B. C. Papazachos

Seismicity of the Aegean and surrounding area

Abstract Seismicity parameters based on instrumental and historical data are given for 36 seismic sources of shallow earthquakes and for 6 seismic sources of intermediate depth earthquakes in the Aegean and surrounding area (34°N–43°N, 18°E–30°E). These parameters are of two categories. Parameters of the first category are useful for seismic hazard evaluation (a and b values of the frequency-magnitude relation, maximum earthquake magnitude, number of shocks larger than certain magnitude per year) while parameters of the second category are proper for describing the spatial distribution of Seismicity (mean return period, most probable magnitude in a certain time period, moment rate). Shallow Seismicity is highest along the convex side of the Hellenic arc but close to the coast (Ionian Islands, south Peloponnesus, south of Crete, south of Karpathos and Rhodos), in Central Greece (Patraikos-Corinthiakos-Evoikos gulfs, Thessalia) and along a seismic belt which includes the northwestern Anatolia fault zone, the northernmost part of the Aegean Sea and the Serbomacedonia zone. The intermediate depth seismic activity is distributed in two parts of the Benioff zone, which dips from the convex side (Eastern Mediterranean) to the concave side (Aegean Sea) of the Hellenic arc. The one part shows a low dip angle and includes earthquakes with focal depths between 70 km and 100 km. This part has a high seismicity (earthquake magnitudes up to 8.0) and is situated below the inner slope of the sedimentary part of the Hellenic arc (Eastern Peloponnesus-north of Crete-Rhodos). Based on the fact that this part of the Benioff zone is a continuation of the seismic zone of shallow earthquakes along the Hellenic arc and that both zones have the same seismicity, it is concluded that they constitute a continuous seismic belt on which coupling occurs between the subducted eastern Mediterranean lithosphere and the overthrusted Aegean lithosphere. The other (inner) part of the Benioff zone shows low seismicity (maximum earthquake magnitude up to 7.1) with earthquakes of focal depths between 100 km and 180 km. It dips at a mean angle of 38° below the volcanic arc of the southern Aegean Sea and is the result of the subduction of the front part of the eastern Mediterranean lithosphere without coupling with the Aegean lithosphere.

THE CEPHALONIA TRANSFORM FAULT AND ITS EXTENSION TO WESTERN LEFKADA ISLAND(GREECE)

The central area of the Ionian Islands is dominated by the existence of a major tectonic structure called the Cephalonia Transform Fault (CTF). Its main part (Cephalonia segment) has been identified by previous work based on the spatial distribution of earthquake foci, fault plane solutions of strong earthquakes, active tectonics, structural studies and geodetic measurements. This part (Cephalonia segment) which exhibits strike-slip motion with a thrust component, strikes in a northeast direction, dips to the southeast and has a length of ∼90 km. In the present paper information concerning new fault plane solutions, orientation of isoseismals, sea bottom topography and recent GPS results are used to further check the properties of this southern part of the CTF and to explore its northeastward prolongation to Lefkada Island. It is shown that the CTF is extended to the western coast of Lefkada. This northern branch (Lefkada segment) of the CTF which is also characterized by strike-slip motion with a thrust component, strikes in a north-northeast direction, dips to the east-southeast and has a length of ∼40 km. These two segments of the CTF form a major kinematic boundary where the slip rate is 2–3 cm/yr.

Evidence for transform faulting in the Ionian sea: The Cephalonia island earthquake sequence of 1983

Accurate locations of aftershocks of the January 17, 1983 (Ms=7.0) main shock in the Ionian islands have been determined, as well as fault plane solutions for this main shock and its largest aftershock, which are interpreted as a right-lateral, strike-slip motion with a thrust component, on a fault striking in about a NE-SW direction.This is considered as a transform fault in the northwesternmost part of the Hellenic arc.

Time-Independent and Time-Dependent Seismic Hazard in Greece Based on Seismogenic Sources

Abstract An improved procedure is followed to assess time-independent andtime-dependent hazard for 144 broad sites (cities, towns, villages) of Greece in termsof expected macroseismic intensities at each one of these sites. In this procedureseveral new results are taken into account: source parameters based on new researchwork on seismicity and on other relative information (new regionalization and im-proved seismicity data parameters and anisotropic radiation at the seismic source), anew relation for the attenuation of macroseismic intensities, and the effect of localconditions (site effects) on the strong-ground motion.Analytic relations and tables, determined by this procedure, permit the reliableassessment of time-independent hazard, in terms of macroseismic intensity (in MMscale), peak-horizontal-ground acceleration, and velocity at each one of these 144sites and at any probability level.Time-dependent hazard has been also assessed for these 144 broad sites in Greece,in terms of the probability for the occurrence of strong ground-motion with macro-seismic intensity I

Seismic faults in the Aegean area

VII (in MM scale) at each one of these sites during the period1996–2010. Comparison of the assessed time dependent hazard with observed ma-croseismic intensities of the period 1950–1995 indicates that the time-dependentmodel gives very reasonable results.Introduction

Foreshocks and earthquake prediction

Abstract Reliable fault plane solutions of shallow earthquakes and information on surface fault traces in combination with other seismic, geomorphological and geological information have been used to determine the orientation and other properties of the seismic faults in the Aegean and surrounding area. Thrust faults having an about NW-SE strike occur in the outer seismic zone along western Albania-westernmost part of mainland of Greece-Ionian Sea-south of Crete-south of Rhodes. The inner part of the area is dominated by strike-slip and normal faulting. Strike-slip with an about NE-SW slip direction occurs in the inner part of the Hellenic arc along the line Peloponnesus-Cyclades-Dodecanese-southwest Turkey as well as along a zone which is associated with the northern Aegean trough and the northwesternmost part of Anatolia. All other regions in the inner part of the area are characterized by normal faulting. The slip direction of the normal faults has an about SW-NE direction in Crete (N38°E) and an about E-W direction (N81°E) in a zone which trends N-S in eastern Albania and its extension to western mainland of Greece. In all other regions (central Greece-southern Yugoslavia and Bulgaria, western Turkey) the slip of the normal faults has an about N-S direction.

Tectonic stress field and seismic faulting in the area of Greece

Abstract Results of a statistical investigation of the magnitude and time distributions of foreshocks in the area of Greece are reported. Further evidence is presented that the parameter b , in the frequency—magnitude relation, has a smaller value before than after the main shock, and that the time distribution of foreshocks follows a statistical law similar to that followed by aftershocks. The difference in magnitude between the main shock and the largest foreshock seems to be independent of the magnitude of the main shock. The average of this difference has been found equal to about two magnitude units. The significance of these results to the problem of statistical prediction of earthquakes is noted.

Long-term earthquake prediction in the Aegean area based on a time and magnitude predictable model

Abstract Fault plane solutions of 89 shallow earthquakes which occurred in the region of Greece between 1947 and 1966 have been used to derive statistical results about the direction of the acting stress component and orientation of faulting. The acting stress component is tensional in most places and its direction is almost horizontal. The faults are almost vertical in most of the cases and in about 90% of the cases the motion is strike slip. The null axes of each sub-region lie on a plane. Evidence exists that this plane coincides with the fault plane in each small region.

Regional Focal Mechanisms for Earthquakes in the Aegean Area

AbstractThe Aegean and surrounding area (34°N–43°N, 18°E–30°E) is separated into 76 shallow and intermediate depth seismogenic sources. For 74 of these sources intervent times for strong mainshocks have been determined by the use of instrumental and historical data. These times have been used to determine the following empirical relations: