Ikuo Katsura

Magnetotelluric survey of an active fault system in the northern part of kinki district, southwest japan

Abstract Combined geological and geophysical surveys are effective in finding hidden active faults and estimating the dimensions of their fracture zones in the northern part of the Kinki District, southwest Japan. The magnetotelluric method using electromagnetic energy in the ELF frequency band can define the dimensions of a fault and suggest whether it is active or not. The results of the survey show that individual active strike-slip faults may be linked through new faults found in unexposed areas. The distribution of major active faults suggest that they form conjugate sets defining boundaries to tectonic blocks. At least seven tectonic blocks are recognized with different patterns of occurrence of microearthquakes and distributions of active faults. Historically large earthquakes, magnitude >5 as determined by maximum velocity amplitude, occur at the boundaries of the blocks. These patterns are caused by the different stress fields applied to each block.

Origin of stable remanent magnetization of siliceous sediments in the central equatorial Pacific

Abstract Siliceous sediments distributed in the central equatorial Pacific have a stable remanent magnetization regardless of age or depth. This is in contrast with the unstable remanence of unfossiliferous red clay (defined as “pelagic clay” here) which accumulates in the middle latitudes of the Pacific. We have conducted a rock-magnetic study of the siliceous sediments to clarify the magnetic carriers of the stable remanence. For this purpose, we combined magnetic granulometry by the suspension method and observation of magnetic grains with a transmission electron microprobe (TEM). The advantage of the former is that the magnetic extraction procedure is not required for obtaining size distribution of the magnetic grains in sediments. The TEM observations revealed that most of the magnetic extracts are identical in size and shape to bacterial magnetosomes. The suspension method proved that the magnetic assemblages of the siliceous sediments of both Quaternary and early Miocene age have mean diameters of about 0.05 μm, which is within the single-domain range of magnetite, and have narrow size distributions compared with pelagic clay. These characteristics can be explained if the magnetofossils observed by TEM are the major constituent of the magnetic grains in the sediments, and have been preserved for a long period of time. The difference in the stability of the remanent magnetization between the siliceous sediments and the pelagic clay can be explained by the difference in the size distribution of magnetic grains, which would reflect differences in their sources, i.e., biogenic vs. detrital (eolian).

Magnetic glasses in the Azuki and AT volcanic ashes in Japan

Abstract Magnetic properties of volcanic glass shards have been described on the widespread Azuki and AT ashes in Japan. Thermomagnetic analysis and isothermal remanent magnetization (IRM) acquisition experiments were performed on both bulk samples and separated glass samples of the Azuki volcanic ash. Together with progressive thermal demagnetization of natural remanent magnetization (NRM), the results demonstrate that titanomagnetite contained in volcanic glass shards is the sole contributor to the stable NRM of the Azuki ash. The results of a Lowrie-Fuller test suggest that the remanence carrier of the ash layer is single-domain grains of titanomagnetite, which can be seen as submicron particles using electron microscopy. On the basis of progressive IRM acquisition in samples with various contents of AT volcanic glass, it is also suggested that titanomagnetite in glass shards is a significant magnetic carrier of NRM for the AT ash. Since most of the glass shards are tens of microns in size, they are expected to acquire depositional remanent magnetization (DRM) without experience of succeeding post-depositional process under aqueous conditions. A redeposition experiment with the Azuki ash shows that stable DRM, whose intensity is comparable to that of the NRM of the water-laid Azuki ash, is acquired during several hours.

Resistivity Structure of the Hanaori Fault, Southwest Japan, using ELF-MT method

A magnetotelluric survey was carried out at the Hanaori fault area in northeastern Kinki district. Apparent resistivities at four frequencies of ELF band (7.8, 14.0 and 20.4Hz) and VLF band (17.4kHz) were measured along two survey lines perpendicular to the strike of the fault. Two-dimensional analysis using the finite-element method gives an underground resistivity model across the Hanaori fault. The model indicates that a low resistivity zone is 1.4km wide and extends to a depth of more than 800m. We interpret this zone as resulting from fault activities.