Frequency-magnitude,depth, and time relations for earthquakes in an island arc: North Island,New Zealand |
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| Authors: | SJ Gibowicz |
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| Institution: | Seismological Observatory, Geophysics Division, Department of Scientific and Industrial Research, WellingtonNew Zealand |
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| Abstract: | Analysis of over 1400 earthquakes in the North Island of New Zealand from 1955 to 1969, comprising all shocks with ml ? 4.3 for shallow, and ML ? 4.0 for deep events, reveals several empirical relationships between the depth and the equivalent radius of the area occupied by shocks, the number and density of the shocks, and the coefficient b and the maximum magnitude. The coefficient b increases linearly with depth from 1.0 for shallow earthquakes to 1.4 for those at a depth of 120 km, and then decreases to 0.75 at 300—350 km. The variation with depth shows clear inverse correlation with the distribution of maximum stress along the downgoing slab, calculated for several slab models by Smith and Toksöz. Similarly, the maximum magnitude at different depths correlates distinctly with the distribution of the principal stress. Time variations of the coefficient b and the rate of earthquake occurrence, for both shallow and deep earthquakes, have an oscillatory character, with a period of 7–8 years. These variations also imply that shallow and deep seismicity are mutually dependent. |
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