Microseismicity Study Of The Central Ionian Islands (Greece) Accompanied By A Focal Mechanism Clustering Investigation

Abstract:

The central Ionian region (Greece) constitutes the most seismically active area in the Mediterranean, with the main tectonic characteristic being the Kefalonia Transform Fault Zone (KTFZ), consisted of two main branches that are linked by a step-over region. A dense seismic network allows intensive microseismicity investigation, detailing the geometry and kinematics of the major along with secondary structures capable of producing moderate earthquakes, adding to the seismic hazard assessment of the area. Earthquake relocation can highlight the complex geometry of the fault network, while the heterogeneity of the local stress regime can be investigated by analysis of the focal mechanisms.

A catalog of 16331 earthquakes was compiled by manual picking of P- and S- phases. Earthquakes were relocated using a 1D velocity model along with station delays to account for lateral crustal variations, and the application of the double difference method using cross correlation differential times. Fault plane solutions were determined using either waveform inversion or P-wave first onset polarities. A density-based clustering algorithm (DBSCAN) was applied to assess the mechanism variability. The Kagan angle was used as a similarity metric between focal mechanisms and the clustering was controlled by the minimum number of mechanisms with Kagan angle less than a specific threshold.

The relocation detailed the characteristics of several en-echelon secondary faults on the northern (Lefkada) branch, exhibiting minor dip change along depth, while a more complex focal distribution combining the strike-slip characteristics of KTFZ along with plate subduction can be observed in the southern (Kefalonia) branch. Clustering analysis of the fault plane solutions, applied in the northern branch of the KTFZ, revealed detailed features of the small-scale fault structures, showing great potential for an application in the broader region.

Acknowledgments

This research is co-financed by Greece and the European Union (European Social Fund- ESF) through the Operational Program «Human Resources Development, Education and Lifelong Learning 2014-2020» in the context of the project “Kinematic properties, active deformation and stochastic modelling of seismogenesis at the Kefalonia - Lefkada transform zone” (MIS-5047845).