Kinematics and deformation of central and southwestern Greece from historical triangulation data and implications for the active tectonics of the Aegean

Abstract

Changes in the coordinates of 38 stations, 32 of which were common to three historical triangulation surveys (1889–1905, 1927–1930 and 1950–1970) in central Greece were computed based on conventional adjustment techniques and the assumption of a nearly fixed baseline length. Some of the calculated coordinate changes are significant against the a posteriori network adjustment errors and are likely to indicate tectonic motions consistent with those deduced from geological and geophysical data and other geodetic studies (comparison of the first survey network with GPS data). More explicitly, the historical geodetic data confirm the rotational character of the deformation in the area, but they show that the strain pattern is likely to have changed after 1930: between 1890 and 1930 strain was changing smoothly, as if no strain discontinuities existed, while the Peloponnesus was under contraction from the northeast, probably reflecting accommodation of strain from the arc. Significant left-lateral shear in the gulf of Corinth and N-S extension in the whole of the study area were observed only after 1930. Estimates of strain are consistent with those deduced from comparison of historical triangulation and GPS data, corrected for the scale error that exists in the terrestrial geodetic data in Greece, and estimates of strain from seismological data. The change in the kinematic pattern after 1930 seems to be confirmed by changes in the shear strain computed directly from changes in observed angles between stations and from changes in the seismicity rates; such changes, although not unusual in the geological record, are not easy to explain, but may reflect elastic rebound effects in area with an extremely complicated tectonic fabric or hysteresis in the accommodation of E-W compression from the arc by N-S stretching.