Recent work on time dependent seismicity is mainly based on the so-called “regional time predictable model”, which is expressed by the relation:
logT_t = cM_p + a
where T_t is the interevent time, i.e. the time between two successive mainshocks of a seismogenic region, and M_p is the magnitude of the precedent mainshock. Parameter a is a function of the magnitude of the minimum earthquake considered and of the tectonic loading and c is a positive (≈0.3) constant. A problem of the method, as it has been applied till now, is its dependence on the zonation, that is, on the definition of the seismogenic regions, which is subjective to some extent. In the present work a different approach, which assumes no a-priori regionalization of the area, is attempted in order to check the validity of the model. Grids of equally spaced points at 0.5° have been created for Greece and Japan and the mainshocks located within each circle with center at a point and radius varying between 30 and 150 km were considered. When the number of mainshocks within the circle was four or larger, regression was performed and the c value was calculated. In about 75% of the cases for Greece with sufficient data and 80% for Japan the parameter c was found to be positive. This result strongly supports the validity of the model.

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Asmall-scale steam explosion occurred on Karanuma crater on the summit of Akita-Yakeyama volcano on August 16, 1997 after a dormancy of 46 years. Chemical compositions of the fumarolic gases at the summit and hot spring waters around the volcano were monitored before the eruption
Obvious changes in the composition and outlet temperatures of the fumarolic gases were not detected, neither before nor after the 1997 eruption. Hydrogen and oxygen isotopic ratios of the gas condensates and hot-spring waters at the Yunuma crater indicated that a hydrothermal reservoir, where the fumarolic gases separated from the hot-spring waters at 150°C, existed in a shallow place beneath the crater.Smectite, kaolinite and pyrophyllite were identified in the clay fraction of the volcanic ejecta. Although pyrophyllite should have been formed at about 1 km beneath the summit, it was not directly derived from the deep zone during the 1997 eruption but had been ejected by previous eruptions. The Cl/S values of the water leachates of the ejecta were about 0.7, which indicated that the volcanic gas which caused the eruption was rich in HCl. However, the fumarolic gases and the water samples collected from the summit area contained little chloride. The source of the water-soluble chloride might be high-temperature magmatic gases that have been estimated as the source of Cl-SO₄ type thermal water. Such magmatic gases might have caused the 1997 eruption.

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With the establishment of the new geomagnetic field observations in the Ocean Hemisphere Network Project (OHP) in Japan, minutes values of geomagnetic components, H D and Z have been recorded. The hourly mean values were used to study the variations in these three components at these new equatorial electrojet regions. The results of the analysis carried out revealed that the amplitude of dH has diurnal variation which peaks during the day at about local noon in all the three equatorial electrojet regions. This diurnal variation in H with Sq(H) enhancement in all the three regions are attributed to the enhanced dynamo action at these regions. Diurnal variation as observed in D indicates that the equatorial electrojet current system has both east-west and north-south components. The pronounced magnitude of Z variation as observed in Kiritimati is attributed mainly to sea induction. Also some abnormal features were observed on 23rd of January at Huancayo, in the components. Seasonal variations with more pronounced equinoctial maximum were observed in H than in Z. D component showed no consistent seasonal variation in all the regions. The equinoctial maximum is due to enhanced equatorial electron density at equinox. More research work, if carried out in these new regions will be useful in making more new contributions to the field of the dynamics of the equatorial electrojet region.

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Five geoelectric potential difference (electric field, here after) monitoring stations have been in operation since September 1997 in an area near Liwa town, southern Sumatra, Indonesia, to examine the relationship between electric field changes and earthquakes. Short-term electric field variations were found to correspond mainly to geomagnetic activity, while long-term variation was mostly gradual shift and was clearly correlated neither precipitation nor ground water level variations. Co-seismic electric field changes ranging between 1 and 8 mV were observed for five mb > 5 earthquakes at multiple stations during September-December 1997. The epicenters of the earthquakes were in the Indian Ocean within about 170 km from the monitoring sites.

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This paper presents a case study of Electromagnetic (EM) signals associated with earthquakes due to the piezoelectricity of crustal rocks. For a simple model of crustal structure with a subsurface piezoelectric body, a mathematical expression was obtained that describes the behavior of piezoelectric EM signals due to incidence of a teleseismic bodywave. Using this expression, we evaluated expectedEMsignals with physical parameters reasonable for crustal rocks. Results of the frequency domain analysis suggested that the intensity of the signal decreases with decreasing frequency due to decreasing stress rate at lower frequencies, and decreases with increasing frequency due to EM attenuation in the conducting medium at higher frequencies. However, the latter (the skin effect) was shown to be negligible at the dominant frequency range of seismic waves so far as a shallower piezoelectric body is concerned. Numerical results also indicated a resonant feature of the piezoelectric EM signals corresponding to geometry of the subsurface piezoelectric body. However, numerical calculations suggested that such signals cannot be detected except for strong motions. If detected, on the other hand, their spatial and frequency characteristics will provide information on the geometry of the subsurface piezoelectric body.

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During the first part of a new set of airglow observations in the OH (6-2) and the O₂ (0-1) Atmospheric bands from El Leoncito, Argentina, that started in August 1997, periods of surprisingly low intensities stand out. For many nights, mean O₂ intensities reach only 30%, or less, of what we have seen in previous campaigns, from 1984 to 1994, at the same site, or a similar latitude. Such an anomaly is also observed, to a lesser degree, in OH intensities. An instrumental artifact in the intensity results can be ruled out by using the signal from the stellar background for a“field calibration”. A relation to the strong ENSO event with its major climate impact in 1997/8, may, of course, be suspected, although a statistical inference would be weakened by the scarceness of historical precedents of a similar magnitude.

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Behavior of fast-magnetosonic-mode MHD signals in the inner magnetosphere that are driven by the impulsive eastward current is investigated as a model of the Pi2 signal at midnight. The magnetosphere is treated as an axisymmetric coldMHDregime with dipole magnetic fields and has the plasmaspheric structure of the Alfven speed distribution. MHD perturbation is assumed to be axisymmetric. Numerical calculation revealed the following: 1) The impulsive current induces the plasmasphere virtual resonance oscillation; 2) The compressional magnetic field perturbation is confined near the equator; 3) The waveform of the compressional magnetic perturbation in the plasmasphere depends on the spatial extent of the source current, its temporal variation, as well as its location; 4) The typical Pi2 waveform in the plasmasphere is obtained when the source current is located near the plasmapause (L≤10); 5) When the source current is not located on the equator, the compressional component and the poloidal component have different waveforms.

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The current density carried by photoelectrons emitted from the GEOTAIL spacecraft is estimated from the electric potential of the spacecraft measured in the single probe mode of GEOTAIL/EFD and plasma density and temperature obtained by GEOTAIL/LEP during the period from September 14, 1993 to October 31, 1998, by assuming balance of the currents carried by photoelectrons and ambient thermal electrons. Behaviour of the photoelectron current as a function of spacecraft potential is consistent with the current profile predicted by Grard (1973), and the emitted photoelectrons consist of several components with different temperatures. The saturation density of the low energy component of the photoelectron current is 85±33×10^-6 [Am^-2]. Number density of the photoelectrons is estimated to be 2.9±1.4×10⁹ [m^-3] at the surface of the spacecraft, and the average energy of the photoelectrons is 2.1±0.5 [eV]. These values are higher than the prediction by Grard but consistent with previous in-flight measurements from GEOS-1, ISEE-1 or Viking.

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On August 17, 1999, a destructive earthquake occurred in the western part of the North Anatolian Fault Zone, Turkey. The earthquake source region has been designated as a seismic gap and an M7-class earthquake has been supposed to occur someday in the future so as to fill this seismic gap. So far we have undertaken various kinds of observations in this area and we could obtain some valuable data before, during and after the mainshock. Here we report some of the preliminary results of our recent studies, which include field work started in late July this year and continued during and after the earthquake occurrence just in the earthquake source region and its vicinity, in addition to seismic observations carried out for several years before the mainshock. Much emphasis is put on magnetotelluric field data acquired during the mainshock; in fact, large variations caused by seismic waves were recorded. Such variations could be interpreted in terms of electromagnetic induction in the conducting crust caused by the velocity field interacting with the static magnetic field of the Earth. In particular, the first motion of seismic wave could be identified in the records and used for precise determination of the hypocenter of the mainshock.

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Magmatic reflectors have been detected beneath some volcanic regions by analysing some reflection phases of seismograms. In this letter we consider a problem to determine the location and tangential plane of such a reflector from the ray parameter, back-azimuth and travel time of a reflection phase, and present a scheme to solve it. It is then shown that the problem is essentially a one-dimensional root-finding problem whose independent variable is only the depth of the reflection point, and thus it can be solved by a line search method such as the one-dimensional grid search. While homogeneous velocity structure models have been used in the conventional analyses, the present scheme employs a vertically inhomogeneous one and can also easily treat converted phases at the reflector.

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