Techreport,
Energy partitioning for seismic events in Fennoscandia and NW Russia
DOE/SF/22636-1. NORSAR, Kjeller, Norway, (2006)
DOI: 10.2172/875774
Abstract
In this project we have addressed the problem of energy partitioning
at distances ranging from very local to regional for various kinds
of seismic sources. 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. 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 3-D finite difference wave
propagation simulations in a realistic mine model to investigate
the physical mechanisms that partition seismic energy in the near
source region in and around the underground mine. The results from
modeling and analysis of local and regional data show that mean S/P
amplitude ratios for explosions and natural events differ at individual
stations and are in general higher for natural events and frequency
bands above 3 Hz. 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
only assist the discrimination between an explosion or a natural
event. This observation is supported by synthetic seismograms calculated
for simple 1-D models which demonstrate that explosions also generate
shear-wave energy if they are fired close to an interface with a
strong material contrast (as is the case for most explosions), e.g.,
free surface or the ocean bottom. The larger difference in S/P ratios
between earthquakes and explosions for higher frequencies can be
explained by the fact that at low frequencies (larger wavelengths),
discontinuities and structural heterogeneities in the explosion source
region are stronger generators of converted S energy. The S*-phase,
for example, is most efficiently generated whenever an explosion
source is located close (within one wavelength) to a strong discontinuity.
The Pyhäsalmi explosions have generally lower S/P ratios than
the rockbursts for all frequencies, but the difference is far too
small to be significant for classification purposes. The maxima for
the explosion distributions are all below 2, whereas they are all
above 2 for the rockbursts. The rockbursts also have a wider distribution
of S/P ratios, which can be explained by the variability of the radiation
patterns from the rockburst sources. S/P ratios for explosions and
rockbursts located in the same small area of the mine show results
very similar to those for the full data set. This indicates that
the observed differences in S/P ratios between explosions and rockbursts
are due to differences in the source characteristics, and not due
to propagation effects along paths in the mine. 3-D finite-difference
simulations were used to model seismic events within the Pyhäsalmi
mine. In particular, a January 26, 2003 rockburst was modeled at
frequencies of 50 Hz (4 meter grid) and 100 Hz (2 meter grid). We
were able to match the characteristics of the observed data at 50
Hz particularly well, and the characteristics of the 100 Hz data
reasonably well. These results help validate the 3-D geologic mine
model and the reliability of our simulations. The simulations showed
that significant shear-energy can be produced due to the geologic
and structural heterogeneities within the mine. In fact, mode-converted
shear-energy generated from mine heterogeneity can dominate the compressional
energy from an explosive source. A strong correlation is observed
between the distance of a source from a mine heterogeneity and the
magnitude of generated shear-energy. The ratio of shear to compressional
energy is about a factor of two larger when the source is located
within one wavelength from a mine heterogeneity. The simulations
also suggest that excavated mine volumes are significantly stronger
contributors to shear-energy generation than geologic heterogeneities.
However, the simulations reveal that the magnitude of shear-energy
generated as part of a shear-producing source mechanism (e.g., rockburst,
mine collapse) can be as large or larger than that caused by heterogeneity
within the mine.
