G-Akis Tselentis

Iterative Deconvolution of Regional Waveforms and a Double-Event Interpretation of the 2003 Lefkada Earthquake, Greece

The moment tensor inversion for multiple point sources, based on Kikuchi and Kanamori (1991), was extended to full waveform data at regional (or local) distances. The new code proved to be efficient for retrieving major source contributions of the 2003 Lefkada, Greece, earthquake. The source model was derived from five three-component regional stations (epicentral distances <140 km), at periods 10–20 s. Two main events dominated the rupture process, one at the Lefkada Island (comprising three subevents of total moment 0.9 × 1018 N m) and the other at the Cephalonia Island (comprising one subevent of 0.5 × 1018 N m). Their spatial and temporal separation is 40 km and 14 s, respectively. They can be understood as two earthquakes. The uncertainty estimate based on reduced data sets (repeatedly excluding a station) shows that the Cephalonia subevent and the major Lefkada subevent are very well resolved regarding their position, time, and focal mechanism. The source model explains well the aftershock distribution, characterized by two clusters at the Lefkada and Cephalonia Islands, respectively. The focal mechanisms of the two main subevents are predominantly right-lateral strike slip of south-southwest–north-northeast orientation. The Cephalonia subevent occurred on a less steeply dipping fault with a small thrust component. Large deviations from pure double couple were found but interpreted as artifacts. The new software developed in this article (Fortran code and Matlab graphic user interface) is freely available. Online material: Color graphics and 3D visualization of the 2003 Lefkada earthquake sequence.

Engineering Ground-Motion Parameters Attenuation Relationships for Greece

Engineering ground-motion parameters can be used to describe the dam- age potential of an earthquake. Some of them correlate well with several commonly used demand measures of structural performance, liquefaction, and seismic-slope stability. The importance of these parameters comes from the necessity of an alter- native measure to the earthquake intensity. In the proposed new attenuation relation- ship we consider peak values of strong motion, spectral acceleration, elastic input energy at selected frequencies, root-mean-square acceleration, Arias intensity, char- acteristic intensity, Fajfar index, cumulative absolute velocity, cumulative absolute velocity integrated with a 5 cm/sec 2 lower threshold, and spectrum-intensity energy. This article describes the steps involved in the development of new attenuation re- lationships for all the preceding parameters, using all existing, up-to-date Greek strong-motion data. The functional form of the empirical equation is selected based on a theoretical model, and the coefficients of the independent variables are deter- mined by employing mixed effects regression analysis methodologies.

Empirical Relationships between Modified Mercalli Intensity and Engineering Ground-Motion Parameters in Greece

Abstract New relationships between modified Mercalli intensity (MMI) and engineering ground-motion parameters are developed for Greece. The ground-motion parameters investigated were peak ground acceleration (PGA), velocity, displacement, Arias intensity, and cumulative absolute velocity. The observed earthquake intensity is quantified in terms of the observed MMI at the recording station and the data set consists of 310 time histories recorded from 89 Greek earthquakes. The selected records were found to be characterized by high-frequency, low-energy content and short duration. Two sets of empirical relationships between MMI and the selected ground-motion parameters were derived. The first set of MMI predictive equations are independent of magnitude and epicentral distance, and they were derived by fitting the mean values of the ground-motion parameters using a weighted least-squares regression technique. The influence of magnitude, epicentral distance, and the local site conditions were incorporated into the second MMI predictive model, resulting in a decrease of the model variance. The lowest standard deviation observed for the first MMI predictive model was for PGA, while for the second MMI predictive model, Arias intensity exhibited the smallest variability. Another finding of the present study was that the local site effect has a little influence on the MMI predictive model for peak ground velocity (PGV). The proposed predictive equations are valid for MMI values IV–VIII, and some of them might be used for rapid assessment of the ground shaking and mapping damage potential.

Northwestern Turkey Earthquakes and the Crustal Structure Inferred from Surface Waves Observed in Western Greece

Records of several earthquakes occurring in Turkey in 1999 obtained at broadband seismic stations in western Greece have been used to study the dispersion of surface waves, mainly Love waves. The observed group-velocity dispersion curves have been inverted into horizontally layered models of the Earths crust by a modified method of the single-parameter variation. As compared with a previous model for the territory of Greece, the dispersion data require significantly lower velocities in the uppermost crust and smaller crustal thickness. In particular, the resulting model displays S -wave velocities between 1.3 and 2.4 km/sec in the upper 2 km and a crustal thickness of about 33 km. Manuscript received 31 July 2000.

The Athens Earthquake of 7 September 1999

Based on detailed aftershock monitoring, the first model of the Athens earthquake is formulated, which is consistent with global, regional, and local strong-motion data of the mainshock, and fits with geological setting. The 30-station temporary network located 450 aftershocks. During the first 20 observation days the aftershocks identified the mainshock fault plane dipping 52° and striking 117°, consistently with the teleseismic fault-plane solution. A formal upward continuation of the fault plane intersects the surface close to the Fili fault. Numerical modeling of the broadband regional data at 10 stations (epicentral distances 140 to 370 km) estimates the centroidal source depth of 10 km and yields an average source duration of 5 to 6 sec. The interstation variability of the apparent duration indicates source directivity. The empirical Green9s function modeling at the closest broadband station suggests a fault length of 20 or 10 km. Both the numerical and empirical modelings give a very short rise time of 0.1 to 0.3 sec. The short rise time seems to favor the nearly complete stress release of an asperity. A 10 km asperity (stress drop of 2.7 M Pa) is in agreement with a gap, identified during the first 12 observation days between two aftershock clusters. The strong-motion accelerograms in Athens also indicate a short apparent duration due to directivity (about 3 sec), and confirm an abrupt rupture beginning. There is no evidence for an abrupt stopping. The short rise time and short apparent source duration were two principal factors determining the damaging ground motions in Athens.

The Egion June 15, 1995 (6.2M L ) earthquake, western Greece

On June 15, 1995 at 00:15 GMT a devastating earthquake (6.2ML) occurred in the western end of the Gulf of Corinth. This was followed 15 min later by the largest aftershock (5.4ML). The main event was located by the University of Patras Seismological Network (PATNET) at the northern side of the Gulf of Corinth graben. The second event (5.4ML) was located also by PATNET near the city of Egion, on a fault parallel to the Eliki major fault that defines the south bound of the Gulf of Corinth graben. A seismogenic volume that spans the villages of Akrata (SE) and Rodini (NW) and extends to Eratini (NE) was defined by the aftershock sequence, which includes 858 aftershocks of magnitude greater than 2ML that occurred the first seventeen days. The distribution of hypocentres in cross section does not immediately suggest a planar distribution but rather defines a volume about 15 km (depth) by 35 km (NW-SE) and by 20 km (NE-SW).

Intrinsic and Scattering Seismic Attenuation in W. Greece

Intrinsic (Q i −1) and scattering (Q s −1) attenuation parameters have been determined in the seismically active region of W. Greece, which is continuously monitored by the University of Patras microearthquake network. One hundred and twenty-three local and shallow earthquakes close to the recording stations have been used and Wennerberg’s (1993) approach has been adopted. Results for 1 to 12 Hz range show that Q i −1 is higher than Q s −1 and coda Q values are close to Q i , indicating that coda Q can be a reasonable estimate of intrinsic Q.

Local high-resolution passive seismic tomography and Kohonen neural networks — Application at the Rio-Antirio Strait, central Greece

A high-resolution passive seismic investigation was performed in a 150 km 2 area around the Rio-Antirio Strait in centralGreeceusingnaturalmicroearthquakesrecordedduringthreemonthsbyadense,temporaryseismicnetworkconsisting of 70 three-component surface stations. This work was part of the investigation for a planned underwater rail tunnel, and it gives us the opportunity to investigate the potential of this methodology. First, 150 well-located earthquake events were selected to compute a minimum 1D velocity model for the region. Next, the 1D model served as the initialmodelfornonlinearinversionfora3DP-andS-velocity crustal structure by iteratively solving the coupled hypocenter-velocity problem using a least-squares method. The retrieved Vp and Vp/Vs images were used as an input to Kohonen self-organizing maps SOMs to identify, systematicallyandobjectively,theprominentlithologiesintheregion. SOMs are unsupervised artificial neural networks that map the input space into clusters in a topological form whose organization is related to trends in the input data. This analysis revealed the existence of five major clusters, one of which may be related to the existence of an evaporite body not shown in the conventional seismic tomography velocity volumes. The survey results provide, for the first time, a 3D model of the subsurface in and around the Rio-Antirio Strait. It is thefirst time that passive seismic tomography is used together with SOM methodologies at this scale, thus revealing themethod’spotential.

An Attempt to Define Curie Point Depths in Greece from Aeromagnetic and Heat Flow Data

The objective of this study is to understand the nature and extent of the regional geothermal system at depth beneath the area of Greece by constructing the Curie isotherms.Spectral analysis of aeromagnetic data in conjunction with heat flow information revealed an almost inverse linear relation between heat flow and Curie depths and was used to construct the Curie isotherms from the existing heat flow data.The results showed that Curie depths in the area range from about 20 km in western Greece, up to 1 km beneath the Hellenic volcanic arc. These results are consistent with the existing geothermal and geotectonic regime in the area.

Seismicity and Seismotectonics in Epirus, Western Greece: Results from a Microearthquake Survey

During a twelve-month passive tomography experiment in Epirus, in northwestern Greece, a total of 1368 microearthquakes were located. The most accu- rately located events and focal mechanisms are used here to understand the seismo- tectonics of the area. The seismicity shows a clear association with the main, previously defined deformation zones. A total of 434 well-defined focal mechanisms were also used for the determination of the stress pattern in the area. The computed stress-field pattern is quite complex close to the surface and almost homogeneous at depths below 15 km. For these depths, the stress field is purely compressional in a west-southwest direction, whereas for shallow depths it is transpressional or even extensional for some smaller areas. The abrupt change in the stress pattern, which occurs as depth increases, suggests the existence of a detachment surface, which is provided by the evaporites that have intruded into the upper layers through the thrust zones. The presence of the evaporites and their lateral extent is mapped by the seismicity distribution and con- firmed by seismic tomography. Based on the findings, we estimate a possible total evaporite thickness of almost 10 km at least for the central part of the study area. Such a result is important for the oil exploration efforts that have just started in Epirus.