The volcanoseismic crisis of 1996-97 in Nisyros, SE Aegean Sea, Greece

After a repose time of several years, intense, shallow seismic activity sta rted at the end of 1995 in the volcanic region of Nisyros island. By the en d of October 1997, hundreds of shocks had been recorded, the largest one be ing that of 27 August 1997 with M-s = 5.3. The activity is highly clustered in space and time, while the b-value of the magnitude-frequency relation w as found equal to 1.67. Similar b-values were determined from Nisyros seism icity registered between 1911 and 1980 and from a 1997 microearthquake data set. These properties seem to be a characteristic of the region, rather tha n precursor of a forthcoming eruption and make Nisyros similar to other lar ge calderas of the Earth where the episodic unrest with local swarm-like ea rthquake sequences separated by months or years is a typical pattern that d o not culminate in a volcanic eruption.