Taking all things of earthquake protection into consideration, the dynamic properties of buildings are extremely important, because the defect of them may lead to the cause of heavy casualties. We have investigated the dynamic properties of many residences (about 110 buildings), namely the natural period T₀, quality factor Q, and maximum amplification ratio R₀, by use of microtremors analysis of the ground and buildings. The earthquake resistant capacity index C has been introduced such an empirical formula as
C=100 · T₀² · √Q · R₀.
The validity of this method has been examined through the investigation of the dynamic properties of small-scale buildings of four construction types, being referred to the wooden residences damaged from Kobe earthquake in 1995.

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It is useful to employ jacking method for construction of lifelines. We have developed an electromagnetic positioning system of underground pipes at the depth between 10 and 15 m. This system consists of a receiver coil on the ground surface and a transmitter coil in the subsurface pipe. The used frequency is approximately 1 kHz and the magnetic moment of the transmitter coil is around 50 ATm².
The prototype system estimates a transmitter position as a cross point of detected magnetic lines at some receiver points. The estimation error of 0.5∼2.0 m resulted from the field tests is higher than the requested one. The improved system synchronizing a receiver clock with a transmitter and limiting a search area small over the transmitter was developed. This system has two measuring modes of rough and detail search. In the first mode, the maximum voltage induced in the vertical receiver coil indicates the transmitter position roughly. In another mode, an exact position is found as a point indicating zero voltage in the horizontal receiver coil. An exact point is shown visibly and easily in the graphic screen on the receiver.
Results obtained in some jacking pipe sites proved that the improved system achieved the tolerance below 5 cm horizontally.

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Airgun arrays are often used in marine seismic experiments, almost of which are tuned to have a strong vertical directivity of source energy for effective observation of tectonic structures just beneath the arrays. The tuned-array, however, does not work effectively in some seismic experiments, such as a wide-angle seismic reflection and refraction surveys. In such surveys, the obliquely radiating wave filed is more important than a vertically down-going one, because the reflections from some targeting reflectors have large incident angle. To develop a marine seismic source having variable directivity, we tested the beam forming method to control the directivity of an airgun array of eight 1500 cu. in. (25 l) guns in a seismic experiment off Boso. By increasing the shooting delay times among guns, the directivity of the source was successfully controlled from vertical to oblique. Thus, the experiment demonstrated high potentiality of the source directivity control by the airgun array using the beam forming. The directivity control may have a great advantage for seismic reflection surveys with long source-receiver offset and refraction surveys using ocean bottom seismometer (OBS) in deep structural studies targeting the seismogenic zone, the lower crust, the crust-mantle boundary and so on.

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It has been almost three decades since geoelectromagnetic observation at the seafloor was enabled. The objective of this article is to make an overview of the present status of the seafloor electromagnetic observation and its potential in the future. Specifically, it consists of: (1) Characteristics of the natural electromagnetic fields at the seafloor with a special reference to the difference between those on land. (2) Reviews on one- or two-dimensional electrical conductivity models derived by seafloor magnetotelluric array measurements, which were recently conducted around Japan and overseas. (3) Future challenges of geophysical exploration at sea, viz., controlled-source marine electromagnetic prospecting and rather difficult instrumentation in shallow seas.

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Submarine electric sounding, which is conducted by towing a measuring cable behind a survey vessel, is a kind of electrical survey methods below sea surface. This method is applied to investigate dredging areas of channels, geological structures and mineral resources at offshore areas in foreign countries. In Japan, only some field tests of the method were carried out during 1950-1953 and in 1965. A new survey and analysis system is developed using the latest innovation techniques. After some field studies, this system has been improved for the practical use and is proved usefulness for marine engineering surveys. This paper describes the outline of the developed system and case studies of it.

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The forearc crust along the Nankai Trough is characterized as an accretionary prism. The unconsolidated sediments on the subducting Philippine Sea plate are underplated to the southwest Japan arc, so that large amount of fluid is supplied to the accretionary prism. Such fluid could be related to various geological phenomena in the accretionary prism such as the accumulation of methane hydrate, expected as new energy resource. However, fluid distribution below the seafloor, in the forearc crust, had not been understood clearly. Marine magnetotelluric soundings around the Nankai Trough and the Kumano Basin were carried out in 2002-2003 to elucidate fluid distribution in the forearc crust. Both time series data of horizontal electric and magnetic fields were obtained at seven sites, and only electric field data were obtained at two sites. Apparent resistivity and impedance phase at each site were calculated from these observed data with removal of spike noises, and a resistivity model below the seafloor was estimated. The model resembles to a seismic reflection section. By interpreting the estimated model, fluid distribution in the accretionary prism and the upper oceanic crust is discussed: high fluid content within the accretionary prism is interpreted, especially along the splay fault. This result implies a pass way of fluid along the splay fault.

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