ENERGY PARTITIONING FOR SEISMIC EVENTS IN FENNOSCANDIA AND NW RUSSIA

We address the problem of energy partitioning at distances ranging from very local to regional for various kinds of seismic sources, and are now in the last year of this three-year effort. On the small scale we have focused on analysis of observations from an in-mine network of 16-18 sensors in the Pyhasalmi mine in central Finland. This analysis has been supplemented with 3-D finite difference wave propagation simulations to investigate the physical mechanisms that partition seismic energy in the near source region in and around the underground mine. On the local and regional scale (20-220 km) we have targeted events from the region offshore Western Norway where we have both natural earthquake activity as well as frequent occurrence of underwater explosions carried out by the Norwegian Navy. Since the previous reporting of this project at the 2004 Seismic Research Review (Bungum et al., 2004), we have extended the finite difference simulations in the 3-D geological model of the Pyhasalmi mine. This model, which encompasses a geologic volume 500 meters in each direction, includes 3-D representations of the ore bodies, excavated regions, tunnels, and voids. The model is discretized on both 2 and 4 meter grids making it possible to simulate seismic energy up to 100-200 Hz. We perform a variety of sensitivity tests to determine the mechanisms that produce shear energy in an underground mine environment. For example, we conduct a suite of 15,000 (2-D) explosive source simulations to quantify the influence of source location on the amplitude of generated shear energy. In fact, most of the shear energy appears to be generated within 10-20 meters from the source (at frequencies of 50 Hz). Examination of waveforms reveals that both geologic heterogeneity and the structural influences of the mine are contributors to the near-source generation of shear energy. There is some suggestion that the effects of geologic inhomogeneity are significant early in the wavetrain, whereas the mine structure is likely to produce scatter and be more significant later in the waveforms. As a validation measure, the synthetic waveforms are compared with observed data from single and multi-component instruments located in the mine. The simulated data match the amplitude and character of the observed waveforms particularly well, especially at frequencies at and below 50 Hz. This suggests that we can reliably infer energy partitioning phenomena based on these simulations. A database of underwater explosions and earthquakes from the region offshore Western Norway, recorded at seven selected stations of the National Norwegian Seismic Network (NNSN), were analyzed for differences in the S/P amplitude ratios. In order to separate the path and source effects for the two event populations, we have investigated the station, distance, and frequency dependencies of the recorded data in detail. The results indicate that the mean S/P amplitude ratios for both underwater explosions and natural events vary from station to station but are, in general, higher for natural events. For frequencies above 3 Hz, the difference in S/P ratios between explosions and natural events is higher than for lower frequencies. However, the distributions of S/P ratios for explosions and natural events overlap in all analyzed frequency bands. Thus, for individual events in our study area, S/P amplitude ratios can assist the discrimination between an explosion or a natural event, but other measures such as spectral analysis should be included in the interpretation.