Seismic response to electromagnetic sounding of the Earth's lithosphere

The seismic catalogues of 1967-2008 for the Bishkek geodynamical test site are analyzed for the purpose of studying the response of seismic activity to the electromagnetic sounding of the Earth's crust during two series of field experiments with high-power controlled sources. The first series of the experiments, which were carried out in 1982-1990, utilized the pulses provided by a magnetohydrodynamic (MHD) generator. The sounding signals in the second series of the experiments (2000-2005) were generated by the capacitor-thyristor source ERGU 600-2. In these experiments, temporal variations of the set of statistical parameters characterizing the seismicity, which are typically used in the studies of the background and transient modes of seismicity, were investigated in a selected spatial domain within 150 km from the current electrodes. In terms of time, the analysis was conducted on two levels of detail. The study on a temporal scale of a few years was focused on the variations that preceded, accompanied, and followed the series of the experiments, while the day-scale analysis considered variations that were observed within 10 days after each sounding event. The day-scale analysis yields the following results. The slope of the frequency-magnitude diagram of the earthquakes (b value) during the sounding events is substantially larger than its background value. The slope of the graph gradually becomes gentler within about a day and a half after termination of sounding. The seismic activity slightly enhances during the interval of sounding and abates after its termination to a minimum, which corresponds to the interval of decreasing b value. This character of variations in seismicity differs from the scenario previously established for other transitional seismic regimes. The analysis on a temporal scale of a few years revealed variations in the studied parameters of the seismicity, some of which fall in both sounding intervals of 1983-1990 and 2000-2005. However, these variations are not unique; their character and durations suggest their being associated with the processes of preparation and after effects of the strong earthquakes that occurred in the vicinity of the sounding dipole.