The pattern of diurnal periodicity of weak earthquakes in Iran

The catalogue of earthquakes recorded in Iran during 2006–2010 by a dense network of digital telemetric seismic stations is analyzed. The spectrum of the time series of these earthquakes contains a sharp maximum at 24 h. The corresponding curve of diurnal periodicity constructed by the superposed epoch method has a clear double-peak maximum near noon(11 a.m.-5 p.m. local time), which exceeds the level of seismic activity observed during the rest time of the day by a factor of 3-3.5. In the same time interval, the average hourly magnitudes of seismic events sharply drop from M = 2.15 to M = 1.95. The ratio of the normalized number of earthquakes in the daytime to those at other times of the day, which was determined within a moving window half a square degree in size, has seven distinct compact spatially isolated maxima whose magnitudes attain several dozens to a hundred units. These maxima are probably caused by industrial activity, such as road building and quarry explosions. We also note the presence of the weekend effect when the daytime maximum in the weekly curve of diurnal variations in seismic activity almost completely disappears on Friday, which is the weekend in Muslim countries. At the same time, elimination of the supposed noise component from the catalogue by the approved technique changes nothing for the daytime maximum in the daily pattern of earthquakes in Iran. In order to account for this inconsistency, we suggest invoking additional information on the technogenic seismicity and considering weak earthquakes induced by quarry explosions and vibrations of industrial machines, in particular, power units of numerous hydroelectric power stations distributed over the territory of Iran.