SOURCE PARAMETERS OF MICROEARTHQUAKES FROM THE 1.5 KM DEEP

The Pyhäsalmi ore mine in Finland is instrumented with 6 three-component and 12 one-component geophones in a depth between 1.2 and 1.4 km. The monitoring network (Integrated Seismic System International Ltd.) has been operational in a triggered mode since January 2003, and about 24,000 events were recorded until August 2004. About two third of the events are identified as production blasts from the mining operations, while the remaining 8000 seismic events are induced by stress changes that are related to the ongoing production. We have developed a microseismic monitoring software that automatically determines microearthquake hypocenters and their source parameters. The recorded microseismic events occur throughout the mining area and they cluster in newly excavated tunnel systems. In moment magnitude (Mw), they vary from about –1.8 to 1.2, while the bulk of events have Mw from –1.5 to –0.5. We estimate the seismic moment and the corner frequency using a spectral integration method and alternatively we fit a theoretical Brune’s shape spectra to the stacked spectra. For two clusters of microseismic events we apply the multiple empirical Green’s function method (MEGF), which allows us to determine source parameters without assuming path and site effects. We determined the ratio between the radiated seismic energy and the seismic moment and obtained a value of about 5·10-7 for a magnitude range from Mw –1.2 to 0. Compared to tectonic seismic events analyzed in other studies, our results for the energy-to-moment ratio are lower by about two orders of magnitude. This discrepancy may partly be explained by a more tensile fault mechanism of the mining events compared to earthquakes that are dominated by shear faulting