Journal of Seismology 2: 257–265, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 257 Simultaneous inversion for velocity structure and hypocenters in Slovenia A. Michelini * , 1 M. Živˇ ci´ c 2 & P. Suhadolc 3 1 Dipartimento di Scienze della Terra, Universit` a degli Studi di Trieste, Via E. Weiss, 1, 34127 Trieste, Italy 2 Geophysical Survey of Slovenia, Observatory, Pot na Golovec 25, SI-1000 Ljubljana, Slovenia 3 Dipartimento di Scienze della Terra, Universit` a degli Studi di Trieste, Via E. Weiss, 1, 34127 Trieste, Italy Received 22 July 1997; accepted in revised form 24 March 1998 Key words: Seismic tomography, Adria, Dinarides, Alps Abstract We use nearly 2100 P-wave arrival times from 166 local earthquakes to investigate the 3-D compressional velocity structure of the upper crust of Slovenia using the simultaneous inversion algorithm developed by (Michelini and McEvilly, 1991). Remarkable and stable features of the resolved model are the relatively high velocities in western Slovenia and the low velocities in central Slovenia, SE of the Ljubljana basin. The boundary between these two anomalies follows approximately the NNW-SSE direction that coincides with the general strike of the External Dinarides. We interpret this feature as the upper crustal expression caused by the tectonic processes occurring along the active margin of the Adria promontory/microplate. Introduction Slovenia lies at the northeastern boundary of the Adria promontory/microplate. Its territory comprises three major geotectonic units: the Alps, the Dinarides and the Pannonian basin. On the south lies the undeformed Istrian platform which gradually evolves into the Ex- ternal Dinarides (Figure 1a). They both form a unique Adriatic-Dinaric carbonate platform of Mesozoic to Paleogene age and their contact consists of a folded zone with a minor reverse oblique right lateral com- ponent of tectonic offset (Jurkovšek et al., 1996). The transition from the External Dinarides in the south to the Southern Alps (Internal Dinarides) in the north is also gradual. The Southern Alps consists mostly of a zone of Mesozoic deep water sediments originally de- posited in a regional depression known as the Slovene trough. In Eocene time, the trough sedimentary units were thrust up onto the Adriatic-Dinaric carbonate platform. The northern rim of the Slovene basin is the Julian platform that has been thrust southward over the Slovene basin during post-Eocene time. To the East, * Now at Osservatorio Geofisico Sperimentale, PO Box 2011 Opicina, 34016 Trieste, Italy the Slovene basin and the Julian platform are covered by Tertiary molasse sedimentary rocks which form the Pannonian basin. All these geotectonic units were gradually brought together as a result of continen- tal collision between the Adria promontory/microplate and Eurasia along the Periadriatic lineament that lies to the north of the Southern Alps. Since the Quater- nary, these units have been bound in an unique and complex structural mosaic (Ravnik et al., 1995, and references therein). Recent plate motion reconstruc- tions (e.g. Ward, 1994; Albarello et al., 1995) suggest a transpressive right-lateral type of motion along the NE boundary of the Adria promontory, manifested by a system of NW–SE oriented faults. A regional scale tomographic velocity inversion of the upper crust in Slovenia can provide additional in- formation on the relations between the different struc- tural units at depth. Thus, the imaging of the velocity structure beneath the major mountain belts to the south of the Adria-Eurasia plate boundary can contribute to the understanding of the complex regional tectonics. Simultaneous inversion for hypocenters and ve- locity structure is a well-established technique (e.g., Aki and Lee, 1976; Crosson, 1976; Thurber, 1983)