Seismic hazard of Egypt

Earthquake hazard parameters such as maximum expected magnitude,M _max, annual activity rate,, andb value of the Gutenberg-Richter relation have been evaluated for two regions of Egypt. The applied maximum likelihood method permits the combination of both historical and instrumental data. The catalogue used covers earthquakes with magnitude 3 from the time interval 320–1987. The uncertainties in magnitude estimates and threshold of completeness were taken into account. The hazard parameter determination is performed for two study areas. The first area, Gulf of Suez, has higher seismicity level than the second, all other active zones in Egypt.b-values of 1.2 ± 0.1 and 1.0 ± 0.1 are obtained for the two areas, respectively. The number of annually expected earthquakes with magnitude 3 is much larger in the Gulf of Suez, 39 ± 2 than in the other areas, 6.1 ± 0.5. The maximum expected magnitude is calculated to be 6.5 ± 0.4 for a time span of 209 years for the Gulf of Suez and 6.1 ± 0.3 for a time span of 1667 years for the remaining active areas in Egypt. Respective periods of 10 and 20 years were reported for earthquakes of magnitude 5.0 for the two subareas.     

Tsunamis in and near Greece and their relation to the earthquake focal mechanisms

The major earthquake-induced tsunamis reliable known to have occurred in and near Greece since antiquity are considered in the light of the recently obtained reliable data on the mechanisms and focal depths of the earthquakes occurring here. (The earthquake data concern the major shocks of the period 1962–1986.) First, concise information is given on the most devastating tsunamis. Then the relation between the (estimated) maximum tsunami intensity and the earthquake parameters (mechanism and focal depth) is examined. It is revealed that the most devastating tsunamis took place in areas (such as the western part of the Corinthiakos Gulf, the Maliakos Gulf, and the southern Aegean Sea) where earthquakes are due to shallow normal faulting. Other major tsunamis were nucleated along the convex side of the Hellenic arc, characterized by shallow thrust earthquakes. It is probably somewhere there (most likely south of Crete) that the region's largest known tsunami occurred in AD 365, claiming many lives and causing extensive devastation in the entire eastern Mediterranean. Such big tsunamis seem to have a return period of well over 1000 years and can be generated by large shallow earthquakes associated with thrust faulting beneath the Hellenic trench, where the African plate subduces under the Euroasian plate. Lesser tsunamis are known in the northernmost part of the Aegean Sea and in the Sea of Marmara, where strike-slip faulting is observed. Finally, an attempt is made to combine the tsunami and earthquake data into a map of the region's main tsunamigenic zones (areas of the sea bed believed responsible for past tsunamis and expected to nucleate tsunamis in the future).     

Late Miocene normal faulting beneath the northern Nile Delta: NNW propagation of the Gulf of Suez Rift

The north Egyptian continental margin has undergone passive margin subsidence since the opening of Tethys, but its post-Mesozoic history has been interrupted by tectonic events that include a phase of extensional faulting in the Late Miocene. This study characterizes the geometry and distribution of Late Miocene normal faulting beneath the northern Nile Delta and addresses the relationship of this faulting to the north–northwestwards propagation of Red Sea–Gulf of Suez rifting at this time. Structural interpretation of a 2D grid of seismic reflection data has defined a Tortonian–Messinian syn-rift megasequence, when tied to well data. Normal fault correlations between seismic lines are constrained by the mapping of fault-related folds. Faults are evenly distributed across the study area and are found to strike predominantly NW–SE to NNW–SSE, with some N–S faults in the north. Faults are interpreted to be <10 km in length, typically in the range 3–6 km. This suggests that rifting in the northern Nile Delta did not proceed beyond a continental rift initiation phase, with distributed, relatively small-scale faults. This contrasts with the Gulf of Suez Rift, where faulting continued to a more evolved fault localization phase, with block-bounding faults >25 km in length. Results suggest that future studies could quantify fault evolution from rift initiation to fault linkage to displacement localization, by studying the spatial variation in faulting from the northern Nile Delta, south–southeastwards to the Gulf of Suez Rift.     

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.     

Tectonic geomorphology of the easternmost extension of the Gulf of Corinth (Beotia, Central Greece)

The easternmost sector of the Gulf of Corinth, the Beotia area in Central Greece, is an area with active normal faults located between the two major rift structures of Central Greece, the Gulf of Corinth and the North Gulf of Evia. These active normal faults include WNW to E–W and NE to ENE-trending faults affect the landscape and generate basin and range topography within the Beotia. We study four normal fault zones and drainage basin geometry in the easternmost sector of the Gulf of Corinth to document the impact of active tectonics on the landscape evolution. Fault and drainage geometry are investigated based on detailed field mapping and high-resolution digital elevation models. Tectonic geomorphic analysis using several parameters of active tectonics provides information concerning the relative tectonic activity and fault growth. In order to detect areas of lateral stream migration that could indicate recent tectonic activity, the Transverse Topographic Symmetry Factor and the Asymmetry Factor are used to analyse drainage basin geometry in six large drainage basins and a drainage domain covering the study area. Our results show that vertical motions and tilting associated with normal faulting influence the drainage geometry and its development. Values of stream-gradient indices (S_L) are relatively high close to the fault traces of the studied fault zones suggesting high activity. Mountain-front sinuosity (S_mf) mean values along the fault zones ranges from 1.08 to 1.26. Valley floor width to valley height ratios (V_f) mean values along the studied fault zones range between 0.5 and 1.6. Drainage basin shape (B_S) mean values along the fault zones range from 1.08 to 3.54. All these geomorphic parameters and geomorphological data suggest that the analyzed normal faults are highly active. Lateral fault growth was likely produced by primarily eastward propagation, with the WNW to E–W trending faults being the relatively more active structures.     

Greek tectonics and seismicity

The validity of existing tectonic models for the area of Greece is examined in the light of the new recalculated parameters for earthquakes of the region (Makropoulos and Burton, 1981). Relocated hypocentral positions are extracted from the catalogue to form radial and vertical distance-depth cross-sections centred on a reference point near the mid-point of the Aegean Volcanic arc, and these are used to form a three dimensional topography of the base of earthquake occurrence below 60 km. Isodepth maps are extracted from this topography as both three and two dimensional map presentations. These maps reveal several significant features of deep-seated tectonic processes in the region. Isodepths exceeding 150 km are seen in the northwest Aegean, and these are more closely linked to the Sporadhes and Gulf of Thermaicos, rather than the North Aegean trough. The 150 km isodepths are also seen in the northeast Aegean straddling the Dardanelles; in the northeastern part of the Peloponnesus, Gulf of Saronikos and eastern Gulf of Corinth in the southern Peloponnesus towards Crete; and extending from north of Kos to south of Rodos. The largest extent of deepest activity is seen south of Rodos and this continues towards southwest of Turkey. The subduction zone of the Hellenic arc is clear, but smooth Benioff zones are not the norm, and these data show that structural complexity is more readily observed. It is concluded that none of the proposed tectonic models completely explain the observed activity over the whole area, and rather than propose yet another model places where further work is still particularly necessary are identified.     

Seismic coupling and uncoupling at subduction zones

Seismic coupling has been used as a qualitative measure of the “interaction” between the two plates at subduction zones. Kanamori (1971) introduced seismic coupling after noting that the characteristic size of earthquakes varies systematically for the northern Pacific subduction zones. A quantitative global comparison of many subduction zones reveals a strong correlation of earthquake size with two other variables: age of the subducting lithosphere and convergence rate. The largest earthquakes occur in zones with young lithosphere and fast convergence rates, while zones with old lithosphere and slow rates are relatively aseismic for large earthquakes. Results from a study of the rupture process of three great earthquakes indicate that maximum earthquake size is directly related to the asperity distribution on the fault plane (asperities are strong regions that resist the motion between the two plates). The zones with the largest earthquakes have very large asperities, while the zones with smaller earthquakes have small scattered asperities. This observation can be translated into a simple model of seismic coupling, where the horizontal compressive stress between the two plates is proportional to the ratio of the summed asperity area to the total area of the contact surface. While the variation in asperity size is used to establish a connection between earthquake size and tectonic stress, it also implies that plate age and rate affect the asperity distribution. Plate age and rate can control asperity distribution directly by use of the horizontal compressive stress associated with the “preferred trajectory” (i.e. the vertical and horizontal velocities of subducting slabs are determined by the plate age and convergence velocity). Indirect influences are many, including oceanic plate topography and the amount of subducted sediments.All subduction zones are apparently uncoupled below a depth of about 40 km, and we propose that the basalt to eclogite phase change in the down-going oceanic crust may be largely responsible. This phase change should start at a depth of 30–35 km, and could at least partially uncouple the plates by superplastic deformation throughout the oceanic crust during the phase change.     

Lithostratigraphic and sedimentary evolution of the Kom Ombo (Garara) sub-basin,southern Egypt

Southern Egypt is mostly covered by clastic sediments belonging to the Paleozoic and the Mesozoic. The Precambrian basement rocks bound the Etbai area to the east and Gabgaba area to the west. The basement extends further west forming dissected small and major exposures in southern Egypt, south of latitude 23° 30′ N but are covered by Cretaceous-Lower Tertiary sediments further north, the Western Limestone Plateau. The clastic sediments in southeast Egypt, on the western side of the basement rocks in-between latitudes 22° N and 24° 35′ N, built two sub-basins, Kom Ombo (Garara) sub-basin in the north and south Nile Valley sub-basin in the south. These are separated by a dissected basement wall. The two sub-basins have different lithostratigraphic successions, Paleozoic (Early to Late) in the south Nile Valley sub-basin whereas Late Paleozoic-Mesozoic-Tertairy in the Kom Ombo sub-basin. The platform clastic sediments within both sub-basins were possibly supplied from an easterly located Paleotethys extending to North Gondwana. The Oxfordian opening of the Indian Ocean associated with rise in sea level supplied more waters to the north and sediments by passed the filled southern Nile Valley sub-basin and reached the adjacent Kom Ombo sub-basin defining a depositional shift. On the other hand, during the Jurassic, Northern Egypt received Neotethys waters that filled deeper sub-basins (e.g., the Maghara sub-basin), hence the difference in lithology between Jurassic northern and southern sediments. Since the Jurassic, most of Egypt received Tethys waters. In the drilled wells studied, the younger top sediments surrounding the well sites are related to the Tethys geostratigraphy. The sub-basins in southern Egypt are controlled by N-S faults defining constant subsiding basins. The E-W Guinea–Nubia Lineament bounds the northern side of the Kom Ombo sub-basin, where it is closed by a northern basement arch.     

Active faulting and natural hazards in Armenia, eastern Turkey and northwestern Iran

Active fault zones of Armenia, SE Turkey and NW Iran present a diverse set of interrelated natural hazards. Three regional case studies in this cross-border zone are examined to show how earthquakes interact with other hazards to increase the risk of natural disaster. In northern Armenia, a combination of several natural and man-made phenomena (earthquakes, landslides and unstable dams with toxic wastes) along the Pambak-Sevan-Sunik fault (PSSF) zone lowers from 0.4 to 0.2–0.3g the maximum permissible level (MPL) of seismic hazard that may induce disastrous destruction and loss of life in the adjacent Vanadzor depression.

In the Ararat depression, a large active fault-bounded pull-apart basin at the junction of borders of Armenia, Turkey, Iran and Azerbaijan, an earthquake in 1840 was accompanied by an eruption of Ararat Volcano, lahars, landslides, floods, soil subsidence and liquefaction. The case study demonstrates that natural hazards that are secondary with respect to earthquakes may considerably increase the damage and the casualties and increase the risk associated with the seismic impact.

The North Tabriz–Gailatu fault system poses a high seismic hazard to the border areas of NW Iran, eastern Turkey, Nakhichevan (Azerbaijan) and southern Armenia. Right-lateral strike–slip motions along the North Tabriz fault have given rise to strong earthquakes, which threaten the city of Tabriz with its population of 1.2 million.

The examples illustrate how the concentration of natural hazards in active fault zones increases the risk associated with strong earthquakes in Armenia, eastern Turkey and NW Iran. This generally occurs across the junctions of international borders. Hence, the transboundary character of active faults requires transboundary cooperation in the study and mitigation of the natural risk.     

Local seismic array observations at north Evoikos, central Greece, delineate crustal deformation between the North Aegean Trough and Corinthiakos Rift

In order to better constrain and define the microseismic activity at the north Evoikos Gulf and its surrounding area we deployed an onshore/offshore seismic array consisting of 31 three-component seismic digital stations. The array was active from 30 June to 24 October 2003, and covered an area of 2500 km². We located more than 2000 seismic events ranging from 0.7 to 4.5 M_L by using six stations as a minimum in order to define the foci parameters. Recorded seismicity delineated three major zones of deformation: from south to north, the Eretria–Parnis–eastern Corinthiakos zone, the Psachna–Viotia zone, and the Northern Sporades–North Evia–Bralos zone. Alignments of the recorded seismicity follow the tectonic trends and their orientation in the above zones. The whole area accommodates the stress field between the North Aegean Trough and the Corinthiakos Gulf. Rate of deformation intensifies from north to south, as revealed also by historical and instrumental seismicity. The successive change of orientation between the two stress fields fragments the crust in relatively small units and the fault systems developed do not permit the generation of major earthquakes in the north Evoikos area and its immediate vicinity. This is also supported by the instrumental seismicity of the last century. Larger events reported in historical times are probably overestimated.Most seismic activity is crustal. Subcrustal events were recorded mainly below the Lichades area and are interpreted as the consequence of the subduction of the Ionian oceanic lithosphere below the Hellenides. The Lichades volcano is the most northern end of the Hellenic volcanic arc.At present the highest seismic activity is associated with the Psachna region of north Evia that has been continuously active since 2001. Considering, however, the development of the seismic activity during the last decade, there has been a sequence of large events, i.e., Parnis in 1999, Skyros in 2001 and Psachna in 2001–2003. This demonstrates the fact that the tectonic deformation in all this area is intense and important for the accommodation of the stress field of the North Aegean Trough to that of the Corinthiakos Rift.     

Relation between electrical resistivity and earthquake generation in the crust of West Anatolia, Turkey

In this paper, we present a relation between the earthquake occurrence and electric resistivity structures in the crust, in West Anatolia and the Thrace region of Turkey. The relationship between magnetotelluric georesistivity models and crustal earthquakes in West Anatolia, during a period from 1900 to 2000, is investigated. It is found that most of the large crustal earthquakes occurred in and around the areas of the highest electrical resistivity in the upper crust, although rare small magnitude earthquakes are observed in some parts of the conductive lower crust in West Anatolian extensional terrain. The high-resistivity zones may represent rocks that are probably mechanically strong enough to permit sufficient stress to accumulate for earthquakes to occur in western Anatolia and the Thrace region. However, some recent studies state that the generation of a large earthquake is not only a pure mechanical process, but is closely related to fluid existence. We also reviewed recent world-wide researches including results from the Anatolian data for the first time and discussed all general findings in combination. Our findings show that the boundary between the resistive upper crust and the conductive lower crust correlates well with the cutout depth of the seismicity in West Anatolia and Thrace. This boundary is also attributed to the fluid bearing brittle–ductile transition zone in world literature. Fluid migration from the conductive lower crust to the resistive upper crust may contribute the seismicity in resistive zones. Alternatively, the upper–lower crust boundary may act as a stress concentrator and fluids may help to release strain energy in brittle parts of lower crust, by small magnitude earthquakes, whereas they may help in focusing strain in mechanically strong and electrically resistive zones for large earthquakes to occur.     

Lithofacies zoning and titanium-zirconium potential of the Mansi and North Sos’va areas,Transural placer region

The lithofacies and morphostructural analyses of the Mansi and North Sos’va areas of the Transural placer region allowed us to establish factors of the spatial and lithostratigraphic control of titanium-zirconium placer potential. It was established that the placers are localized in sediments of the upper stage of the Oligocene-Miocene complex and rocks of Oligocene basin shows facies zoning. The most promising are the paleolittoral and shoal facies formed under moderate hydrodynamic activity. In tectonic aspect, the position of promising areas and their lithofacies composition are controlled by flank zones of local domes that experienced epeirogenic movements. Regressive regime in the Oligocene basin suggests the highest probability for the discovery of titanium-zirconium placers of the littoral type in inner zones of neotectonic domes.     

Statement of editorial and publication policies

The new European Macroseismic Scale 1992 (EMS) is applied to intensity estimation of data collected in field investigations of the 26 June 1993 Pollina (northern Sicily) earthquake and the results are compared with those obtained using the MSK-81 scale. The highest intensity estimated (VI–VII) fits with the low magnitude value (M _L = 4.7) and the low resistance level of buildings is responsible for some unexpected severe damage, thus raising the seismic vulnerability of the area. The tectonic framework shows significant elements of recent activity accounting for the frequent seismicity. The study also evaluated the validity of using vulnerability classes of buildings as defined in the EMS scale in the intensity assessment of historical earthquakes of the area.     

Fluoride in Quaternary groundwater aquifer,Nile Valley,Luxor, Egypt

The occurrence of fluoride in ground water is the focus of the public and has attracted the attention of many scientists all over the world due to its importance in public health. Deficiency or increase of fluoride uptake is considered a public health problem due to the narrow permissible limit which should not exceed 1.5 mg/l according to the World Health Organization (WHO). The range of fluoride tolerance and toxicity is narrow. Deviation from the optimal levels therefore results in dental health effects such as caries and fluorosis. Many studies have found fluorosis to be invariably associated with high concentrations of fluoride in drinking water. Fluorosis is a considerable health problem in many areas of the world including Brazil, China, East Africa, Ghana, India, Kenya, Korea, Malawi, Mexico, Pakistan, South Africa, southeastern Korea, Spain, Sri Lanka, Sudan, Taiwan, Tanzania, and Turkey. Fluoride in groundwater of Quaternary aquifer of the Nile Valley, Egypt, does not gain the attention of the authors in the Nile Valley which makes the public health status of fluoride is not certain. The present work aims at investigating the fluoride concentration of Quaternary groundwater aquifer at Luxor as a representative area of the Nile Valley to be a base line for subsequent studies and criteria for public health. Ground water samples were collected from Quaternary groundwater aquifer at Luxor area, Egypt and analyzed for the purpose of investigating fluoride content. The results showed that fluoride concentration in the study area ranges between 0.113 and 0.452 with an average of 0.242 mg/l. Sources of fluoride in the study area can result from the natural dissolution from fluoride-rich minerals, fertilizers and from groundwater recharge. It is worth mentioning that low fluoride content in the study area is considered a public health threat specially limited growth, fertility, and dental caries. Corrective measures should be taken to avoid the public health impacts of fluoride deficiency at Luxor area as well as similar areas in the Nile Valley. A public health program should be initiated to account for the deficiency of fluoride in groundwater and deal with the other supplementary fluoride sources in food or fluoridation of drinking water supplies.     

Maximum Likelihood Estimation of Seismic Hazard for Sweden

The maximum magnitude, the activity rate, and the Gutenberg-Richterb parameter as earthquake hazard parameters, have been evaluated for Sweden. The maximum likelihood method permits the combination of historical and instrumental data. The catalog used consists of 1100 earthquakes in the time interval 1375–1989. The extreme part of the catalog contains only the strongest historical earthquakes, whereas the complete part is divided into several subcatalogs, each assumed complete above a specified threshold magnitude. The uncertainty in magnitude determination was taken into account. For southern Sweden, the calculations giveb-values of 1.04 (0.05) for the whole area south of 60° N and 0.98 (0.06) for a subregion of enhanced seismicity in the Lake Vänern area. For the whole area north of 60° N, theb-value is 1.35 (0.06) and for the seismicity zone along the Gulf of Bothnia 1.26 (0.06). The number of annually expected earthquakes with magnitude equal to or larger than 2.4 [M_L(UPP) or M_M(UPP)] is 1.8 for the whole southern Sweden, 1.3 for the Lake Vänern region, 3.7 for northern Sweden, and 2.4 for the region along the Gulf of Bothnia. The maximum expected regional magnitude is calculated to 4.9 (0.5) for a time span of 615 years for southern Sweden and the Lake Vänern subregion, and 4.3 (0.5) for a time span of 331 years for northern Sweden and the Gulf of Bothnia subregion. However, several historical earthquakes with magnitude above 5 in nearby areas of Norway indicate that the seismic potential may be higher.     

Alexandria-Nile Delta coast,Egypt: update and future projection of relative sea-level rise

Previous studies have indicated that the Nile River deltaic plain is vulnerable to a number of aspects, including beach erosion, inundation, and relatively high rates of land subsidence. This issue motivates an update and analysis of new tide-gauge records, from which relative sea-level changes can be obtained. Estimated rates from five tide gauges are variable in terms of magnitude and temporal trend of rising sea level. Analysis of historical records obtained from tide gauges at Alexandria, Rosetta, Burullus, Damietta, and Port Said show a continuous rise in mean sea level fluctuating between 1.8 and 4.9 mm/year; the smaller rate occurs at the Alexandria harbor, while the higher one at the Rosetta promontory. These uneven spatial and temporal trends of the estimated relative sea-level rise (RSLR) are interpreted with reference to local geological factors. In particular, Holocene sediment thickness, subsidence rate and tectonism are correlated with the estimated rates of relative sea-level change. From the relatively weak correlation between them, we presume that tectonic setting and earthquakes, both recent and historical ones, contribute more to accelerated RSLR than that of dewatering and compression/dewatering of Holocene mud underlying the Nile Delta plain. As a result, large areas of the coastal plain have been subsided, but some sectors have been uplifted in response to tectonic activities of thick underlying older strata. Projection of averaged sea-level rise trend reveals that not all the coastal plain of the Nile Delta and Alexandria is vulnerable to accelerated sea-level rise at the same level due to wide variability of the land topography, that includes low-lying areas, high-elevated coastal ridges and sand dunes, accretionary beaches, and artificially protective structures. Interaction of all aspects (tectonic regime, topography, geomorphology, erosion rate, and RSLR rate) permitted to define risk areas much vulnerable to impacts of sea incursion due to accelerated sea-level rise.     

西秦岭北缘一个新生代伸展型角度不整合及其地质意义

通过对西秦岭北缘漳县地区渐新统—中新统含盐红层地层与下伏造山带地层之间的角度不整合及其之上的砾岩、地层序列、沉积旋回等特征研究,提出了该角度不整合为伸展型角度不整合的认识。该伸展型角度不整合的存在指示了西秦岭北缘漳县渐新统—中新统含盐红层盆地具有伸展断陷盆地的属性,意味着在渐新世—中新世漳县含盐盆地形成和沉积充填时期,青藏高原东北缘(至少西秦岭北缘)一直处于伸展构造环境。这与印度板块与欧亚板块碰撞汇聚动力学作用向东北缘扩展形成的以挤压缩短和隆升为主的构造环境不协调,也就是说,青藏高原东北缘在渐新世—中新世可能尚未卷入现今青藏高原构造系统。     

THE PROBLEM OF SEISMIC REGIONALIZATION

Seismic regionalization maps, delimiting zones wherein earthquakes of given intensity are to be expected, have been prepared for many countries. Such maps of U.S.S.R., Japan, Germany, Rumania, Hungary, India, Turkey and China are included herein. Existing methods of preparing seismic zonation maps suffer the following shortcomings: 1) No information is presented on how soon an earthquake can be expected in any given zone; 2) No information is presented on the characteristics of ground motion within the zones, nor on the type of vibrations transmitted; 3) There has been insufficient accuracy in delimiting seismic regions. Consequently, a new two-stage principle of seismic regionalization has been proposed. The first stage is the preparation of a “seismicity prognosis” map outlining zones of occurrence of earthquakes of a given intensity at given depths of focus and giving the probability of their occurrence. The second stage is preparation of a seismic effect prognosis map which would give information on intensity at the surface, spectral composition, character of vibration in the near surface materials, and earthquake probability. Methods of calculating the degree of seismic activity and of calculating the probability of occurrence of earthquakes, are also discussed.—R. E. Goodman     

Coastal neotectonics in Southern Central Andes: uplift and deformation of marine terraces in Northern Chile (27°S)

Neotectonic observations allow a new interpretation of the recent tectonic behaviour of the outer fore arc in the Caldera area, northern Chile (27°S). Two periods of deformation are distinguished, based on large-scale Neogene to Quaternary features of the westernmost part of the Coastal Cordillera: Late Miocene to Early Pliocene deformations, characterized by a weak NE–SW to E–W extension is followed by uppermost Pliocene NW–SE to E–W compression. The Middle Pleistocene to Recent time is characterized by vertical uplift and NW–SE extension. These deformations provide clear indications of the occurrence of moderate to large earthquakes. Microseismic observations, however, indicate a lack of shallow crustal seismicity in coastal zone. We propose that both long-term brittle deformation and uplift are linked to the subduction seismic cycle.     

Effect of the depositional environment on the compositional variations among the phosphorite deposits in Egypt

Late Cretaceous economic phosphorites from the Red Sea, Nile Valley, and Abu Tartur areas, Egypt, show distinct variations in the lithology of associated sediments, mineralogy of nonphosphatic constituents, and distributions of major and trace elements. In the Red Sea area, the phosphorite beds are intercalated with laminated black shales, and the nonphosphatic constituents are detrital quartz and calcite, ankerite, and pyrite cements. In the Nile Valley, the phosphorite beds are intercalated with chert, marl, and sandstone and the nonphosphatic constituents are detrital quartz and calcite and chalcedony cements. In the Abu Tartur Plateau, the phosphorite beds are intercalated with laminated black shales, and the nonphosphatic constituents are detrital quartz and ankerite and pyrite cements. The phosphorites studied also show distinct variations in major- and trace-element concentrations. The Abu Tartur phosphorites have higher contents of TiO₂, Al₂O₃, Fe₂O₃, K₂O, Co, Nb, Pb, Sr, Th, Y, and Zr and lower SiO₂, Ba, and U contents as compared to those in the Red Sea and Nile Valley areas. The positive correlations between Al₂O₃ and TiO₂, K₂O, Nb, Y, and Zr suggest the detrital origin of these constituents.Similarity in the phosphatic constituents, which were derived from outside the depositional sites, and variations in the lithology of associated sediments and the mineralogy and geochemistry of the nonphosphatic constituents, which reflect the conditions at the depositional sites, suggest that the variations in the depositional environment of the phosphorites are the potential controlling factor of the compositional variations among these phosphorites. The abundance of black shales in the Red Sea and Abu Tartur areas, as well as the occurrence of ankerite and pyrite as cementing materials for the phosphatic constituents, might reflect reducing conditions in these areas, while the abundance of siliciclastic sediments and calcite and chalcedony cements suggests oxidizing conditions in the Nile Valley. The reducing conditions in the Red Sea and Abu Tartur areas were probably developed within the pre-existing depressions in a shelf environment. These depressions might have formed as a result of a change in the movements of the North Atlantic, Eurasian, and African Plates during the late Santonian, which led to transgressive inversion of rifts along northern Egypt and consequent folding in the continental interior. The higher contents of detrital components in the Abu Tartur phosphorites compared to the Red Sea and Nile Valley areas suggest more detrital inputs during the deposition of the phosphorites in Abu Tartur. The products of the diagenesis and weathering of these deposits also reflect the variations in the depositional conditions.