V. Karakostas

The Ms = 6.2, June 15, 1995 Aigion earthquake (Greece): evidence for low angle normal faulting in the Corinth rift

We present the results of a multidisciplinary study of the Ms = 6.2, 1995, June 15, Aigion earthquake (Gulf of Corinth, Greece). In order to constrain the rupture geometry, we used all available data from seismology (local, regional and teleseismic records of the mainshock and of aftershocks), geodesy (GPS and SAR interferometry), and tectonics. Part of these data were obtained during a postseismic field study consisting of the surveying of 24 GPS points, the temporary installation of 20 digital seismometers, and a detailed field investigation for surface fault break. The Aigion fault was the only fault onland which showed detectable breaks (< 4 cm). We relocated the mainshock hypocenter at 10 km in depth, 38 ° 21.7 ′ N, 22 ° 12.0 ′ E, about 15 km NNE to the damaged city of Aigion. The modeling of teleseismic P and SH waves provides a seismic moment Mo = 3.4 1018 N.m, a well constrained focal mechanism (strike 277 °, dip 33 °, rake − 77°), at a centroidal depth of 7.2 km, consistent with the NEIC and the revised Harvard determinations. It thus involved almost pure normal faulting in agreement with the tectonics of the Gulf. The horizontal GPS displacements corrected for the opening of the gulf (1.5 cm/year) show a well-resolved 7 cm northward motion above the hypocenter, which eliminates the possibility of a steep, south-dipping fault plane. Fitting the S-wave polarization at SERG, 10 km from the epicenter, with a 33° northward dipping plane implies a hypocentral depth greater than 10 km. The north dipping fault plane provides a poor fit to the GPS data at the southern points when a homogeneous elastic half-space is considered: the best fit geodetic model is obtained for a fault shallower by 2 km, assuming the same dip. We show with a two-dimensional model that this depth difference is probably due to the distorting effect of the shallow, low-rigidity sediments of the gulf and of its edges. The best-fit fault model, with dimensions 9 km E–W and 15 km along dip, and a 0.87 m uniform slip, fits InSAR data covering the time of the earthquake. The fault is located about 10 km east-northeast to the Aigion fault, whose surface breaks thus appears as secondary features. The rupture lasted 4 to 5 s, propagating southward and upward on a fault probably outcropping offshore, near the southern edge of the gulf. In the shallowest 4 km, the slip – if any – has not exceeded about 30 cm. This geometry implies a large directivity effect in Aigion, in agreement with the accelerogram aig which shows a short duration (2 s) and a large amplitude (0.5 g) of the direct S acceleration. This unusual low-angle normal faulting may have been favoured by a low-friction, high pore pressure fault zone, or by a rotation of the stress directions due to the possible dip towards the south of the brittle-ductile transition zone. This fault cannot be responsible for the long term topography of the rift, which is controlled by larger normal faults with larger dip angles, implying either a seldom, or a more recently started activity of such low angle faults in the central part of the rift.

3D crustal structure from local earthquake tomography around the Gulf of Arta (Ionian region, NW Greece)

During summer of 1995 local seismicity was recorded in the area around the Gulf of Arta in northwestern Greece by a dense temporary seismic network. Of the 441 local events observed at 37 stations, 232 well locatable events with a total of 2776 P-phase readings were selected applying the criteria of a minimum of 6 P-observations and an azimuthal gap less than 180°. This data set is used to compute a minimum 1D velocity model for the region. Several tests are conducted to estimate model stability and hypocenter uncertainties, leading to the conclusion that relative hypocenter location accuracy is about 500 m in latitude and longitude and 1 km in depth. The minimum 1D velocity model serves as initial model in the non-linear inversion for three-dimensional P-velocity crustal structure by iteratively solving the coupled hypocenter–velocity problem in a least-squares sense. Careful analysis of the resolution capability of our data set outlines the well resolved features for interpretation. The resulting 3D velocity model shows generally higher average crustal velocities in the east, and the well resolved area of the eastern Gulf of Arta exhibits a homogeneous velocity around 6 km/s for the whole upper crust. A pronounced north–south trending zone of low velocities in the upper 5–10 km is observed in the area of the Katouna fault zone (KFZ). At greater depths (below 10 km) the KFZ is underlain by high-velocity material. E–W profiles suggest a horst–graben structure associated with the KFZ.

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.

Uncertainties in the estimation of earthquake magnitudes in Greece

Instrumental magnitudes in Greece have been reported as: a) Mmagnitudes based on the records of the Wiechert or Mainka seismographs,b) MLGR magnitudes based on the records of the Wood-Anderson(WA) seismographs (To = 0.8 sec, Veffective ∼ 1000) or othershort period seismographs calibrated against WA records and,c) MLSM magnitudes based on strong motion records(accelerograms). Comparison of such magnitudes with momentmagnitudes, Mw, for 329 earthquakes, with epicenters in thebroader Aegean area, performed in this study, showedthat M, MLGR+0.5 and MLSM are practically equalto Mw, with a small overall standard error (σ = 0.23).Therefore, equivalent moment magnitudes, Mw*,estimated from these magnitudes and reported in the catalogues of theGeophysical Laboratory of the University of Thessaloniki are equal tomoment magnitudes for all practical purposes with reasonable uncertainties.It has been further shown that surface wave magnitudes, Ms,for Ms <6.0, can be also transferred into momentmagnitudes, Mw*, but the larger uncertaintiesencountered make its use rather problematic.

The 2014 Kefalonia Doublet (MW6.1 and MW6.0), Central Ionian Islands, Greece: Seismotectonic Implications along the Kefalonia Transform Fault Zone

The 2014 Kefalonia earthquake sequence started on 26 January with the first main shock (MW6.1) and aftershock activity extending over 35 km, much longer than expected from the causative fault segment. The second main shock (MW6.0) occurred on 3 February on an adjacent fault segment, where the aftershock distribution was remarkably sparse, evidently encouraged by stress transfer of the first main shock. The aftershocks from the regional catalog were relocated using a 7-layer velocity model and station residuals, and their distribution evidenced two adjacent fault segments striking almost N-S and dipping to the east, in full agreement with the centroid moment tensor solutions, constituting segments of the Kefalonia Transform Fault (KTF). The KTF is bounded to the north by oblique parallel smaller fault segments, linking KTF with its northward continuation, the Lefkada Fault.

Episodic occurrence of strong (Mw≥6.2) earthquakes in Thessalia area (central Greece)

Abstract Between 1954 and 1957, three seismic sequences struck the southern margin of Thessalia basin (central Greece). The proximity in time (∼3 yr) and space (∼100 km) between these three seismic manifestations suggests a possible link between them. Their interconnection is then investigated by resolving changes of Coulomb failure function (ΔCFF) during the 20th century. Coulomb stress changes were calculated assuming that earthquakes can be modelled as static dislocations in an elastic half-space, and taking into account both the coseismic slip in strong ( M w ≥6.2) earthquakes and the slow tectonic stress buildup associated with major fault segments. The study area constitutes one of the most active fault zones of the Greek mainland where several strong events have occurred in the past as it is inferred from both historical and instrumental information. However, no earthquake with M w ≥6.2 occurred there for about two centuries before the last activity. This fact led us to investigate the occurrence mode of strong events in the Thessalia area since the 16th century, when adequately reliable historical information exists for the stronger events. Forasmuch as they occur in clusters both in space and time, as it is the case during the 20th century, the episodic occurrence of strong events in this area is substantiated, when these episodes are separated by longer quiescent periods. Since during the 20th century all the earthquakes have occurred in stress-enhanced areas, by extending the stress calculations to 2025 and, provided that no additional large ( M w ≥6.2) earthquake occurs between 2002 and 2025, assessment for the future seismic hazard is given.

A Homogeneous Earthquake Catalog for Western Turkey and Magnitude of Completeness Determination

Abstract A catalog for earthquakes that occurred in western Turkey during the period 1964–2010 is compiled for achieving homogeneity for magnitudes. Data are obtained from the International Seismological Center (ISC), where earthquake magnitudes are reported in different scales and come from a variety of sources. For establishing a common magnitude expression, namely an equivalent moment magnitude , new relations correlating the different magnitude scales with each other are derived from converting as many as possible of the magnitudes reported in the ISC bulletins. After magnitude conversions, the completeness magnitude M c is sought by modifying the goodness‐of‐fit method of Wiemer and Wyss (2000) to become more appropriate for datasets with smaller sample size and higher M c thresholds. The study region is divided into four smaller regions on the basis of spatial data homogeneity, while different periods of similar seismic network performance are recognized and tested to seek spatiotemporal variation of M c . The results derived in each case are compared with those yielded by the application of both the original goodness‐of‐fit and maximum curvature methods and are found to be quite similar, although there are still cases with a difference exceeding 0.3 magnitude units. The goodness‐of‐fit method is very sensitive in the selection of the desirable percentage of fitting a power law (90% or 95%), whereas the proposed modification makes it independent of this level selection, and performing better especially for datasets that include events before 1990, when higher completeness magnitudes are evident. Online Material: Earthquake catalog with equivalent moment magnitude for western Turkey.

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.

THE KOZANI-GREVENA (GREECE) EARTHQUAKE OF MAY 13, 1995, A SEISMOLOGICAL STUDY

Abstract We present a detailed seismological study of the Kozani earthquake. We relocate the mainshock with regional data at depth of 14.2km beneath the Vourinos massif. We compute the focal mechanism by body waveform modeling at teleseismic distance and find a normal fault striking N240 ° and dipping 40 ° toward the NW with a centroid depth of 11 km. We installed a dense network of portable seismographs around the epicentral region and located several hundreds of aftershocks. The main cluster of aftershock seismicity defines a plane dipping north at an angle of about 35 °, consistent with the main-shock mechanism, while some seismic activity is also seen on an antithetic fault. Our results suggest the active fault plane to be the Deskati fault which dips at a constant angle and therefore branches on the Paleohori fault where surface breaks were observed. We also compute 181 focal mechanisms which mostly show normal faulting.