Test of the empirical Green's function deconvolution on Vrancea (Romania) subcrustal earthquakes

Vrancea is one of the few singular seismic regions of the world where inter mediate-depth earthquakes are permanently generated (around 10 events/month with M-L > 3) within an extremely confined focal volume. This particularit y and the relatively large number of short-period waveforms recorded by the Romanian local network provides us the opportunity to test the performance of the empirical Green's function technique in retrieving the source time function and source directivity of the Vrancea earthquakes. Three earthquak es that occurred on March 11, 1983 (M-L = 5.4), April 12, 1983 (M-L = 5.1) and August 7, 1984 (M-L = 5.1) in the lower part of the subducting lithosph ere (h approximate to 150 km) were analyzed. A set of 28 adjacent events (3 .0 < M-L < 4.4) which occurred between 1981 and 1997 were selected as corre sponding empirical Green's functions. To test the confidence of the retriev ed source time function, we compare the deconvolved pulses using Green's fu nctions of different sizes and recorded simultaneously by short-period and broad-band instruments. Our tests show that the durations of the source tim e function is well-constrained and is not affected by the limited frequency range of the short-period instruments, or by the relative difference in th e focal mechanism between the main event and Green's event. The apparent du ration of the source time function outlines source directivity effects, and when these effects are sufficiently strong, they can identify the real fau lt plane. Relatively short source duration and correspondingly high stress drop values are in agreement with other previous results emphasizing a spec ific seismic regime in the lower part of the Vrancea subducting lithosphere .