Anisotropy of magnetic susceptibility (AMS) was studied in three Plio-Pleistocene turbiditic mudstone sequences accumulated in the foreland basin of southwestern Taiwan. These formations were incorporated in the front units of the collision belt and underwent folding and thrusting during the last 2 Ma. Five types of fabrics were identified from more than 3, 000 samples collected in 352 sites, with 251 sites allowing determination of a magnetic lineation. NNE-SSW trends are predominant, minor N-S and NE-SW trends are present. Magnetic lineations are widespread in the lower section where folds are tight, and scarce in the youngest sediments where folds are gentle. The strong correlation between the structural features and the AMS orientations suggests a tectonic origin for most magnetic lineations superimposed on the initial flattening that results from sediment compaction. This is confirmed by tectonic studies based on structural analysis and paleostress tensor reconstructions. The tectonic studies reveal a major WNW-ESE compression, which provide orientations of compressive tectonic regimes consistent for resulting the magnetic lineations. In contrast, the hypothesis of a sedimentary origin can be ruled out in most cases, because the orientations of magnetic lineations and those of depositional fabrics (paleocurrents, sediment supply directions and even slumps) are oblique at a variety of angles. Furthermore, based on magnetostratigraphy, we conclude that this compression culminated about 0.9-1 Ma ago. Earlier minor events, NW-SE and W-E compression, have also been found and we propose that they have occurred in approximately 1 and 2 Ma ago, respectively. Thus, the main cause of AMS trend is thought to be the WNW-ESE Quaternary compression responsible for major folding and thrusting. In addition, the magnetic fabric of tectonic origin is absent, or poorly marked, in formations younger than about 0.9 Ma to the north. However, it is still recognized but decreased after about 0.7 Ma ago to the south. This indicates that the WNW-ESE compression propagated southward between 0.9 and 0.7 Ma ago, consistent with the migration of folding and thrusting during the last Taiwan collision.

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Magnetotelluric observations were carried out in the southernKyushu district of southwestern Japan to investigate the characteristics of the electrical resistivity structure of a high-angle subduction zone. We constructed a 2-D resistivity model parallel to the subducting plate motion by using the inversion technique with the Akaike Bayesian Information Criterion (ABIC) smoothness constraint. The general features of the obtained resistivity structure are as follows: (1) a conductive block (below 1 Ω·m) is found beneath the volcanic zone and is widespread bilaterally below 40 km depth, (2) a resistive block (about 1000 Ω·m) distributes from 10 to 25 km depth in the forearc region and (3) a conductor (1-30 Ω·m) is embedded beneath the resistive block, which may correspond to the negative Bouguer gravity anomaly observed in this region. We propose the following for the high-angle subduction zone: A serpentinized block is generated in the lower crust of the forearc region and a partial melting and hydrothermal fluid are well developed beneath the volcanic front.

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An all sky optical imaging system was operated from Sriharikota rocket range (SHAR) (14°N, 80°E, 5.5°N dip latitude) during January-March, 1993 to observe ionospheric plasma depletions through 630 nm and 777.4 nm night glow emissions. Strong plasma depletions were observed only on four nights viz., 14, 17, 19 and 21 February, 1993. Except the 17 February, which was a magnetically disturbed day, all the other nights pertained to magnetically quiet period. A number of plasma depletion parameters such as, degree of depletion, east-west extent, tilt with respect to the geomagnetic field, inter-depletion distance, drift velocity and plasma enhancements or brightness patterns were estimated. Some of the important results are: (a) It was found that the east-west extent of plasma depletions varied with the degree of depletion; for the 630 nm images the degree of depletion ranged between 6-9% per 100 km east-west extent and for 777.4 nm images it was 3% per 100 km east-west extent, (b) The average inter-depletion distance (IDD) was in the range of 1500±100 km during the magnetically disturbed period and 740±60 km during quiet periods. This is suggestive of gravity wave modulation of the bottom side of the F-region. While the large scale gravity waves (1500±100 km) of auroral origin could be responsible during magnetically disturbed period, smaller scale gravity waves (740±60 km) having their origin in the lower atmosphere could produce initial perturbation in the bottom side of the F-region, (c) Plasma depletions are observed to have an eastward tilt in the range of 10-15° with respect to the geomagnetic field. It has been suggested here that these tilts are associated with the variation of plasma drift with altitude, (d) plasma depletions are observed to be moving eastwards with drift velocities in the range of 40-190 ms^-1, and (e) Strong plasma enhancements or brightness patterns were observed on three nights. The degree of enhancement was by a factor of 1.4-3.8. These enhancements lasted for more than 15 minutes. Although, prima facie, these observations look similar to the transient brightness wave reported by Mendillo et al. (1997a), the high degree of enhancement and an extended duration of more than 15 minutes, observed in the present case, need to be understood.

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The dependence of the energy of ions on their mass could be a useful tool for the identification of possible acceleration processes in the Martian plasma sheet. The study is based on a few cases when heavier ions and protons were registered simultaneously in the plasma sheet by the TAUS energy spectrometer, and on published data of the ASPERA instrument. The analysis of mass dependence of the ion energy in the plasma sheet suggests that frictional preacceleration of ions probably in the pole regions of the Martian magnetosphere can be responsible for not more than 40% of the energy gain of heavy ions in the plasma sheet, the cross-tail current sheet acceleration yields up to 70%, and from 30 to 70% of the heavy ion energy is provided by the acceleration due to magnetic field line stresses (and/or electric field aligned acceleration).

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The Yamasaki fault system is a well known major, active fault in South-Western Japan. Large historical earthquakes have occurred along it. Several resistivity models of the Yamasaki fault have been proposed as the result of intensive electromagnetic investigations during the 1970's and 1980's. In 1996, a trench study was made of the cross-section of the Yasutomi fault which is the kernel of the Yamasaki fault system. The cross-section showed the occurrence of past major paleo-earthquakes. To clarify the deeper structure of the fault, multiple electrode resistivity soundings were conducted across the fault in 1997. Although the target sounding depth of the previous EM survey is different, the resistivity measurements show the boundary structure of the central part of the fault beneath the trench survey area. They also show the extent of a shallow conductive zone across the fault.

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Seismic activity suddenly started on June 26, 2000 beneath Miyakejima, a small volcanic island located on the Izu-Bonin island arc. Then a series of swarm activity migrated toward the northwest to adjacent volcanic islands, Kozushima and Niijima, with intermittent quiet periods and spatial shifts of active areas. During the period from June 26 to the end of August, numerous earthquakes occurred, including five earthquakes of magnitude ≥ 6.0. Here we present the outline of the swarm activity, in particular its spatio-temporal characteristics.

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In the end of June, 2000, intense crustal activity took place in Miyake-jima, Niijima, Kozu-shima and their vicinity. Here we report on the crustal deformation in the area during the period from June 24 to September 4, 2000, detected with the nationwide Global Positioning System (GPS) array operated by the Geographical Survey Institute. The deformation in this area during the above period is characterized by the deflation of Miyake-jima and the extension of the crust in the northeast-southwest direction over a wide area.

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The Miyakejima observation network had been constructed by the National Research Institute for Earth Science and Disaster Prevention mainly until early 1999. This observation network has provided the crustal deformation data by tiltmeters and GPS and the seismic data by short-period and broadband seismometers in association with the 2000 Miyakejima eruption. The subsurface magma movement at the first stage of the present activity, during the period from June 26 to 27, was successfully detected mainly by the tilt measurements. The tilt change observed at five stations indicates the migration of magmas from the eastern part of Miyakejima to the western part. The most distinctive phenomenon appearing after the first stage is tilt steps, which started on July 8 with the first eruption from the summit crater. Each tilt step indicates an abrupt uplift of the summit area. These tilt steps continued until the eruption of August 18, which is the largest eruption up to early September, 2000. 45 tilt steps in total were observed in this period. The seismic data show a variety of seismograms including VT (volcano-tectonic) earthquakes, LF (low frequency) earthquakes and volcanic tremor. At the time of the tilt steps, very long period events with predominant periods of about 100 s were detected by the broadband seismometers. As the activity has still continued, this report summarizes the observation during June, July, and August, 2000.

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