Extended abstract of talk given at the 1st International Symposium on Deformations in Turkey, September 1994


Proc. 1st Turkish International Symposium on Deformations, 746-747.

Geodetic Studies of Aspects of Strain Accumulation and Release during the Seismic Cycle:
Results from the Gulfs of Korinthos and Argos

Peter Clarke (1), Andrew Curtis (1), Iordanis Galanis (2),
Harris Billiris (2), Philip England (1), Demitris Paradissis (2),
Barry Parsons (1) and George Veis (2)

(1) Department of Earth Sciences, University of Oxford, U.K.
(2) Higher Geodesy Laboratory, National Technical University of Athens, Greece

We have studied accumulation of strain in and around the Gulfs of Korinthos and Argos, using a combination of GPS and the results of conventional ground surveys. Here we present two aspects of these studies based on small (30 km-scale) networks around individual fault segments.

In 1981 a series of damaging earthquakes affected the region around the eastern end of the Gulf of Korinthos. The region surrounding one of these, the Mb=5.9 Platea-Kaparelli earthquake, had been surveyed as part of the Greek National Survey in 1969. Twenty-nine monuments of the 1969 survey were re-surveyed within a few months of the earthquake. We carried out a third occupation of eight of these monuments in 1991 using GPS. Processing of the GPS observations was carried out using the Ashtech proprietary GPPS software, using broadcast ephemerides, and version 3.4 of the Bernese GPS software, using NGS precise ephemerides. The results from the two methods agree to within 12 mm (rms) in north and 8mm (rms) in east components of relative positional vectors, after a 2.5 ppm scale difference between the solutions is removed.

In order to produce displacements from the 1969 and 1981 angle measurements, scale and orientation assumptions must be made. We assume that post-seismic displacements in the interval 1981-1991 are negligible compared with co-seismic displacements in the interval 1969-1981, so scale and orientation can be taken from suitable stations in the 1991 GPS survey. Using a forward model from the seismic moment tensors for the 1981 earthquake sequence determined by Taymaz et al. [1991] and the fault breaks mapped by Jackson et al. [1982] we determine which baselines will have least strained and rotated co-seismically, and hold one azimuth and one length fixed at the 1991 values. Baselines crossing any known active fault break are not considered for this purpose.

Using the 1969-1981 displacements we invert for the strike, dip and rake of the Platea-Kaparelli earthquake, and the position and length of the fault break. The event is modeled as a dislocation on a finite rectangular plane extending to a depth of 10 km into an elastic half-space, after the formulation of Okada [1985]. The solution for two separate events on neighbouring fault segments is a good fit to the geodetic data: the rms weighted residual is less than 1.2, and the predicted fault break is a close match to the mapped fault break. However, the moment of the event required to fit the geodetic displacements 4.8x10^18 N m) is greater than the moment obtained from seismological studies [Taymaz et al., 1991; Ekstrom and England, 1989] by a factor of 1.7. Fault slip vectors predicted by the model are in reasonable accord in both magnitude and direction with mapped slip vectors, but differ from slip vectors predicted by the seismic moment tensor solutions.

We compare the strains in the intervals 1969-1981 and 1981-1991, for polygonal regions. The strains in the first interval are largest across the mapped fault break, and therefore reflect predominantly the co-seismic deformation, but smaller extensional strains elsewhere cannot be simply related to the parameters of that earthquake determined from surface faulting and the seismic moment tensor. The smaller, but significant, strains in the interval 1981-1991 are interpreted as a superposition of a post-seismic relaxation of strain (compressional in the region of the fault break, extensional elsewhere) upon a regional accumulation of extensional strain.

References

Ekstrom, G. and P. England (1989). Seismic strain rates in regions of distributed continental deformation. J. Geophys. Res., 94, 10231-10257.

Jackson, J.A., J. Gagnepain, G. Houseman, G.C.P. King, P. Papadimitriou, C. Soufleris and J. Virieux (1982). Seismicity, normal faulting and the geomorphological development of the Gulf of Corinth (Greece): the Corinth earthquakes of February and March 1981. Earth Planet. Sci. Lett., 57, 377-397.

Okada, Y. (1985). Surface deformation due to shear and tensile faults in a half-space. Bull. Seis. Soc. Am., 75, 1135-1154.

Taymaz, T., J. Jackson and D. McKenzie (1991). Active tectonics of the north and central Aegean Sea. Geophys. J. Int., 106, 433-490.


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