J. Geomag. Geoelectr., 43, 741-754, 1991

Ground Magnetic Anomalies in the Tanna Fault and Their Implications

Yasukuni OKUBO, Keiko MIZUGAKI, and Hiroshi KANAYA

Geological Survey of , Tsukuba 305, Japan

(Received April 9, 1991; Revised June 10, 1991)

The Tanna fault is a left lateral fault extending in the middle of the Izu Peninsula. We carried out a detailed ground magnetic survey at a grid spacing of 50 to 150 m along the Tanna fault and constructed a magnetic model based on the ground survey with the help of drilling data in the Tanna basin and measurement of rock magnetism. Total intensity distribution obtained by the ground magnetic survey represents a distinguished contrast forming relative magnetic highs in the west to the fault and lows in the east. The Tanna fault is a remarkable border terminating magnetic highs. Drillings revealed that formations filling the Tanna basin are mainly the middle Pleistocene Taga volcanics which possess high susceptibility as a whole. A recent work suggests that the Taga volcanics were produced in normal polarity and had strong remanent magnetization. Therefore, remanent magnetization is inferred to be dominant in the total magnetization. The magnetic contrast forms almost a north-south trend throughout the study area. The magnetic modeling using E-W profiles delineated strong normally magnetized sources of 6 A/ m confined to the west to the fault. Three drillings indicated that the east formations to the fault were wholly disintegrated and the west formations, by contrast, included fresh lavas. These evidences suggest that the major cause of magnetic contrast is not the difference of geologic unit but instead the weakened remanent magnetization in the east adjacent area to the fault. Seismic reflection and shallow drillings indicate the possibility of existence of shallow wrench fault. Bending usuallly appears along a strike slip and actually several basins such as Tanna basin and the Tashiro basin exist along the fault, and some magnetic lows are supposed to be caused by sinking of volcanic layer or pull-apart movements. The striking magnetic discontinuity, on the other hand, implies that the fault surface been displaced ever since Pleistocene has always been the known Tanna fault. In conclusion, the cause of relative magnetic lows in the east to the fault is responsible for effects of, either remanent magnetization reduced, and sinking and truncating of magnetic sources deformed, by the fault activities.

1. Introduction

The N-S trending Tanna fault extends in the middle of the Izu Peninsula (Fig. 1). The Tanna fault is a north fault of the Kita-Izu Fault system which extends for as much as 35 km (MATSUDA,1972). The Izu Peninsula is in the north margin of the Philippine Sea plate which has been subdutced under the Honshu Island (Fig. 1). The subduction of the Philippine Sea plate is thought to be deforming the Kita-Izu fault system at present as an active strike slip fault. The Kita-Izu of magnitude 7.3 occurred in 1930 and several parts of the Kita-Izu fault system were displaced. Major displacements caused by the earthquake

741 742 Y. OKUBO et al.

Fig. 1. Location map and geologic map (KUNO, 1972) of the study area. Location of faults was taken from THE RESEARCHGROUP FOR ACTIVE FAULTS (1980). Ground Magnetic Anomalies in the Tanna Fault and Their Implications 743 emerged along the Tanna fault and the lateral movement of the fault was 1-2 m (GEOLOGICALSURVEY OF JAPAN, 1932). The Tanna fault is about 1 km left lateral fault displaced since middle Pleistocene (KUNO, 1936b). The rate of movement was estimated to be 2 m/1000 year (MATSUDA, 1975). The Tanna fault is surrounded by the young volcanics produced by Quaternary activities of the Fuji volcanic zone (Fig. 1). Two tunnels in east-west direction penetrate the subsurface beneath the Tanna basin which the Tanna fault crosses. One is for usual train and the other is for Shinkansen. The drillings of the tunnel for usual train revealed that the Tanna basin was filled by the volcanics of Taga volcano unconformably covering the Hata basalt (KUNO, 1936a). In the southern San Francisco Bay region, aeromagnetic anomaly map was used to find relation of linear anomalies to the active faults (BRABBand HANNA, 1981). Several elongate magnetic anomalies define steeply dipping ribbons of supentinite within fault zone. However, BRABBand HANNA(1981) noted that low level survey was required to map overall distribution of linear anomalies throughout the study area. The ground magnetic survey along the North Anatolian fault defined a number of coherent magnetic anomalies supposed to be caused by sheet-shaped sources (ISIKARAet al., 1985). A detail and low level or ground magnetic survey has a high possibility to define fault structure which can not be identified by other approaches. In this paper, we show the ground magnetic map along the Tanna fault and discuss a Tanna fault structure inferred from the ground magnetic survey with the help of drilling data and rock magnetism.

2. Ground Magnetic Survey

Ground magnetic measurements were carried out at a grid spacing of 50 to 150 m. The study area extends about 1.5 km from east to west and about 5 km from north to south and the Tanna fault runs through the middle of the study area. Number of measured point is 780 (Fig. 2). Figures 2 and 3 show the total intensity distribution map. We divided these points into several profile lines and observed total magnetic intensity with a proton magnetometer by walking point by point. We walked a profile line and walked back again on the same profile line on the same day so that we measured twice at one measured point. A difference of total intensity between forward and backward going measurements was mainly caused by time dependent variation. The differences resulted in less than 100 nT except for the data above the Tanna tunnels. Above the tunnels the differences often exceed 100 nT, which are probably due to superimposed artificial noise such as tunnels, moving trains, and so on (Fig. 4). Since total intensity distribution in the study area exhibits high amplitude variation of over 500 nT as shown on Figs. 2 and 3, it is eventually not necessary to remove such relatively small differences of less than 100 nT. Then, the average value of two measurements was regarded as a final observed data. The survey was divided into two blocks. The first survey in the Tanna basin was performed on March, 1990 and the remaining survey was performed on August, 1990. The diurnal differences between profile lines were checked at cross points with tie lines in the Tanna basin and with the main profile line where all profile lines of the remaining area started and ended. All differences of final observed data at the cross points were 744 Y. OKUBO et al.

Fig. 2. Total intensity magnetic map of the ground survey in nT and location of measured point . Location of Tanna fault was drawn from GEOLOGICALSURVEY OF JAPAN (1932) , OTUKA(1933), and THE TANNAFAULT TRENCHINGRESEARCH GROUP (1983). Ground Magnetic Anomalies in the Tanna Fault and Their Implications 745

Fig. 3. Colored map of the total intensity obtained from the ground survey overlain by the geologic map shown in Fig. 1. 746 Y. OKUBO et al.

Fig. 4. Examples of the measured magnetic profiles. (a) Ordinary profile of double measurement. (b) Profile superimposed artificial noise. Location of profiles is shown in Fig. 2. within 100 nT. Consequently, we did not correct diurnal variations in as much as they were relatively small in comparison with the high amplitude total intensity distribution. As illustrated in Fig. 3, a distinguished difference of more than 1000 nT in magnetic intensity between the west and the east to the Tanna fault can be found. The fault terminates the relative magnetic highs, therefore, the fault is a remarkable border of magnetic distribution. A typical example can be found in the Tanna basin. Ground surface in the Tanna basin is almost flat (Fig. 5), but a magnetic high occurs just in the west basin to the fault line.

3. Rock Magnetism of Drilling Cores and Cuttings

The New Energy and Industrial Technology Development Organization (NEDO) drilled three holes in the Tanna basins. The one named TN-lS (500 m in drilled depth) was located at a distance of 70 m east of the Tanna fault and the remaining two drillings named TN-2S (500 m in depth) and TN-3 (600 m in depth), respectively, were at a distance of 100 m west of the fault (Fig. 2). TN-1S was drilled partially by core boring from 120 m to 290 m. Cores of TN-2S were sampled at two levels of 200 m and 500 m. Cores of TN-3 were sampled at 7 levels of 56 m, 101 m, 216 m, 263 m, 471 m, 510 m, and 602 m. All of their drillings provided cuttings. Ground Magnetic Anomalies in the Tanna Fault and Their Implications 747

Fig. 5. Shaded relief map of topography. Brighter tone represents relatively higher elevation. Digital data of topography used was created by NAKAZAWA and YAMAGUCHI(1983). 748 Y. OKUBO et al. Ground Magnetic Anomalies in the Tanna Fault and Their Implications 749

Figure 6 represents geologic columns of three drillholes. Volcanic sand, gravel, and mudstone which form the top layer in the geologic column probably correspond to the

Quaternary lake deposits. Drilling of Tanna tunnel for usual train at a depth of about 170 m below the surface revealed that the Tanna basin was filled by lavas and tuffs of the Taga volcano unconformably covering the Hata basalt of Tertiary Usami volcano

(KuNO, 1936a). Therefore, most of the layer below the lake deposits are probably volcanics of the Taga volcano. Stratigraphies of TN-2S and TN-3 are correlative with each other. Their geologic columns include fresh lava flows. The formations contain a high ratio of lava of about 45% in drillhole of TN-2S and of about 50% in TN-3. In contrast, few lavas reside in drillhole of TN-IS and the formations were prominently brecciated and wholly disintegrated (NEDO,1990). In addition, its geologic column does not correlate to the other two drillings. We measured susceptibility, remanent magnetization and normal or reversal magne- tization using cores and cuttings. The result is shown in the geologic columns of Fig. 6. Susceptibility ranges from 0.34•~10-3 cgs/g to 2.07•~10-3 cgs/g. This represents about 1•~10-2 to 5•~10-2 in SI units. The susceptibility is dominantly high and a distinguished difference is not discernible among the drillholes. Remanent magnetization ranges widely from 0.39 A/m to 15.33 A/m. Number of sample was so short that any significant difference of remanent magnetization can not be apparently identified among the drillholes. Under assumption of the average total intensity of 46000 nT as an ambient magnetic field of the earth, Q-ratio or Koenigsberger ratio defined by "natural remanent magnetization"/ "induced magnetization" results to be mostly greater than 1. Measure- ments of exposed rocks revealed that the Taga volcanics possess strong remanent magnetization of 2 to 8 A/m (Koyama, personal communication). Therefore, remanent magnetization is dominant in the total magnetization of rocks filling the Tanna basin. The cores of three drillholes available for measurements are mainly normal in polarity. Since age of Taga volcanics was estimated to be around 0.7 Ma (KURASAWA, 1972) and the Taga volcanics proved to possess predominantly normal magnetization

(KOYAMA and UMINO, 1991), the Taga volcanics was produced during Brunhes normal polarity (0.73-0 Ma) and the formations of three drillholes are supposed to be mostly the Taga volcanics which are younger than 0.73 Ma. Susceptibility measurements of exposed rocks resulted in the Yugawara volcanics which is the younger formation than the Taga volcanics exhibits high susceptibility ranging from 1.8•~10-2 to 2.8•~10-2 (SI), suggesting that the Yugawara volcanics has generally a strong magnetization.

4. Discussions

The main subject discussed here is why magnetic highs occur largely in the west adjacent area to the Tanna fault and magnetic lows conversely appear in the edge of east block. The topographic relief of Fig. 5 represents a N-S trending elongate valley. The west edge of the valley roughly corresponds to the Tanna fault. The pattern of magnetic distribution does not correlate well with the topographic relief and we must seek the cause of magnetic discontinuity beneath the surface. Magnetic modeling using east-west profiles indicates that strong magnetized bodies are confined in the west side to the Tanna fault (Fig. 7). The two-dimensional models of 750 Y. OKUBO et al. Ground Magnetic Anomalies in the Tanna Fault and Their Implications 751 both the south of the Tashiro basin (F-F' profile) and the Tanna basin (G-G' profile) were assumed to possess uniform magnetization of 6 A/m. Induced magnetization calculated from the measured susceptibilities of volcanics is 1 A/m at the most. Therefore, such high values inferred from the modeling lead to conclude that the main cause of relative magnetic highs is the remanent magnetization of magnetic sources confined in the edge of west block to the fault. All evidences of drillhole data, rock magnetism, and magnetic modeling suggest that the cause of the magnetic highs in the Tanna basin is mainly remanent magnetization of the Taga volcanics produced during Brunhes normal polarity. In the northern part of the study area, the cause of magnetic highs in the west is also believed to be the remanent magnetization of lavas of the Yugawara volcano which widely crops out around the Tashiro basin. No correlation between drillhole of TN-IS and the other drillholes implies that characteristics of formations are completely differentiated by the fault. This is an evidence of the displacement. The magnetic contrast differentiated by the fault may be due to the geologic difference. However, there are several evidences to deny the possibility. The magnetic contrast forming almost a north-south trend extends for 5 km throughout the study area. In addition to that, magnetic anomalies show distinctively different values over the same geologic units crossing the fault. The west outcrops of the Taga volcanics, for instance, produce relative magnetic highs but the east outcrops represent relative magnetic lows (see Fig. 3). Such a distribution confirms that geologic difference is not a main cause of the magnetic contrast. There was no strong evidence on the basis of the measurement of rock magnetism that difference of remanent magnetization appears between the east and the west drillholes to the fault. The vicinity of the drillhole of TN-IS exceptionally marks a magnetic high (see Fig. 2). No difference among the drillholes may imply that the site of TN-IS is an exceptional place. Anyhow, the east drillhole (TN-IS) are disintegrated and include few lava flows. Furthermore, a number of NW-SE trending composite faults appear in the east side to the Tanna fault while few faults are in the west side (HOSHINOet al., 1978). These facts suggest the possibility of the east side formations being alternatively wholly weathered or altered by water convection along the branching faults. We believe that weathering causes a magnetic mineral to alter and reduces remanent magnetization, and that weathering is possible to accelerate alternatively to lose magnetization of the east fault dominant block. Otherwise, brecciation and disintegration lost the east block a uniform orientation of remanent magnetization. This effect must cause a weak remanent magnetization of the east block as a whole. Horizontal displacement was estimated to be about 1 km since the middle Pleistocene. This was evidenced by the displacement of middle Pleistocene Yugawara volcanic lava (KUNO, 1936b). The activities of the Taga volcano and the Yugawara volcano range from about 0.7 Ma to 0.4 Ma so that east volcanics have subsequently been displaced northward at least 1 km. The geologic map in Fig. 1 shows such features. In the magnetic map of Fig. 3, on the other hand, a magnetic high related closely to the Taga volcanics in the west side disappear beyond the fault, and in the east side we can find again a small magnetic high on the Taga volcanic outcrop region displaced northward for about 1 km at a distance of about 500 m east from the fault. This offset is 752 Y. OKUBO et al.

Fig. 8. Sketch map of Tanna fault inferred from the ground magnetic survey. inferred to be caused by the left lateral movements. Some basins appear along the Tanna fault. Strike slip produces basins along the boundary between major tectonic plates, especially if the boundary is a complex zone of branching faults (CROWELL, 1974). A strike slip along the fault with a sharp bend generates a sharp pull-apart basin or folds and thrust faults. A strike slip on the fault with marked double bends results in pull-aparts at releasing bends and deformation and uplifts at restraining bends. A strike slip on gently curved fault produces a crowding and rising within convexities of deformable lithosphere plates and a stretching and sinking within concavities. In the case of the Tanna fault, there are several evidences that bends along the fault emerged at the terrain surface. Investigation by shallow drillings of about 20 m revealed that the Tanna basin is composed of two different scale deformation; the larger one is the downwarping of a few kilometers in extent and the smaller one is the pressure ridge of the upper part of the basin fills (YAMAZAKI,1988). Seismic reflection represents a flower structure from the surface to about 500 m depth along the fault (NEDO,1991). A wrench fault generally marks a flower structure, which is a pattern of fault cluster shaped like a flower opening upward usually displayed in the seismic section. At the 1930 earthquake, in fact, a vertical displacement of 0.4 to 0.7 m in addition to the lateral displacement was observed in the Tanna basin. The vertical displacement was so complex that both rising and sinking occurred in the same side of the fault (MATSUDA,1972). Therefore, the fault must have a wrench component in addition to lateral movement. The remarkable border of magnetic highs and lows implies that fault movement always occurred on the same fault surface in the course of repeated fault activities. A wrench fault which causes a rising and sinking expresses a shallow stress field, while the main fault goes down to a deep part. The subsidence of basin was possibly caused by the releasing bend or the stretching and sinking along the strike slip fault with complex zone of branching faults accompanied by surface wrench faults. In the Tashiro basin, the west block may remain almost fixed in Ground Magnetic Anomalies in the Tanna Fault and Their Implications 753 shape as the other east block participates in most of the bending. Otherwise, the basin is a pull-apart fault itself. The magnetic low in Tashiro basin is inferred to represent the sinking of the Yugawara volcanic lava or the pull-apart. The Tanna basin apparently differs from that type. The both east and west blocks seem to share almost equally in the bending (Fig. 8). The remarkable discontinuity of magnetic intensity must be responsible for weak remanent magnetization in the edge of east block.

5. Conclusions

The ground magnetic survey along the Tanna fault represents a contrasting distribution differentiated by the fault. With the help of the drillings in the Tanna basin and measurements of rock magnetism, we conclude that the main cause of the mangetic contrast is a difference of remanent magnetization intensity of Quaternary volcanics produced in the normal polarity. The remanent magnetization of volcanics had been strong initially but was reduced by weathering and reorientations. Rising and sinking of the surface generally appeared along the strike slip fault and these surface bends create basins. Tashiro basin and the Tanna basin were supposedly generated by surface bends. Some magnetic lows east to the fault is inferred to be the effects of surface sinking and pull-apart movement. Seismic reflection represents the occurrence of wrench fault along the Tanna fault. But remarkable discontinuity of magnetic distribution suggests that the fault surface displaced ever since middle Pleistocene was the Tanna fault which was displaced by the 1930 earthquake. The wrench fault is probably an evidence of shallow stress field caused by the strike slip. The relative magnetic lows in the east to the fault are attribute to reducing remanent magnetization by weathering, reorientation of formations and sinking or truncating of the magnetic sources.

We wish to express our appreciation to T. Miyazaki and M. Kawamura of the Geological Survey of Japan who helped us to obtain financial support and gave much encouragement, Y. Honkura of the Institute of Technologywho encouragedus about magnetic surveyof active faults, H. Yamazaki who provided much information about the Tanna fault, S. Shinada and K. Okazaki of the Japan Petroleum Exploration Co., Ltd. who gave the idea of susceptibility measurement of cuttings and helped us to obtain the data, M. Koyama of the Shizuoka University who provided much information about rock magnetism, S. Rokugawa of the University of Tokyo and Y. Aoki of the Japan Petroleum Exploration Co., Ltd. who discussedthe seismic structure of the Tanna basin at a series of meetings, K. Kishimoto of the Geological Survey of Japan who helped us to make the colored magnetic map and the shaded topographic map, C. Finn of the U. S. Geological Survey and guest researcherof the Geological Survey of Japan who gave comments on implication of magnetic anomalies along the lateral fault, the staffs of the Sogo Geophysical Exploration Co., Ltd. who performed the high quality ground magnetic survey,the staffs of NEDO who endeavoured to make drillings and seismicdata available, the staffs of the Association for the Development of Earthquake Prediction who promoted our investigation, and the two anonymous referees who gave helpful comments. 754 Y. OKUBO et al.

REFERENCES

BRABB,E. E. and W. F. HANNA,Maps showing aeromagnetic anomalies, faults, earthquake epicenters, and igneous rocks in the southern San Francisco Bay Region, California, U.S. Geological Survey Geophysical investigations Map GP-932, scale 1;125,000, 1981. CROWELL,J. C., Origin of late Cenozoic basins in southern California, in Tectonics and Sedimentation, edited by W. R. Dickinson, pp. 190-204, Society of Economics Paleontologists and Mineralogists, Oklahoma, 1974. GEOLOGICALSURVEY OF JAPAN,Report on Kita-Izu earthquake, Rep. Geol. Survey Japan, 112, 111 pp., 1932 (in Japanese). HOSHINO,K., H. HASHIMOTO,and T. MATSUDA,Active faults in Izu Peninsula, Tectonic Map Series 4-1, Geol. Survey Japan, 1978. ISIKARA,A. M., Y. HONKURA,N. WATANABE,N. ORBAY,D. KOLCAK,N. OHSHIMAN, O.GUNDOGDU, and H. TANAKA,Magnetic anomalies in the western part of the north Anatolian Fault zone and their implications for active fault structure, J. Geomag. Geoelectr., 37, 541-560, 1985. KOYAMA,M. and S. UMINO,Why does the Higashi-Izu monogenetic volcano group exist in the Izu Peninsula?: Relationships between late Quaternary volcanism and tectonics in the northern tip of the Izu-Bonin Arc, J. Phys. Earth, 39, 391-420, 1991. KUNO,H., The geologic section along the Tanna Tunnel, Bull. Earthq. Res. Inst., 14, 92-103, 1936a. KUNO,H., On the displacement of the Tanna fault since the Pleistocene, Bull. Earthq. Res. Inst., 14, 619-631, 1936b. KUNO,H., Geologic Map of Volcano and the Adjacent Areas, Okubo-Syoten, Tokyo, 1972. KURASAWA,H., Volcanoes and volcanic rocks in Izu Peninsula, Japan-with regard to genesis of volcanic rocks, in Izu Peninsula, edited by M. Hoshino and H. Aoki, pp. 155-184, 1972 (in Japanese with English abstract). MATSUDA,T., Surface faults associated with Kita-Izu earthquake of 1930 in Izu Peninsula, Japan, in Izu Peninsula, edited by M. Hoshino and H. Aoki, pp. 73-93, 1972 (in Japanese with English abstract). MATSUDA,T., Magnitude and recurrence interval of from a fault, Zisin, 28, 269-283, 1975 (in Japanese with English abstract). NAKAZAWA,S. and Y. YAMAGUCHI,On radar image simulation, Report on Showa 57 Fiscal Year Sunshine Project, Basic Study on Nation-wide and Regional Geothermal Base Map, pp. 117-131, 1983 (in Japanese). NEWENERGY AND INDUSTRIAL TECHNOLOGY DEVELOPMENT (NEDO), Confirmation study of the effectiveness of prospecting techniques for deep geothermal resources, development of exploration technology for fracture type reservoir, using elastic wave (drilling, logging, well-test, and characterization of fracture), Report abstract, 1990, 1991 (in Japanese). OTUKA,Y., The geomorphology and geology of Northern Idu Peninsula, The earthquake fissures of Nov. 26, 1930, and the pre- and post-seismic crust deformations, Bull. Earthq. Res. Inst., 11, 530-574, 1933. THE RESEARCHGROUP FOR ACTIVEFAULTS, Active Faults in Japan-Sheet Maps and Inventories, 363 pp., University of Tokyo Press, 1980. THE TANNAFAULT TRENCHING RESEARCH GROUP, Trenching study for Tanna Fault, Izu, at Myoga, , Japan, Bull. Earthq. Res. Inst., 58, 797-830, 1983 (in Japanese with English abstract). YAMAZAKI,H., Subsurface geology of Tanna Basin in Izu Peninsula, Central Japan, J. Geography, 97, 69-84, 1988 (in Japanese with English abstract).