Geophysical Journal zyxwvutsrqpon (1989) zyxwvutsrq 98, 515-524 zyxwvutsrq A microearthquake study in the Gulf of Patras region, western Greece, and its seismotectonic interpretation N. S. Melis, M. Brooks and R. G. Pearce Department of Geology, University of Wales College of Cardiff, PO Box 914, Cardiff CF1 3YE, Wales, zyxw UK Accepted 1989 March 16. Received 1989 March 16; in original form 1988 June 1 zyxwvutsr SUMMARY The seismicity of the Gulf of Patras region was monitored from 1983 May to 1984 April by a six-station microearthquake network with 30 km aperture. Over 650 events were located using the revised version of the HYPO71 computer program for determining hypocentral locations. Of these, a subset of 73 well-located events was identified for detailed study. Analysis of these 73 events reveals a local pattern of seismicity with two main features: firstly, a gap in seismicity in the central and western parts of the Gulf, despite the local presence of active faulting affecting the seabed; secondly, a zone of high seismicity in the northeast part of the Gulf where offshore seismic surveys reveal a zone of intense structural complexity. A northeast dipping, diffuse zone of seismicity extending to a depth of about 25 km has been identified in the northeastern comer of the Gulf, centred on the Rio-Antirrio Channel between the Gulfs of Patras and Corinth. It is postulated that the active faults in the central part of the Gulf, which are apparently aseismic, are linked via a shallow decollement surface under the Gulf to a deep, WNW-ESE trending fault zone defining the southern margin of the Corinth graben, and extending westwards to Trichonis Lake. The dipping zone of seismicity is related to the latter fault zone, and aseismic faulting in the Gulf of Patras is attributed to the shallow surface of decollement into which the faults sole. A transfer fault zone trending approximately NE-SW is postulated to account for the offset of the Corinth and Patras grabens. Key words: Microearthquake, Gulf of Patras, seismotectonics 1 INTRODUCTION The Gulf of Patras contains a WNW-ESE trending neotectonic graben structure (Ferentinos, Brooks zyxwvut & Doutsos 1985). It lies at the western end of a regional system of three grabens (Patras-Corinth-Saronicos; Fig. l), extending from the vicinity of the western Hellenic trench to the Cyclades volcanic arc of the southern Aegean. Both of these major geological features are associated with active subduction of the eastern Mediterranean lithosphere beneath the Aegean domain. The graben system is a localized manifestation of crustal extension in the Aegedn marginal basin behind the active subduction zone of the outer Hellenic arc. Available fault plane solutions for the last 30 yr show the prevailing pattern of crustal stress to involve approximately N-S extension over much of the Aegean region, including the present study area (Ritsema 1974; McKenzie 1978; Drakopoulos & Delibasis 1982; Papazachos et al. 1984; Papadopoulos et al. 1986; Fig. 2). North and south of the Gulf, a thin and discontinuous cover of Plio-Quaternary sediments unconformably overlies tectonized Mesozoic and Cenozoic rocks of the Gavrovo and Ionian isopic zones of the outer Hellenides. In particular, these are Cretaceous through Eocene limestones with intervening outcrops of Oligo-Miocene flysch, and they are underlain by evaporites of presumed Permo-Triassic age (British Petroleum Co. Ltd 1971). Seismic profiling at 3.5 kHz in the Gulf (Ferentinos et al. 1985, figs 2 and 3) has shown that offshore Plio-Quaternary sediments are affected by widespread active faulting with a WNW-ESE trend. In the northeast part of the Gulf, profiling records show that concealed faulting affects Pleistocene sequences beneath an undeformed Holocene surface layer. In the immediate vicinity of the Rio-Antirrio Channel (Fig. 3), there is a complicated system of active faults with several trends (Perisoratis, Mitropoulos & Aggelopoulos 1986; Chronis et al. 1987). Onshore neotectonic data from around the Gulf show two groups of normal faults trending WNW-ESE and ENE-WSW (Ferentinos et al. 1984, 1985; Doutsos, Kontopoulos & Ferentinos 1985; Doutsos, Kontopoulos & Frydas 1987). To the east of the Gulf NNW-SSE and NNE-SSW normal faulting is observed (Kontopoulos & Doutsos 1985; Doutsos et al. 1987). In addition, near-vertical faults have been observed in the same area that cannot be characterized as either dip-slip or strike-slip for lack of fault-plane lineations. In marked contrast to this N-S extensional regime, large-scale compressional tectonics on NW-SE trending lines characterize the area to the west, in the Kefallinia- 515 Downloaded from https://academic.oup.com/gji/article-abstract/98/3/515/646411 by guest on 16 June 2020