Previous studies such as that of Meybeck (1987) estimated the CO2 flux from atmosphere to riverwater due to chemical weathering by assuming that the rate of weathering of silicate rocks and carbonate rocks is proportional to the surface areas of rocks (silicate rocks : carbonate rocks = 7 : 3). However, the dissolution rate of carbonates (calcite and dolomite) is in two to four orders of magnitude higher than silicate (feldspar). This may imply that Meybeck's and other previous approaches lead to a large uncertainty in the estimate of CO2 flux. However, their estimates are nearly similar to that of Gaillardet et al. (1999), who estimated the contribution of the weathering of silicates and carbonates to riverwater chemistry based on a large volume of analytical and runoff data of the world's 60 major rivers. The thermochemical calculation in the present study indicates that the chemistry of riverwater having a small runoff is controlled by the solubilities of calcite and Na ·Ca-feldspar (Na : Ca = 1 : 1) at atmospheric PCO2 (10 -3.5 atm) and the world-wide average riverwater chemistry plot is close to the Na ·Ca feldspar-calcite-riverwater equilibrium point. This result reasonably explains the similar estimated values of CO2 flux obtained by Meybeck (1987) and others and Gaillardet et al. (1999). The conditions for riverwater saturated with Na ·Ca feldspar and calcite were derived based on a dissolution kinetics-fluid flow coupling model and were expressed as functions of τ (residence time of groundwater) and A/M (A : surface area of mineral, M : mass of water).
The detection rate of eruption cloud with Geostationary Meteorological Satellite (GMS ; HIMAWARI in Japanese) is 12.1%, but GMS can detect and track, at a high rate of 81.5%, eruption cloud from a large explosive eruption higher than 10 km which may threaten aviation safety. Estimates of the top altitude of eruption clouds within the tropopause based on cloud-top temperature show fairly high values compared to those obtained by ground observations. Growth of vapor clouds over eruption clouds induced by strong ascending currents with eruption-onset may be the reason. Apparent dislocation of eruption clouds on GMS images due to the parallax of GMS is clarified for the case of the 1986 Izu-Oshima Eruption, Japan, and the underestimation of cloud-top is possibly due to warming of the cloud-surface by radiation from internal hot material. From inspections of the pattern of cloud-extent, type, strength, decay, and duration of eruption activity can be evaluated. Differential Thermal Infrared Imagery of GMS-5 is very effective for discriminating ashbearing cloud from ambient atmospheric cloud, but cannot clearly separate an eruption cloud with an extremely high content of water-vapor with phreatic/phreatomagmatic eruption.
Seismic activity in and around the Japanese islands was conspicuously high in the mid1990s. Further, a number of large earthquakes occurred around the Philippine islands in the early 1990s, and then, around Kalimantan, Sulawesi, Java, and western New Guinea islands. We show that the seisimic activity in the western peripheral region of the Pacific Ocean in 1990s was so remarkable that it contributed substantially to the notable increase of seismic energy released in 1995-1996 worldwide. The activity seems to have started in the region near the Philippine islands. It is improbable, however, that the successive occurrence of large earthquakes in the western peripheral region of the Pacific Ocean was triggered by the impact of one particularly big event such as the 1990 Philippine earthquake or the huge 1991 eruption of Mt. Pinatubo. Instead, we think that the activity might be produced by a slow tectonic event that lasted for a few years and had a spatial scale of several thousands of kilometers.
We studied the relations between the lithological properties of bedrock, soil layers, and dimensions of soil slips (depth and slope angle) in slopes on Jurassic granite and Precambrian gneiss in the suburbs of Seoul, Republic of Korea. The slope angle before slippage and the average depth at which slippage initiates are estimated to be about 35-40° and 70-90 cm in granite. The values for gneiss scars are 27-33° and 140-190 cm. Regolith (weathering products) is coarse on slopes underlain by granite and fine on slopes underlain by gneiss, reflecting the grain size of the minerals in the underlying bedrock. Soil layers at the slip (shearing) plane on the granite slope are coarser grained with a larger angle of internal friction (φ) and a smaller cohesion (c) compared to soil layers on the gneiss slope. A slope stability analysis indicates that these properties of soil, themselves derived from weathering, control the difference in dimensions of soil slips found between granite and gneiss.