Inproceedings,
Energy partitioning for seismic events in Fennoscandia and NW Russia
28th Seismic Research Review: Ground-based nuclear explosion monitoring technologies, page 570--579. Orlando, Florida, (September 2006)
Abstract
In this three-year project we have been addressing 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 of Western
Norway, where we have both natural earthquake activity and frequent
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 and a natural
event. This observation is supported by synthetic seismograms calculated
for simple 1-D models, which demonstrate that explosions 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 to (within one wavelength) a strong discontinuity.
High-frequency (50-400 Hz) S/P ratios for mine blasts (explosions)
and rockbursts recorded at the Pyhäsalmi in-mine network do not
show any significant dependency on the distance to the events, which
ranges between 40 and 400 m. 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. 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 to propagation effects along
paths in the mine. Three-dimensional 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 m grid) and 100 Hz (2 m 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. 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 are geologic heterogeneities.
