Distinguishing Between Source and Site Effects for High-Frequency Radiation

Using seismic data recorded in a deep South Africa mines at stations with close distances to earthquakes, we examined waveforms to try and separate the source and path contributions in the high-frequency radiation. We studied the waveforms from small events recorded at several closely spaced stations, and also studied the waveforms from closely spaced events at single stations. We used cross correlations of the waveforms over a range of frequencies from 2 to 20 hz, to estimate the degree of similarity of the recorded data.

This dataset provides an excellent opportunity to investigate the high-frequency characteristics of the seismic waveforms because stations are located at distances of several hundred meters to a few kilometers and installed on hard rock sites at 1 to 3 km depth in the mine, which minimizes the site effects. We also compared the waveforms from the subsurface mine stations with stations located on the surface.

Preliminary results indicate that for the lower frequencies, there is relatively more contribution from the source effects, while at higher frequencies the path effects are larger. High frequency content due to both the source and path are likely related to the wavelengths of the geological heterogeneities of the region.