Source parameters of intermediate and deep focus earthquakes in the Tonga arc☆
Abstract
The source dimensions, stress-drops and apparent average stresses of 19 Tonga-Kermadec earthquakes with depth greater than 79 km were estimated from the amplitude spectra of body waves. The results for each event were based on a combination of spectra from short and long period records of several (an average of five to six) WWSSN stations. In all cases the focal mechanism was known.
The radii for intermediate and deep earthquakes are approximately 10 km for mb (CGS) = 5.8 and the stress drops are between 4 and 70 b. The stress drops do not change much with depth, but appear to be slightly larger at intermediate depths than at great depths. The apparent stresses are larger at intermediate depths. These data suggest that the shear stress necessary to cause earthquakes may also be higher at intermediate depths. At greater depth, the increasing temperature and possible phase changes and dehydration may be factors that weaken the source material and that produce a change in the material properties within the slab near a depth of about 450 km.
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Lamont-Doherty Contribution No. 1880.