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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.

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The distribution of granitic rocks occupies 12 percent of the Japanese Islands, but the physical properties of the granitic rocks such as density, porosity, natural remanent magnetization (NRM), and ultrasonic velocity have only been reported for small limited areas with an exception of magnetic susceptibility for the regional areas. The aim of this study is to clarify the physical properties of granitic rocks in the Japanese Islands by the systematic measurements under the same precision and to try to discuss the characteristics of each exposure area and the differentiation process of magma. Samples were collected at about 570 outcrops from four areas (Kitakami Mountains, Taiheizan, Kurikoma-Narugo and Murakami areas) in the northern Tohoku region. All the samples were measured on density, magnetic susceptibility and NRM. The results are summarized as follows: 1. Density The mean density of these areas varies from 2.66 to 2.77(g/cm 3 =10 3 kg/m 3), the lowest one is Murakami area, the second Kurikoma-Narugo area, the third Taiheizan area and the highest is Kitakami Mountains. 2. Porosity There is no large difference between the four areas. The variation range is about 0.4 0.5(%). 3. Magnetic susceptibility All the samples can be sectioned within a zone bounded by two straight lines, in a densitymagnetic susceptibility chart: The samples in these four areas lie in each characteristic zone. Magnetic susceptibilities near along the lower line are subject to the content of paramagnetic rock-forming iron minerals in granitic rocks. 4. Natural remanent magnetization (NRM) The relation between the density and NRM shows a pattern similar to that between density and magnetic susceptibility but less correlative. Königsberger ratio (Q n) of 70 percent or more granitic rocks shows less than 0.4 in this region. Consequently, NRM is negligible for a case of analysing magnetic anomaly in the field.

Several linear magnetic anomalies over continental crust have been identified in and around the Japanese Islands. The anomalies are probably related to island arc tectonic structures, but identifying specific sources has been difficult. Several deep holes were drilled in and around Aso caldera, where a linear anomaly occurs along an active fault. One drillhole located on the linear anomaly encountered a zone of highly magnetized and altered basement rocks at least 100 m thick at a depth of -1000 m. The other hole was located away from the anomaly and did not encounter any high-magnetic zones. Rocks from the zone have exceptionally strong remanent magnetization (several tens of A/m) sub-parallel to the present field. AF demagnetization experiments indicated that the magnetization is hard and stable. Magnetic modeling indicates that the linear anomaly is caused mainly by this layer. Microscopic examination of core samples shows that the highly magnetized zone includes secondary magnetic minerals and abundant hydrothermal alterations. Temperatures determined by fluid inclusions and down-hole temperatures show that the temperature of the highly magnetized zone was elevated in the past relative to surrounding rocks. The high temperature could destroy primary magnetic minerals and replace them with secondary magnetic minerals. Thus, the past hydrothermal system may have enhanced thermo-chemical remanent magnetization. The results can produce a model indicating that there was a past hydrothermal system related to the tectonic structure.