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Energy partitioning for seismic events in Fennoscandia and NW Russia
27th Seismic Research Review: Ground-based nuclear explosion monitoring technologies, стр. 529--538. Rancho Mirage, California, (сентября 2005)
Аннотация
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 Pyhäsalmi mine in central Finland.
This analysis has been supplemented with 3D 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 3D geological model of the Pyhäsalmi 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 15000 (2-D) explosive source simulations
to quantify the influence of source location on the amplitude of
generated shear energy. We find that shear energy generation is particularly
prevalent when the source is located near a geologic or structural
boundary of the mine. 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 geologic inhomogeneity is 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.
