A Two‐Fault Model of the 2003 Leucas (Aegean Arc) Earthquake Based on Topological Inversion of GPS Data

Abstract

The fault pattern of the 2003 M w 6.2 Leucas (Lefkada, Lefkas) earthquake at the northwest edge of the Aegean arc, an area of very high seismicity rates, was modeled using Global Positioning System (GPS) data. The rupture process of this earthquake is a matter of debate because of the complexities in the aftershock distribution and in the waveforms of the mainshock, as well as the kinematics of the area are not well understood, especially the characteristics of the Cephalonia (Kefalonia, Keffalinia) transform fault and its relationship with major, discordant active thrusts representing the tectonic fabric of western Greece. Our analysis was based on the TOPological INVersion (TOPINV) algorithm, which uses a deterministic hypergrid search to identify one or more clusters of n ‐dimensional points around the true solution of the system of observation equations. From these clusters, optimal estimators of the variables defining Okada‐type faults and their variance–covariance matrices are computed as statistical moments, and results are validated by minimum mean misfits. Faulting of the 2003 seismic sequence was approximated by two stepping, essentially strike‐slip segments with a separation of ∼10 km. This pattern is in agreement with fault rupture models based on inversion of seismological data, and their main difference is a thrust component in the major segment, well constrained by Interferometric Synthetic Aperture Radar and GPS observations of uplift. This thrust component may reflect either postseismic, evaporite/salt‐controlled slip, or uncertainties in the inversion models. The 2003 and other seismic sequences, among them the 1953 M 7.2 Cephalonia catastrophic earthquakes that were associated with major uplift, seem to confirm previous views for an ∼100‐km‐wide shear zone superimposed on compression along the edges of the Aegean arc. Online Material: Description of analytical inversion methodology (the TOPological INVersion algorithm [TOPINV]), including a figure showing projection of hypergrids on planes.