Costas Papazachos

Tomography of the crust and upper mantle in southeast Europe

Compressional velocity structure of the crust and the upper mantle in southeastern Europe (broader Aegean area) is studied by inverting residuals of the first P arrivals from earthquakes in this region (16°E-31°E, 34°N-43°N). The data used are from regional events recorded by the permanent network of stations during the period 1971-1987, enriched with data from experiments with portable seismographs in four regions of this broad area. This study confirms the strong variations of crustal thickness in this area as well as the subduction of the eastern Mediterranean lithosphere under the southern Aegean and gives further detailed information on the crustal and upper mantle structure of the area. Important new information is the existence of a low-velocity crustal layer in western Greece and Albania and that the velocity anomaly in the mantle under the southern Aegean extends much farther and deeper to the northeast than the Benioff zone of the intermediate depth earthquakes indicates. Furthermore, evidence is presented about the possible existence of older subduction in the northern Aegean and about the influence of the tectonic regime on the velocity field.

Properties of the 2003 Lefkada, Ionian Islands, Greece, Earthquake Seismic Sequence and Seismicity Triggering

On 14 August 2003, Lefkada Island (Central Ionian) was strongly af- fected by an Mw 6.2 earthquake. A dense temporary seismic network was installed one day after and accurately located hundreds of aftershocks that defined in detail the main rupture, as well as the activity distribution in the neighboring fault segments. The main rupture occupied the northwestern part of the coastline and trends north- northeast-south-southwest in agreement with regional tectonics. Regional network locations were appropriately calibrated using the local network data, allowing the relocation of the mainshock and strong (M 4.5 or larger) aftershocks during the first day. Intense aftershock activity took place up to 40 km beyond the southern end of the main rupture. Theoretical static stress changes from the mainshock give a preliminary explanation for the aftershock distribution aside from the main rupture, as well as triggering of seismicity in the nearby Kefalonia fault, providing evidence for future seismic hazard ensuing from this fault.

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.

A large scale geophysical survey in the archaeological site of Europos (northern Greece)

Abstract The results of a large scale exploration of an archaeological site by geophysical means are presented and discussed. The operation took place in the site where the ruins of the ancient city of Europos are buried. This site is in northern Greece. Resistivity prospecting was employed to detect the remnants of wall foundations in the place where the main urban complex of the ancient city once stood. The data were transformed in an image form depicting, thus, the spatial variation of resistivity in a manner that resembles the plane view of the ruins that could have been drawn if an excavation had taken place. This image revealed the urban plan of the latest times of the life of the city. Trial excavations verified the geophysical result. Magnetic prospecting in the same area complemented the resistivity data. The exact location of the fire hearths, kilns and remnants of collapsed roofs were spotted. Magnetic gradient measurements were taken in an area out of the main complex of the ancient city and revealed the location of several kilns. One of these locations was excavated and a pottery kiln was discovered. The resistivity prospecting in one of the graveyards of the ancient city showed anomalies which were expected and corresponded to monumental tombs. The locations of a few of them were excavated and large burial structures were revealed. Ground probing radar profiles were measured over the tombs which showed pronounced resistivity anomalies, so far unearthed. The relatively high resolving ability of the method assisted the interpretation in the sense that a few attributes were added. In the presented case, it was concluded that a particular tomb consists of two rooms and that it is roofless.

Accelerating Seismic Crustal Deformation in the Southern Aegean Area

A region of intense accelerating seismic crustal deformation has been identified in the southwestern part of the Hellenic arc (broader area of Cythera island). The identification is performed using a detailed parametric grid search of the broader southern Aegean area for accelerating energy release behavior. The identified region has similar properties with past preshock (critical) regions, which have been identified for strong mainshocks in the Aegean area. Based on such observations, which suggest that this region is at a critical state that can lead to a critical point, that is, to the generation of a mainshock, an estimation is made of the possible epicenter coordinates, magnitude, and origin time of this oncoming large (M ∼7.0) earthquake. The estimation procedure is validated on the basis of retrospective analysis of strong events in the Aegean area, as well as by appropriate application on synthetic random catalogs. These results, the existence of similar observations of accelerating seismic deformation in eastern part of southern Aegean and independent information on the time distribution of large earthquakes (M ≥6.8) for the whole southern Aegean indicate that the generation of strong earthquakes in this area in the next few years must be considered as very probable.

Along-Arc and Back-Arc Attenuation, Site Response, and Source Spectrum for the Intermediate-Depth 8 January 2006 M 6.7 Kythera, Greece, Earthquake

An M 6.7 intermediate-depth (66 km), in-slab earthquake occurring near the island of Kythera in Greece on 8 January 2006 was well recorded on networks of stations equipped with acceleration sensors and with broadband velocity sensors. All data were recorded digitally using recording instruments with resolutions ranging from almost 11 to 24 bits. We use data from these networks to study the distance dependence of the horizontal-component Fourier acceleration spectra (FAS) and horizontal-component pseudoabsolute response spectral acceleration (PSA). For pur- poses of simulating motions in the future, we parameterize the distance decay using several forms of the geometrical-spreading function, for each of which we derive Q as a function of frequency. By extrapolating the distance decay back to 1 km, we obtain a reference spectrum that can be used in future simulations. This spectrum requires a more complicated spectral shape than the classic single-corner-frequency model; in particular, there appears to be an enhancement of motion around 0.2-0.3 Hz that may be due to the radiation of a 3-5 sec pulse from the source. We infer a κ0 value of about 0.055 sec for rock stations and a stress parameter in the range of 400-600 bars. We also find distinctive differences in the site response of stations on soft soil and soil; both the FAS and the 5% damped PSA amplifications have similar peak amplitudes (about 2 and 4 for soil and soft-soil sites, respectively, relative to the rock sites) at similar frequencies (between about 0.4 and 2.0 Hz, with the soft-soil amplifications peaking at somewhat lower frequencies than the soil amplifications). One of the most distinctive features of the data is the clear difference in the motions for along-arc and back-arc stations, with the former being significantly higher than the latter over a broad range of frequencies at distances beyond about 250 km. The motions from the Kythera earthquake are roughly comparable to those from intermediate-depth earthquakes elsewhere, but they appear to be significantly higher than those from re- cordings of shallow earthquakes in Greece of comparable magnitude and hypocentral distance.

Three‐dimensional shallow structure from high‐frequency ambient noise tomography: New results for the Mygdonia basin‐Euroseistest area, northern Greece

We examine the use of ambient noise cross-correlation tomography for shallow site characterization using a modified two-step approach. Initially, we extract Rayleigh wave traveltimes from correlation traces of vertical component seismic recordings from a local network installed in Mygdonia basin, northern Greece. The obtained Rayleigh wave traveltimes show significant spatial variability, as well as distance and frequency dependence due to the 3-D structure of the area, dispersion, and anelastic attenuation effects. The traveltime data sets are inverted through a surface wave tomography approach to determine group velocity maps for each frequency. The proposed tomographic inversion involves the use of approximate Fresnel volumes and interfrequency smoothing constraints to stabilize the results. In the last step, we determine a final 3-D velocity model using a node-based Monte Carlo 1-D dispersion curve inversion. The reliability of the final 3-D velocity model is examined by spatial and depth resolution analysis, as well as by inversion for different model parameterizations. The obtained results are in very good agreement with previous findings from seismic and other geophysical methods. The new 3-D VS model provides additional structural constraints for the shallow sediments and bedrock structure of the northern Mygdonia basin up to the depth of ∼200–250 m. Present work results suggest that the migration of ambient tomography techniques from large scales (tens or hundreds of km) to local scales (few hundred meters) is possible but cannot be used as a black box technique for 3-D modeling and detailed geotechnical site characterization.

Site‐Response Study of Thessaloniki (Northern Greece) for the 4 July 1978 M 5.1 Aftershock of the June 1978 M 6.5 Sequence Using a 3D Finite‐Difference Approach

The 3D wave-propagation characteristics of the 4 July 1978 aftershock (M 5.1) of the 20 June 1978 strong mainshock (M 6.5) that struck the city of Thessaloniki are studied using a 3D finite-difference approach. Synthetics are esti- mated for a dense grid of receivers and compared with available accelerograms from soft-soil sites in the city of Thessaloniki, exhibiting a good agreement both in time and frequency domain for the frequency band studied (0.7-3 Hz). Moreover, the spatial distribution of various measures of ground motion (peak values, spectral values) is used for the quantitative study of site effects in the broader city area. Comparisons show that the coastal zone, including the city harbor and large areas of the eastern parts of the city, exhibit high values of ground motion (and significant site amplifications), in good qualitative correlation with the observed damage distribution of the main- shock of the 1978 seismic sequence. Finally, the 3D synthetics are compared with available 2D simulations, as well as amplifications derived from macroseismic infor- mation for three typical cross sections spanning the urban area of the city. The com- parisons confirm the strong spatial variability of ground motion throughout the Thessaloniki area, as well as the superiority of 3D modeling of actual recordings against previous modeling attempts. These results verify the practical usefulness of 3D wave-propagation tools for hazard mitigation, especially of specific target events, in complex geometry sedimentary basins such as the Thessaloniki area.

Recent reliable observations and improved tests on synthetic catalogs with spatiotemporal clustering verify precursory decelerating–accelerating seismicity

We examined the seismic activity which preceded six strong mainshocks that occurred in the Aegean (M = 6.4–6.9, 33–43° N, 19–28° E) and two strong mainshocks that occurred in California (M = 6.5–7.1, 32–41° N, 115–125° W) during 1995–2010. We find that each of these eight mainshocks has been preceded by a pronounced decelerating and an equally easily identifiable accelerating seismic sequence with the time to the mainshock. The two preshock sequences of each mainshock occurred in separate space, time, and magnitude windows. In all eight cases, very low decelerating seismicity, as well as very low accelerating seismicity, is observed around the actual epicenter of the ensuing mainshock. Statistical tests on the observed measures of decelerating, qd, and accelerating, qa, seismicity against similar measures calculated using synthetic catalogs with spatiotemporal clustering based on the ETAS model show that there is an almost zero probability for each one of the two preshock sequences which preceded each of the eight mainshocks to be random. These results support the notion that every strong shallow mainshock is preceded by a decelerating and an accelerating seismic sequence with predictive properties for the ensuing mainshock.

Stochastic Strong Ground Motion Simulation of the Southern Aegean Sea Benioff Zone Intermediate‐Depth EarthquakesStochastic Strong Ground Motion Simulation of the Southern Aegean Sea Intermediate‐Depth Earthquakes

We employ the stochastic finite-fault modeling approach of Motazedian and Atkinson (2005), as adapted by Boore (2009), for the simulation of Fourier amplitude spectra (FAS) of intermediate-depth earthquakes in the southern Aegean Sea subduction (southern Greece). To calibrate the necessary model parameters of the stochastic finite-fault method, we used waveform data from both acceleration and broadband-velocity sensor instruments for intermediate-depth earthquakes (depths ∼45–140 km) with M 4.5–6.7 that occurred along the southern Aegean Sea Wadati–Benioff zone. The anelastic attenuation parameters employed for the simulations were adapted from recent studies, suggesting large back-arc to fore-arc attenuation differences. High-frequency spectral slopes (kappa values) were constrained from the analysis of a large number of earthquakes from the high-density EGELADOS (Exploring the Geodynamics of Subducted Lithosphere Using an Amphibian Deployment of Seismographs) temporary network. Because of the lack of site-specific information, generic site amplification functions available for the Aegean Sea region were adopted. Using the previous source, path, and site-effect constraints, we solved for the stressparameter values by a trial-and-error approach, in an attempt to fit the FAS of the available intermediate-depth earthquake waveforms. Despite the fact that most source, path, and site model parameters are based on independent studies and a single source parameter (stress parameter) is optimized, an excellent comparison between observations and simulations is found for both peak ground acceleration (PGA) and peak ground velocity (PGV), as well as for FAS values. The final stress-parameter values increase with moment magnitude, reaching large values (> 300 bars) for events M ≥ 6:0. Blind tests for an event not used for the model calibration verify the good agreement of the simulated and observed ground motions for both back-arc and alongarc stations. The results suggest that the employed approach can be efficiently used for the modeling of large historical intermediate-depth earthquakes, as well as for seismic hazard assessment for similar intermediate-depth events in the southern Aegean Sea area. Electronic Supplement: Input parameter file for program EXSIM_DMB and tables of site amplification factors.