Talk given at the European Union of Geosciences VIII, Strasbourg, April 1995.
TERRA abstracts, Abstract supplement No. 1 to TERRA nova 7, 35-35.
We have studied co-seismic and post-seismic strains in the Alkyonides region, using a combination of GPS and conventional ground surveys of a small-scale network around an individual, currently active fault segment.
The region surrounding the 4 March 1981 Mb=5.9 Platea-Kaparelli earthquake had been surveyed by triangulation in the national survey of 1969. Nearby monuments were re-surveyed by triangulation and trilateration a few months after the earthquake. A third occupation of a some of these monuments was carried out in 1991 using GPS.
We invert for the focal mechanism from the strains in the interval 1969-1981, using a static-elastic half-space dislocation model. These strains should predominantly represent the co-seismic deformation, but a double-couple source is unable to fit the data to within 2 sigma. A complex mechanism consisting of two double-couple sources on neighbouring fault segments fits the geodetic data much better, and the resultant fault breaks and slip directions agree with field observations. This complex solution is also a better match to the CMT solution than is the best-fit double-couple.
The strain in the interval 1981-1991 is interpreted as a combination of post-seismic relaxation of strain upon a regional accumulation of extensional strain. Model post-seismic strains are computed using a viscoelastic-gravitational model of a purely elastic layer overlying a Maxwell half-space. It is necessary to include a contribution from the nearby February 1981 Perahora earthquakes. Agreement between modeled and observed strain is generally good.