SCALING OF SOURCE PARAMETERS FOR VRANCEA (ROMANIA) INTERMEDIATE-DEPTHEARTHQUAKES

The stress drop scaling of the Vrancea (Romania) intermediate depth ea rthquakes with seismic moments from 10 to 20 dyne cm is investigated. The empirical Green's function technique is used in order to retrieve the seismic source parameters for the smaller earthquakes (10(21) < M( 0) < 10(23) dyne cm). Locally recorded digital waveforms from 101 even ts occurred between 1982 and 1992 are examined to select suitable cand idates for applying this technique. Finally, the deconvolved source ti me function is obtained in case of eight earthquakes (4.2 less than or equal to M less than or equal to 5.4) located in the depth range from 80 to 160 km within the subducting slab. For all these earthquakes th e deconvolved source time function was demonstrated to be a simple uni polar pulse with an omega(-2) spectrum. To extend the scaling analysis to large seismic moment values (M(0) > 10(26) dyne cm), available sou rce parameters for the major Vrancea earthquakes that occurred in 1940 , 1977, 1986 and 1990 (M(W) greater than or equal to 6.9) as determine d from teleseismic and local data were included. A circular source mod el approximates well the deconvolved source time functions and source spectra for the analyzed earthquakes. In case of me large earthquakes an equivalent circular area is considered corresponding to the zone of aftershocks, The static stress drop is determined from seismic moment and source radius for Brune's model. A dependence of the stress drop on the seismic moment as Delta sigma similar to M(0)(1/4) is outlined over the entire magnitude range.