Modeling the ML4.7 mainshock of the February-July 2001 earthquake sequence in Aegion, Greece

Abstract

An earthquake sequence comprising almost 2000 events occurred in February-July 2001 on the southern coast of the Corinth Gulf. Several location methods were applied to 171 events recorded by the regional network PATNET. The unavailability of S-wave readings precluded from reliable depth determination. For the mainshock of April 8, ML = 4.7, the depth varied from 0 to 20 km. The amplitude spectra of complete waveforms at three local stations (KER, SER, DES; epicentral distances 17, 26 and 56 km) were inverted between 0.1 and 0.2 Hz for double-couple focal mechanism and also for the depth. The optimum solution (strike 220 ◦ ,d ip 40 ◦ , rake –160 ◦ , and depth of 8 km) was validated by forward waveform modeling. Additionally, the mainshock depth was further supported by the P- and S-wave arrival times from the local short-period network CRLNET (Corinth Rift Laboratory). The scalar seismic moment was 2.5e15 Nm, and the moment rate function was successfully simulated by a triangle of the 0.5 second duration. This is equivalent to a 1–1.5 km fault length, and a static stress drop 2–6 MPa. This value is important for future strong ground motion simulation of damaging earthquakes in Aegion region, whose subevents may be modeled according to the studied event. The T axis of the mainshock (azimuth 176 ◦ and plunge 67 ◦ ), is consistent with the regional direction of extension ∼N10 ◦ . However, none of the nodal planes can be associated to an active structure seen at the surface. The relationship of this earthquake sequence with deeper faults (e.g. possible detachment at about 10 km) is also unclear.