Seismic faults in the Aegean area

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.     

Seismotectonics of Portugal and its adjacent area in the Atlantic

New elements on the seismicity of Portugal and new focal-mechanism solutions of earthquakes with epicentres situated off the coast of the Portuguese mainland and in the Azores region are presented. Historical seismicity data show that in the territory of the Portuguese mainland there are active faults that are responsible for earthquakes that have caused important damage and many casualties. However, most of the intraplate earthquakes with epicentres situated in the Portuguese mainland or near the shore are normally of small magnitude and this renders difficult their interpretation in the light of focal mechanisms. A solution for one earthquake, with magnitude 5 and epicentre at the Nazaré submarine canyon, is presented.Southwestwards of Cape St. Vincent there is an important seismic zone responsible for high-magnitude earthquakes such as that of 1 November 1755. This zone is situated in the region where the extension of the Messejana fault into the ocean joins with the Azores-Gibraltar fault.The seismicity of the area situated between the western coast of the Portuguese mainland and the Azores increases approximately along the 15°W meridian, from the latitude of the Azores-Gibraltar fault up to 44°N. Focal mechanisms of earthquakes with epicentres situated along this line show very similar solutions.The interpretation of the focal mechanism solutions of the earthquakes with epicentres situated in the studied area shows that the stress field trends approximately NW-SE. It is assumed that this stress field results from the interaction of the Eurasian and African plates; however, this direction is not maintained in the Azores region.     

Perceptible earthquakes in the broad Aegean area

A probabilistic estimate of seismic hazard can be obtained from the spatial distribution, of earthquake sources, their frequency–magnitude distribution and the rate of attenuation of strong ground motion with distance. We calculate the earthquake perceptibility, i.e. the annual probability that a particular level of ground shaking will be generated by earthquakes of particular magnitude, by weighting frequency–magnitude data with the predicted felt area for a given level of ground shaking at a particular magnitude. This provides an earthquake selection criterion that can be used in the anti-seismic design of non-critical structures. We calculate the perceptibility, at a particular value of isoseismal intensity, peak ground acceleration and velocity, as a function of source magnitude and frequency for the broad Aegean area using local attenuation laws. We use frequency–magnitude distributions that were previously obtained by combining short-term catalogue data with tectonic moment rate data for 14 tectonic zones in Greece with sufficient earthquake data, and where contemporary strain rates are available from satellite data. Many of the zones show a ‘characteristic earthquake’ distribution with the most perceptible earthquake equal to the maximum magnitude earthquake, but a relatively flat perceptibility between magnitudes 6 and 7. The maximum perceptible magnitude is in the fastest-deforming region in the middle of the Aegean sea, and tends to be systematically low on the west in comparison to the east of the Aegean sea. The tectonic data strongly constrain the long-term recurrence rates and lead to low error estimates (±0.2) in the most perceptible magnitudes.     

Current accelerating seismic excitation along the northern boundary of the Aegean microplate

According to previous observations [Geophys. Res. Lett. 27 (2000) 3957], the generation of large (M≥7.0) earthquakes in the western part of the north Anatolian fault system (Marmara Sea) is followed by strong earthquakes along the Northern Boundary of the Aegean microplate (NAB: northwestermost Anatolia–northern Aegean–central Greece–Ionian islands). Therefore, it can be hypothesized that a seismic excitation along this boundary should be expected after the occurrence of the Izmit 1999 earthquake (M=7.6). We have applied the method of accelerating seismic crustal deformation, which is based on concepts of critical point dynamics in an attempt to locate more precisely those regions along the NAB where seismic excitation is more likely to occur. For this reason, a detailed parametric grid search of the broader NAB area was performed for the identification of accelerating energy release behavior.Three such elliptical critical regions have been identified with centers along this boundary. The first region, (A), is centered in the eastern part of this boundary (40.2°N, 27.2°E: southwest of Marmara), the second region, (B), has a center in the middle part of the boundary (38.8°N, 23.4°E: East Central Greece) and the third region, (C), in the westernmost part of the boundary (38.2°N, 20.9°E: Ionian Islands). The study of the time variation of the cumulative Benioff strain in two of the three identified regions (A and B) revealed that intense accelerating seismicity is observed especially after the occurrence of the 1999 Izmit mainshock. Therefore, it can be suggested that the seismic excitation, at least in these two regions, has been triggered by the Izmit mainshock.Estimations of the magnitudes and origin times of the expected mainshocks in these three critical regions have also been performed, assuming that the accelerating seismicity in these regions will lead to a critical point, that is, to the generation of mainshocks.     

Seismotectonics of Taiwan

High-quality seismicity data and focal mechanism solutions obtained during 1973–1983 by the permanent Taiwan Telemetered Seismographic Network and several temporary local seismographic networks are used for a detailed study of the seismotectonics of the Taiwan area. Seismicity distribution in southern Taiwan clearly reveals an east-dipping Benioff zone which has a thickness of about 30 km and begins to deepen along 121°E at a dip angle of 55°–60°. The leading edge of this Benioff zone reaches a depth of about 180 km between 21°N and 22°N, but tapers off to a shallower depth of about 100 km from 22°N to 23°N. The presence of this seismic zone implies that subduction of the South China Sea plate under the Philippine Sea plate extends from Luzon northward to about 23°N. The position of the northern boundary of the South China Sea plate, as tentatively defined according to the seismicity distribution, passes through southern Taiwan from the offshore area in the Taiwan Strait west of Kaohsiung in an east-northeast direction to the Taitung area where a triple junction probably lies. Seismicity is found to disappear abruptly below a certain depth in many parts of Taiwan. This phenomenon may be attributed to the frictional to quasiplastic transition in the crust or upper mantle. Comparison of shallow seismicity with surface faults and fractures shows that all areas of active shallow seismicity are marked by densely-developed faults and fractures. However, the converse is not necessarily true. This may be partly due to the relatively short duration of seismicity data and partly due to excessive weakening of some of the severely faulted and fractured areas. Finally, focal mechanism solutions for west central Taiwan and the Kuangfu-Fuli area in eastern Taiwan predominantly show a maximum horizontal compression in the SE-NW direction which can be related to collision between the Eurasian and Philippine Sea plates. However, focal mechanism solutions for both the Hualien area in eastern Taiwan and the Tainan area in southwestern Taiwan show remarkable irregularities which may result from local tectonic complexities.     

Fate of the Evros River suspended particulate matter in the northern Aegean Sea

Evros River is the most important river flowing into the North Aegean Sea (eastern Mediterranean) in terms of freshwater discharge, and the second largest one of Eastern Europe after the Danube River. Salinity and temperature measurements, together with suspended particulate matter concentrations were obtained in various depths at 14 stations in the adjacent Alexandroupolis Gulf during four seasons (June 1998, September 1998, February 1999 and March 2000) in order to investigate the particle dynamics and distributions in the northern Aegean Sea. Analysis of the collected data, together with particle observations under the scanning electron microscope and study of satellite images showed that, under certain circumstances driven by the hydrological and wind regime of the area, the Evros River particulate matter, with the associated pollutants, can be transferred far away from the estuary and implicitly comprise a hazardous factor for the environmental status of the northern Aegean Sea. This fact, combined with the future construction of the Burgas-Alexandroupolis pipeline, may cause a negative impact on the studied natural ecosystem.     

Seismotectonics of the Calabrian arc

Relocation of intermediate and deep earthquakes of Tyrrhenian Sea area through joint hypocenter determination for the period 1962–1979, has allowed a more detailed definition of the geometry of this peculiar Benioff zone. Earthquakes dip along a quasi-vertical plane to 250 km depth; there is a 50° dip in the 250–340 km depth range, and a low dip angle to 480 km depth. The structure sketched from the hypocenters is almost continuous, but most energy has been released in the 230–340 km depth interval. An evaluation of fault plane solutions of intermediate earthquakes in this area indicates predominance of down-dip compressions in the central part of the slab. At the border, strike-slip motion occurs independent of depth. Some earthquakes that occurred at intermediate depth (less than 100 km) along the Ionian margin of Calabria show predominance of reverse faulting, with the P-axis oriented SE-NW. However, shallow earthquakes in the Calabria-Sicily region indicate a more complex motion, with predominance of normal faulting. A possible interpretation of these features according to the available geological history, which involves subduction of continental lithosphere, is discussed.     

Seismotectonics and Earth tides

The results of the processing and analysis of the global earthquake distribution (more than 250000 events based on the ISC catalog) and the study of moonquakes distribution (about 900 events based on the published materials) are presented. It was found that the number of events and the energy for both cases show a bimodal distribution with maximums in the middle latitudes, zero values at the polarcaps, and a local minimum in the vicinity of the equator. The probable influence of tectonic processes on the revealed character of the seismic event distribution is analyzed, and the role of Earth tides in the activation of the seismicity in the symmetric zones on both sides of the equator is shown.     

Seismotectonics of the Baikal rift zone

High seismicity in the Baikal rift zone is controlled by the development of conjugate rising and subsiding block structures. Many types of seismological phenomena resulting from large earthquakes are manifested in the rift zone and include seismotectonic (regional, zonal and local), gravity-seismotectonic and seismogravitational deformations. Impulsive as distinct from gradual seismogenetic crustal movements play a dominant role in the recent development of the Baikal geomorphology.     

Seismotectonics and tectonic history of the andaman sea

The Andaman Sea is considered as an actively spreading back-arc basin. Seismicity and newly determined focal-mechanism solutions in the Andaman Sea area support this view. The tectonic history of the region is inferred from magnetic lineations in the northeastern Indian Ocean and the northward motion of Greater India. The mid-oceanic ridge which migrated northward along the east side of the Ninetyeast Ridge collided with the western end of the “old Sunda Trench” in the Middle or Late Miocene (10–20 m.y. B.P.). This ridge—trench collision released much of the compressional stress in the back-arc area and the continued northward movement of India that collided with Eurasia exerted a drag on the back-arc region, causing the opening of the Andaman Sea. In appearance, the subducted ridge jumped to the back-arc area. Thus, the Andaman Sea is not an ordinary subduction-related back-arc basin, but probably a basin formed by oblique extensional rifting associated with both ridge subduction and deformation of the back-arc area caused by a nearby continental collision.     

Seismotectonics of the Hindukush and Baluchistan arc

A seismicity map of that part of the Pakistan-Afghanistan region lying between the latitudes 28° to 38°N and longitudes 66° to 75°E is given using all available data for the period 1890–1970. The earthquakes of magnitude 4.5 and above were considered in the preparation of this map. On the basis of this map, it is observed that the seismicity pattern over the well-known Hindukush region is quite complex. Two prominent, mutually orthogonal, seismicity lineaments, namely the northvestern and the north-eastern trends, characterize the Hindukush area. The northwestern trend appears to extend from the Main Boundary Fault of the Kashmir Himalaya on the southeast to the plains of the Amu Darya in Uzbekistan on the northwest beyond the Hindukush. The Sulaiman and Kirthar ranges of Pakistan are well-defined zones of intermontane seismicity exhibiting north-south alignment.Thirty-two new focal-mechanism solutions for the above-mentioned region have been determined. These, together with the results obtained by earlier workers, suggest the pre-dominance of strike-slip faulting in the area. The Hazara Mountains, the Sulaiman wrench zone and the Kirthar wrench zone, as well as the supposed extension of the Murray ridge up to the Karachi coast, appear to be mostly undergoing strike-slip movements.In the Hindukush region, thrust and strike-slip faulting are found to be equally prevalent. Almost all the thrust-type mechanisms belonging to the Hindukush area have both the nodal planes in the NW-SE direction for shallow as well as intermediate depth earthquakes. The dip of P-axes for the events indicating thrust type mechanisms rarely exceeds 35°. The direction of the seismic slip vector obtained through thrust type solutions is always directed towards the northeast. The epicentral pattern together with these results suggest a deep-seated fault zone paralleling the northwesterly seismic zone underneath the Hindukush. This NW-lineament has a preference for thrust faulting, and it appears to extend from the vicinity of the Main Boundary Fault of the Kashmir Himalaya on the southeast of Uzbekistan on the northwest through Hindukush. Almost orthogonal to this NW-seismic zone, there is a NE-seismic lineament in which there is a preference for strike-slip faulting.The above results are discussed from the point of view of convergence of the Indian and Eurasian plates in the light of plate tectonics theory.     

Seismotectonics of the passive continental margin of Egypt

The results of focal mechanisms determination for the recent seismic activity (earthquakes of 1951, 1955, 1987, 1988, and 1998) in the passive continental margin of Egypt may shed some light on the local stress field in this area. Moreover, studying the source mechanism of these events provides an opportunity to understand the structural style of the passive margin of Egypt, as well as the tectonic setting beside its variation in space and time. This study reveals that there are two types of tectonic regimes which caused these mechanisms. The first is a tensional regime, represented by NW oblique (normal-dextral) faults and the second is a compressive one represented by E–W to ENE (reverse-sinstral) faults. These fault trends probably indicate rejuvenation of inherited E–W Mesozoic and NW Oligo-Miocene faults.     

Geology of the northern side of the Gulf of Edremit and its tectonic significance for the development of the Aegean grabens

This paper describes the Neogene evolution of northwestern Anatolia based on geological data collected in the course of a new mapping program. The geological history of the region, as recorded by the Neogene sedimentary and magmatic rocks that overlie the Paleozoic–Triassic basement, began after a lake invasion during the Early Miocene period with the deposition of shale-dominated successions. They were accompanied by calc-alkaline intermediate lavas and pyroclastic rocks ejected through NNE trending fractures and faults. The Lower–Middle Miocene successions were deformed under a compressional regime at the end of the Middle Miocene. The deposition of the overlying Upper Miocene–Lower Pliocene successions was restricted to within NE–SW trending graben basins. The graben bounding faults are oblique with a major strike-slip displacement, formed under approximately the N–S extension. The morphological irregularities formed during the Miocene graben formations were obliterated during a severe erosional phase to the end of the deposition of this lacustrine succession. The present E–W graben system as exemplified from the well-developed Edremit graben, postdates the erosional phase, which has formed during the Plio-Quaternary period.     

Geology of the northern side of the Gulf of Edremit and its tectonic significance for the development of the Aegean grabens

Abstract

This paper describes the Neogene evolution of north-Western Anatolia based on geological data collected in the course of a new mapping program. The geological history of the region, as recorded by the Neogene sedimentary and magmatic rocks that overlie the Paleozoic-Triassic basement, began after a lake invasion during the Early Miocene period with the deposition of shale-dominated successions. They were accompanied by calc-alkaline intermediate lavas and pyroclastic rocks ejected through NNE trending fractures and faults. The Lower-Middle Miocene successions were deformed under a compressional regime at the end of the Middle Miocene. The deposition of the overlying Upper Miocene-Lower Pliocene successions was restricted to within NE-SW trending graben basins. The graben bounding faults are oblique with a major strike-slip displacement, formed under approximately the N-S extension. The morphological irregularities formed during the Miocene graben formations were obliterated during a severe erosional phase to the end of the deposition of this lacustrine succession. The present E–W graben system as exemplified from the well-developed Edremit graben, postdates the erosional phase, which has formed during the Plio-Quaternary period. © 2001 Éditions Scientifiques et médicates Elsevier SAS     

Seismotectonics and large earthquake generation in the Himalayan region

Bounded by the western and eastern syntaxes, the Himalayan region has experienced at least five M ~ 8 earthquakes during a seismically very active phase from 1897 through 1952. However, there has been a paucity of M ~ 8 earthquakes since 1952. Examining of various catalogues and seismograms from the Gottingen Observatory, it is established that this quiescence of M ~ 8 earthquakes is real. While it has not been possible to forecast earthquakes, there has been a success in making a medium term forecast of an M 7.3 earthquake in the adjoining Indo-Burmese arc. Similarly we find that in the central Himalayan region, earthquakes of M > 6.5 have been preceded by seismic swarms and quiescences. In the recent past, based on GPS data, estimates have been made of the accumulated strains and it is postulated that a number of M ~ 8 earthquakes are imminent in the Himalayan region. We examine these estimates and find that while earthquakes of M ~ 8 may occur in the region, however, the available GPS data and their interpretation do not necessarily suggest their size and time of occurrence and whether an earthquake in a particular segment will occur sooner in comparison to that in the neighboring segment. We also comment on the inference of occurrence of M ~ 8 earthquakes based on M8 algorithm for the region. We conclude that while an M ~ 8 earthquake could occur any time anywhere in the Himalayan region, there is no indication as of now as to where and when it would occur. We impress on the need for preparedness to mitigate the pending earthquake disaster in the region.     

Trace element distribution in waters of the northern catchment area of Lake Linneret,northern Israel

Waters of the northern watershed of Lake Kineret, sampled during the period 1978–1983, were analyzed for their major and trace element contents. The trace element concentrations of the major water sources of the watershed (the Dan and Banias springs) represent background values. After emergence, the waters are subjected to human activity. In crossing the populated and cultivated Hula Basin in man-made canals, the major and trace element contents increase. In comparison to the trace element concentrations, those of the major elements have narrow ranges and small temporal fluctuations. Trace element concentrations varied by 3 orders of magnitude, and temporal variations were large but not neccessarily seasonal. Point sources of trace elements were urban effluents, fish pond wastes, and peat soil drainage. The trace element concentrations decrease in the waters of the last segment of the Jordan River. All measured trace elements were below the criteria levels established by regulatory agencies. Several, however, were of the same order of magnitude. Addition of wastes from enhanced recycling, and morphologic modification of the final course of the Jordan River could result in increase in the trace element concentrations in the water.     

Seismotectonics and seismicity of the Silakhor region, Iran

This paper deals with seismotectonic and seismicity of the Silakhor region that shows high seismic activity in western Iran. Silakhor is a vast plain with several villages and cities of Dorud and Borujerd and a small town of Chalanchulan that were destroyed and/or damaged many times by large earthquakes. This paper addresses the historical and instrumental earthquakes and their causative faults, seismotectonic provinces and seismotectonic zones of the region. Available seismic data were normalized by means of time normalization technique that resulted in the magnitude-frequency relation for the Silakhor area and estimation of the return period of earthquakes with different magnitudes. Some active faults in this region include the Dorud fault, the main Zagros thrust, the Galehhatam fault, the Sahneh fault and others. Among them, the Dorud fault is an earthquake fault and is the cause for most of the large and intermediate earthquakes in the region. The return period of large earthquakes with magnitudes greater than 7.0 (Ms) is very low, however, the occurrence of destructive earthquakes is greater in the region than in the neighboring provinces. The study proves the high seismicity of this zone and it is required to develop an accurate national plan for future building and reinforcement of the existing buildings in this region.     

New aspects on the interconnection between precursory seismic electric signal lead time and geodynamics in the northern Aegean Sea

New insights on the deformation process of a confined region in the northern Aegean Sea corroborates the claims that the observed long lead times of the precursory seismic electric signals of the two largest earthquakes which occurred within the last 15 years in the area are attributed to specific geodynamics. Possible underlying physics are also discussed.