Intense Crustal Deformation Rates on Nissyros Island (Greece), Deduced from GPS Studies, May Foreshadow a Forthcoming Volcanic Event

Abstract

Nissyros Volcano lies at the SE part of the Hellenic Volcanic Arc (HVA) and, together with the Thera (Santorini) Volcano comprise the most active volcanoes in the Aegean Sea. The last major episode including freatic explosions and gas emanation tool place during the last century. Since then there was not any kind of similar major activity, until early 1997, when an intense local seismic activity broke out with a simultaneous reactivation of an older fault passing through Mandrakion at the NW part of the island.

However, the observed seismic activity is not closely associated with the former fault, but seems to be spreading out to the north, encompassing the whole marine area between Nissyros and Western Kos islands. A GPS network was established on Nissyros to monitor the crustal deformation of the area in June 1997. This network, as part of the implemented methods for the surveillance of Nissyros Volcano, was then remeasured three times, the last one in May 1998. It was found that significant displacements were observed in the coordinates of the stations, in both vertical and horizontal, with respect to a GPS station in eastern Kos. The overall observed displacements for the period June 1997 — May 1998 show intense ground deformation rates until six months after its establishment. A general uplift was measured at all stations of the network ranging from 14(±5)mm to 45(± 10)mm (Sept. 1997), which in the subsequent remeasurements showed a slight decline, maintaining though the uplifted character of the deformation. The amplitude of the horizontal deformation was ranging from 13mm to 37mm (average error estimate about ±5mm) for the period of only three months (June-Sept. 1997), which increased for the next eight months, but not analogously, up to 53mm. The directions of the horizontal deformation observed at the GPS stations (June’97-May’98) indicate that the island is “opening up” along two major faulting zones, both commencing from the centre of the island, with one extending to the NW and the other one to the SW.

It is believed that the observed seismicity on Nissyros and the broader area to the north, a great part of which is of volcanic character (Stavrakakis, personal communication), may be attributed to gradually upwelling magma, which due to the change of the regional regime, produces:

• The reactivation of older faults, which subsequently are responsible for the observed seismicity of tectonic character

• The defined relatively large uplift rates at almost all stations on Nissyros

• The fault-controlled “opening” of Nissyros along the two major faulting zones, introducing again relatively large rates of horizontal deformation.

A reactivation of an older volcanic centre (150,000yr) could possibly be taking place, located in the middle between Nissyros and West Kos, consistent to the local picture of the seismicity for the last two years.