Distribution Of Earthquake Foci Research Articles

Distribution of Earthquake Foci with Depth as a Manifestation of the Nature of Deformation of the Continental Crust

Distribution of Earthquake Foci with Depth as a Manifestation of the Nature of Deformation of the Continental Crust

Active tectonics of central-western Caucasus, Georgia

This work contributes to a better knowledge of potentially seismogenic faults of the Georgia Greater and Lesser Caucasus by evaluating the distribution of earthquake foci, active tectonic stress field, kinematics and geometry of main fault planes. We consider all the information coming from field structural geology, geomorphology, seismological data from historical and instrumental catalogues, seismic reflection sections, as well as new focal mechanism solutions. These data enable recognizing some active ENE-WSW reverse faults in the core of the Greater Caucasus that are parallel to the mountain range. At the southernmost front of the Greater Caucasus, a series of main thrusts dipping towards NNE are active, with up to hundreds-km-long segments; along this thrust zone, a potentially locked segment is present, about 90km long. The studied section of the Lesser Caucasus has active structures along the northern front given by south-dipping thrusts, as well as in the central core where strike-slip and oblique faults coexist. The Transcaucasian depression between the two mountain ranges shows an ongoing inversion tectonics of the central part of the Rioni Basin where active N- to NE-dipping reverse faults are present, accompanied by clear evidence of uplift of a wide area. The data are coherent with a N-S to NNE-SSW contraction of the central-western Greater Caucasus and Lesser Caucasus. Although in general the seismicity decreases westward in terms of number of earthquakes and magnitude, seismological and geological structural data in the Rioni Basin indicate here a Quaternary propagation of deformation towards the west.

Розрахунок та аналіз азимутальних коефіцієнтів qi та уточнення швидкісної моделі Закарпатського сейсмоактивного регіону

The aim of this paper is to clarify the velocity model of the Transcarpathian seismic region. The model will further be implemented in neural-network modelling to calculate and verify the depth and distribution of earthquake foci. There has been carried out an analysis of seismic wave propagation in different directions across the Transcarpathian seismic region. Being an important parameter indicative of the direction of wave propagation in a natural medium, the azimuthal coefficient q³ has proved to be efficient in developing a training neural network set. Two methods of selecting sectors have been shown, based either on the location of a seismic station or a seismic event area. We have calculated average values of the azimuthal coefficient q³ for sectors with close values of q³ for one-, two- and three-layered media according to the depth of earthquake foci in each of the three layers. With three-layered media covering earthquake foci depths of 8,000-9,000 m, the calculations accurately reflect local seismic events in the Carpathians. An average layer thickness h and an average layer velocity v were calculated separately for each E-S pair (epicenter - seismic station). Conventional combining of layers was used as a method of calculating the third layer azimuth coefficient q³. The calculations were made for direct P-waves (similar calculations can be made for S-waves). We have suggested an interpretation of the obtained results and their practical implications. It has been demonstrated how the azimuthal coefficient can be used in analysing the parameters of media.

Tectonic implications of spatial variation of b-values and heat flow in the Aegean region

The Aegean region is tectonically a complex area characterized mainly by the subduction of African oceanic lithosphere beneath the Aegean continental lithosphere including extensional subbasins and mantle driven block rotations. In this study, spatial distribution of earthquakes, b-value distribution, and heat flow data have been analyzed to reveal the deep structural features of the Aegean region. b-value distributions show two low NE–SW and NW–SE trending b-anomaly zones in the western and eastern side of the Crete, implying slab tear within the Aegean slab. Earthquake foci distribution indicates that the Aegean slab steepens in the eastern side of the Crete, compared to its western side. Earthquake foci reach maximum depth of 180 km along the Cycladic arc axis, suggesting northward subducted slab geometry. The low seismic activities and high b-value anomalies within Aegean basin, except North Aegean Trough, can be compared to higher heat flow. We concluded that collision-induced westward mantle flow beneath Turkey followed by hard collision between Arabian-Eurasian continental plates played a major role in the evolution of clockwise rotational retreat of the Aegean slab and slab steepening to the east of the Crete.

Present-day stress state of the Shanxi tectonic belt

The Shanxi tectonic belt is a historically earthquake­abundant area. For the majority of strong earthquakes in this area, the distribution of earthquake foci was controlled by the N-S oriented local structures on the tectonic belt. Studies of the present stress state of the Shanxi tectonic belt can contribute to the understanding of the relationship between strong earth­ quakes' occurrence and their structural distribution and also facilitate assessments of regional seismic danger and determina­ tion of the regions wherein strong earthquakes may occur in future. Using the Cataclastic Analysis Method (CAM), we per­ formed stress inversion based on the focal mechanism data of earthquakes which took place in the Shanxi tectonic belt from 1967 to 2010. Our results show that orientations of the maximum principal compressive stress axis of the Shanxi tectonic belt might have been variable before and after the 2001 Kunlun MS=8.1 strong earthquake, with two different superior trends of the NW-SE and NE-SW orientation in different periods. When the maximum principal compressive stress axis is oriented in the NE-SW direction, the pattern of the space distribution of the seismic events in the Shanxi tectonic belt shows a trend of their concentration in the N-S oriented tectonic segments. At the same time, the stress state is registered as horizontal shea­ ring and horizontal extension in the N-S and NE-SW oriented local segments in turn. When the maximum principal com­ pressive stress axis is NW-SE oriented, the stress state of the N-S and NE-SW oriented tectonic segments is primarily regis­ tered as horizontal shearing. Estimations of plunges of stress axes show that seismicity in the Shanxi belt corresponds prima­ rily to the activity of low­angle faults, and high­angle stress sites are located in the NE-SW oriented extensional tectonic seg­ ments of the Shanxi belt. This indicates that the stress change of the Shanxi belt is caused by adjustment of the regional stress field, rather than by the diverse seismic activities.

Seismically active column and volcanic plumbing system beneath the island arc of the Izu-Bonin subduction zone

A detailed spatio-temporal analysis of teleseismic earthquake occurrence (mb > 4.0) along the convergent margin of the Izu-Bonin-Mariana arc system reveals an anomalously high concentration of events between 27° and 30.5°N, beneath a chain of seamounts between Tori-shima and Nishino-shima volcanoes. This seismicity is dominated by the 1985/1986 earthquake swarm represented in the Engdahl—van der Hilst—Buland database by 146 earthquakes in the body wave magnitude range 4.3–5.8 and focal depth range 1–100 km. The epicentral cluster of the swarm is elongated parallel to the volcanic chain. Available focal mechanisms are consistent with an extensional tectonic regime and reveal nodal planes with azimuths close to that of the epicentral cluster. Earthquakes of the 1985/1986 swarm occurred in seven time phases. Seismic activity migrated in space from one phase to the other. Earthquake foci belonging to individual phases of the swarm aligned in vertically disposed seismically active columns. The epicentral zones of the columns are located in the immediate vicinity of seamounts Suiyo and Mokuyo, recently reported by the Japanese Meteorological Agency as volcanically active. The three observations—episodic character of earthquake occurrence, column-like vertically arranged seismicity pattern, and existence of volcanic seamounts at the seafloor above the earthquake foci—led us to interpret the 1985/1986 swarm as a consequence of subduction-related magmatic and/or fluid activity. A modification of the shallow earthquake swarm magmatic model of D. Hill fits earthquake foci distribution, tectonic stress orientation and fault plane solutions. The 1985/1986 deep-rooted earthquake swarm in the Izu-Bonin region represents an uncommon phenomenon of plate tectonics. The portion of the lithospheric wedge that was affected by the swarm should be composed of fractured rigid, brittle material so that the source of magma and/or fluids which might induce the swarm should be situated at a depth of at least 100 km in the aseismic part of the subduction zone.

Depth distribution of earthquakes through latitudinal belts of the Pacific region: General trends

The geological sciences have recently demonstrated a tendency to move away from considering the Earth as an immobile system and from explaining global seismic processes only by endogenic interrelationships. Nowadays, some geologists take into consideration external forces and irregularities in the orbital parameters of the Earth in the Earth‐Sun‐Moon system when analyzing seismic processes [2]. The study of global regularities in the latitudinal distribution of earthquake foci revealed their distinct irregularity. It has been shown [3, 4, 6] that seismic activity is practically absent at polar and near-polar latitudes and rapidly increases in the middle latitudes to reach a maximum at 40 ° ‐50 ° N and 20 ° ‐30 ° S with a stable local minimum along the equator. The quantitative distribution of seismic events through different latitudes with account for hypocenter depths at four depths intervals is considered in [7], although its authors performed no quantitative analysis of their two-dimensional distribution (through depths and latitudes) and two-dimensional distribution of released energy.

Seismological and GPS constraints on Sinai sub-plate motion along the Suez rift

This work presents new seismological and Global Positioning System (GPS) results aimed at understanding the nature and rate of strain associated with the opening of the Suez rift that separates the Sinai sub-plate from the African plate. The Sinai sub-plate has played a significant role in the tectonic evolution of the northern Red Sea and the Eastern Mediterranean region. Most small, moderate and large earthquakes occur within belts associated with the geologically documented borders of this sub-plate including the Dead Sea fault (DSF) system in the east, the Cyprian arc (CA) in the north, and the Suez rift (SR) to the southwest. The DSF and CA are well defined; however, the SR is only partially defined. Earthquake foci distribution supports the idea that the SR is seismically active, and this earthquake activity cannot be ignored throughout the kinematics evaluation of northern Red Sea region. The earthquake activity is relatively higher in the southern part of the SR and gradually decreases northward. The high seismicity is mainly attributed to the presence of the Sinai triple junction. Earthquake focal mechanisms in the SR are dominated by oblique normal faulting with left-lateral strike-slip components on NW trending fault planes consistent with regional kinematics. The extensional semi-principal stress axes derived from fault plane solutions are oriented NNE-SSW in good agreement with the current stress field obtained from borehole breakouts along the SR as well as results from GPS surveying. Recent survey-mode GPS observations provide evidence for coherent northerly motion of the Sinai sub-plate that varies between 2 and 5 mm/yr. Moreover, strain analysis indicates that the southern SR is dominated by extension while its northern segment is characterized by constriction, inconsistent with earthquake focal mechanisms and regional tectonic models. The overall northward motion of the Sinai sub-plate indicates that slab-pull rather than ridge-push is the dominant force controlling regional kinematics. Based on the low rate of extension and lack of oceanic crust, the SR can be considered an incipient plate boundary between the Sinai sub-plate and the Nubian plate.

Internal tectonic structure of the Central American Wadati‐Benioff zone based on analysis of aftershock sequences

Relocated Engdahl et al. (1998) global seismological data for 10 aftershock sequences were used to analyze the internal tectonic structure of the Central American subduction zone; the main shocks of several of these were the most destructive and often referenced earthquakes in the region (e.g., the 1970 Chiapas, 1983 Osa, 1992 Nicaragua, 1999 Quepos, 2001 El Salvador earthquakes). The spatial analysis of aftershock foci distribution was performed in a rotated Cartesian coordinate system (x, y, z) related to the Wadati‐Benioff zone, and not in a standard coordinate system (ϕ, λ, h are latitude, longitude, focal depth, respectively). Available fault plane solutions were also transformed into the plane approximating the Wadati‐Benioff zone. The spatial distribution of earthquakes in each aftershock sequence was modeled as either a plane fit using a least squares approximation or a volume fit with a minimum thickness rectangular box. The analysis points to a quasi‐planar distribution of earthquake foci in all aftershock sequences, manifesting the appurtenance of aftershocks to fracture zones. Geometrical parameters of fracture zones (strike, dip, and dimensions) hosting individual sequences were calculated and compared with the seafloor morphology of the Cocos Plate. The smooth character of the seafloor correlates with the aftershock fracture zones oriented parallel to the trench and commonly subparallel to the subducting slab, whereas subduction of the Cocos Ridge and seamounts around the Quepos Plateau coincides with steeply dipping fracture zones. Transformed focal mechanisms are almost exclusively (>90%) of normal character.

Estimation of an optimum velocity model in the Calabro-Peloritan mountains-assessment of the variance of model parameters and variability of earthquake locations

SUMMARY Accurate earthquake locations are of primary importance when studying the seismicity of a given area, they allow important inferences on the ongoing seismo-tectonics. Both, for standard, as well as for earthquake relative location techniques, the velocity parameters are kept fixed to a priori values, that are assumed to be correct, and the observed traveltime residuals are minimized by adjusting the hypocentral parameters. However, the use of an unsuitable velocity model, can introduce systematic errors in the hypocentre location. Precise hypocentre locations and error estimate, therefore, require the simultaneous solution of both velocity and hypocentral parameters. We perform a simultaneous inversion of both the velocity structure and the hypocentre location in NE-Sicily and SW-Calabria (Italy). Since the density of the network is not sufficient for the identification of the 3-D structure with a resolution of interest here, we restrict ourselves to a 1-D inversion using the well-known code VELEST. A main goal of the paper is the analysis of the stability of the inverted model parameters. For this purpose we carry out a series of tests concerning the initial guesses of the velocity structure and locations used in the inversion. We further assess the uncertainties which originate from the finiteness of the available data set carrying out resampling experiments. From these tests we conclude that the data catalogue is sufficient to constrain the inversion. We note that the uncertainties of the inverted velocities increases with depth. On the other hand the inverted velocity structure depends decisively on the initial guess as they tend to maintain the overall shape of the starting model. In order to obtain an improved starting model we derive a guess for the probable depth of the Moho. For this purpose, we exploit considerations of the depth distribution of earthquake foci and of the shear strength of rock depending on its rheological behaviour at depth. In a second step we derive a smooth starting model and repeated the inversion. Strong discontinuities tend to attract hypocentre locations which may introduce biases to the earthquake location. Using the smooth starting model we obtaine again a rather smooth model as final solution which gives the best traveltime residuals among all models discussed in this paper. This poses severe questions as to the significance of velocity discontinuities inferred from rather vague a priori information. Besides this, the use of those smooth models widely avoids the problems of hypocentre locations being affected by sudden velocity jumps, an effect which can be extremely disturbing in relative location procedures. The differences of the velocity structure obtained with different starting models is larger than those encountered during the bootstrap test. This underscores the importance of the choice of the initial guess. Fortunately the effects of the uncertainties discussed here on the final locations turned out as limited, that is, less than 1 km for the horizontal coordinates and less than 2 km for the depth.

Earthquake occurrence along the Java trench in front of the onset of the Wadati‐Benioff zone: Beginning of a new subduction cycle?

Spatial distribution of earthquake foci in the central part of the Sunda Arc was analyzed using global seismological data. The analysis revealed the existence of a distinct strip of earthquakes distributed along the Java trench, separated by a trench‐parallel, 50–150 km wide aseismic link from seismicity belonging to the Wadati‐Benioff zone of the recently subducting slab. The seismicity pattern at the plate margin corresponds well with the morphology of the seafloor: The along‐trench seismicity correlates with the position of the Java trench and the onset of the Wadati‐Benioff zone correlates with the Java trough. The GPS measurements performed in three analogous structural units of the neighboring Sumatran convergent margin point to substantial differences in velocities of movement of the respective units and correlate with the spatial distribution of earthquake foci. We interpret the distinct along‐trench seismicity as a consequence of the onset of a new subduction cycle due to the rapid convergent movement of the Indo‐Australian plate relative to the aseismic lithospheric link.

Seismically active fracture zones in the continental wedge above the Andean subduction zone in the Arica Elbow region

In the continental lithosphere overlying the Wadati-Benioff zone in the Arica Elbow region eleven seismically active fracture zones were delineated on the basis of distribution of earthquake foci. The position and width of the trace, thickness, dip and maximum depth of individual fracture zones were estimated and complemented by tectonic and geological evidence. Existing fault plane solutions were attributed to the pertinent fracture zones and confronted with their orientation, inclination and tectonic function. The analysis shows normal faulting in the eastward dipping fractures, bordering the Andes in the west, and reverse faulting in the westward dipping fractures, limiting the Andes in the east. Fault plane orientations and slip directions indicated by earthquake focal mechanisms were used for the analysis of the state of stress in the fracture zones. The results point to the necessity of presence of fluids for the generation of earthquakes along faults in the western fracture zones. The position, inclination and stress conditions in the westernmost fracture system might influence the shift of active volcanism westwards from the surface projection of the intermediate depth aseismic gap in the Wadati-Benioff zone and cause the channeling of magma through tectonically disintegrated seismically active fracture zones.

Seismic anisotropy and its relation with crust structure and stress field in the Reggio Emilia Region (Northern Italy)

SUMMARY Shear wave splitting is measured at 14 seismic stations in the Reggio Emilia region above local background seismicity and two sequences of seismic events. The good quality of the waveforms together with the favourable distribution of earthquake foci allows us to place strong constraints on the geometry and the depth of the anisotropic volume. It is about 60 km 2 wide and located between 6 and 11 km depth, inside Mesozoic age carbonate rocks. The splitting results suggest also the presence of a shallower anisotropic layer about 1 km thick and few km wide in the Pliocene‐Quaternary alluvium above the Mesozoic layer. The fast polarization directions (N30 ◦ E) are approximately parallel to the maximum horizontal stress (σ 1 is SSW‐NNE) in the region and also parallel to the strike of the main structural features in the Reggio Emilia area. The size of the delay times suggests about 4.5 per cent shear wave velocity anisotropy. These parameters agree with an interpretation of seismic anisotropy in terms of the extensivedilatancy anisotropy model which considers the rock volume to be pervaded by fluid-saturated microcracks aligned by the active stress field. We cannot completely rule out the contribution of aligned macroscopic fractures as the cause of the shear wave anisotropy even if the parallel shear wave polarizations we found are diagnostic of transverse isotropy with a horizontal axis of symmetry. This symmetry is commonly explained by parallel stress-aligned microcracks.

A plausible model for the present day seismicity and tectonic activity in the Hindukush complex zone

The Hindukush zone is situated on the northern boundary of the Indian Plate along its northwestern flanks. Fifteen years (1974–1988) of seismic data published by United States Coast and Geodetic Surveys (USCGS) and focal mechanism solutions of the subducted lithosphere, reported by different scientists, have been used to model the seismic and tectonic characteristics of the Hindukush complex zone. The data indicate that the zone can be separated into two seismic segments, OP and PQ, with a V-shaped pattern, striking E–W and NE–SW, respectively. The two segments do not represent well-defined subduction zones, as the depth and frequency of events is not uniformly distributed. The E–W striking segment extends from 68.2°E to 70.5°E at about 36.5° N, whereas the northeast–southwest segment extends from 36.5°N, 70.5°E to 38°N, 73°E. The frequency and depth of earthquakes are greatest at 36.5°N, 70.5°E decreasing towards the W and NE. Focal mechanism solutions (thrust and normal faulting) for the Hindukush zone suggest that the directions of stress may be grouped into: 1. Compressive forces acting NE–SW for the east–west striking segment. For the NE–SW striking seismic segment compressive forces are directed towards the NW; 2. Compressive forces are also acting along the strike of the two seismic segments; and 3. The direction of the tensile stress is different in each segment and extends to a depth of up to 221 km. A model is proposed to explain the distribution of earthquake foci in terms of the V-shaped subduction of oceanic lithosphere, with tensile, and compressive stresses in the two seismic belts being due to the upper mantle flow of the Indian Plate and the Tibetan Plateau. The sinking of the mantle in the Hindukush zone is responsible for the stretching of the seismic slab in a vertical direction, giving rise to the present earthquake frequency and focal depth distribution pattern.

Active Tectonics in the Aegean and surrounding area

The purpose of the present article is to summarize the current scientific knowledge related to the active tectonics of the Aegean and surrounding area (active deformation, lithospheric plate-motions, etc.), as well as describe the main information (data, methods, etc.) which were used to obtain this knowledge. It is pointed out that the understanding of active tectonics has not only theoretical but also practical interest, as it contributes to the solution of problems of direct social impact such as the problem of earthquake prediction. It is shown that most of our present knowledge relies on geophysical, geological and geodetic data. Due to the fact that the Aegean exhibits a variety of geomorphological structures and on going geophysical processes, it has been one of the modern "natural laboratories" where scientists from different parts of the world are working and verify various hypotheses related to our current view of World Tectonics. The Aegean exhibits the typical characteristics of a subduction area, such as the Hellenic Arc (a typical island arc), the Aegean Sea (a marginal sea with typical geomorphological characteristics) and the Collision Zone between the Balkan peninsula and the southwestern Adriatic. A large number of results concerning the Aegean area relies on the use of the spatial distribution of earthquake foci. Accurate data of the last two decades showed that most shallow earthquakes are generated on the shallowest part of the crust (upper 20km) and only along the southern Aegean subduction zone can their depth reach up to 60km. Papazachos and Comninakis (1969/70, 1971) were the first to determine the depth of 109 intermediate-depth events using PcP phases and showed that their foci lied on an amphitheatrically-shaped Benioff zone, which dips from the outer arc (Hellenic Trench) towards the concave part of the Hellenic Arc. This has been confirmed by recent studies, showing that the subduction is separated in a shallower (20-100km), small-dip (-20-30°) section where the lithospheric coupling takes place and events up to M = 8.0 occur, and a deeper (100-180km) part with higher dipping angle (-45°) where events up to M=7.0 occur. Fault plane solutions which have been constructed since the 60s were used for the study of the active tectonics in the Aegean. Their use allowed the detection of reverse faulting along the Hellenic Arc (Papazachos and Delibasis 1969), the Rhodes sinistral fault (Papazachos 1961), as well as the domination of a strong ~N-S extension field throughout the whole back-arc Aegean area (McKenzie 1970, 1972, 1978). The identification of the dextral transform Cephalonia fault (Scordilis et al., 1985) was also of significant importance for the understanding of the Aegean tectonics. This understanding was enhanced by the results obtained about the geophysical lithospheric structure of the Aegean, using either traditional or tomographic methods. These results showed strong crustal thickness variations in agreement with isostasy, detected the presence of a high-velocity subducted slab under the Aegean, with low-velocity/low-Q material in the mantle wedge above the slab, as usually anticipated for a subduction zone. The active deformation of the Aegean has been studied by seismological, geodetic and palaeomagnetic methods. The obtained results allowed the determination of various models describing the active crustal deformation in the Aegean area, showing a anticlockwise motion for Anatolia and a fast southwestern motion of the Aegean microplate at an average rate of ~3.5cm/yr relative to Europe. Similar studies have been performed for the subducted slab. The derivation of such models is further supported by geophysical and geological studies that led to the identification and classification of a large number of active faults, which are related to several strong shallow events in the broader Aegean area. In general, active seismic faults in the Aegean area can be separated in ten main groups, which exhibit different type of faulting. The active deformation and faulting characteristics of the broader Aegean area is the base of the understanding of the driving mechanisms, which control the Aegean active tectonics. In general, the convergence of Africa and Eurasia is responsible for the eastern Mediterranean subduction under the Aegean. The Arabian plate pushes the Anatolia microplate towards the Aegean, thus affecting the active tectonic setting in the Northern Aegean where the dextral motion along the northern Anatolia border continues. Also, the Apulia (Adriatic) anticlockwise rotation results in convergence along the coastal Albania and NW Greece, with trust faulting. However, the main controlling force of the active tectonics in the Aegean is the fast southwest Aegean motion and its overriding of the Mediterranean lithosphère, which is responsible for the large thrust events along the Hellenic Arc, as well as for the large seismicity of the Cephalonia (dextral) and Rhodes (sinistral) faults that are the contact between the Aegean microplate and Apulia and the eastern Mediterranean (east of Rhodes) plates, respectively.

Seismically active fracture zones and distribution of large accumulations of metals in the central part of Andean South America

The analysis of the geometry of distribution of earthquake foci in the central part of Andean South America between 18° and 34°S made the delineation of several seismically active fracture zones in the continental wedge overlying the subducting Nazca plate possible. Correlation of their position with the distribution of hypogene accumulations of metals revealed that the majority of large mineral deposits and mining districts are situated in the outcrops of these fracture zones. We present geometrical documentation (map of epicentres, vertical and longitudinal cross sections) of the most important fracture zones and data on mineralogical composition, genetic type and available radiometric ages of mineral deposits. Sixteen mining districts in Chile, and 24 in Argentina, were attributed to the seismically active fracture zones. Major mining districts and individual large mineral deposits occur in six seismically active fracture zones roughly parallel to the axis of the Peru-Chile trench (Carachas-Portillo, Choquelimpie, Iquique, Domeyko, Rio Blanco-Los Bayos and Farellones F.Z.), in six fracture zones roughly perpendicular to the trench (El Salvador, Maricunga, Jaroma, Ujina, Tumbaya and Incahuasi-Leon Muerto F.Z.) and in two fracture zones oriented at an angle of about 45° in relation to the direction of the presently active Andean subduction (Aconcagua and Sierra del Volcan F.Z.). The occurrences of large mineral deposits of different ages show that these fracture zones were also active in the geological past and represent sites of permanent re-opening of paths allowing ore-bearing solutions and long-term accumulation of large amounts of metals to occur in relatively restricted domains of the Earth's crust. The mining districts with dated mineral deposits are arranged into four periods of hypogene mineralization: Upper Miocene-Pliocene, Upper Oligocene-Middle Miocene, Upper Eocene-Middle Oligocene, Lower Paleocene-Upper Eocene. These periods of metallogenic activity correlate well with four supposed Andean subduction cycles active in the Tertiary. The occurrence of mineral deposits of different ages in recently active fracture zones can be used as an important evidence in favour of long-term spatial permanence and activity of these zones and as a guide for the discovery of further mineral deposits hidden under young sedimentary and volcanic cover in the fracture zones.

Refined Locations of The Swarm Earthquakes In The Nový Kostel Focal Zone and Spatial Distribution of The January 1997 Swarm In Western Bohemia, Czech Republic

In order to improve the accuracy of the spatial distribution of earthquake foci in the principal Novy Kostel focal zone, refined focal locations of about 1500 micro-earthquakes of the 1991 – 1997 period were determined using the relative Master-Event location method. To estimate the reduction in the scatter of located hypocentres, the results were compared with those obtained by routinely used FASTHYPO method and cluster analysis (the nearest-neighbour method) was applied to the located foci to evaluate the spatial distribution of the foci. Based on the results of refined location and of the cluster analysis, a concept of seismic energy release in space and time in the main focal zone was developed. Especially the January 1997 earthquake swarm was studied in detail: 946 events were located with the Master-Event location method, and the dimensions and geometry of focal clusters were determined. Type analysis was applied to waveforms to divide approximately 800 located events into eight multiplet groups to each of which a characteristic source mechanism was assigned. The spatial distribution of the foci as well as of the eight types of source mechanisms was revealed in this way and also the planes fitting the clusters of foci with two predominant source mechanisms were determined fairly well.

The Cephalonia Transform Fault and its extension to western Lefkada Island (Greece)

The central area of the Ionian Islands is dominated by the existence of a major tectonic structure called the Cephalonia Transform Fault (CTF). Its main part (Cephalonia segment) has been identified by previous work based on the spatial distribution of earthquake foci, fault plane solutions of strong earthquakes, active tectonics, structural studies and geodetic measurements. This part (Cephalonia segment) which exhibits strike-slip motion with a thrust component, strikes in a northeast direction, dips to the southeast and has a length of ∼90 km. In the present paper information concerning new fault plane solutions, orientation of isoseismals, sea bottom topography and recent GPS results are used to further check the properties of this southern part of the CTF and to explore its northeastward prolongation to Lefkada Island. It is shown that the CTF is extended to the western coast of Lefkada. This northern branch (Lefkada segment) of the CTF which is also characterized by strike-slip motion with a thrust component, strikes in a north-northeast direction, dips to the east-southeast and has a length of ∼40 km. These two segments of the CTF form a major kinematic boundary where the slip rate is 2–3 cm/yr.

Discontinuous Nature of the Lower Part of the South American Wadati-Benioff Zone in The Arica Elbow Region

The Arica Elbow region represents that part of Andean South America where the azimuth of the strike of the Peru-Chile trench changes from 150° to 190°. The area under study is roughly bounded by latitudes 17 °S and 23 °S, The shape of the Wadati-Benioff zone was studied in terms of the distribution of ISC hypocentres dated between 1964 and 1993. A system of 22 vertical cross-sections, perpendicular to the trench axis, and a map of epicentres was used to derive the detailed shape of the Wadati-Benioff zone of the presently descending slab. The distribution of earthquake foci indicates a fingerlike shape of the lower part of the Wadati-Benioff zone beneath the aseismic gap. The slab length shows small changes around 350 km in the northern sections, pronounced length oscillations between 350 and 750 km in the neighbouring central sections and a constant value of 650 km in the southern sections. The dip and thickness of the Wadati-Benioff zone are practically constant in all sections. Fault plane solutions, separated spatially into three zones, were used to estimate the state of stress in the slab.

Lateral variations of thermo-mechanical properties in the Tyrrhenian–northern Apennine region

We present lithospheric strength profiles calculated for a transect running from the Island of Corsica, in the west, to the Apennine foredeep in the east. Strength profiles are constructed at locations selected on the basis of the availability of seismic, heat flow, gravity data and earthquake foci distribution. Rheological modelling points to the presence of strong lateral variations in the thermo-mechanical state of the lithosphere. The Tyrrhenian–Tuscany area has a low rigidity in relation with the intense stretching active from the Early Miocene, and with very high heat flow associated with asthenospheric rise. On the contrary, hinterland (Corsica) and Apenninic foreland areas (Adriatic area) show a higher level of lithospheric strength. Effective elastic thickness estimates confirm this scenario, exhibiting a strong reduction from the northern Tyrrhenian to the inner Apennines and then an abrupt increase in correspondence of the Adriatic plate. The distribution of earthquake focal depths confirms the presence of a brittle rheology in the first kilometres of the crust in the Tuscany area, whereas close to the plate boundary much lower thermal gradients, partly induced by the westward-plunging Adriatic plate, enable seismic activity to occur at larger depths.