Travel time inversion of local earthquake data was used to improve a one-dimensional (1-D) P-wave velocity structure model for the northern Red Sea area, including the entrance of the Gulf of Suez and the Gulf of Aqaba. The 1-D velocity model and station corrections were obtained by using 1538 P-arrivals from 216 well-located earthquakes recorded at ten local seismic stations from August 1994 to January 1999. Earthquakes used in the present study were located in depths less than 28 km so that the upper 20 km of the 1-D velocity model was well constrained. The resulting P-velocity model indicated velocities from 5.0 to 6.0 km/sec down to a depth of 10 km, and velocities from 6.0 to 6.8 km/sec with a nearly constant gradient in a depth range from 10 to 20 km. This 1-D velocity model reduced the RMS residual by 47% from 0.21 to 0.11 sec and improved routine earthquake locations, which clearly indicated clusters of hypocenters in the southern tip of the Sinai Peninsula and the entrance of the Gulf of Suez. The resulting station corrections suggested irregular surface geology in the area. Negative and positive station corrections were obtained; the minimum value (-0.35) was observed at the station MAZR, while the maximum value (0.24) at SHRM.

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Since autumn 1993 the BIFROST project has provided daily GPS solutions of geodetic positions from a network of more than 40 stations covering a large area of the Baltic shield. This area is expected to show large vertical motion due to glacial isostatic rebound following the deglaciation at the end of the Pleistocene. This paper will discuss the inference of three-dimensional rates of crustal motion at the GPS stations with respect to (1) a plate-fixed average for the horizontal components; (2) a geocentric reference in order to infer absolute changes of sea level from vertical crustal motion and models of geoidal rebound. We show that the horizontal strain rate pattern is largely dominated by unilateral extension and not exhibiting horizontal shear to an important extent. In regard to the vertical motion a crucial issue is the stability of the geocentre in the GPS frame. We show results from an Empirical Orthogonal Function analysis that attenuates regionally correlated noise. In all components our observations suggest reasonably close agreement with forward computions on the basis of postglacial isostatic adjustment. A dominant tectonic signal would lead to a certain fraction of the batch of baselines to exhibit shortening. A tectonic process leading to a similar pattern of horizontal motion as expected from postglacial rebound can safely be dismissed in the context of the currently accepted plate tectonic setting. Thus, our baseline rate comparison will be a critical first order test of the prevailing style of deformation.

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A numerical simulation of Newtonian viscous flow without inertia terms in a 3-D spherical shell driven by the negative buoyancy due to the slabs has been conducted to understand the effects of weak plate margins on the plate motions. Density loads are inferred from the seismicity and the reconstruction of the subduction history. The toroidal energy of plate motion comparable to the poloidal energy appears, when γ (ratio of the viscosity at margins to that of interiors) becomes O(0.01). For the whole mantle density model, all the plates move too fast relative to the Pacific plate. The direction of major plate motions is generally improved by the inclusion of weak plate boundaries. The density loads in the upper mantle appear to explain the overall plate motions, although some of the plate motions may require hidden and/or deeper density anomalies to be consistent with the observations. As γ decreases, the geoid anomalies associated with the upper mantle slabs change their signs. This reversal affects the long-wavelength components of the geoid anomalies. A considerable part of the horizontal stress field shows a horizontal extension suggesting that another type of density anomalies is necessary to explain the general compressional field of the real Earth.

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A change in the chemical compositions of volcanic gases is one of the noticeable phenomena that frequently occurs prior to an eruption. Analysis of the water-soluble components adhering to volcanic ash is available for remote monitoring of volcanic gases from inaccessible volcanoes. It is a secure method for monitoring volcanic activity without using particular devices. Prolonged volcanic eruption at the Unzen Fugendake volcano from 1990 to 1995 started with a phreatic eruption after 198 years of dormancy. Volcanic activity changed from a phreatic and phreatomagmatic eruption to a magmatic eruption with pyroclastic flows inMay 1991. The relationship between the chemical composition of volcanic ash and the contents of the water-soluble components adhering to it are discussed in relation to the early stage of the long-term eruption. Volcanic ash ejected by phreatic and phreatomagmatic eruption before dome formation was the product of the alteration in the volcanoclastic materials beneath the surface. The ash had a high content of water-soluble components, which was caused by the absorption of hydrogen chloride and sulfur dioxide gases from magma into wet debris before dome formation. Volcanic ashes which were generated by pyroclastic flows after dome formation were fresh lava fragments. While the contents of water-soluble sulfate adhering to the ash noticeably decreased, those of water-soluble chloride adhering to the ash hardly decreased. The considerable decrease in the contents of water-soluble sulfate was caused by the reaction of volcanic gases with dry lava fragments. Contrary to this, the concentration of hydrogen chloride gas in ash clouds was extremely high, which obstructed the decrease in the water-soluble chloride content in the ash. Volatility of chlorine and sulfur from volcanic rock suggests that the inner temperature of pyroclastic flows was higher than 600-700°C at least.

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Central Honshu, Japan, is conspicuous for a northward-convex form of zonal geologic structures. To clarify the formation of curvature of the zonal structures in the west of the convex form and further to discuss the tectonic evolution of central Honshu, we carried out a paleomagnetic study of dated (-15 Ma) dike rocks of the Shitara basin. Samples were collected from 25 basalt dikes trending north-south with a nearly vertical intrusion surface. After stepwise alternating-field and thermal demagnetization, 24 site-mean directions were determined. Three dikes gave anomalous directions, probably resulting from instantaneous recording of a field transition or excursion. The other 21 site-means produced a mean direction, D = 9.7°, I = 54.5°, α₉₅ = 5.2°, and a paleomagnetic pole at 82.3°N, 216.6°E, A95 = 6.2°. Although the reversal test is negative at the 5% significance level, the mean direction and pole are time-averaged ones in which the secular variation is averaged out. This was confirmed by studying the angular standard deviation of virtual geomagnetic poles. We conclude that the central Honshu curvature formed when the southwest Japan arc rotated clockwise between 17 and 15 Ma in relation to the opening of the Japan Sea, associated with differential rotation of the eastern part of the arc with respect to the central part. The differential rotation probably resulted from a sinistral shear on the eastern margin of the rotating arc. The formation of the curvature seems not to have borne on the collision of the Izu-Ogasawara arc with Honshu, although the collision probably caused large-scale clockwise rotation of the Kanto Mountains in the east of the northward-convex structure.

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Anewoptical instrument for studying upper atmospheric dynamics, called the Multicolor All-sky Imaging System (MAIS), has been developed. The MAIS can obtain all-sky images of airglow emission at two different wavelengths simultaneously with a time resolution of several minutes. Since December 1991, imaging observations with the MAIS have been conducted at the Zao observatory (38.09°N, 140.56°E). From these observations, two interesting events with wave structures have been detected in OI 630-nm nightglow images. The first event was observed on the night of June 2/3, 1992 during a geomagnetically quiet period. Simultaneous data of ionospheric parameters showed that they are caused by propagation of the medium-scale traveling ionospheric disturbance (TID). Phase velocity and horizontal wavelength determined from the image data are 45-100 m/s and -280 km, and the propagation direction is south-westward. The second event was observed on the night of February 27/28, 1992 during a geomagnetic storm. It is found that a large enhancement of OI 630-nm emission is caused by a propagation of the large-scale TID. Meridional components of phase velocities and wavelengths determined from ionospheric data are 305-695 m/s (southward) and 930-5250 km. The source of this large-scale TID appears to be auroral processes at high latitudes.

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We studied the local time dependence of the direction of cosmic noise absorption drift using a large data set obtained with a 16×16-antenna imaging riometer in Poker Flat, Alaska (65.1°N, 147.5°Win geographic coordinates; 65.4°N, 100.7° in geomagnetic coordinates). The drift velocities were determined using regression lines for the absorption peaks, which were automatically detected. Based on the characteristics of the motion of the drift region, the events were categorized into three magnetic local time (MLT) groups: I (13-19 MLT), II (21-01 MLT) and III (01-07 MLT). Most events in group I move southwestward with steady velocities. Events in group II had various velocities and directions. Events in group III also had various velocities, but the tilt angles of their vector velocities from the eastward axis were between -90° and 90°, meaning that they moved eastward. These results are consistent with some found in previous studies. A polar plot of the drift had a pattern with features similar to that of magnetospheric convection, which has been reported on several studies: equatorward in the pre-midnight sector and sunward in the morning and evening sectors.

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In the previous paper (Sekiya and Ishitsu, 2000), the shear instability in the dust layer of the solar nebula was investigated by using the constant Richardson number solution (Sekiya, 1998) as the unperturbed state, and the growth rate of the most unstable mode was calculated to be much less than the Keplerian angular velocity as long as the Richardson number was larger than 0.1. In this paper, we calculate the growth rate using different unperturbed states: (1) a sinusoidal density distribution, and (2) a constant density around the midplane with sinusoidal transition regions. An unperturbed state of this paper is considered to correspond to the first stage of shear instability that occurs as a result of dust settling in a laminar phase of the solar nebula. On the other hand, the unperturbed state of the previous paper corresponds to the quasi-equilibrium state (Sekiya, 1998) which is attained by the turbulent mixing. The results show that the growth rate is much larger than the Keplerian angular velocity, in contrast to the previous result.

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This is the third paper of a series of our papers, in which we have investigated explosive nucleosynthesis of the X-elements (Li, Be, and B) and the CNO-elements in supernova explosions. We concentrate here on evaluating the varieties of the isotopic/elemental ratios of the light elements due to the mixing process between the He-layer and the H-rich envelope of a supernova taking account of nuclear reprocessing after the mixing. Almost all of the X-elements are influenced strongly by the nuclear reprocessing after the mixing; the X-elements produced in the He-layer are decomposed by protons in the H-rich envelope even if the He-component mixes with a bit of the H-component. The p-rich isobars of ⁷Li and ¹¹B, namely ⁷Be and ¹¹C, are not decomposed in the mixture, so that the ¹¹B/⁷Li ratio is determined by the ¹¹C/⁷Be ratio after ⁷Li and ¹¹B are completely decomposed. On the other hand, the CNO-elements are almost free from the nuclear reprocessing after the mixing. The small ratios of ⁶Li/⁷Li and ⁹Be/⁷Li, and the diagram of ¹¹B/⁷Li-¹²C/¹³C obtained in the previous studies are still valid.

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