Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.s11b1145b&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #S11B-1145
Other
7205 Continental Crust (1242), 7215 Earthquake Parameters, 7230 Seismicity And Seismotectonics, 7294 Instruments And Techniques
Scientific paper
Intensity derived focal depths can be a useful parameter for historic tremors because of the lack of constraint from instrumental data. The well known Bath equation: Io=3log[(r2+h2)/h2]+2 tends to give foci that are too deep. A newer modified equation: Io=2.8log[(2.79Av/pi+h2)/h2]+2-6/1.2h+(6-Io)/6, was empirically developed from a data base of 24 earthquakes that had well constrained instrumental depths and intensity maps. The average difference between instrumental depths and modified equation depths was 3 kilometers. Half of the events differed from the instrumental depth by 2 kilometers or less. The modified equation was applied to tremors of M5.0 or greater in Arizona that occurred between 1906 and 1959. These tremors had no depth estimates or ones that were inaccurate. A calibration test of the modified equation was performed on a 1976 Arizona event that had instrumental depth estimates of 10-15 kilometers. The intensity depth from the modified equation was 11-12 kilometers. The modified equation was then used to calculate depths for the following earthquakes in northern Arizona: 1-25-1906, M6.2, depth=30 \pm 3 kilometers; 8-18-1912, M6.2,depth=20 \pm 3 kilometers; 7-25-1959,M5.75, depth=11 \pm 3 kilometers. The 1959 tremor had a published depth of 0.5 kilometers from instrumental data. The intensity derived depth of 11 kilometers appears more reasonable. The 1906 and 1912 tremors did not have instrumentally determined depths due to lack of data. The mid to lower crustal depths of these two events suggests a thick seismogenic layer, unusual in continental areas. A cold, thick Colorado Plateau crust and upper mantle is documented by other tremors of more than 40 kilometers depth and relatively low heat-flow values.
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