The analysis and processing of 1135 microearthquakes, recorded during the time period May 1993 – December 1994, indicated that the North Aegean Trough and the Trough of Skyros are the boundaries of, respectively, the NW and SE side of a tectonic Block in the shape of a trapezium, whose large base is the NW one. ΝW of the island of Lemnos, the North Aegean Trough changes direction from ENE-WSW to NE-SW. This point operates as a joint, on either side of which a differentiation of the areas is visible. In the SW of the troughs, two seismic gaps, which seem to be connected to a presently shaping channel of a NW-SE trend, were observed. The two southern branches of the North Anatolia Fault will constitute, in future, the northern and southern boundary of the Trough of Skyros. The migration of earthquakes in the North Aegean Sea follows a right lateral direction, however this direction changes to left lateral east of Chios Island.
Milos island belongs to the Greek volcanic arc and is part of a small number of hot spots in the South Aegean Sea. It is an area that stands out for the diversity in volcanic formations and chemical composition of rocks. The island is characterized by the presence of a high enthalpy geothermal field and a non continuous microseismic activity, expressed both by tectonic earthquakes and swarm activity. During the period of our study (May-December 1999), this activity, which was recorded by a small, portable, seismological network has been very small and limited in the SE part of the island. From the chemical analyses performed, no significant alterations in the composition of the hydrothermal fluids was detected, except from slight changes in the content of K, Ca and Li and even smaller changes in Mg and Fe. This fact certainly implies a possible relation between microseismic activity and alterations in the content of various chemical elements, but it still needs further verification, which means that our research should be prolonged for a time period of at least two years
Significant changes in the groundwater radon concentration, were observed on the 29/7/2003, at Kammena Vourla. The radon concentration changes can be attributed to the temporary enrichment of the groundwater by radon from the surrounding rocks due to the action of stress release or stress accumulation prior an earthquake. EPPO was immediately informed for the existence of these changes. On the 3/8/2003, a significant number of earthquakes in Northern Evia were recorded in an interval of a few hours. The epicentres of the earthquakes were located in the conceivable line between the epicentre of the strong earthquake of Skyros island (July 2001) and the site of the station of radon measurement at Kammena Vourla, on the extension of the Northern Aegean fault. In the 23/1/2003, in the same station, 3 significant changes of radon concentration were measured EPPO was immediately informed for impending seismic activity in Northern Evia. On the 29th of January three earthquakes were recorded. Their epicentres were located in N. Evia, close to the radon measurement, station. On the 2/2/2003, a new significant radon change occurred at Kammena Vourla followed by an earthquake (7/2/2003). In the 12/2/2003 in a letter that we addressed to the EPPO administration, the beginning of a new radon change was reported which also led to an earthquake with the same epicentre. The changes that are recorded in the system of radon measurement of Kammena Vourla are precursors of earthquakes, related to the extension of the N. Aegean fault towards the Sterea Hellas and have their epicentre in N. Evia. It seems that the method can be an important tool for local, short term earthquake prediction.
Significant precursory changes in the radon concentration of groundwater were observed, 5-6 days prior all the earthquakes (M>2.5) occurred in the area of the Atalanti fault during the period from 22/9/1998-17/1/1999. The radon concentration measurements of ground water,were obtained at one hour intervals by the use of a ZnS(Ag) scintillation chamber. When no earthquake occurs, the radon concentration of the groundwater shows considerable stability. Prior of an earthquake radon concentration increases significantly at the beginning and then dropped to an also significant lower value. These radon concentration changes can be attributed to the temporary enrichment of the groundwater by radon from the surrounding rocks due to the action of stress release or stress accumulation prior an earthquake. It seems likely that the method can be an important tool for the earthquake prediction in local scale.
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