Extension of active fault zones on Nisyros volcano across the Yali-Nisyros Channel based on onshore and offshore data

摘要

Nisyros island is a volcano at the eastern edge of the Aegean volcanic arc within the Hellenic arc and trench system along the convergence zone of the Eurasian and African plates. Several fault zones have been mapped and analyzed on the island with fault displacements reaching 100–150 m as deduced from the morphology and the offset of the stratigraphic formations of the volcano. Seismic activity during 1995–1998 affected the island with damage along the western edge of the Mandraki town, related to the Mandraki fault. The geological, tectonic and morphological data on land show that the Mandraki fault throw is 80–100 m and its length about 2 km. Its continuation northwards under the sea was studied within a systematic survey of the broader area of the Kos-Nisyros-Tilos islands; and the bathymetric and lithoseismic data showed the existence of some active tectonic structures. In the area of the Yali-Nisyros Channel the prolongation of the Mandraki fault has a 100 m high submarine scarp between the two sides of the fault. Morphological slopes along the fault are high between 20 and 50% in contrast to slopes of 1–5% observed on top of the two adjacent tectonic blocks. The general structure both on land and offshore shows a westward tilt contemporaneous to the extension in the E-W direction observed in this area. Observations of the submarine fault during a dive with submersible Thetis showed spectacular landslides and loose rocks along the fault scarp and very abrupt linear topographic change along the strike of the fault. The synthesis of the onshore and offshore data on a digital topographic map shows that the Mandraki fault is a secondary structure of the major F3 fault zone of Nisyros which separates the neotectonic block/horst of Prophitis Ilias in the west from the Emborio/Nikia block in the east. The GPS data from the period 1997–2001 show excellent agreement with the neotectonic block structure of Nisyros. The seismic hazard of the F3/Mandraki fault zone is discussed together with the volcanic hazard of Yali-Nisyros area with the general conclusion that the expected seismic magnitude of 6.1–6.3 is significantly higher than that observed in 1995–1998. The ascent of magma from a chamber 7.5–8.5 km deep between the Yali and Nisyros islands may trigger tectonovolcanic activity similar to that observed at the end of the 19th century.