A three-component digital seismic station is in operation at Heinrich Robert coal mine in the Ruhr basin, at a depth of 890 m. A cluster of seismic events occurred at the beginning of 1987 at a distance of about 5 km from the station and continued in 1988. Source parameters of 46 tremors selected from the cluster, with local magnitude from 1.2 to 2.3, are studied. The ratio of P-wave to S-wave corner frequency is variable, implying its dependence on the angle of observation and in turn the variability of focal mechanism with time.
The scaling relations indicate that characteristic tremors, with a fault length of about 200 m, are possibly generated in the area, but the evidence is not conclusive. The seismic energy for a given seismic moment can vary by a factor of 20, and the ratio of S-wave to P-wave energy ranges from 1.5 to 30.
Stress release estimates show low values, not exceeding a few bars, typical for tremors induced by mining. A comparison of the apparent stress with the static stress drop divides the selected tremors into high and low apparent stress events. The latter events are characterized by high P-wave energy and almost constant and large ratio of P-wave to S-wave corner frequency, which differs from the lower ratios of other events, implying similar focal mechanism. These high P-wave energy tremors are the most likely candidates for nondouble-couple events.