JOURNAL OF PHYSICS OF THE EARTH, Vol. 17, No. 1, 1969 55 The Subsurface Structure of Oosima Volcano, Izu Izumi By YOKOYAMA Geophysical Institute, Hokkaido University The subsurface structure of Oosima Volcano is discussed mainly from the geophysical standpoints on the basis of the available data. First the author summarizes the results of gravimetric surveys and explosion-seismic observations carried out on and around Oosima Island to discuss the deeper structure of the volcano and then proceeds to a discussion of the caldera structure utilyzing the results of the drillings, electric prospecting and gravity measurements. Finally a tentative model of the shallow subsurface structure of Oosima Volcano is presented. Oosima Volcano is equal to whole Oosima Island which is situated at about 100km southwest of Tokyo and less than 30km distant from the Izu Peninsula. It is the northern part of a volcano chain extending from the Mariana Islands to central Honsyu via the Seven Izu Islands. Oosima Volcano is a basaltic stratovolcano with a summit caldera measuring about 4km in diameter, within which stands an active central cone Mihara. The basement rocks of Oosima Volcano are the Yugasima and the Senzu Groups and the main cone (the somma) is mainly composed of the Older Oosima Group. After the formation of the summit caldera, the Younger Oosima Group including Mihara has covered the older group. The recent largest The results of the recent investigations of Oosima Volcano were summarized by Kuno et al. on Special Issue of the Bulletin of Volcanological Association of Japan, 1958, from the standpoints of geology, geophysics and geochemistry. In 1963, also on the same Bulletin, Isshiki et al. reported "the structure of Oosima Caldera based on the results of drilling". In 1964, Nakamura published "the volcano -stratigraphic studies of Volcano subsurface structure of Oosima Volcano will be discussed from the standpoints of geophysics on referring to the above-mentioned papers and more recent data. The gravity measurement on whole Oosima Island was first made by the present author and Tajima [1957] in 1956 and since then, it was supplemented in 1964, 65 and 66; 35 new gravity points at the eastern part of the island and 2 measuring routes across the Oosima" on the Bulletin of Earthquake Research Institute. In the present paper, the Fig. 1. Locality map. Contour interval is 100 meters.
Fig. 2. Distribution of gravity points. Solid circles and triangles denote the bench marks for precise levels and the triangulation points respectively. eastern rim of the summit caldera were set, The distribution of these gravity points are shown in Fig. 2 and the results of the supplementary measurements are tabulated in Table 1. Prior to the summary discussion of all the measured values, the previous observations in 1956 are analyzed as follows: the gravity value at height h is generally expressed by (1) value of gravity, the mean density, topographic correction and Bouguer anomaly respectively. Our aim is to get the distribution of the subterranean density by knowing the gravity anomalies, but it is rather contradicting that we have to know the mean density in order to calculate the Bouguer anomalies. As an approximation at Oosima Island, the mean density is assumed to be 2.4g/cc for the topographic corrections and the Bouguer anomalies not to be correlative with height, and the observed values are applied to the relation (1). By the method of least square, the coefficient of h is obtained as The distribution of the Bouguer anomaly on Oosima Island deduced from the available data adopting the coefficient (2), is shown in Fig. 4. The present results are almost the same as the previous one in the western part but clearer in detail in the eastern part due to the increase in number of the observation. In the figure the gravity anomaly increases, from the west to the east and the high anomalies at the caldera and at the eastern coast are noticeable. In Section 4, these anomalies will be referred in connection with the structure of the caldera. The high a- nomalies prevailing at the regions of Funo-falls and Hude-sima Island suggest correspondency with the andesitic and basaltic lavas exposed at these points. The north-eastern coast between Senzu and Gyozyakutu belongs to a local high anomaly region, but not so conspicuous. The Okata basalts exposed near Okata harbour are not accompanied with any remarkable anomalies. In connection with Oosima Island, it may be interesting to review the distribution of gravity anomalies at the northern part of the Seven Izu Island. On Nii-zima Island and Hatizyo Island, Fujii et al. [1964 a, b] measured gravity and on Miyake Island, the present author and Okada [1964] did.
The Subsurface Structure of Oosima Volcano, Izu 57 Table 1. Results of the supplementary gravity surveys on Oosima Island. For reference, the distribution of the Bouguer anomaly, not corrected for topography, on Miyake Island is reproduced in Fig. 5, which is rather simple comparing with that on Oosima: The regional anomaly decreases from the north-west toward the east. At the relics of the explosion craters on this island, high anomalies are observed. This is the same to Habu harbour at the southern end of Oosima Island. On the sea around the Seven Izu Island, Y. TOMODA (personal communication) meas-
58 Izumi YOKOYAMA Fig. 5. Distribution of the Bouguer gravity anomalies on Miyake Island (not corrected for topography). Unit is mgal. ured gravity with a surface ship gravitymeter. The all-compiled distribution of the Bouguer anomaly in the northern district of the Seven Izu Islands is shown in Fig. 6: At the north of Oosima Island, the gradient increasing southward is very steep and on the island there is a local high of 150 mgal and the general trend of the anomaly connecting Oosima and Miyake Island is the contour of 140 mgal. Fig. 4. Distribution of the Bouguer gravity anomalies. Unit is mgal. At the northern part of the Seven Izu Islands, including the Izu Peninsula and the north off Hatizyo Island, three observations of explosion seismology have been carried out. The Research Group of Explosion Seismology made observations of a series of the artificial explosions along the longitude line observation line terminated at Kawazu of the Izu Peninsula as the southernmost shot point and simultaneously the eruptions of Miyake Volcano which occurred on 24 Aug. 1962 and accompanied earthquake swarms, were observed by this Group. The results were analyzed by Hotta et al. [1964] as follows: at the Izu Peninsula, the superficial layer is about
The Subsurface Structure of Oosima Volcano, Izu 59 Fig. 6. Distribution of the Bouguer gravity anomalies in the northern part of the Seven Izu Islands. Unit is mgal. The values on the sea were obtained by Y. TOMODA. 1.3km thick (P-wave velocity 2.83km/sec.) and the first layer is about 20km thick (P-wave velocity 6.00km/sec.). The second layer (Pwave velocity 6.82km/sec.) is about 30km in the second layer corresponding to "basaltic layer" may thin southward and be almost zero near Miyake Island. Ludwig et al. [1966] carried out a two-ship seismic survey about 40km north off Hatizyo with a length of 60km in the east-west direction (without a reverse observation) as a part of research of the Japan trench in 1964. The location is shown in Fig. 6. According to their results, two sedimentary layers of which P-wave velocities are 2.0 and 3.0km/sec. are underlied by the basement of 5.4km/sec. and further by two layers of 6.0 and 6.8km/sec. The depth of the boundary between the last two layers is about 8km below sea level. Namely, Ludwig et al. suggest the existence of "basaltic layer" at this region. Geological Survey of Japan carried out an explosion seismic observation near Oosima in 1967 in order to select one suitable site for deep drilling of UMP. The measuring line connected the northernmost point of Oosima and the extremity of the Booso Peninsula, the length being about 60km and the explosions were made on the lands (Fig. 6). This observation was not aimed properly to study the structure of Oosima but its results are very useful 6.6km/sec., the boundary between the last two layers being about 15km in depth. At each point of the above seismic observations, the Izu Peninsula, Oosima Island and the north off Hatizyo Island, the Bouguer anomalies are known as shown in Fig. 6. Assuming equilibrium at a depth of 30km in this district, one can calculate the depth of the Moho layer at each point with reference to one below the Izu Peninsula which is approximately known as 30km. In this calculation, the relation between the P-wave velocity and the density is based upon the experimental one compiled by NAFE and DRAKE. As the result, at observation line No. 1, north off Hatizyo Island, the depth of the Moho
60 Izumi YOKOYAMA Fig. 7. Gravity anomalies and structure of seismic P-wave velocities between Izu Peninsula and the north of Hatizyo Island. Numerals in the parentheses denote density in g/cc. (a), (b) and (c) denote three examples of the solutions beneath Oosima Island. No. 1 is the seismic refraction profile by Ludwig et al. layer is obtained as about 24km on the basis of the density distribution shown in Fig. 7. Beneath Oosima Island, the present knowledge is not sufficient to determine the depth of the Moho layer there uniquely: if we assume that the depth varies linearly from the Izu Peninsula to the east and south, the depth beneath Oosima Island is 28km. Then each layer is determinable on the assumption of equilibrium at 30km in depth as previous and under the following assumptions. In this case, of course, each layer in Fig. 7 does not necessarily correspond to geological classifications. a) If the layer of P-wave velocity 6.6km/sec. in depth referring to the results of the observations by the Geological Survey, we can not interprete the observed gravity anomalies on Oosima Island (about 143 mgal). b) The first layer is assumed to be 5km in depth and 2.3g/cc in density which cor- ty, and the second layer is assumed to be 4.7km/sec. not 6.0km/sec. in P-wave velocity. Then the third layer of 6.6km/sec. is 22km in thickness. c) Assuming a layer of P-wave velocity 6.0km/sec (density 2.8g/cc) with reference to the result of observation line No. 1 north off Hatizyo Island, we get its thickness as 4km and that of the third layer as 19km. Of the discussions of subsurface structures of Oosima Volcano, formation of the caldera must be very interesting and most characteristic. If the structure of the caldera is clarified, the whole structure of Oosima Volcano may be understood and some clues to the mechanisms of volcanic eruptions will be afforded. The caldera is distorted semi-circular and about 10kmkm2 in area, the bottom being less than 100m in relative depth. According to volcano-stratigraphic studies about the caldera rim by Nakamura [1964], formation of the caldera, as far as the present One of the characteristics of Oosima Caldera is that its north-eastern rim is not clear in topography. Although some geologists believe that this part is hidden beneath the later erupted material, this must, the author thinks, not be accidental but have a certain relation with the subsurface structure. In this Section, the structure thereabout, the results of the drillings, geoelectric and gravimetric measurements principally at the north-eastern part of the caldera will be discussed. Results of drillings at the rim of the caldera. Isshikiet al. [1963] discussed the structure of the caldera as revealed by drilling. They studied petrological and physical proper-
The Subsurface Structure of Oosirna Volcano, Izu 61 ties of the cores of five bores drilled near the caldera rim, of which sites are shown in Fig. 10. They find a tuff layer once covered common to four drillings and they deem the base of this layer as the very bottom of the caldera immediately after its formation and estimate the depth of subsidence as about 160m. Strictly speaking, the present author thinks, one can not have any sufficient base to determine the depth of subsidence unless one can find some layers corresponding to the above tuff layer or to the adjacent layers, at the earth-surface (at the outcrops or in the drillings) outside the caldera. In fact, Isshiki et al. themselves described an unexpected fact that the lava specimen beneath the tuff layer of the drillings is not similar to the thick lava flows which are exposed at the caldera wall adjacent to the drilling sites. Nakamura [1964] presents a schematic profile of Oosima Volcano as shown in Fig. 8. Fig. 8. Schematic section of Oosima Island from the summit caldera to the coastal area after Nakamura. Considering this figure, the present writer supposes that the existence of an approximately horizontal layer of tuff in a few drillings within the caldera, may mean only a repose period of volcanic activity. Isshiki et al. define the caldera bottom as the earthsurface within the caldera immediately after its formation. However, it may be possible that the caldera was not be formed by one action; Was the caldera formation completed by one action of subsidence by 160m in depth? The distribution of the gravity a- nomaly characteristic to Oosima Volcano as shown in Fig. 4 should have a causal relation with the formation of the caldera; this anomaly should be interpreted by a process of the caldera formation. In the following, an alternative idea about the formation of the caldera of high gravity anomaly type such as Oosima and Kilauea, Hawaii, will be qualitatively stated: Repeating lava flows, eruptive activities and the successive accumulation of the overflowed lavas, the summit part became denser than the surroundings and hence might subside continuously or stepwise. And further, into this subsided region, the new lavas might be overflowed. The repetitions of such processes the present surface. The conception that calderas of high gravity anomaly type such as Oosima would be formed by one action in short time at a certain epoch, may be prejudiced by a traditional assumption of shallow "magma reservoirs", of which substance has not yet been clarified, especially from the geophysical standpoint. "Magma reservoir" which is situated near the earth-surface in the present sense, may mean a condition of magma which are kept at a depth by a certain amount of pressure head and permeate laterally into the fissures beneath volcanoes. It is not always necessary to assume a spherical space. Now we return back to the discussion of the drilling cores. According to Kuno [1953], these rocks of the post-caldera age represent an advanced stage of crystallization of the pre-caldera one, there being no change between the rock series of the pre- and postcaldera ages. Isshiki et al. discuss the variations with depth of chemical components of 8 specimens obtained from drilling OY2: They plot the results of chemical analyses of the specimens on the triangular diagram, MgO-(total iron as FeO)-(Na2O+K2O) (weight percentage) and connect all plots successively from the low to high horizons. As shown in Fig. 9, the plots do not show any systematic changes and all except one drop in the range
62 Izumi YOKOYAMA properties to the lavas of the Older Oosima Group which are exposed at Hunoo-falls, the eastern coast. This may suggest, the present author thinks, a continuous mass from the border of the caldera to the east. Electric prospecting. Prior to the abovementioned drillings around the caldera, Ono et al. (1961) made electric prospectings at the area including the Oosima Park, Yuba, and the eastern part of the caldera in 1953 and 1954. The configuration of the east-west measuring lines in the eastern caldera is shown at the upper part of Fig. 10 and the vertical distribution of specific resistivity along each line at the lower part. In the figure, a layer of low resistivity a little above sea level is noticeable. It may be an aquifer below which there exists an impermeable layer, possibly compact lavas. The broken lines drawn by the present author in Fig. 10, denote a layer of high resistivity 4000-4600- of the post-caldera lavas. This means that the chemical components of the lavas do not change systematically at the boundary layer of the tuff (Fig. 14). In other words, it is not positively supported that the tuff layer is "the bottom immediately after the caldera formation"; the "true bottom" may be deeper. Concerning the chemical components of the drilled cores. Isshiki et al. point also that the may be also rather compact lavas. In this resistivity profile, there is no clear discontinuity of structure which has a direct relation with the caldera formation. Gravity measurements. In order to study the structure of the buried rim at the northeastern part of the caldera, two measuring lines in the east by north direction (AB and CD lines in Fig. 11) and a north-south line connecting the above two, were set in 1964-
The Subsurface Structure of Oosima Volcano, Izu 63 Fig. 11. Gravity points at the eastern part of the caldera. Contour unit is meter. 66. Along these lines, precise levels were carried out and at every turning point, the gravity was measured. The Bouguer anomalies without topographic corrections are shown in Fig. 12 and the two profiles in the east-west direction in Fig. 13. Along profile AB, the topographic corrections are indicated by the bars. The distribution of the Bouguer anomaly on whole Oosima Island previously shown in Fig. 4 is obtained in consideration of Fig. 12. At the two profiles shown in Fig. 13, the topographic correction takes the highest value of about 14 mgal at the east and decreases to about 10 mgal towards the centre. Because these corrections do not change abruptly along the topographies shown in the figure, discontinuities of the gravity anomalies, if any, may be found. Along profile AB, there is a gravity low near to point A on the northward extension of the caldera rim. This low may probably show a characteristic of scoria cones. And the high anomaly which continues to the east may form a plateau anomaly together with a high anomaly in profile CD. These indicate the existence of a large mass but decrease towards the east coast as already shown in Fig. 4. The contour of 150 mgal in Fig. 4, namely one of 137 mgal (+ topographic correction 13 mgal) along profile AB or one of 138 mgal (+ topographic correction 12 mgal) along profile CD, seems to show a vertical fault, if any, or the outline of an anomalous mass, which is indi- Fig. 12. Distribution of the Bouguer gravity anomalies without topographic corrections at the eastern part of the caldera. Unit is mgal. Fig. 13. Profiles of the gravity anomalies and topographies along measuring lines AB and CD in Fig. 11. Bars along profile line AB denote the topographic corrections. cated by arrows in Fig. 13. In order to get a clue for distribution of the subterranean mass, the previous drillings are utilized as follows: at the columnar figures which appear in the paper of Isshiki et al. [1963], one can calculate the mean densities of the drilled cores as shown in Fig. 14, adopting
64 Izumi YOKOYAMA gravity anomalies. Magnetic anomalies. By analyses of geomagnetic anomalies, it is aimed to get knowledges of the distribution and intensity of magnetic material and further, to discuss the subsurface structures of volcanoes. As for the geomagnetic anomalies observed at the earth surface of Oosima Island, the present author [1957] has already discussed. For example, the distribution of the declination on the island is shown in Fig. 15. Generally Fig. 14. Distribution of the mean density in g/cc of the drilled cores at the north-eastern corner of the caldera, calculated from the data given by Isshiki et al. cording to the material. In the figure, at drillings OY3, OY1 and OY2, the densities of the upper and lower parts of the tuff layer are shown in the parentheses respectively. It may be said that at OY4 which is supposed to be within the caldera and at OY3, especially its deeper part, there are more compact lavas than at other drillings. According to the previous discussion of the formation of Oosima Caldera, drillings OY1 and OY2 probably may be situated not within the caldera, but at the debris which were induced to collapse by the caldera formation. Now, the author would like to say that asymmetrical subsidence of calderas might be possible, especially at the calderas of high gravity anomaly type. approximation of the aboves, it may be possible to interprete the distribution of the Of the subsurface structure of Oosima Volcano, the deeper parts than 5km below sea observed anomalies such as shown in Fig. 15 level are discussed in Section 3 by using the results of the explosion seismic observations and gravity anomalies. In this Section, principally the shallower parts will be discussed on the basis of the magnetic and Fig. 15. Distribution of the westerly declination for 1956.0 on Oosima Island. speaking, there are several methods for approximation of a volcano as a magnetized body: those by a dipole, one or plural ellipsoids, and a uniformly magnetized cone. By any and, in fact, there is no distinct prominence among the above approximations. However the intensity of magnetization assumed in each case to interprete the anomaly must be the most important criterion. Originally, magnetization of volcanoes is caused by both natural remanent magnetiza-
The Subsurface Structure of Oosima Volcano, Izu 65 speaking, volcanoes consist of lava flows which are strong in thermal remanent magnetization (T.R.M.) and also of pyroclastics which are effective only in induced magnetization (I.R.M.). Therefore, the mean magnetization of a volcano as a whole, should drop between the two magnetizations. Although there is no concrete proof of the ratio of volume between them, the mean magneti- shallow and basaltic intrusions, and just agrees with the dipole previously mentioned by the author. Gravity anomalies. In Fig. 4 showing the distribution of the Bouguer anomaly on Oosima Island, two profiles in the east-west and north-south directions, are constructed: The east-west profile (Fig. 17): The gravity Fig. 17. Profiles of the gravity and topography along line WE shown in Fig. 4. anomaly increases from west to east. This is a regional anomaly which was already shown in Fig. 6. At the inside of the western caldera rim, the anomaly increases sharply but decreases gradually towards the eastern coast. At the eastern caldera rim, which is shown by letter R in the figure, there is no conspicuous discontinuity of both the topography and anomaly but precisely speaking, as already discussed in Fig. 13, there may be a discontinuity of gravity about the contour of 150 mgal in the eastern part. The north-south profile (Fig. 18): The local Fig. 16. Magnetic material responsible for the geomagnetic anomalies on Oosima Island. On the other hand, Kato et al. [1962] carried out aeromagnetic surveys over Oosima Island in 1958 and analyzed the vertical components at the altitude of 3000m and at the earth surface. They attribute the observed anomalies to two dipoles, one at a depth of 6.5km Fig. 18. Profiles of the gravity anomaly and topography along line SN shown in Fig. 4. anomaly having relations with the caldera superposes upon the regional one. Near the central cone Mihara, the anomalies are locally low: the Bouguer anomalies at No. 38 and No. 39 around the crater are 139.6 mgal and 137.5 mgal including 14.7 mgal and 18.2 mgal
66 Izumi YOKOYAMA also the parasitic cones in Miyake. Judging from the above facts, it may be concluded that the structure of the high anomaly which was symmetrical in the north-south direction within the caldera has been destructed by the eruptive activities of Mihara. The contours of the Bouguer anomaly (Fig. 4) and both the profiles (Figs. 16 and 17) indicate that there are high gravity anomalies amounting to about 7 mgal at two circular regions of about 2.5km in each diameter within the caldera and in its adjacent east (Fig. 19). If we assume a cylindrical model Fig. 19. Distibution of the subsurface dense material around the caldera. of anomalous mass which produces an anomaly of 7 mgal at the centre of the upper end, the relation among the radius, length and density contrast is shown in Fig. 20. The length of the cylinder is estimated as about 1km (to a depth of about 500m below sea level) and about 500m (to sea level) for the density summit part became denser than the surroundings and subsided due to their own weight resulting in the caldera. A part of the accumulated lavas has been replaced by the coarse material because of the activities of the central cone Mihara. Concerning the north-eastern part, some geologists deem it as also a part of the caldera and imagine that the original caldera was gourd-shaped and its north-eastern rim has been hidden by the later ejecta from Mihara. The present author would like to take another interpretation that the anomalous mass is a part of the dense rocks of the Older Oosima Group and the north-eastern rim is not clear because the subsidence there was hindered by this mass. In other words, the caldera structure is asymmetrical as clearly shown in both the topographical and gravimetrical profiles, Fig. 17. It may be possible to imagine that this Older Oosima Group (density is 2.6g/cc) is mainly covered with the ejecta from the Younger Oosima Group and partly outcrops at Hunoo-falls and Hudesima Island at the eastern coast. According to Nakamura [1964], the distributions of vents, dykes and exposed basements are shown in Fig. 21 where the eruption fissure of the Sasikizi Formation (S2) of the Younger Oosima Group runs on a line near the eastern coast. This fissure is just tangential to the region of the
The Subsurface Structure of Oosima Volcano, Izu 67 Fig. 21. Distribution of vents and dykes in Oosima Island after Nakamura. above anomalous mass where inside there are no vents through the period of the Younger Oosima Group although the vent of the Yuba Formation (Y3) is not clear. An overall schematic section of Oosima Volcano along the west-east line shallower than 2km below sea level deduced from seismological, gravimetrical and geomagnetic observations is shown in Fig. 22. It is constructed so as to be compatible with geological knowledge as far as possible. The double circle denotes the position of a dipole which can interprete the geomagnetic anomalies observed on the surface of Oosima Volcano. About here or below, there may be a magma stop which is connected to the lower magma origin by the conduits. The regional gravity anomaly 9 mgal/8km increasing towards the east can not be explained enough by the inclination of the Moho layer but is due to the existence of the shallow basement (BV) in the eastern part or due to the inclination of the substratum as shown by a broken line in Fig. 22. In conclusion, the author realizes that we are possessed of but a meagre knowledge of the subsurface structure of Oosima Volcano in spite of many pionners' efforts. Geomagnetic and gravimetric studies have yielded the Fig. 22. Schematic section of Oosima Volcano at the depth shallower than 5km. The numerals denote density in g/cc.
68 Izumi YOKOYAMA valuable results at the present level. Explosion seismic observations should be carried out with the aim of proper studies of the subsurface structure of Oosima Island. Further, in future, the dynamic methods should be applied to this volcano from the various standpoints though some of them have been already tried: The crustal deformation of the island as a whole and the deformation of the summit caldera in correlation with the activities will give a suggestion concerning a "hearth" of the volcano. The observations of the earth-tides will offer some clues to elastic properties of the material beneath the volcano. The observations of geomagnetic variations of short periods will provide some information relating to electrical conductivity and temperature beneath the volcano. The observations of the anomalous secular changes in geomagnetic field and gravity values, may enable us to discuss heat and mass-transfer and to locate "magma reservior", if any. The same holds good of the observations of distribution of the seismic wave velocities and of attenuation of the waves beneath the volcano. Acknowledgements. The author wishes to express his thanks to Miss R. Yashiro for her assistance in preparing the text figures. References FUJII, Y., S. KOGURE, K. INOUE and K. SAKAMOTO: 1964 "Gravity survey in Niijima and Ooshima" (in Japanese), Journ. Geodetic Soc. Japan, 10, 83-93, FUJII, Y., S. KOGURE and K. KITADA: 1964 "Gravity survey in Hachijo and Torishima Islands" (in Japanese), Journ. Geodetic Soc. Japan, 10, 24-35. HOTTA, H., S. MURAUCHI, T. USAMI, E. SHIMA, Y. MOTOYA, and T. ASANUMA: 1964 "Crustal structure in Central Japan along explosion seismic observations. Part 2. Crustal structure", Bull. Earthq. Res. Inst., 42, 533-541. ICHIKAWA, K.: 1967 "Seismic prospecting around Oosima Island, Izu (UMP)" (in Japanese), Abstract of Spring Meeting, Seism. Soc. Japan, 47. ISSHIKI, N., K. NAKAMURA, M. HAYAKAWA, T. YUKUTAKE, Y. ARAI and B. IWASAKI: 1963 "Structure of caldera of Oshima Volcano, Izu, as revealed by drilling" (in Japanese), Bull. Volc. Soc. Japan, Ser. 2, 8, 61-106. KATO, Y., MATSUO and A. TAKAGI: 1962 "Aeromagnetic surveys over the Oshima Island", Sci. Rep. Tohoku Univ. V Ser., 14, 65-80. KUNO, H.: 1953 "Formation of calderas and magmatic evolution", Trans. Amer. Geophys. Union, 34, 267-280. KUNO, H., R. MORIMOTO, I. YOKOYAMA and I. IWASAKI ET AL.: 1958 "O-sima Special Issue" (in Japanese), Bull. Volc. Soc. Japan. Ser. 2, 3, 1-97. LUDWIG, W.J., J.I. EWING, M. EWING S. MURAUCHI, N. DEN, S. ASANO, H. HOTTA, M. HAYAKAWA, T. ASANUMA, K. ICHIKAWA and I. NOGUCHI: 1966 "Sediments and structure of the Japan trench", Journ. Geophys. Res., 71, 2121-2137. NAGATA, T.: 1943 "The natural remanent magnetism of volcanic rocks and its relation to geomagnetic phenomena", Bull. Earthq. Res. Inst., 21, 1-196. 1951 "Magnetic properties of the lavas of Volcano Mihara" (in Japanese), Journ, Geography (Tokyo), 60, 144-145. NAKAMURA, K.: 1964 "Volcano-stratigraphic study of Oshima Volcano, Izu", Bull. Earthq. Res. Inst., 42, 649-728. ONO, Y., J. SUYAMA and S. TAKAGI: 1961 "On the electric prospecting by the direct current method in Izu-Oshima Island", Bull. Geol. Survey Japan, 12, 1-18. YOKOYAMA, I.: 1957 "Geomagnetic anomaly on volcanoes with relation to their subterranean structure", Bull. Earth. Res. Inst., 35, 327-357. YOKOYAMA, I, and H. OKADA: 1964 "A gravity survey on Miyake Island by means of a LaCoste and Romberg gravity meter" (in Japanese), Geophys. Bull. Hokkaido Univ., 12, 91-98. YOKOYAMA, I. and H. TAJIMA: 1957 "A gravity survey on Volcano Mihara, Ooshima Island by means of a Worden gravimeter", Bull. Earthq. Res. Inst., 35, 23-33. (Received Jan. 27, 1969)