We deployed an on/offshore seismic network consisting of 36 three-component stand-alone digital stations in the Saronikos Gulf and surrounding area and observed the microseismic activity from 17 August to 9 December 2001. The geometry and location of the network were designed to observe the offshore activity to connect known active faults onshore with those offshore. An average of 15 events per day were recorded by a minimum of six stations. The focal parameters of 739 events could be defined. Within the network area we located 306 events with an rms travel-time residual of about 0.2 sec. The epicenters in the Saronikos Gulf are aligned mainly along an east-west trending fault with a length of nearly 30 km. This fault is located north of Aegina and could produce earthquakes as large as Ms 6.5. It is the offshore continuation of a seismic zone located in east Corinthia. A second active fault was identified east of Aegina, trending northeast-southwest. It is probably linked to the northeast-southwest striking zone of the Salamis-Fili seismic region, activated by the September 1999 event of Athens. Outside the network we mapped an active area at the Parnis mountain, still active after the Ms 5.9 event, and a second one further northeast at the western coast of Evia. This zone is close to Chalkis and the recently active Psachna area. The network also mapped intense activity at the northeastern coast of Evia, close to the city of Kymi and the area of Skyros, which was seriously damaged by an Ms 5.8 earthquake in 2001. The onshore/offshore local network identified several regions of very high seismic activity not detectable by regional networks, and demonstrated once more the advantage of deploying local seismic networks for mapping active deformation in very short periods of time.

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