G.-A. Tselentis

Periodic variations in the Earth's electric field as earthquake precursors: Results from recent experiments in Greece

Abstract The results obtained during an extensive investigation of the variations in the Earths electric field at three tectonically active areas in Greece, and their relation to the occurrence of earthquakes are described. In all three cases, a 24 h periodic oscillation of the Earths electric field was observed prior to large imminent earthquakes, one of which was the September 13th, 1986 (measuring Ms = 6.2 R) earthquake which devastated the city of Kalamata. Finally, two physical processes, which could explain the above phenomenon are proposed.

The Patras earthquake (14 July 1993): relative roles of source, path and site effects

A damaging earthquake occurred on 14 July 1993 in Patras, Western Greece. The mainshock (local magnitude 5.1) was followed on the same day by two aftershocks of magnitudes 4.4 ML and 3.6 ML, respectively. The strong motion record of the mainshock is studied, based on the teleseismically determined seismic moment and focal mechanism. The Discrete Wavenumber (DW) and Empirical Greens Function (EGF) methods are used. The main conclusion is that the 1993 Patras mainshock had a complex S-wave group mainly due to structural (path and site) effect. However, some effects of the rupture stopping on the peak ground acceleration (0.2 g in the so-called S3 phase) cannot be ruled out. Two values of the source radius are suggested: R = 1.9 and 3.0 km. The strong motion record better agrees with R = 1.9 km. If the latter is true, the stress drop was of the order of 20 MPa, i.e., higher than often reported for comparable events in Western Greece. Regardless of the true source radius, the ratio of stress drops between the mainshock and aftershocks was about 1–2. The aftershock waveforms indicate significant lateral heterogeneities around Patras. Therefore, the ground-motion predictions of strong events in the area will remain highly non-unique until weak events from an immediate neighbourhood of the particular fault are recorded.

Relocation of the 2001 Earthquake Sequence in Aegion, Greece

The western part of the Corinth Gulf attracts attention because of its seismically active complex fault system and considerable seismic hazard. Close to the city of Aegion, damaged by the ML 6.2 earthquake of 1995, a sequence of small earthquakes occurred from February to May 2001. The sequence, comprising 171 events of ML 1.8 to 4.7, was recorded by a short-period network of the University of Patras, PATNET. As most stations have single component-recording, the S-wave arrival time readings were scarce. A sub-set of 139 events was recorded by at least 5 stations, and in this study we limit ourselves just to that sub-set. A preliminary location is performed by a standard linearized kinematic approach, with several starting depths and crustal models. Then the mainshock is re-located, and finally it is used as a master event to locate the remaining events. The mainshock relocation is performed by a systematic 3D grid search, and the trade-off between depth and origin time is eliminated by a special procedure, the so-called station difference (SD) method. In the SD method, instead of inverting arrival times directly, their intra-station differences are employed. The station corrections, determined from the master event, are also used. As a result, the sub-set is imaged as a relatively tight cluster, occupying space of about 5 by 5 km horizontally and 10 km vertically, with the mainshock inside (at a depth of 7 km). The results should be interpreted with caution, mainly as regards the “absolute” depth position of the cluster. A more accurate location would require a local network with both P and S readings.

Trans-Aegean Seismicity, Seismic Hazard and Defences

Observation and analysis of both micro- and macroearthquakes is vital to understanding and preparation of multi-facetted and in-depth earthquake defences. Therefore the paper revisits simple, yet fundamentally important maps of macroearthquake felt effects in Turkey which modern techniques extend to statistically interpretable hazard and zoning maps. Such mapping should ultimately be extended well beyond peak ground acceleration to a larger, more representative suite of earthquake hazard parameters for advisory urposes. Both vigilance and rapid reaction dominate exploitation of microearthquakes. Rapid reaction resulted in a seismic network around the Athens 1999 September 15 earthquake, and 450 locally recorded aftershocks identify the fault plane (strike 117°, dip 52°) consistently with teleseismic observations. Longer term monitoring in central Greece associated with swarms at Pavliani and with Egion 1995 June 15 aftershocks provides physical properties during rupture: stress drop, coseismic slip length etc. and scaling relations between seismic moment and slip. In the case of Pavliani coseismic slip is mapped in 2D throughout a pseudo fault-zone that lacks a history of any major earthquake. The ultimate vigilance and rapid reaction in defence are embodied by earthquake early warning. An embryonic early warning demonstration system associated with liquid natural gas storage near Athens is introduced.