TEST OF THE EMPIRICAL GREEN'S FUNCTIONDECONVOLUTION ON VRANCEA (ROMANIA) SUBCRUSTAL EARTHQUAKES MIHAELA POPA AND MIRCEA RADULIAN National Institute for Earth Physics, Bucharest, Romania* Summary: Vrancea is one of the few singular seismic regions of the world where intermediate-depth earthquakes are permanently generated (around 10 events/month with ML > 3) within an extremely confined focal volume. This particularity 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 earthquakes that occurred on March 11, 1983 (ML = 5.4), April 12, 1983 (ML = 5.7) and August 7, 1984 (ML = 5.1) in the lower part of the subducting lithosphere (h =750 km) were analyzed. A set of 28 adjacent events (3.0 < ML < 4.4) which occurred between 1981 and 1997 were selected as corresponding empirical Green's functions. To test the confidence of the retrieved source time function, we compare the deconvolved pulses using Green's functions of different sizes and recorded simultaneously by short-period and broad-band instruments. Our tests show that the durations of the source time function is well- constrained and is not affected by the limited frequency range of the short-period instruments, or by the relative difference in the focal mechanism between the main event and Green's event. The apparent duration of the source time function outlines source directivity effects, and when these effects are sufficiently strong, they can identify the real fault plane. Relatively short source duration and correspondingly high stress drop values are in agreement with other previous results emphasizing a specific seismic regime in the lower part of the Vrancea subducting lithosphere. K e y w o r d s : deconvolution , empirical Green's function, Vrancea earthquakes 1. INTRODUCTION The empirical Green's function (EGF) deconvolution introduced by Hartzell (1978) has been widely applied to earthquakes using local network data, strong motion data, teleseismic body and surface waveforms (e.g., Mueller, 1985; Frankel et al., 1986; Frankel and Wennerberg, 1989; Mori and Hartzell, 1990; Kanamori et al., 1992; Velasco et al., 1994). The deconvolution procedure is an unstable process and the main technical problem was how to reduce as much as possible the instability. Several alternative techniques, including both time domain and frequency domain deconvolution, have been developed to this purpose, such as the simulated annealing inversion (Courboulex et al., 1996) or spectral deconvolution with different filtering strategies (Mueller, 1985; Amman et al., 1993; Mori, 1993). As shown by our previous studies (Radulian and Popa, 1993;1996a), the EGF deconvolution is a useful approach to retrieve the seismic source parameters in the case of Vrancea (Romania) intermediate-depth earthquakes using short-period records. The main goals of the present study are to find out how confident the retrieved source parameters are and how to optimize the application of the EGF technique to constrain the source of moderate-size (ML 4-5) Vrancea earthquakes when the frequency bandwidth of the recording instruments is limited to 0.1 - 12.5 Hz. *Address: P.O. Box MG-2, 76900 Bucharest, Romania, Fax: +40-1-4930118, e-mail: mihaela@infp.ro; mircea@infp.ro Studia geoph. et geod. 44 (2000), 403-429 © 2000 StudiaGeo s.r.o., Prague 403