High precision gravity observations on Crete, Greece

The position of Crete within the Hellenic arc-trench system is important for understanding the tectonics and geodynamics of the area. In 1983–1984, a high precision network of 52 reoccupied gravity stations was established on Crete for studies of vertical crustal movement and the detection of precursory phenomena associated with a large magnitude earthquake (M > 7) forecase just northwest of Crete first by Wyss and Baer (1981). The standard deviation of the network adjustment is 0.045 and 0.065 g.u. for 1983 and 1984, respectively, after applying a newly developed adjustment algorithm.There were not statistically significant gravity changes for the stations of the network during its remeasurement in 1984. However, there were differences at some stations of more than twice the level of their standard error. Stations established along the sedimentary basins of east and central Crete, such as Perama, Heraklio and lerapetra grabens, have a tendency to subsidence. Perama, Cheliana, Tymbaki, Stavraki have subsided from 37 ± 22 to 72 ± 23 mm. On the other hand, nearly all stations established on horsts tend to show uplift. Malia, Chania and Heraklio Airport were uplifted from 58 ± 20 to 65 ± 29 mm. These elevation change values were obtained using the Bouguer gravity gradient of −0.2 g.u./mm.Almost all the stations west of Rethymno have a tendency for uplift, regardless of their tectonic setting. It is argued that the gravity data provide initial evidence that present-day Earth movements are consistent with longer term tectonic observations of uplift in Western Crete. The occurrence of a strong shallow earthquake (M = 6.3) just offshore and west of Crete, between 1983 and 1984, may have been associated with this movement.