Abstract

To investigate source-scaling relations for small earthquakes (Mw −1.8 to 1.2) we have determined source parameters for numerous events (∼1500) from the 1400-m-deep Pyhäsalmi ore mine in Finland. In addition to a spectral integration approach, we have fitted Brune, Boatwright, and Haskell spectral-shape models to the observed spectra and investigated attenuation influences. Of three different constant Q models (200, 350, and 800), a Q of 350 in combination with the Brune spectral model satisfied the data best. We have also investigated the frequency dependence of Q using the spectral decay method and found that Q increases with frequency. For selected events from two distinct clusters, we compared source parameters derived from constant Q models with source parameters using the multiple empirical Green’s function (megf) approach. By using constant Q models, the apparent stress seems to increase with magnitude, whereas results based on the megf approach indicate constant apparent stress with magnitude. In comparison with results from other studies that cover a larger-magnitude scale, we find apparent stresses that are about 1 to 2 orders of magnitude smaller than most of those. A modified M0fc−(3 + ϵ) scaling relation allows for increasing apparent stress with magnitude and can hence combine this study’s results with apparent stresses found for large earthquakes. However, within the limited-magnitude range of our data, apparent stresses seem constant.

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