E. M. Scordilis

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.

Empirical Peak Ground-Motion Predictive Relations for Shallow Earthquakes in Greece

In the present article new predictive relations are proposed for the peak values of the horizontal components of ground acceleration, velocity, and displace- ment, using 619 strong motion recordings from shallow earthquakes in the broader Aegean area, which are processed using the same procedure in order to obtain a homogeneous strong motion database. The data set is derived from 225 earthquakes, mainly of normal and strike-slip focal mechanisms with magnitudes 4.5 M 7.0 and epicentral distances in the range 1 km R 160 km that have been relocated using an appropriate technique. About 1000 values of peak ground acceleration (PGA), velocity (PGV), and displacement (PGD) from horizontal components were used to derive the empirical predictive relations proposed in this study. A term ac- counting for the effect of faulting mechanisms in the predictive relations is intro- duced, and the UBC (1997) site classification is adopted for the quantification of the site effects. The new relations are compared to previous ones proposed for Greece or other regions with comparable seismotectonic environments. The regression anal- ysis showed a noticeable (up to 30%) variance reduction of the proposed relations for predicting PGA, PGV, and PGD values compared to previous ones for the Aegean area, suggesting a significant improvement of predictive relations due to the use of a homogeneous strong motion database and improved earthquake parameter infor- mation.

The 2001 Skyros, Northern Aegean, Greece, earthquake sequence: off - fault aftershocks, tectonic implications, and seismicity triggering

was identified as a possible site for the occurrence of a strong event by Papadimitriou and Sykes [2001] who applied an evolutionary stress model in the Northern Aegean area. [4] The paper analyzes the details of the earthquakes in the Skyros sequence, aiming to contribute to the understanding of the seismotectonic properties in this area where the western termination of the north Aegean strike slip faulting against the mainland of Greece takes place. The co-seismic stress changes associated with the main shock are computed and the areas of static stress increases are correlated with the aftershock spatial distribution.

Surface Fault Traces, Fault Plane Solution and Spatial Distribution of the Aftershocks of the September 13, 1986 Earthquake of Kalamata (Southern Greece)

A shallow earthquake ofMS=6.2 occurred in the southern part of the Peloponnesus, 12 km north of the port of the city of Kalamata, which caused considerable damage. The fault plane solution of the main shock, geological data and field observations, as well as the distribution of foci of aftershocks, indicate that the seismic fault is a listric normal one trending NNE-SSW and dipping to WNW. The surface ruptures caused by the earthquake coincide with the trace of a neotectonic fault, which is located 2–3 km east of the city of Kalamata and which is related to the formation of Messiniakos gulf during the Pliocene-Quaternary tectonics. Field observations indicate that the earthquake is due to the reactivation of the same fault.A three-days aftershock study in the area, with portable seismographs, recorded many aftershocks of which 39 withMS≥1.7 were very well located. The distribution of aftershocks forms two clusters, one near the epicenter of the main shock in the northern part of the seismogenic volume, and the other near the epicenter of the largest aftershock (MS=5.4) in the southern part of this volume. The central part of the area lacks aftershocks, which probably indicates that this is the part of the fault which slipped smoothly during the earthquake.

Evaluation of the Results for an Intermediate-Term Prediction of the 8 January 2006 Mw 6.9 Cythera Earthquake in the Southwestern Aegean

During the past few decades the critical earthquake model, which is based on observations concerning accelerating seismic deformation and concepts of the critical point dynamics, has been proposed by various seismologists as a useful tool for intermediate-term earthquake prediction. A refined approach of this model has been previously applied to search for preshock (critical) regions in the southern Aegean, using all available data until the middle of 2002. A critical region corre- sponding to a large mainshock had been identified (Papazachos et al., 2002a,b) in the southwestern part of the Aegean, near the Cythera island. The predicted (in 2002) parameters for this ensuing earthquake are u 36.5� N, k 22.7� E for the epicental geographic coordinates (with a model uncertainty of 120 km), focal depth 100 km, moment magnitude M 6.9 0.5, and origin time tc 2006.4 2.0. The generation of the strong Cythera earthquake on 8 January 2006 with M 6.9, epicenter coordinates u 36.2� N and k 23.4� E and a focal depth of h 65 km satisfies this intermediate-term prediction. The region where significant macroseismic effects were anticipated from the predicted mainshock (Cythera, south Peloponnesus, west Crete, and west Cyclades) corresponds to the area where damage by the 8 January 2006 strong earthquake has been observed. The verification of this prediction is strong evidence that the intermediate-term prediction of strong earthquakes is potentially feasible, but additional forward testing of the model is needed to validate this result.

Earthquake triggering in the north and east Aegean plate boundaries due to the Anatolia westward motion

Historical and instrumental data of the last five centuries show that major earthquakes in the Marmara Sea area are followed by high strong-earthquake seismicity in the Aegean area. During the first phase of this excitation period, which lasts about 3 years, strong shallow earthquakes concentrate almost exclusively along the Northern Boundary of the Aegean plate. Within 1–5 years after the first strong earthquake along this boundary, strong-earthquake seismicity also increases in the Eastern Aegean Boundary. On the basis of current ideas on active tectonics and the previous observations, we expect the generation of several mainshocks with a mean magnitude of M=6.5 along the Northern Aegean Boundary during the next 3 years, following the recent large Izmit (NW Turkey) earthquake (1999.8.17, M=7.4).

Accelerating seismic crustal deformation before strong mainshocks in Adriatic and its importance for earthquake prediction

Time accelerating Benioff strain releasebefore the mainshock has been observed inall five cases of strong (M > 6.0) shallowmainshocks, which have occurred during thelast four decades in the area surroundingthe Adriatic Sea. This observation supportsthe idea that strong mainshocks arepreceded by accelerating seismic crustaldeformation due to the generation ofintermediate magnitude shocks (preshocks).It is further shown that the values ofparameters calculated from these datafollow appropriately modified relations,which have previously been proposed asadditional constraints to the criticalearthquake model and to the correspondingmethod of intermediate term earthquakeprediction. Thus, these results show thatthe identification of regions wheretime-accelerating Benioff strain followssuch constraints may lead to usefulinformation concerning the epicenter,magnitude and origin time of oncomingstrong mainshocks in this area. Theprocedure for identification of thetime-acceleration is validated byappropriate application on synthetic butrealistic random catalogues. Largerdimension of critical regions in Adriaticcompared to such regions in the Aegean isattributed to an order of magnitude smallerseismic deformation of the crust in theformer in comparison to the latter.

Perspectives for earthquake prediction in the Mediterranean and contribution of geological observations

Abstract Accelerating seismic strain caused by the generation of intermediate-magnitude preshocks in a broad (critical) region, accompanied by decelerating seismic strain caused by the generation of smaller preshocks in the seismogenic region are systematically observed before strong mainshocks. On the basis of this seismicity pattern a model has been developed that seems promising for intermediate-term earthquake prediction, called the ‘Decelerating in-Accelerating out Seismic Strain Model’. Recent seismological data for the Mediterranean region are used here for backward and forward testing of this model. The selection of the broader Mediterranean region as a test area was motivated not only by the interest of time-dependent seismic hazard assessment in a high-seismicity and highly populated region but also by the fact that the Mediterranean is a natural geophysical and geological laboratory where both complex multi-plate and continuum tectonics are found in a more or less convergent zone. Within this complex geotectonic setting several geological phenomena such as subduction, collision, orogen collapse and back-arc extension take place, leading to the generation of a broad spectrum of mainshocks, reaching MW = 8.0 or greater for subduction-related thrust events and a variety of corresponding seismicity levels and neotectonic activity ranging from very low (e.g. large parts of Iberian peninsula) to very high (broader Aegean area). The backward procedure shows that all six strong (M ≥ 6.8) mainshocks that have occurred in the Mediterranean since 1980 had been preceded by preshock sequences that followed this seismicity pattern and satisfy all model constraints. Application of the model for future mainshocks has led to the identification of nine regions (in the Pyrenees, Calabria, NE Adriatic, Albania, Northern Greece, SE Aegean, NW Anatolia, western Anatolia, NE Anatolia) where current intermediate-magnitude seismicity satisfies the constraints of the model and corresponds to strong (M ≥ 6.2) mainshocks. The magnitudes, epicentres and origin times of these probably ensuing mainshocks, as well as their corresponding uncertainties, are estimated, so that it is possible to evaluate the model potential during the next decade (2006–2015). Furthermore, it is shown that geological observations of surface fault traces can contribute to the accurate location of the foci of future strong mainshocks in the Mediterranean and to an estimation of their sizes. For this purpose, globally valid relations between fault parameters based on geological observations (surface fault length, LS, and fault slip, uS) and measures of mainshock size (mainshock magnitude, subsurface fault length, L, and fault slip, u) are proposed.

Reviewing the active stress field in Central Asia by using a modified stress tensor approach

Central Asia and its surroundings constitute a geodynamically complicated region, where almost all types of tectonic patterns can be observed. A triple junction, collision, and subduction zones, as well as extended fault systems of all types prevail in different parts of this region and compose one of the most interesting and complex geotectonic environments on Earth. This complicated setting is also associated with intense deformation, resulting in a large number of high seismicity zones, where numerous strong earthquakes occur, also extending, in some cases, to intermediate depths. Several previous studies have focused on specific seismotectonic zones in order to assess the active tectonic setting and the associated stress regime. We attempt to provide a unified but detailed picture of the stress field variability for the entire central Asia region, using the well-known inversion method proposed by Gephart and Forsyth (1984), modified in the present work on the basis of Fisher statistics. For this application, we employ a large number of focal mechanisms, spatially separated in 138 data groups. Τhe proposed modified algorithm (FD-BSM) examines the Fisher distribution of all possible stress principal axes solutions and select the one that: (a) shows the smallest angle variation from the Fisher mean in all three principal distributions and (b) minimizes the difference between the theoretical and observed slip vectors of the employed fault plane solution data. Synthetic tests and comparison of the corresponding results with real data show that, in cases where the stress regime is not clearly uniform or the number of available data is rather small, the models selected by the modified approach (FD-BSM) are more robust and show better spatial coherence compared to the initial Gephart and Forsyth (1984) method or alternative techniques such as the method of Michael (1984, 1987) as adapted by Vavrycuk (2014).

Crustal and upper mantle structure of the Kozani-Grevena area obtained by non-linear inversion of P and S travel times

Abstract The present study focuses on the P and S crustal and uppermost mantle velocity structure in the broader Kozani-Grevena area. The velocity structure is derived from the inversion of travel times of local events. The main data source is the travel times from the aftershock sequence of the large event of 13 of May 1995 (Mw = 6.6) which occurred in the study area. An appropriate preconditioning of the final linearized system is used to reduce ray density effects on the results. An attempt is made to interpret the features and details of the crustal structure in terms of the geotectonic setting of the area. The observed features of the deeper crustal and uppermost mantle structure are in very good agreement with previous results. Specifically, a crustal thickening is observed along a ENE-WSW direction, perpendicular to the well-known Dinaric trend (NNW-SSE) of the geological formations of the area, in accordance with the theoretical expectation of a thicker crust under the accretion prism which starts at the SW edge of the study area.