Abstract
Many precursory phenomena, both geophysical and biological, were reported in relation to the Haicheng (M=7.3, 1975) and Tangshan (M=7.8, 1975) earthquakes in China. Precursor times and epicentral distances of these premonitory effects are analysed by making use of the Weibull distribution which was extensively utilized by RIKITAKE (1975, 1978a, 1978b, 1979a, 1979b) and RIKITAKE and SUZUKI (1979).
Three kinds of precursor, each having a mean precursor time amounting to 418 days, 2.8 hours and 4.2 days respectively are found from the analysis for the geophysical precursors of the Haicheng earthquake. These precursors seem to coincide with the first, second and third precursors as defined by RIKITAKE (1979a). On the other hand, the second or imminent precursor seems to have been missing for the Tangshan earthquake although the other two precursors having mean precursor times respectively amounting to 497 and 21 days seem to have been observed.
Mean precursor times for anomalous animal behavior are estimated as 7.8 and 0.6 days for the Haicheng earthquake, while only one peak of precursor time is obtained at 0.4 days for the Tangshan earthquake.
Maximum frequency of the epicentral distance of precursors seems to occur within a range 120-140km for both the earthquakes.
The results of the present analysis do not differ much from those so far obtained for the distribution of precursor times for world-wide data and a few particular earthquakes. It may be that we may accomplish an earthquake prediction provided careful detection of possible precursors can be performed with a dense array of observation instruments covering an extensive area.
