Diurnal periodicity of weak earthquakes and high-frequency underground noise in Kamchatka

We have analyzed the data on the times and number of weak earthquakes (M = 0–2.5) included in the up-to-date (final version) Catalog of Kamchatka Earthquakes for 1995–2008, and the intensity of highfrequency underground noise measured in the deep borehole near Petropavlovsk-Kamchatskiy (according to the literature data). We calculated the spectra of the seismic time series within the range of periods from 1 to 48 hours with a step of one minute. It was found that the spectrum of the earthquake data series contains a significant high Q extremum in a period of 24 hours that can be linked with a similar period in the high-frequency underground noise in Kamchatka and in the Russian Platform. There are some grounds to suggest that the weak earthquakes and the underground noise (seismoacoustic emission) have the same nature. In both cases, the shapes of the curve of the diurnal periodicity are found to depend on the duration of light during the day. The probable reasons for the solar impact on the seismic emission processes are discussed.