Final Technical Report, 30 SEPTEMBER 2002 - 31 JANUARY 2006; ENERGY PARTIONING FOR SEISMIC EVENTS IN FENNOSCANDIA AND NW RUSSIA

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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 Pyhasalmi mine in central Finland. This analysis has been supplemented with 3-D finite difference wave propagation ... continued below

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Bungum, H.; Kvaerna, T. & Larsen, S. January 31, 2006.

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Description

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 Pyhasalmi 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 Pyhasalmi 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 Pyhasalmi 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.

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  • Report No.: DOE/SF/22636-1
  • Grant Number: FC03-02SF22636
  • DOI: 10.2172/875774 | External Link
  • Office of Scientific & Technical Information Report Number: 875774
  • Archival Resource Key: ark:/67531/metadc875040

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  • January 31, 2006

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  • Sept. 21, 2016, 2:29 a.m.

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Bungum, H.; Kvaerna, T. & Larsen, S. Final Technical Report, 30 SEPTEMBER 2002 - 31 JANUARY 2006; ENERGY PARTIONING FOR SEISMIC EVENTS IN FENNOSCANDIA AND NW RUSSIA, report, January 31, 2006; United States. (digital.library.unt.edu/ark:/67531/metadc875040/: accessed August 23, 2017), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.