Panagiotis Hatzidimitriou

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.

Seismic faults in the Aegean area

Abstract Reliable fault plane solutions of shallow earthquakes and information on surface fault traces in combination with other seismic, geomorphological and geological information have been used to determine the orientation and other properties of the seismic faults in the Aegean and surrounding area. Thrust faults having an about NW-SE strike occur in the outer seismic zone along western Albania-westernmost part of mainland of Greece-Ionian Sea-south of Crete-south of Rhodes. The inner part of the area is dominated by strike-slip and normal faulting. Strike-slip with an about NE-SW slip direction occurs in the inner part of the Hellenic arc along the line Peloponnesus-Cyclades-Dodecanese-southwest Turkey as well as along a zone which is associated with the northern Aegean trough and the northwesternmost part of Anatolia. All other regions in the inner part of the area are characterized by normal faulting. The slip direction of the normal faults has an about SW-NE direction in Crete (N38°E) and an about E-W direction (N81°E) in a zone which trends N-S in eastern Albania and its extension to western mainland of Greece. In all other regions (central Greece-southern Yugoslavia and Bulgaria, western Turkey) the slip of the normal faults has an about N-S direction.

The seismic parameter b of the frequency-magnitude relation and its association with the geological zones in the area of Greece

Abstract The parameter b of the frequency-magnitude relation has been accurately calculated for each of the 21 seismic zones into which the Aegean and surrounding area (34°N–43°N, 18°E–30°E) has been divided on the basis of several seismotectonic criteria. These 21 seismic zones have been geographically separated into three (A, B, C) groups (regions), according to the values of the parameter b . The outer region (A) is characterized by a relatively high value (= 1.03) the middle region (B) by an intermediate value (= 0.84) and the innermost region (C) by a low value (= 0.60). The boundaries of these three groups of zones almost coincide with the boundaries of well-known geological zones. This observation is interpreted in terms of mechanical heterogeneity of the material, and of structural and stress conditions in the area.

The Hellenic subduction beneath the peloponnesus: first results of a microearthquake study

Abstract A preliminary examination of the 1070 earthquake locations, determined from 6 weeks of recording in 1986 by 46 stations, show that the seismicity is spread over a wide area of the Peloponnesus and the western Hellenic arc and throughout the whole crust. No clear individual faults can be identified from the seismicity, but clusters of activity are observed in some places. Seismicity is concentrated above 40 km and deeper earthquakes were not numerous. Only 28 of the 466 events with uncertainties in depth less than 5 km occurred deeper than 40 km. Seismicity deeper than 30 km defines a flat zone at a depth between 40 km and 70 km, starting from the trench to about 200 km towards the northeast. Further northeast, the dip of the seismic zone abruptly changes to 45°. Fault plane solutions for the deeper events, generally indicate T-axes plunging northeast, within the subducted slab. Therefore, we interpret the seismicity deeper than 30 km as due to the superposition of two different causes: (1) the steep zone is due to the subduction of the African litospheric plate beneath the Aegean, and (2) the shallow flat zone located between the trench and the Argolide is partly due to the loading of the overriding Aegean plate which is deforming above the African plate.

A microearthquake study of the Mygdonian graben (northern Greece)

During March and April 1984, a temporary network of 29 portable stations was operated in the region of the Mygdonian graben near Thessaloniki (northern Greece), where a destructive earthquake (Ms = 6.5) had occurred in the Summer of 1978. During a period of six weeks we recorded 540 earthquakes with magnitudes ranging from −0.2 to 3.0. From this set of data, 254 events are selected which according to us have a precision in epicenter and depth better than 1.5 km. A total of 54 single-event focal mechanisms have been determined. The seismicity and focal mechanisms show a rather complex pattern. There are no clear individual faults, but the E-W and NW-SE striking zones show N-S extension. Zones striking NNE-SSW show dextral strike-slip motion but NW-SE zones with sinistral strike-slip are also observed. In the center of the graben where the 1978 earthquake was located, we observe several thrust mechanisms distributed in two groups showing either NW-SE or E-W compression; these earthquakes seem to be located 2 km above the earthquakes showing normal mechanisms. The mean direction of the T-axes, found from the focal mechanisms, trends N15° and dips sub-horizontal. We propose a model for the formation and evolution of a complex graben system comprising several stages. In the initial stage the deformation occurs along pre-existing NW-SE or NNE-SSW faults, with normal or strike-slip movements. In the second stage, a new, E-W trending group of normal faults is formed over the ancient fault network. These new faults have a direction perpendicular to the mean T-axis and accommodate better the actual state of stress. At this stage the initial faults adjust to the deformation produced by the E-W trending new faults, and may constitute geometric barriers to the evolution of the new normal faults.

Source Spectral Scaling and Stress Release Estimates Using Strong-Motion Records in Greece

A data set of 50 horizontal strong-motion accelerograms from 18 main events from moderate and large earthquakes (5.2 Mw 6.9) and distances (9 km R 116 km) occurred at three different tectonic settings (normal, strike-slip, and thrust faulting), were used to estimate stress parameters. Three different definitions of stress release were adopted: the classical Brune, the dynamic-rms (root mean square), and the apparent stress. Corner frequencies, f0, based on the Andrews (1986) methodology were computed and, using these calculations, seismic moments were estimated from independent sources (teleseismic data). The calculated corner fre- quencies seem to be consistent, based on seismic moments, with previous estimates for the same area. Based on those source parameters, a theoretical x-square spectrum was derived from describing source radiation in terms of earthquake size. Frequency- dependent site amplifications were included in the simulations, separating the re- cordings in three different categories (B, C, and D) corresponding to (NEHRP) Na- tional Earthquake Hazards Reduction Program site classification. The theoretical and observational spectra are in good agreement, in a frequency range from 0.05 to 25 Hz, except for the cases where steep topography affects the recorded data. A poor fit is also observed in cases where the distance of the recorded data is greater than 100 km and the attenuation model seems to be inadequate to describe the theoretical model. The calculated parameters present a mean value of 55 � 16 bars for Brune stress, 72 � 26 bars for dynamic-rms stress, and 17 � 10 bars for apparent stress. For all determined stresses, the thrust-faulting earthquakes exhibited higher values than the calculated normal-and strike-slip-fault seismic events.

Subcrustal microearthquake seismicity and fault plane solutions beneath the Hellenic Arc

Subcrustal seismicity recorded in the southern Aegean sea during a 7-week microearthquake study was low compared with shallow seismicity. Most intermediate depth seismicity occurred beneath the western and eastern ends of the Hellenic arc. This distribution confirms that a slab of lithosphere is being subducted at a very shallow (<15°) angle for 200 km beneath the western end (Peloponnese) but more steeply beneath the eastern end (Dodecanese). We could locate only one intermediate depth event beneath the central pan of the arc, where teleseimically located intermediate depth earthquakes also are infrequent. T axes for most of the 22 focal mechanisms of subcrustal earthquakes are roughly parallel to the local dip direction of the seismic zone. Between depths of 40 and 80 km, the mechanisms are more confused than at greater depth, perhaps because some of these earthquakes did not occur within the downgoing slab. Earthquakes deeper than 80 km, and within the subducted slab, have nearly horizontal P axes that trend NNE-SSW in the eastern part and NNW-SSE in the western part of the arc. These deeper mechanisms show horizontal P axes along strike, perhaps in response to the contortion of the slab or to the westward motion of Turkey, as well as lengthening downdip, probably in response to gravity acting on excess mass in the slab. Thus the short slab, both downdip and along strike, subducting beneath the Aegean is subjected to a more complex set of forces than the long slabs of the Pacific.

Attenuation of Coda waves in Northern Greece

The single scattering model has been applied for the estimation of codaQ values for local earthquakes that occurred in northern Greece during the period 1983–1989 and recorded by the telemetered network of the Geophysical Laboratory of the University of Thessaloniki. CodaQ estimations were made for four frequency bands centered at 1.5 Hz, 3.0 Hz, 6.0 Hz and 12.0 Hz and for the lapse time windows 10–20 sec, 15–30 sec, 20–45 sec, 30–60 sec and 50–100 sec. The codaQ values obtained show a clear frequency dependence of the formQc=Q0fn, whileQ0 andn depend on the lapse time window.Q0 was found equal to 33 andn equal to 1.01 for the time window of 10 to 20 sec, while for the other windowsQ0 increased from 60 to 129, withn being stable, close to 0.75. This lapse time dependence is interpreted as due to a depth dependent attenuation. The high attenuation and the strong frequency dependence found are characteristic of an area with high seismicity, in agreement with studies in other seismic regions.

Source and short-distance propagation of the July 9, 1956 southern Aegean tsunami

Abstract Properties of the tsunamigenic source as well as the attenuation of the tsunami intensities of the large tsunami of July 9, 1956 in the southern Aegean are investigated. This tsunami was generated by a shallow earthquake with a surface wave magnitude of 7.5 which was caused by a normal fault. Fault-plane solution, isoseismal lines and bathymetric information indicate that the tsunamigenic source had an elliptical shape with the large axis equal to 110 km and oriented in an ENE—WSW direction. This is also the strike of the normal fault which produced the earthquake and it is probable that the southern block was downthrown. Linear relations between: (a) the tsunami intensity and the epicentral distance for distances up to 330 km; (b) the observed tsunami height and the epicentral distance; and (c) the tsunami height and intensity, have been determined. This information, concerning the source parameters of tsunamis and the attenuation of the tsunami intensity and tsunami height with distance, are of great importance for determining the tsunami hazard in this area.

Attenuation of shear-waves in the back-arc region of the Hellenic arc for frequencies from 0.6 to 16 Hz

Abstract Q β for shear-waves is determined for the inner part of the Hellenic arc, the back-arc area, as a function of frequency in the range 0.6–16 Hz. We used 314 digital records from 32 earthquakes with magnitudes ( M w ) ranging from 3.9 to 5.1. Epicentral distances ranged from 65 to 515 km. The data were obtained in 1997 during a 6-month operation of a digital portable network in Greece. The Q β estimates were made for five frequency bands centred at 0.8, 1.5, 3.0, 6.0 and 12.0 Hz and the Q β values obtained were 47, 79, 143, 271 and 553, respectively. The results show that Q β for S-waves increases with frequency taking the form Q β =55 f 0.91 (or Q β −1 ≃0.018 f −0.91 ). The high attenuation and the strong frequency dependence found, which is close to the frequency dependence of coda Q for Greece, are characteristic of an area with high seismicity, rapid extension, and in agreement with other similar studies in Greece.