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- Brown, R. James
(författare)
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Microearthquakes and seismic risk
- 1978
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Ingår i: Geologiska föreningens i Stockholm förhandlingar. - : Informa UK Limited. - 0016-786X. ; 100:3, s. 307-313
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Tidskriftsartikel (refereegranskat)abstract
- This paper is presented in the nature of a review of the potential of microearthquake surveys in seismotectonic studies, particularly seismic-risk analyses. Microearthquakes are generally considered as shocks with magnitude less than 3. They radiate, in general, higher frequencies than larger earthquakes and therefore microearthquake seismographs must be sensitive to frequencies typically up to 30 or 40 Hz. Mircroearthquakes are useful in seismotectonic studies because: (1) a greater number of small than large shocks occur in the same time, (2) extension to lower magnitudes of frequency-of-occurrence curves can yield more reliable seismic-risk statistics, (3) due to the reduced scale, both in space and time, very precise focal location is possible, often enabling mapping of fracture surfaces, (4) focal-mechanism solutions can further determine the fault surface and, in addition, the direction of slip, and (S) continuous monitoring of seismic activity is possible through the means of a permanent network of microearthquake systems.
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| 2. |
- Brown, R.J., et al.
(författare)
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Weighted vertical stacking in crustal seismic reflection studies on the Canadian shield
- 1977
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Ingår i: Geophysical Prospecting. - : Wiley. - 0016-8025 .- 1365-2478. ; 25:2, s. 251-268
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Tidskriftsartikel (refereegranskat)abstract
- Seismic reflection methods are being developed at the University of Manitoba to aid in determining fine crustal structures in the Precambrian of Manitoba and northwestern Ontario. Present-day environmental concern as well as mineshaft conditions necessitate the detonation of several smaller charges repeated, say, I times and followed by ''vertical'' stacking. To obtain the familiar ..sqrt..I improvement in signal-to-noise (S : N) amplitude ratio applying the straight-sum (SS) method, one assumes, among other things, that both S : N ratio and signal variance are the same on all traces. Dropping these assumptions, as we must for our data, it becomes necessary to apply weighting coefficients to optimize the S : N ratio of the stacked trace. We still assume the signal shapes to be the same for repeated shots, so for the jth trace on the record of the ith shot we model the time series as: t/sub ij/ = a/sub i/(s/sub j/ + n/sub ij/); where a/sub i/ is a scaling factor. The proper weights w/sub i/ are then shown to be proportional to sigma/sub si//sigma/sup 2//sub ni/ where sigma/sup 2/ is variance, or to ..gamma../sub i//a/sub i/ where ..gamma../sub i/ is S : N power ratio. Applying the weighted-stack (WS) method gives S : N amplitude ratios which are, on average, 55% of the optimal ratios expected from WS theory compared with only 24% for the SS method. The 45% shortfall in WS performance is ascribed mainly to trace-alignment (or time-delay) errors. Varying noise levels on individual traces, slight dissimilarity of signal shape, and correlated noise may also contribute to a lesser extent (in decreasing order of significance). This WS method appears to strike a good practical balance between S : N improvement and processing efficiency.
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