[1] Combined P and S receiver functions from seismograms of teleseismic events recorded at 65 temporary and permanent stations in the Aegean region are used to map the geometry of the subducted African and the overriding Aegean plates. We image the Moho of the subducting African plate at depths ranging from 40 km beneath southern Crete and the western Peloponnesus to 160 km beneath the volcanic arc and 220 km beneath northern Greece. However, the dip of the Moho of the subducting African plate is shallower beneath the Peloponnesus than beneath Crete and Rhodes and flattens out beneath the northern Aegean. Observed P-to-S conversions at stations located in the forearc indicate a reversed velocity contrast at the Moho boundary of the Aegean plate, whereas this boundary is observed as a normal velocity contrast by the S-to-P conversions. Our modeling suggests that the presence of a large amount of serpentinite (more than 30%) in the forearc mantle wedge, which generally occurs in the subduction zones, may be the reason for the reverse sign of the P-to-S conversion coefficient. Moho depths for the Aegean plate show that the southern part of the Aegean (crustal thickness of 20-22 km) has been strongly influenced by extension, while the northern Aegean Sea, which at present undergoes the highest crustal deformation, shows a relatively thicker crust (25-28 km). This may imply a recent initiation of the present kinematics in the Aegean. Western Greece (crustal thickness of 32-40 km) is unaffected by the recent extension but underwent crustal thickening during the Hellenides Mountains building event. The depths of the Aegean Moho beneath the margin of the Peloponnesus and Crete (25-28 and 25-33 km, respectively) show that these areas are also likely to be affected by the Aegean extension, even though the Cyclades (crustal thickness of 26-30 km) were not significantly involved in this episode. The Aegean lithosphere-asthenosphere boundary (LAB) mapped with S receiver functions is about 150 km deep beneath mainland Greece, whereas the LAB of the subducted African plate dips from 100 km beneath Crete and the southern Aegean Sea to about 225 km under the volcanic arc. This implies a thickness of 60-65 km for the subducted African lithosphere, suggesting that the Aegean lithosphere was not significantly affected by the extensional process associated with the exhumation of metamorphic core complexes in the Cyclades.

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A particle velocity‐stress, finite‐difference method is developed for the simulation of wave propagation in 2-D heterogeneous poroelastic media. Instead of the prevailing second‐order differential equations, we consider a first‐order hyperbolic system that is equivalent to Biot’s equations. The vector of unknowns in this system consists of the solid and fluid particle velocity components, the solid stress components, and the fluid pressure. A MacCormack finite‐difference scheme that is fourth‐order accurate in space and second‐order accurate in time forms the basis of the numerical solutions for Biot’s hyperbolic system. An original analytic solution for a P‐wave line source in a uniform poroelastic medium is derived for the purposes of source implementation and algorithm testing. In simulations with a two‐layer model, additional “slow” compressional incident, transmitted, and reflected phases are recorded when the damping coefficient is small. This “slow” compressional wave is highly attenuated in porous media saturated by a viscous fluid. From the simulation we also verified that the attenuation mechanism introduced in Biot’s theory is of secondary importance for “fast” compressional and rotational waves. The existence of seismically observable differences caused by the presence of pores has been examined through synthetic experiments that indicate that amplitude variation with offset may be observed on receivers and could be diagnostic of the matrix and fluid parameters. This method was applied in simulating seismic wave propagation over an expanded steam‐heated zone in Cold Lake, Alberta in an area of enhanced oil recovery (EOR) processing. The results indicate that a seismic surface survey can be used to monitor thermal fronts.

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SUMMARY We use data from recently installed broad‐band seismographs on the islands of Crete, Gavdos, Santorini, Naxos and Samos in the Hellenic subduction zone to construct receiver function images of the crust and upper mantle from south of Crete into the Aegean Sea. The stations are equipped with STS‐2 seismometers and they are operated by GFZ Potsdam, University of Chania and ETH Zürich. Teleseismic earthquakes recorded by these stations at epicentral distances between 35° and 95° have been used to calculate receiver functions. The receiver function method is a routinely used tool to detect crustal and upper‐mantle discontinuities beneath a seismic station by isolating the P–S converted waves from the coda of the P wave. Converted P–S energy from the oceanic Moho of the subducted African Plate is clearly observed beneath Gavdos and Crete at a depth ranging from 44 to 69 km. This boundary continues to the north to nearly 100 km depth beneath Santorini island. Because of a lack of data the correlation of this phase is uncertain north of Santorini beneath the Aegean Sea. Moho depths were calculated from primary converted waves and multiply reflected waves between the Moho and the Earth's surface. Beneath southern and eastern Crete the Moho lies between 31 and 34 km depth. Beneath western and northern Crete the Moho is located at 32 and 39 km depth, respectively, and behaves as a reversed crust–mantle velocity contrast, possibly caused by hydration and serpentinization of the forearc mantle peridotite. The Moho beneath Gavdos island located south of Crete in the Libyan Sea is at 26 km depth, indicating that the crust south of the Crete microcontinent is also thinning towards the Mediterranean ridge. This makes it unlikely that part of the crust in Crete consists of accreted sediments transported there during the present‐day subduction process which began approximately 15 Ma because the backstop, i.e. the boundary between the current accretionary wedge of the Mediterranean ridge and the Crete microcontinent, is located approximately 100 km south of Gavdos. A seismic boundary at 32 km depth beneath Santorini island probably marks the crustal base of the Crete microcontinent. A shallower seismic interface beneath Santorini at 20–25 km depth may mark the depth of the detachment between the Crete microcontinent and the overlying Aegean subplate. The Moho in the central and northern Aegean, at Naxos and Samos, is observed at 25 and 28 km depth, respectively. Assuming a stretching factor of 1.2–1.3, crustal thickness in the Aegean was 30–35 km at the inception of the extensional regime in the Middle Miocene.

Contractional structures recognised in a recent SW‐NE oriented seismic profile offshore western Greece, between the islands of Zakynthos and Kefallinia (Cephalonia), indicate that this part of the Pre‐Apulian geotectonic zone was involved in Quaternary shortening related to the westward propagation of the Hellenic fold‐and‐thrust system. Deep reflector horizons including the Moho and the top of the crystalline basement were identified on the profile. Shallower reflectors include those corresponding to the contacts between the Mesozoic/Miocene, Upper Miocene/Lower Pliocene, and Pliocene/Pleistocene sedimentary sequences. The Upper Cenozoic to Quaternary sequence rests unconformably upon Mesozoic carbonates. Triassic evaporites wedge‐out in the Paxos geotectonic zone, where the Palaeozoic passes up into Mesozoic deposits. We have identified contractional structures which were reactivated during the Plio‐Quaternary on pre‐existing high‐angle normal faults, and which gave rise to significant topographic anomalies. West‐dipping normal faults were also recognised both within the Palaeozoic and Cenozoic successions, and are related to regional extension during sedimentation. East‐dipping thrust faults which root in the evaporites were also identified on the seismic profile. Due to right‐lateral strike‐slip activity on the Kefallinia Transform Fault, east‐dipping normal faults were formed within the lonion abyssal plain. This abyssal plain together with the Hellenic Trench, an accretionary prism, and a forearc basin can be recognised on the seismic profile. A “triple junction” between the Apulian (African) Platform, the oceanic crust of the Ionian Abyssal Plain and the Eurasian Plate in the west of the line is related to the Kefallinia Transform Fault. Neotectonic structural deformation (i.e. Quaternary‐Holocene) is superimposed on the above‐mentioned structures. Finally, diapiric movement of Triassic evaporites has affected both the Alpine and the late Cenozoic to Holocene sedimentary sequences. Diapiric activity continues at the present day in the eastern part of the profile, in the lonian geotectonic zone. The forearc basin may be prospective for hydrocarbons. Target areas include the lonian channel where a play has already been located, and its extension to the south (the Kyparisiakos Gulf area). Here, thick late Cenozoic to Quaternary deposits may act as a top‐seal above a reservoir consisting of eroded Mesozoic to Eocene carbonates, as at the recent Katakolon discovery.

A novel approach that borrows methods commonly used in environmental geophysics was developed for obtaining the estimates of the aquifer parameters. Specifically, estimates of hydraulic conductivity were obtained from field measurements of the electrical resistivity while accounting for the karsticity of the geological formations in the area of study. Geophysically determined hydraulic conductivity estimates were introduced to a 3-D groundwater numerical simulator (Princeton Transport Code -PTC) to compute the hydraulic heads distribution of the area of interest. The calibration of the numerical model was obtained matching the hydraulic-heads predicted by the simulator with the hydraulic-heads measured at specific well locations. Simulated hydraulic-heads were used with the Chyben-Herzberg equation to approximate the position of the sharp freshwater/saltwater interface of the base of the water supply aquifer. The existence of the faults impacts the groundwater flow and the distribution of the freshwater/saltwater interface.

A tumulus is a construction erected to cover a tomb. Some tumuli are impressively massive and may conceal architectural masterpieces. Seismic refraction is employed to locate the tomb and to allow selective excavation without destroying the tumulus. The detectors are spread along a circular profile on the periphery of the tumulus, and acoustic waves are generated on its top. Time delays observed in the arrivals of the headwaves reveal the position of the monument. The delays are not caused by the monument itself, but are an effect caused by the presence of a ramp that was dug in the undisturbed soil to help in the construction of the tomb. Three case histories in Northern Greece establish the efficiency of the technique. In the first example, an experiment was conducted at a previously excavated tumulus, and time delays attributed to the revealed ramp are observed. The second case study led to the discovery of an impressive monument; 3-D modeling by finite difference verifies the interpretation. A third study is also reported; where, for the most promising portion of the data, 3-D modeling has been performed.