Publisher Summary This chapter focuses on elastic wave propagation in stratified media. The development of the theory of elastic wave propagation in stratified media has been strongly influenced by the problems of seismic wave propagation and the nature of the seismograms recorded from earthquakes. For purely analytic developments of elastic wave propagation, the level of manageable algebraic complexity is reached in a model with one or two uniform layers overlying a uniform half space. This chapter shows how the excitation of elastic waves, within a horizontally stratified structure, can be conveniently developed in terms of reflection and transmission matrices. This procedure has allowed the construction of the full response of the medium or approximations with desired properties so that theoretical seismograms may be calculated for realistic distributions of elastic parameters. Although this development has been for isotropic media, nearly all the results apply directly to the case of full anisotropy if 3 × 3 reflection and transmission matrices allowing coupling between all wave types are employed. This development of the wavefield for both source and receiver within the stratification may be used for other classes of wave propagation.

Seismic Wave Propagation in Stratified Media

Summary.
Over 80 new fault plane solutions, combined with satellite imagery as well as both modern and historical observations of earthquake faulting, are used to investigate the active tectonics of the Middle East between western Turkey and Pakistan.

The deformation of the western part of this region is dominated by the movement of continental material laterally away from the Lake Van region in eastern Turkey. This movement helps to avoid crustal thickening in the Van region, and allows some of the shortening between Arabia and Eurasia to be taken up by the thrusting of continental material over oceanic-type basement in the southern Caspian, Mediterranean, Makran and Black Sea. Thus central Turkey, bounded by the North and East Anatolian strike-slip faults, is moving west from the Van region and overrides the eastern Mediterranean at two intermediate depth seismic zones: one extending between Antalya Bay and southern Cyprus, and the other further west in the Hellenic Trench. The motion of northern Iran eastwards from the Van region is achieved mainly by a conjugate system of strike-slip faults and leads to the low angle thrusting of Iran over the southern Caspian Sea. The seismicity of the Caucasus shows predominantly shortening perpendicular to the regional strike, but there is also some minor elongation along the strike of the belt as the Causcasus overrides the Caspian and Black Seas.

The deformation of the eastern part of this region is dominated by the shortening of Iran against the stable borders of Turkmenistan and Afghanistan. The north-east direction of compression seen in Zagros is also seen in north-east Iran and the Kopet Dag, where the shortening is taken up by a combination of strike-slip and thrust faulting. Large structural as well as palaeomagnetic rotations are likely to have occurred in NE Iran as a result of this style of deformation. North-south strike-slip faults in southern Iran allow some movement of material away from the collision zone in NE Iran towards the Makran subduction zone, where genuinely intermediate depth seismicity is seen.

Within this broad deforming belt large areas, such as central Turkey, NW Iran (Azerbaijan), central Iran and the southern Caspian, appear to be almost aseismic and therefore to behave as relatively rigid blocks surrounded by active belts 200-300 km wide. The motion of these blocks can usefully be described by poles of rotation. The poles presented in this paper predict motions consistent with those observed and also predict the opening of the Gulf of Iskenderun NE of Cyprus, the change within the Zagros mountains from strike-slip faulting in the NW to intense thrusting in the SE, and the relatively feeble seismicity in SE Iran (Baluchistan). This description also explains why the north-south structures along the Iran-Afghanistan border do not cut the east-west ranges of the Makran.

Within the active belts surrounding the relatively aseismic blocks a continuum approach is needed for a description of the deformation, even though motions at the surface may be concentrated on faults. The evolution of fault systems within the active zones is controlled by geometric constraints, such as the requirement that simultaneously active faults do not, in general, intersect.

Many of the active processes discussed in this paper, particularly large-scale rotations and lateral movement along the regional strike, are likely to have caused substantial complexities in older mountain belts and should be accounted for in any reconstructions of them.

Active tectonics of the Alpine—Himalayan Belt between western Turkey and Pakistan

Summary We use the tectonics of Eastern Anatolia to exemplify many of the different aspects of collision tectonics, namely the formation of plateaux, thrust belts, foreland flexures, widespread foreland/hinterland deformation zones and orogenic collapse/distension zones. Eastern Anatolia is a 2 km high plateau bounded to the S by the southward-verging Bitlis Thrust Zone and to the N by the Pontide/Minor Caucasus Zone. It has developed as the surface expression of a zone of progressively thickening crust beginning about 12 Ma in the medial Miocene and has resulted from the squeezing and shortening of Eastern Anatolia between the Arabian and European Plates following the Serravallian demise of the last oceanic or quasioceanic tract between Arabia and Eurasia. Thickening of the crust to about 52 km has been accompanied by major strike-slip faulting on the right-lateral N Anatolian Transform Fault (NATF) and the left-lateral E Anatolian Transform Fault (EATF) which approximately bound an Anatolian Wedge that is being driven westwards to override the oceanic lithosphere of the Mediterranean along subduction zones from Cephalonia to Crete, and Rhodes to Cyprus. This neotectonic regime began about 12 Ma in Late Serravallian times with uplift from wide-spread littoral/neritic marine conditions to open seasonal wooded savanna with colluvial, fluvial and limnic environments, and the deposition of the thick Tortonian Kythrean Flysch in the Eastern Mediterranean. Earthquake hypocentres are scattered throughout the region but large earthquakes are concentrated mainly on the major faults and are mostly shallow, supporting the idea of a brittle elastic lid with hypocentres concentrated towards its base with more ductile deformation in the middle and lower crust. Neotectonic magmatic suites are nepheline-hypersthene normative alkali basalts of mantle origin, and silicic/intermediate/mafic calcalkaline suites, both suites occurring in pull-apart basins in strike-slip regimes and along N-S extensional fissures, and both suites showing a strong change to central activity in the Pliocene. Upper-crustal strains appear to be discontinuous in space and time, with zones of strong shortening representing shoaling of crustal detachment zones flattening between 5 and 10 km. Approximately NW- (dextral) and NE- (sinistral) trending lineaments bound less deformed wedges (low relief seismically ‘dead’ areas) and vary from simple strike-slip faults to complicated braided transform-flake boundaries with pull-apart and compressional segments (N and E Anatolian Transform Faults). Volcanoes lie in grabens on N-S ‘cracks’ that extend into the Arabian Foreland and in transcurrent pull-aparts. Major extensional basins lie at plate (Adana) and flake (Karliova) triple junctions and result from compatibility problems.

Shortening of continental lithosphere: the neotectonics of Eastern Anatolia — a young collision zone

A numerical procedure to obtain the dynamic Green's functions for layered viscoelastic media is presented. The procedure is based on numerical evaluation of certain Hankel-type integrals which appear in an integral representation derived previously by the authors. Comparisons illustrating the accuracy and flexibility of the approach are made with a number of solutions obtained by other methods.

On the Green's functions for a layered half-space. Part II

Summary. The computation of theoretical seismograms for models in which the elastic parameters and density vary only with depth (in a plane, cylindrical or spherical geometry) reduces to the solution of an ordinary differential equation plus the evaluation of inverse transformations. In principle, the problem is straightforward. In practice, many techniques and approximations can be used at each stage and many combinations and variants are possible. In this paper, we discuss a new method of evaluating the inverse transforms. Any method can be used to solve the differential equation and we only discuss a few analytic approximations to illustrate the new method. The inverse transformations are a frequency and wavenumber integral. Essentially four techniques can be used to evaluate these depending on the order of integration and whether the wavenumber integral is distorted from the real axis. Three of these have been widely used, but the technique of evaluating the frequency integral first and keeping the wavenumber real is new. In this paper, we discuss some of the advantages of this combination.

A new method for computing synthetic seismograms

Tectonics, seismicity and structure of the Afro-Eurasian junction — the breaking of an incoherent plate

Previous theoretical results are used to produce synthesized seismograms for realistic crustal models and earthquake dislocation sources. Short-period vertical displacement seismograms at Δ = 240 km are calculated. They show strong resemblance to observed recordings, at least for the first 55 sec after the P n arrival. These results are then used for the study of low-magnitude earthquakes from a known source and the tectonics of the source region south of the Sinai peninsula. The source-mechanism found by Ben-Menahem and Aboodi (1969) and the crustal structures given by Niazi (1968) and Drake and Girdler (1964) are consistent with the time series of many short-period seismograms recorded at Eilat since 1969.

Application of synthetic seismograms to the study of low-magnitude earthquakes and crustal structure in the northern Red Sea region

A variant of the Cagniard-Pekeris inversion technique is extended to nonsymmetric earthquake sources of arbitrary multipolar order with a particular emphasis on shear dislocations. In the present work, we concentrate on the infrastructure of the time-domain displacement field in the presence of a single interface. A series of transformations in the complex plane together with a novel decomposition of the total field into three fundamental constituents enables us to separate the real-time contributions due to head waves, geometrical waves and the so-called non-least-time arrivals.

Extension and interpretation of the Cagniard-Pekeris method for dislocation sources

Observed P and S waves from the respective Soviet and Chinese atmospheric explosions of October 30, and 1961 and October 14, 1970 are found to differ both in their S/P amplitude ratios and the form of the individual wavelets. This difference is attributed to the effect of the local topography on the ensuing ground motion at the source. Asymptotic ray theory indicates that an additional horizontal force component was acting at the source of the Chinese event while the Soviet event could be modeled by a vertical force only. 

Generalized ray theory is then applied to calculate the body wave forms in each case. The agreement between the theoretical results and the observations is excellent.

Modeling of atmospheric nuclear explosions over a mountainous region by vertical and horizontal single forces

Near-field seismograms due to finite faulting in a half-space were calculated. An attempt is made to evaluate near-field fault parameters by using two criteria: (1) agreement of observed and theoretical pulse width; and (2) agreement of observed and calculated amplitude ratios of the horizontal components of motion. Using this method we have investigated possible fault parameters of an earthquake along the San Andreas fault (source depth 12.5 km), recorded at two stations (epicentral distance 2.3, 5.5 km). It is found that computed seismograms are strongly dependent on the point in which a unilateral fracture begins. Assuming different initial failure points within the uncertainty region of the hypocentral location may effect fault parameter solutions. Thus, fault parameters obtained in this study should not be viewed as unique. However, we have shown, that for an assumed point and dynamics model, it is possible to determine rupture direction and source parameters. The seismic moment found in this study is weakly dependent on the dynamics model assumed.

Moshe Vered

Papers

Tectonics, seismicity and structure of the Afro-Eurasian junction — the breaking of an incoherent plate

Application of synthetic seismograms to the study of low-magnitude earthquakes and crustal structure in the northern Red Sea region

Extension and interpretation of the Cagniard-Pekeris method for dislocation sources

Modeling of atmospheric nuclear explosions over a mountainous region by vertical and horizontal single forces

Near-field source parameters by finite-source theoretical seismograms