Papers in Meteorology and Geophysics

We investigated the possibility of forecasting the aftershock activities of large earthquakes occurring in and near the Japanese Islands. On the basis of the Modified Omori formula and the Gutenberg-Richter relation, an intensity function of aftershock occurrence, which contains four parameters, is obtained. We evaluated those four parameters for each aftershock sequence of 47 earthquakes which occurred in the period 1969 through 1990. Then, taking the mean values of the four parameters, and assuming that the aftershock sequence is a non-stationary Poisson process, we calculated the probabilities of the average aftershock occurrence for earthquakes in and near the Japanese Islands. It is shown that the coincidence between the forecasted and the actually observed aftershock activities are fairly well. It is expected that we can further improve the accuracy of forecasting by taking into consideration the regional differences of the four parameters contained in the intensity function of aftershock occurrence.

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   A statistical analysis is made of mesoscale warm fronts (MWFs) and mesoscale cold fronts (MCFs) in the Kanto plain by using hourly surface data covering eleven years. A series of objective criteria are used to select 190 MWFs and 177 MCFs, and composite analyses are made of their local structure, seasonal and diurnal variabilities, and the larger-scale environment.
   MWFs are associated with southeast-southwest winds or northwest winds blowing against a shallow cold-air pool having a depth of a few hundred meters. The MWF associated with southeast-southwest winds corresponds to the warm-air intrusion during the passage of a cyclone, while the MWF associated with northwest winds corresponds to a strong northwesterly surge. The frequency of MWFs is highest between 06 and 12 JST.
   The MCF generally occurs during a cold-air surge. From late autumn to winter, the MCF is mainly associated with northwest winds accompanied by shallow cold air in its leading edge, and is more frequent in the nighttime than in the daytime. From spring to summer, MCFs associated with northeast or east winds are more frequent than those associated with northwest winds, and have a strong tendency to occur in the afternoon.

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