P. Papadimitriou

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.

Shear wave anisotropy in the upper mantle beneath the Aegean related to internal deformation

Seismic anisotropy, deduced from SKS splitting measured at 25 stations installed in the Aegean, does not show a homogeneous pattern. It is not restricted to the North Anatolian Fault but is distributed over a region several hundreds kilometers wide. Little anisotropy is observed in continental Greece or along the Hellenic arc; however, significant anisotropy is observed in the north Aegean Sea. Large values of delay times suggest that anisotropy is due to a long path within the upper mantle and to strong intrinsic anisotropy. Our results, both in fast polarization directions and in values of delay time, do not support the idea that anisotropy is associated with inherited tectonic fabric nor are they consistent with the present-day Aegean motion relative to an absolute frame. In contrast, the direction of fast polarization and the magnitude of delay times correlate well with the present-day strain rate observed at the surface deduced from both geodetic measurements and seismicity. This anisotropy is not horizontally restricted to major surface faults but is spread over a wide region.

Crustal and upper mantle structure beneath the Corinth rift (Greece) from a teleseismic tomography study

We report here the results of a tomographic lithospheric study in the area of the Corinth and Evvia rifts (Greece), designed to constrain the mechanism of continental extension. Sixty seismological stations were deployed in the area for a period of 6 months, and 177 teleseismic events were recorded by more than five stations and gave more than 2000 travel time residuals (P and PKP phases), which were inverted to image the velocity structure down to 200 km depth. We use both a linear and a nonlinear method to invert the data set. The main result is a long-wavelength positive velocity anomaly located in the upper mantle, which is interpreted as the subducted African lithosphere. The subducted lithosphere is well defined from ∼7O km depth down to 200 km. From synthetic tests as well as from the amplitude of the anomaly (more than +7%) we conclude that the subduction continues below 200 km. In addition, a second positive velocity anomaly of about +4% from the surface down to 40 km depth, located north of the Gulf of Corinth, has been found. This is interpreted as the result of a crustal thinning of several kilometers (∼5 km), shifted to the north from the Gulf of Corinth and trending obliquily NW-SE. We suggest that this crustal thinning is mainly related to the Miocene widespread extension in the Aegean and that the Quaternary Corinth rift initiated where the crust was already thinned. The different styles of deformation of the eastern and western part of the rift are consistent with this interpretation. No clear velocity anomaly can be related to the Evvia rift.

The Mw = 6.0, 7 September 1999 Athens earthquake

On 7 September 1999 at 11:56 GMT a destructive earthquake (Mw = 6.0) occurred close to Athens (Greece). The rupture process is examined using data from the Cornet local permanent network, as well as teleseismic recordings. Data recorded by a temporary seismological network were analyzed to study the aftershock sequence. The mainshock was relocated at 38.105°N, 23.565°E, about 20 km northwest of Athens. Four foreshocks were also relocated close to the mainshock. The modeling of teleseismic P and SH waves provides a well-constrained focal mechanism of the mainshock (strike = 105°, dip = 55° and rake = -80°) at a depth of 8 km and a seismic moment M0 = 1.01025 dyn·cm. The obtained fault plane solution represents normal faulting indicating an almost north-south extension. More than 3500 aftershocks were located, 1813 of which present RMS < 0.1 s and ERH, ERZ < 1.0 km. Two main clusters were distinguished, while the depth distribution is concentrated between 2 and 11 km. Over 1000 fault plane solutions of aftershocks were constrained, the majority of which also correspond to N–S extension. No surface breaks were observed but the fault plane solution of the mainshock is in agreement with the tectonics of the area and with the focal mechanisms obtained by aftershocks. The hypocenter of the mainshock is located on the deep western edge of the fault plane. The relocated epicenter coincides with the fringe that represents the highest deformation observed on the differential interferometric image. The calculated source duration is 5 sec, while the estimated dimensions of the fault are 15 km length and 10 km width. The source process is characterized by unilateral eastward rupture propagation, towards the city of Athens. An evident stop phase observed in the recordings of the Cornet local stations is interpreted as a barrier caused by the Aegaleo Mountain.

Evidence of shear-wave splitting in the eastern Corinthian Gulf (Greece)

Abstract The analysis of local earthquakes recorded by the Cornet network and located in the Gulf of Corinth (Greece) has revealed the existence of shear-wave splitting. The visual inspection technique is used to estimate the polarization direction of the fast shear wave and the time delay between the two split shear waves. The selected earthquakes are located close to one of the Cornet stations. Most of these earthquakes are recorded by only one station and their azimuth and angle of incidence are estimated using the covariance matrix decomposition method combined with the P-wave polarization direction. Polarigrams plotted for the horizontal components present a clear linear and almost constant polarization for each station, independent of the azimuth of the earthquake, except for one station where different Sfast polarization directions are observed. The mean direction of the fast shear wave polarization at Paradeisi station is 146°N, at Sofiko station 104°N and at Villia station 142°N. At Desfina station, two different main Sfast polarization directions were observed, one 143°N and the other 55°N. The calculated time delays between the two split shear waves are higher for the stations located in the eastern part of the Gulf. Comparing the mean Sfast polarization direction with the direction of local faulting, we observe that they are approximately parallel at Sofiko station, while at Villia station they are almost perpendicular. In general, the obtained mean Sfast polarization directions at the Cornet stations are perpendicular to the direction of the extension of the Gulf which is NNE–SSW and consistent with the extensive dilatancy anisotropy (EDA) model.

Seismic site characterization at the western Cephalonia Island in the aftermath of the 2014 earthquake series

BackgroundThe site response during a strong earthquake event may be proven crucial for earthquake hazard assessment and risk mitigation. Two moderate magnitude earthquakes that occurred in early 2014 in Cephalonia produced the largest ground motion values ever recorded in Greece, highly exceeding the provisions of the effective seismic code implying for local effects. This motivated the investigation of site response in the epicentral area presented herein.MethodsWe applied the HVSR method on free-field ambient noise measurements obtained during an in situ survey. 68 measurements were adopted for site characterization after their validation using earthquakes and geotechnical data. The site response was approximated by the peak frequency and the amplification ratio of the HVSR curves.ResultsThe majority of measurements exhibit smooth lateral variations in the frequency range 0.7–17 Hz, at a factor up to 7 and they are clearly classified in two bands, a low (0.7–4 Hz) and a high one (5–17 Hz). Some discrepancies that are observed between microtremor measurements and earthquake recordings for peak frequencies <2 Hz and overall underestimated ambient noise HVSR amplification are likely explained by near-source, radiation pattern and/or nonlinear soil effects.ConclusionsHigh frequencies combined with low amplification correlate with damage in the hardest hit areas. Low frequencies are aligned in a NNE-SSW direction in the epicentral area, similar to the strike of the activated fault, indicating that the properties of rocks along the fault zone have possibly been affected by slippage and/or dynamic effects.

Seismicity and Tomographic Imaging of the Broader Nisyros Region (Greece)

Nisyros Island is a Quaternary composite volcano located close to the eastern termination of the South Aegean Volcanic Arc. Large destructive earthquakes have been reported in the study area. Nevertheless, seismic activity during the last decades is moderate to low, consisting of both shallow and intermediate depth earthquakes. The main regions of the broader area with observed spatiotemporally clustered seismicity are between Nisyros and Karpathos, east of Kos and in the gulf of Symi. Major events of intermediate depth have occurred near Karpathos and Rhodes Islands while the most significant zone of deep earthquakes is identified in the western Nisyros basin. Evidence for a non-systematic temporal co-incidence of deep events at different regions as well as increase in shallow seismicity after the occurrence of a strong deep event have been observed. Moment tensor inversion , using recordings in local and regional distances, was applied to determine the focal mechanisms of recent moderate events. The solutions, obtained by minimizing the difference between observed and synthetic waveforms, revealed that shallow events are mainly related to normal faulting, whereas intermediate depth events to reverse faulting with important strike-slip component. A tomography study was performed, using manually located events, and identified two areas of high V p/V s ratio and low velocity perturbations. The first is located SW of Nisyros and can be attributed to magma intrusion of deeper composition containing fluids and melts. The second, reaching 15 km depth, is possibly related to the magmatic chambers that feed the Yali and Strongyli volcanic centers.