Papers in Meteorology and Geophysics

Earthquake swarms repeatedly occurred in Chijiwa Bay and around Unzendake volcano in west Kyushu, Japan. The volcano erupted in November 1990 and lava domes began to emerge in May 1991 after a quiescence of 198 years. It was investigated whether the change of the volcanic activity, such as the occurrences of eruption and lava domes, were easily detected by monitoring the time variations of coda Q^-1 values (Qc^-1s). In this study, Qc^-1s were estimated from the earthquakes around the Unzen area in the period from 1984 to 1991. According to single isotropic scattering approximation, the Qc^-1s were calculated from seismic records with sampling rate 30 Hz at Nagasaki and Unzen stations, which were located 40km and 4km away from the summit of the volcano, respectively. We obtained the following two conclusions. The Qc^-1s for Unzen station were independent of the frequencies in the range from 2 to 3Hz. No remarkable changes were detected for the Qc^-1s, their variances and their frequency dependence at Nagasaki and Unzen stations associated with the changes of the volcanic activity in the period from 1984 to 1991.

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   Long-term temperature trends at 60 stations in Japan were evaluated by using monthly data during 1891-1992. The polynomial trends of minimum and maximum temperature were obtained with a least-squares method which included discontinuous changes due to site changes. The minimum temperature (T_min) is found to show a rising trend with an average rate of about 2-5°C/(100 years) at large cities and about 1°C/(100 years) at medium-sized ones. The rising rate (T^↑_min) tends to be higher after World War II than before, but some cities in Hokkaido show exceptionally high rising trends of T_min in the early part of the analysis period. Changes in maximum temperature (T_max) are smaller, although a weak rising trend before World War II is found for many cities.
   The dependence of the temperature rising rate on the city size (population of cities or towns) was examined. It is found that T^↑_min is positively correlated with population parameters with only slight differences in correlation coefficients according to their functional forms (the logarithm and low powers). There is also a weak correlation between the rising rate of maximum temperature (T^↑_max) and the population parameters.
   A similar analysis was made for the increasing rate of the population parameters. There is a positive correlation between T^↑_min and the net increasing rate of the population adjusted to the present area of the city. On the other hand, there is little correlation between T^↑_min with the nominal increasing rate of population without adjustment for changes in the area of the city.
   Analysis was also made about the relationship between T^↑_min and the daily temperature range (R), which was regarded as a measure of the intensity of nighttime cooling. It is shown that T^↑_min has a positive correlation of 0.3-0.5 with R in old times, such as that in the 1910's, but is hardly correlated with R in recent years.

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