Source Parameters of Microearthquakes from the 1.5 Km Deep Pyh‰salmi Ore Mine, Finland

Abstract

The Pyh‰salmi ore mine in Finland is instrumented with 6 three-component and 12 one-component geophones with sampling rates of generally 3kHz in a depth from 1.2 to 1.4 km. The monitoring network (Integrated Seismic System International Ltd.) has been operational in a continuous mode since January 2003, and until August 2004 about 24,000 events were recorded. About two thirds of the events are identified as explosions from the mining operations, while the remaining 8000 events are seismic and are induced from stress changes that, most likely, are related to the ongoing excavation.

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 fit stacked spectra to theoretical Brune's shape 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. Results from the MEGF approach for the ratio between the radiated seismic energy and the seismic moment is about 5?10-7 for a range in Mw from 1.2 to 0. Compared to 'natural' seismic events that were analysed by the MEGF method, this study's values for the energy-to-moment ratio are about two orders of magnitudes lower. 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.