Midday Effect in the Time Series of Earthquakes and Seismic Noise

摘要

Results of the investigation of the diurnal distribution of earthquakes in the Garm test area (GTA) in 1955-1991 demonstrated that weak earthquakes with energy class K ≤ 6 are characterized by the existence of diurnal periodicity with a maximum in the middle of the night. The analysis of earthquake samples for separate months revealed the existence of an additional specific feature of diurnal seismicity distribution—the midday effect. The essence of this effect is as follows: the general decrease in the number of earthquakes recorded in the daytime is marked by a short-term seismicity intensification, which is distinctly confined to midday (local solar time). We found that such an effect is also manifested in the time series of highly sensitive monitoring data on seismic noise. However, the midday effect in this case is characterized by inverse polarity; i.e., the general increase in the amplitude of daytime noise is complicated by its short-term attenuation.