Electrical structure of the tectonically active Kalabsha Fault, Aswan, Egypt (original) (raw)
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Journal of Seismology ISSN 1383-4649
Within the Spain and Moroccan networks, a large volume of seismic data has been collected and used for investigating the lithosphere in the Betic-Rif Cordillera. The present study has two main goals: (1) Use the most actual seismological data from recent earthquakes in the Betic-Rif arc for investigating the lithosphere through the application of seismic local tomography techniques. (2) Define the possible structural blocks and explain the GPS velocities perturbation in this region. The resolution tests results indicate that the calculated images gave a close true structure for the studied regions from 5-to 60-km depth. The resulting tomographic image shows that the presence of two upper crust body (velocity 6.5 km/s) at 3-to 13-km depth between Iberian Betic and Moroccan Rif in the western and in the middle of Alboran Sea also shows the low velocity favoring the presence of melt in the base of these two bodies. The crustal bodies forms tectonic blocks in the Central Rif and in the Central Betic Cordillera.
Seismic studies of crustal structure
Reviews of Geophysics, 1983
Insroduction DL?•ing the past four years, seismoiogy nas continued its role as the key technique for determination of the structure of the continental and oceanic crust. By and large, nowever, the field and analysis methods used Oear little resemoiance to tnose of ten years ago. A continued push to increase tne n•nOer of recording caanneis has led to tne routine use of ocean oottan seismometers, sonoouoys, and multichannel strewhers in oceanic studies, and to the use of large sets of portaole instruments and multichannel seismic reflection strings on land. In studies using earshqua•es as sources, local and regional arrays provide a capaOility for oO%aining geologically useful two-and tareedimensional information aOout the cruss. Growth in computing capacity has proceeded in parallel; a suostantial numoer of university and governmen5 research groups nave hands-on access to superminicomputers with array processors, and handle data Copyright 1983 by the American Geophysical Union. Paper number 3R0806. 0034-6853/83/003R-0806515.00 lioraries containing hundreds of reels of tape. Theoretical advances now make it possible to generate synthetic seismograms for aii layered models and for a wide class of nonlayered models, for aid in interpretation of data. Techniques of plane wave decomposition and wavefield migration nave opened up many new possioilities for the reduction and interpretation of data collected from arrays wisaout spatial aliasing. In tnis review we take up crustal seismoiogy in terms of the major field metnods: 1) Explosion studies on land, for crustal and upper mantle structure, with networks of port-aOle stations, and using refractions and wide angle reflections. 2) Deep continental reflection studies, using Vioroseis sources, with dense geophone arrays normally shorter than 10 km, using narrow angle reflections. 3) Marine multichannel reflection studies, using an array of airguns and a towed hydrophone streamer normally shorter than 3 Km. 4) Marine long range studies, similar to (1), using ocean Oottom seismometers (OBS), or sonoOuoys as receivers, with explosions or airguns as sources. Generalized inversion for crust and iithos-5) Phinney and Odom: Seismic Studies of Crustal Structure 1319 pheric structure under a local or regional %he daLa. All the da%a were preprocessed to network of seismic sta%ions. Use is made of a remove %he effects of individual instrument varie%y of data cons%rain%s, fr•n ooth %elese-responses Dy BaKer et al. [1982]. Spariin et al. isms and local even%s, sucn as P delays, local [1982] oO%ained theore%ical travel times Oy two %ravel times, surface wave pnase velocities dimensional ray tracing, and developed a gloOally and the like. consis%en% model Oy iteratively refining the We then close wi%n a orief review of the •neoret-model toward agreemen% wi%h %he data. This icai advances which have had an influence on approach, wnile admittedly subjec%ive, proOaDly seismic crus%al s%udies during the quadrennium. makes as much sense in %erms of %he goals of inverse theory as the direc% data processing Continental Explosion Studies aigori%nms which are tne norm in reflection seismoiogy. Pries%iy and OrcutJt [1982] applied Large scale studies by U.S. investigators in %he me%hod of extremal inversion for %he tau(p) %he SnaKe River Plain, Saudi Arabia, and %he function, as described oy Bessonova et al. [1974] Imperial Valley were most noraDie. The firs% was and modified oy Garmany et al. [1979]. They conorganized Dy a group of universi%ies in colia-finned %he interpretation of a high average crupotation with European colleagues and U. S. stai velocity and inferred a velocity reversal in government agencies, wnile the la%ter two were tne eastern end of the SRP, closest to Yellow-sSudies Oy the U.S. Geological Survey, using a stone. Evans [1982], used a damped least squares specially designed se% of 100 portable instru-inversion of %eleseismic P residual dasa for the men%s. SRP %o infer 3-dimensional structure beneath the
Geophysical Journal International, 2012
We provide a new hypothesis for the deep subsurface structures near the Bhuj 2001 earthquake region based on magnetotelluric (MT) investigations carried out close to the epicentre zone. 2-D inversion of broad-band MT data of two profiles of lengths 32 km (AA) and 52 km (BB) revealed a thick (∼3 km) highly conductive (1-4-m) surface layer of fluviomarine Mesozoic-Cenozoic sediments. The models delineate the hypocentre zone located at ∼20-25 km depth that manifests the high resistivity-conductivity transition zone. The accumulation of compressive stresses post-rifting along this weak zone has resulted in the reverse slip of Bhuj 2001 earthquake. The reverse fault (F 1) associated with the earthquake is believed to be an ancient normal fault formed during the rifting phase. Contrary to earlier suggested theories, we suggest that F 1 got initiated along the high resistivity-conductivity transition zone causing the Bhuj 2001 event. The geoelectric models revealed a laterally extending partially resistive zone at 20-30 km depth range showing a tendency to extend further deep. Model calculations using synthetic data also support this observation. Therefore, we hypothesize the presence of a basal detachment, marking the transition zone between the continental crust and the lithospheric upper mantle at ∼40 km depth, intersected by the F 1. The geoelectric models suggest that the crustal thinning caused the asthenospheric upwelling and/or serpentinization leading to the ascent of volatiles and melts. The subsurface geometry in Kachchh basin suggests the thickskinned deformation.
Frontiers in earth sciences: new ideas and interpretation
Annals of Geophysics, 2006
The strict similarity of some Mediterranean tectonic situations arcs, trenches, Wadati-Benioff zones, volcanic and seismic activities to the East Asia ones, which are commonly interpreted as produced by plate conver-gence, provides many clues that are at variance with the ...
Constraints on the seismic properties of the middle and lower continental crust
Geological Society, London, Special Publications, 2011
For the past two decades geodetic measurements have quantified surface displacement fields for the continents, illustrating a general complexity. However, the linkage of geodetically defined displacements in the continents to mantle flow and plate tectonics demands understanding of ductile deformations in the middle and lower continental crust. Advances in seismic anisotropy studies are beginning to allow such work, especially in the Himalaya and Tibet, using passive seismological experiments (e.g. teleseismic receiver functions and records from local earthquakes). Although there is general agreement that measured seismic anisotropy in the middle and lower crust reflects bulk mineral alignment (i.e. crystallographic preferred orientation, CPO), there is a need to calibrate the seismic response to deformation structures and their kinematics. Here, we take on this challenge by deducing the seismic properties of typical mid- and lower-crustal rocks that have experienced ductile deforma...
Crustal Stresses and Seismodynamic Characteristics in the Upper Crust
Open Journal of Earthquake Research, 2014
In this paper an approach to estimate near-surface seismodynamic features by using distanceamplitude reduction with geotectonic characteristics of the upper crust in the Eastern Anatolia is discussed. The data set used in this study consists of 287 regional earthquakes in the magnitude range of 3.0-6.1, epicentral distances between 15 km and 202 km and their focal depths reaching up to 13 km. The entire study area is divided into three tectonic blocks according to the distributions of the earthquakes and the location of the fault segment. The estimated quality factor P-S Q values for the three regions ranged from 28.6 to 65, highlighting the regional differences in the seismodynamics of the crust. In Eastern Anatolia, the relatively low average quality factor Q values (P Q : 37, S Q : 55) show average σ (0.217) and average δ values (δ P : 0.0166, δ S : 0.017). The lowest S P Q Q value 1.39 and the highest p s V V value 1.65 are found at the Mus station. The highest Poisson's ratio σ and lowest absorption coefficient δ were found in the Mus area. The variation in Q , δ and σ indicates that the northern part (Erc and Kem region) of East Anatolia appears to be more active and heterogeneous compared with the southern part (Mus region) of East Anatolia.
Seismicity, focal mechanisms, and tectonics
Reviews of Geophysics, 1983
In the last four years, there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study. New data from many local and regional networks of seismographs established during the previous four-year period, plus the now routine application of improved hypocentral location procedures, long-period body wave modelling, and moment-tensor inversion using digitally recorded data, have increased resolution of both the hypocentral location and the nature of the faulting process. Consequently, research emphasis has been on local tectonics, or those aspects of regional and global tectonics requiring high resolution. I briefly describe below some of the highlights of these studies. Shallow-Dipping Subduct ion A controversy over the dip of the subducting slab beneath central Peru and central Chile was resolved using data from local networks. Barazangi and Isacks (1976;1979) proposed, on the basis of a group of teleseismically determined hypocenters selected carefully for their quality, that the slab dips at an angle of only about 10 ø in these regions before dipping steeply several hundred kilometers inland. James (1978) questioned their event selection, pointing out that their model was based primarily on the seismicity from one local cluster of events. He preferred the model of Snoke et al. (1977;1979) of a slab dipping at about 30 ø , which was based on ScSp conversions within the dipping slab beneath western Peru. Data from local seismic networks (Hasegawa and Sacks, 1981; Suarez et al., 1982) revealed that the slab dips initially at an angle of about 30 ø beneath the coastal region of Peru, but then flattens out &md is nearly horizontal for several hundred kilometers. The cause of the unusually shallow subduction is not clear, but it may be related to the subductio• of a buoyant, aseismic, Nazca Ridge (Pilger, 1981). Plate Rheo logy The maximum depth of earthquakes in both oceanic and continental regions seems to be related to the tectonic age or surface heat flow of the province. Hence, the occurrence of seismic deformation is probably controlled by the temperature of the source region. Careful compilations of reliable focal depths in oceanic lithosphere show that the maximum depth age of the seafloor, with seismicity apparently confined above the 600 to 800øC isotherms (Chen and Molnar, 1982; Wiens and Stein, 1982).