Imaging Fault Structure of the 1995 Kozani-Grevena Earthquake Sequence, Greece Using High Precision Aftershock Locations

Phillip G. Resor, Wesleyan University
G C. Beroza
David D. Pollard

Abstract

The May 13, 1995 Kozani-Grevena earthquake (Mw=6.5) is a natural laboratory for studying crustal normal fault systems. The event and its aftershocks have been well observed geodetically, seismically, and geologically, providing an opportunity to integrate data sets to create a detailed subsurface fault model and investigate triggering and deformation associated with a large normal fault earthquake. Previous modeling of the earthquake has focused primarily on single geodetic data sets (e.g. inSAR - Meyer et al, 1996, GPS - Clarke et al., 1997) and has led to conflicting subsurface fault interpretations. In order to better model the subsurface fault geometry we have relocated aftershocks and use the interpretation of multiple complementary data sets to constrain a 3D boundary-element model of the earthquake sequence. Using the Double-Difference earthquake location algorithm (Ellsworth and Wauldhauser, 2000) we have reduced the hypocentral location error by a factor of ~10, obtaining high-precision aftershock locations for 650 events recorded by a local network (Hatzfeld et al., 1997). Relocated aftershocks cluster into a system of planar structures that reveal the "fine" structure of faults that were active during the earthquake sequence. The master normal fault dips 45° north from 6-14 km depth and extends over a length of ~12 km, consistent with the Harvard CMT solution. Two south-dipping antithetic faults extend from the western half of the master fault, one located at the up-dip tip extending from 4-6 km depth, and the second located at approximately the mid-point of the master fault in cross section from 6-9 km depth. These antithetic faults dip 45° and 35° respectively. At the western end of the rupture is a system of strike-slip faults in an orientation consistent with slip-transfer or segment linking structures. Fault patterns interpreted from the aftershock distribution form the basis of a 3D boundary element model using Poly3D. This code uses a mesh of contiguous triangular dislocation elements to capture the essential features of complex fault geometry in an idealized elastic half space. The mechanical model allows us to test the consistency of fault interpretations based on aftershock locations against surface geodetic and geologic data as well as to understand the mechanics of triggering and strain accommodation associated with the earthquake sequence. An improved fault model for the Kozani-Grevena earthquake is important for assessing seismic hazard and for understanding the mechanics of normal fault earthquakes and normal fault systems.