavenumber value increases for the near surface components. At a wavenumber equal to 1.79 km-1, the curve approaches the Nyquist frequency that describes the noise produced by the digitization errors and finite sample interval as fluctuations about a constant level of energy. Fig. 5 Bouguer gravity map of Ungaran volcano. The black square shows gravity measurement station. Two The depth of the gravity sources can be estimated by profiles A-A’ and B-B’ are used for 2-D modeling. The band-pass filtering. In this paper, the point average calculation boundaries of Old and Young Ungaran rocks are from the slope of the energy curve was used. Five point overlapped on the Bouguer gravity map. moving average window “triangular” filters with weighting factors of 1/9, 2/9, 3/9, 2/9, and 1/9 were used to trace the gravity depths for the Ungaran area. The gravity source depth estimated from the power spectral analysis can also be seen in

Fig. 7 The horizontal gradient map of the gravity data for

Fig. 6 A) Radially averaged power spectrum of the Bouguer Ungaran. The black square shows gravity measurement anomalies of Ungaran. station. Solid black bold lines indicate the maxima of the B) Gravity depth estimation based on five point averages horizontal gradient which are interpreted as faults, the of the slope of the energy spectrum. Layer 1 indicates the yellow lines indicate faults from geologic map, and the Young Ungaran, layer 2 indicates the Old Ungaran and yellow circles show the locations of the geothermal layer 3 indicates Tertiary volcanic rocks. manifestations.

― 112 ― Fig. 6-B. Three different layers can be distinguished. Layer 1 correlated with the horizontal gradient anomalies that are corresponds to the Young Ungaran units, with depths from the surface to 500 m, layer 2 and layer 3 correspond to the older Ungaran rocks, with depths from 500 m to 3,500 m. The amplitude of the horizontal gradient was calculated in the frequency domain. Grauch and Cordell (1987) discussed the limitations of the horizontal gradient method for gravity data. They concluded that the horizontal gradient magnitude maxima could be offset from a position directly over the boundaries if the boundaries are not nearly vertical and are close to each other. The horizontal gradient map of gravity data for Ungaran volcano is presented in Fig. 7. The high gradient values were observed between highs and lows of the Bouguer gravity. There are two possible interpretations; one is to correlate the high gradients with the body of the mountain or intrusions of rock, and the other is to correlate them with the fault structure. The study area may be dissected by circular structure. The most interesting result is that almost all of the hot springs are located on the high gradient value of HG forming the circular structure. The interpretation of the maxima of the HG representing Fig. 8 Two dimensional density model using Talwani’s faults is not in agreement with the mapped faults. This is due algorithm for cross-section A-A’. The patterned line to: 1) the mapped faults are at the surface, and the locations of represents a caldera rim deduced from geological most of them are approximated (using visual interpretation); 2) information. The faults interpreted from the HG could be deeper than the geological ones due to the differences in depths. One interpreted fault matches with a mapped fault near the center of the map. The gravity data was interpreted using Talwani’s 2-D forward modeling method. Two profiles (A-A’ and B-B’) were selected, as labeled in Fig. 5. Based on the measured rock samples, as presented in Table 1, we constructed the 2D model using only 2 layers (Old and Young Ungaran rocks). The density contrast between the two modeled layers is 250 kg/m3, and the calculated density structures along A-A’ and B-B’ are presented in Figs. 8 and 9, respectively. Layer 1 has an average density of 2,390 kg/m3,which is composed of andesitic lava, and correlates with the Young Ungaran volcano, while layer 2 has an average density of 2,640 kg/m3 and is composed of basaltic lava that correlates with the old volcano in the northeastern part of Ungaran volcano. The interpretation presented for the gravity anomalies in the region of Ungaran volcano is based on the distribution of subsurface geological formations and their structures. An Fig. 9 Two dimensional density model using Talwani’s interesting result is that the geothermal features (e.g. algorithm for cross-section B-B’. The patterned Gedongsongo, Banaran, Diwak, Kendalisodo, and Nglimut) line represents a caldera rim deduced from are located on the younger Ungaran volcano and are well geological information.

― 113 ― Quaternary older Ungaran volcanic rocks, consisting of augite-olivine basalt flows (basaltic lava) from the Kaligesik formation, is 2,640 ± 100 kg/m3. Horizontal gradient analysis indicates that the existing geothermal features in the Ungaran region are structurally controlled by circular structure and located in the Young Ungaran volcano. As a result, the horizontal gradient of the regional component of gravity is useful in locating the structure that controls the geothermal manifestations.

Acknowledgements The authors would like to express deep gratitude to Dr. Wahyudi (Gadjah Mada University, Indonesia) for providing valuable gravity data. The authors would like to thank Dr. K. Fukuoka (Faculty of Engineering, Kyushu University, Japan) for suggestions and comments. The authors would like to thank Y. Kohno, M.Sc., for lending rock samples in order to measure rock densities. The authors also would like to thank D.H. Barianto, M.T (graduate student at laboratory of Economy Geology, Kyushu University) for suggestions and comments on the Fig. 10 The conceptual structural model of Ungaran. The geology of Ungaran volcano. The first author gratefully patterned line represents a caldera rim deduced from acknowledges the financial support of the Ministry of Education, geological information. Culture, Science and Technology, Government of Japan in the form of a scholarship. This study was also supported by a Grant-in interpreted as circular structures. This indicates that the aid for Scientific Research (B) No.17404024 from the Ministry of geothermal features at Ungaran volcano are structurally Education, Culture, Sports, Science and Technology of Japan, controlled, especially by the deep gravity sources. which is acknowledged. Finally, a conceptual structural model of Ungaran volcano is presented in Fig. 10. The model includes two periods of References volcanism that have been responsible for the Quaternary volcanoes in the Ungaran region recognized from gravity Baranov, V. (1957) A new method for interpretation of aeromagnetic analysis. The first stage is Old Ungaran that widely distributed maps-pseudogravity anomalies. Geophysics, 22, 359-383. in the northern and western regions of the present caldera. The Bemmelen, R.W. Van. (1949) The geology of Indonesia, General nd second stage of volcanism is the Young Ungaran volcano, geology of Indonesia and adjacent archipelago. 2 Edition, which covered Old Ungaran volcano. Martinus Nilhoff, the Haque, Netherlands, 1A, 555-567. Blakely, R.J., and Simpson, R.W. (1986) Locating edges of source 5. Conclusions bodies from magnetic and gravity anomalies. Geophysics, 51, This paper attempted to provide new insight into the 1494-1496. structural setting of Ungaran volcano using gravity data. A Budiardjo, Nugroho, and Budihardi, M. (1997) Resources high Bouguer anomaly is observed in the northern region of characteristic of the Ungaran field, Central Java, Indonesia. the summit of Ungaran, which correlates with the older Proceedings of the National Seminar of Human Resources Ungaran volcano. Generally, the regional structure of Ungaran Indonesian Geologist. Geological Engineering Mineral Technology volcano is divided into two layers with a contrast density of Faculty, UPN ”Veteran”, Yogyakarta, 139-147. 250 kg/m3. The Quaternary younger Ungaran volcanic rocks Cordell, L. (1979) Gravimetric expression of graben faulting in Santa have an average density of 2,390 ± 120 kg/m3 and consist of Fe Country and the Espanola basin. New Mexico Geological th hornblende-augite-andesite (andesite lava) from the Society Guidebook, 30 Field Conference, New Mexico, 59-64. Gajahmungkur volcanics, while the average density of the

― 114 ― Cordell, L., and Grauch, V.J.S. (1985) Mapping basement Talwani, M., Worzel, J.L., and Landisman, M. (1959) Rapid gravity magnetization zones from aeromagnetic data in the San Juan Basin, computations for two-dimensional bodies with applications to the New Mexico. in Hinze, William J. (ed), The utility of regional Mendocino submarine fracture zone. Journal Geophys. Res., 64, gravity and magnetic anomaly maps: Society of Exploration 49-59. Geophysicists, Tulsa, Oklahoma, 181-197. Thanden, R.E., Sumadirdja, H., Richards, P.W., Sutisna, K., and Amin, Gertisser, R. and Keller, J,(2003) Trace element and Sr, Nd, Pb and O T.C. (1996) Geological map of the and Semarang sheet, isotope variations in medium-K and high-K volcanic rocks from Central Java, scale 1:100.000. Geological Research and Merapi volcano, Central Java, Indonesia: Evidence for the Development Centre, Bandung. involvement of subducted sediments in Sunda arc magma genesis. Widarto, D.S., Wardhana, D.D., Gaffar, E.Z., Yudistira, T., Grandis, H., Journal of Petrology, 44, 457-489. Indarto, S., Arsadi, E.M., Sudrajat, Y., and Djupriono (2005) Grant, F.S., and West G.F. (1965) Interpretation theory in applied Integrated geophysical and geochemical studies of the geophysics. New York, McGaw-Hill Inc. Gedongsongo geothermal field in the southern flank of Ungaran Grauch, V.J.S., and Cordell, L. (1987) Short note, Limitations of volcano. Abstract submitted to the Joint Conference of Indonesian determining density or magnetic boundaries from the horizontal Association of Geophysicists (HAGI) and Indonesian Association gradient of gravity or pseudogravity data. Geophysics, 52, 118-121. of Geologists (IAGI), Surabaya. Hamilton, W. (1979) Tectonic of the Indonesian regions. U.S. Geology Widiyantoro, S. and Van der Hilst, R. (1997) Mantle structure beneath Survey Professional Papers, 1078. Indonesia inferred from high-resolution tomography imaging. Jarrad, R.D. (1986) Relations among subduction parameters. Review Geophysical Journal International, 130, 167-182. of Geophysics, 24, 217-284. Kohno, Y., Taguchi, S., Agung, H., Pri. U., Imai, A., and Watanabe, K. (2006) Geological and geochemical study on the Ungaran 論 文 geothermal field, Central Java, Indonesia: an implication in genesis and nature of geothermal water and heat source. Proceedings of 4th インドネシア・ウンガラン火山の重力異常 International Workshop on Earth Science and Technology, Fukuoka, ：解析と解釈 367- 374. Murata, Y. (1993) Estimation of optimum average surficial density アグス セツヤワン, 江原幸雄, 藤光康宏, 西島 潤, from gravity data: an objective Bayesian approach. Journal of ハキム サイビ, アッサム アバウド Geophysical Research, 98, No B7, 12,097 – 12,109. Olhoeft, G.R. and Johnson, G.R. (1989) Density of rock mineral. in （平成 20 年 6 月 18 日受付，平成 20 年 10 月 2 日受理） Carmichael (ed.) practical handbook of physical properties of rock and minerals, section II, CRC Press, Boca Raton Florida. 概 要 Phillips, J.D. (1998) Processing and interpretation of aeromagnetic ウンガラン山はインドネシア・中央ジャワ地区にある第四紀 data for the Santa Cruz Basin-Patahonia Mountains area, 火山であり，新期火山体と古期火山体から構成されている。新 South-Central Arizona. U.S. Geological Survey Open-File Report, 期火山体は今からおよそ 30 万年前までに形成され，古期火山 02-98. 体は今からおよそ 50 万年以上前に形成されたと言われている。 Phuong, Ng, K, Hendrayana, H., Harijoko, A., Itoi, R., and Unoki, R. 新期火山体は，古期火山活動によって形成されたカルデラ内に (2005) Hydrothermal system of the Ungaran geothermal area, 形成されたと考えられている。本研究におけるブーゲー重力異 Indonesia inferred from geochemical study. Proceedings of 3rd 常解析の目的は，新期火山体および古期火山体の位置を明確に International Workshop on Earth Science and Technology, Fukuoka, するとともに，断層構造と地熱徴候との関係を議論することに 19-28. ある。高重力異常がウンガラン火山を全体的に覆っているが， Reynolds, J.M. (1997) An introduction to applied and environmental より詳細に見ると，高重力異常群は古期火山体に関係する山頂 geophysics. John Wiley & Sons Ltd, 796. 部の北方に位置している。種々の重力解析手法，すなわち，水 Spector, A., and Bhattacharya, B.K. (1966) Energy density spectrum 平一次微分解析，スペクトル解析，2 次元フォワード解析等が and autocorrelation function of anomalies due to simple magnetic 適用された。実測された新期火山体の火山岩の密度は， models. Geophysical Prospecting, 14, 242-272. 2,390±120kg/m3 であり，主として安山岩質溶岩から構成されて Spector, A., and Grant, F.E. (1970) Statistical models for interpreting いる。一方，実測された古期火山体の火山岩の密度は， aeromagnetic data. Geophysics, 35, 293-302. 2,640±100kg/m3 であり，主として，玄武岩質溶岩から構成され

― 115 ― ている。ウンガラン火山の構造の特徴は，東西方向および北西 －南東方向の 2 つ方向の主要な断層の存在であり，ウンガラン 火山の地熱徴候は、環状構造に規制されており，かつ，それら が新期火山体に存在している。

― 116 ―