Seismic slip on a low angle normal fault in the Gulf of Corinth: Evidence from high-resolution cluster analysis of microearthquakes

The Gulf of Corinth in Western Greece is one of the most active extensional zones in the Aegean region. It is still an open question whether extension can be actively accommodated on low angle faulting or if those faults as seen in geological records have been rotated. Whilst numerous fault plane solutions obtained from a dense temporary network deployed in the western part of the gulf in July-August of 1991 showed one of the nodal planes as a subhorizontal plane, slip on the high-angle conjugate plane is equally probable from the focal mechanism data (Rigo et al. 1996). Since part of the activity occurred in spatial clusters with similar focal mechanisms, we used a high resolution cluster analysis to determine the most likely active plane. Exploiting the waveform similarity of these events, relative onset times of P and S waves could be determined at subsample accuracy (less than 0.01 s). The cluster analyzed here contains 12 events, among these 8 have a well constrained normal faulting fault plane solution with a shallow (12-20 degrees) north dipping plane and a steeply south dipping plane. A master event relocation shows that the relocated 12 hypocenter centroids are aligned along the low angle plane showing clear evidence for active low angle normal faulting.