The spatially extended 2006 April Zakynthos (Ionian Islands, Greece) seismic sequence and evidence for stress transfer

SUMMARY The seismic sequence at the southern part of Zakynthos Island (Ionian Sea, Western Greece) during April and May of 2006 is investigated. It consists of four moderate earthquakes (5.3 ≤ M w ≤ 5.7) that were followed by significant seismic activity. Source parameters of the significant events were determined using two different techniques. Teleseismic body‐wave modelling was employed for the major events of the sequence ( M w > 5.6), although for the moderate ones the frequency‐wavenumber integration method was used. The calculated fault plane solutions of the 2006 April seismic sequence revealed compressional regime in all cases. More than 3500 microearthquakes were recorded and located using a local temporary network. Their spatial distribution indicated high concentration of seismic activity within an expanded area, taking into account the magnitudes of the major events. Within this expanded area (approximately 10 × 30 km 2 ), the focal depths varied between 10 and 25 km. Two main clusters were revealed. The major events occurred in an area of low seismic activity that separates the two clusters. Seismic cross‐sections indicated a complex pattern of the hypocentre distribution with the activation of two nearly antithetical faults, consistent with the determined focal mechanisms. The northern cluster can be associated with a fault plane trending NNW–SSE and dipping towards SW, although the southern cluster with a fault plane striking NW–SE and dipping towards ENE. Finally, Coulomb stress analysis was performed to calculate the stress transfer and correlate it with the activated area. Positive lobes with stress more than 0.3 bars were obtained, indicating that these values are large enough to increase the seismicity towards the observed NNW–SSE direction.