VERTICAL DISPLACEMENT ABOVE A SUBDUCTION ZONE (SW CRETE): SPATIAL COINCIDENCE OF CO- AND INTERSEISMIC SURFACE UPLIFT — FROM HISTORIC DATA AND PERSISTENT SCATTERER INTERFEROMETRY ANALYSIS

The occurrence of large earthquakes at subduction zones requires the accumulation and sudden release of large amounts of elastically stored energy, which builds up when the plate interface is locked. This leads to elastic bending and in turn to transient vertical surface displacement of both, the down-going and the upper plate during the seismic cycle. Quantification of this transient vertical deformation behaviour, therefore, provides important constraints on models of the “earthquake cycle”. We collected vertical displacement data on the island of Crete using radar interferometry. The island is an ideal test case to observe the interseismic vertical surface motion due to its proximity to the Hellenic subduction zone. The island was affected by large earthquakes in the past, e.g., in 365 AD a M_S > 8 earthquake occurred along a reverse fault ~70 km SW of Crete, and induced sudden vertical uplift of the SW part of the island of up to 9 m. The recently developed Persistent Scatterer Interferometry (PSI) provides a new tool to quantify vertical motion of the Earth's surface with millimeter accuracy and a wide spatial coverage of hundreds of kilometers. For the PSI method scatterers, which are stable over a long time, are observed. We used the observational PSI system (PSI-GENESIS) for wide areas, developed at the German Aerospace Center (DLR). Data of the ERS-1 & -2 satellites were used for the PSI analysis. The results of the PSI observation show a surface uplift of up to 10 mm/yr of the SW part of the island. Surprisingly, the active uplift pattern coincides with the region that underwent co-seismic uplift of up to 9 m in 365 AD. Our preliminary findings are inconsistent with those expected based on currently accepted models of vertical surface displacement during the seismic cylcle, where opposite motion patterns would be predicted during the co-seismic versus the interseismic phase. A similar pattern has been observed for the Tohoku-Oki earthquake in March 2011. More generally, interseismic strain accumulation in SW Crete implies that the subduction zone interface is at least partly locked and hence may produce another large earthquake in the future.