Periodic spectral characteristics of seismicity before strong earthquakes and their application

Periodic spectral characteristics of earthquake activity in the seismic strengthening areas of 24 earthquakes with M≥6.0 are studied by the maximum entropy spectral method whose superiority is tested. Then the follow results have been obtained: (1) The periodic spectra of seismic activity in seismic strengthening areas are different in different stage in earthquake-generating processes. Long periodic spectra and short ones coexist in normal stage, while only short ones (on average, 43% of long ones) exist and long ones disappear prior to earthquakes. (2) The appearing time of short period before earthquakes has some relations with magnitude. The result shows that decades or even one hundred years is the common value for a great earthquake of M=8.0, 30 years for one with magnitude about 7 and 20–30 years for a strong quake of M=6.0. For the same magnitude earthquakes in different regions the appearing time is also different. For example, it is longer in North China than that in the western part of China. Then the characteristics are preliminarily explained applying the strong body earthquake-generating model. Applying the maximum entropy spectral method, the idea of tendency prediction for strong and great earthquakes is suggested and used into practice, for example, the tendency predictions of the Wuding earthquake with M=6.5 and the Lijiang earthquake of M=7.0 in Yunnan Province got some positive effects. So a new method of tendency prediction of M>-6.0 earthquakes is offered. This research was supported by the Chinese Joint Seismological Science Foundation (198085).