In this paper it is shown that any recorded motion has an imaginary counterpart. Starting from the harmonic input motion where the sine function is the imaginary counterpart of the cosine function, it is shown that all time histories exist in pairs. The imaginary counterpart of records is not needed when the response of a system is described by a real-parameter constitutive model. However, it is shown that when complex-parameter models are used, the imaginary counterpart of records has equal importance as the real-valued record and in this case the input motion should be strictly complex valued. The procedure to construct the imaginary counterpart of any piecewise continuous record is developed. A constructed complex-valued earthquake time history is used as input motion to a system described by a Kelvin model with complex damping constant. The validity of the procedure is demonstrated by comparing the response of the complex-damping Kelvin model against the results obtained from a mathematically equivalent real-valued formulation. 相似文献
Combined gravity and seismic data from Greece and the adjacent areas have been used to explain the high seismicity and tectonic activity of this area. Computed 2-D gravity models revealed that below the Aegean region a large “plume” of hot upper-mantle material is rising, causing strong attenuation of the crust. The hot “plume” extends to the base of the lithosphere and has very probably been mobilized through compressional processes that forced the lithosphere to sink into the asthenosphere. The above model is supported by: high heat flow in the Aegean region; low velocity of the compressional waves of 7.7 km/sec for the upper mantle; lower density than normal extending to the base of the lithosphere; teleseismic P-wave travel-time residuals of the order of +2 sec for seismic events recorded at the Greek seismic stations; volcanics in the Aegean area with a chemical composition which can be explained by assuming an assimilation of oceanic crust by the upper mantle; deep seismicity (200 km) which has been interpreted by various authors as a Benioff zone. 相似文献
The previously published results of a deep seismic refraction study of the Dead Sea—Gulf of Elat rift show crustal thinning underneath the rift and the presence of a 5 km thick velocity transition zone in the lower crust along the rift. The structural interpretation of the first-arrival data was revised using the detailed velocity-depth distribution.The revised crustal thicknesses are 35 km near Elat and 27 km, 160 km south of Elat.The crustal thinning and the presence of the velocity transition zone are interpreted as being the result of intrusion of upper mantle material into the lower crust, possibly representing the initial shape of the processes which have been active further south in the Red Sea since earlier times. 相似文献
An on-/offshore seismic network consisting of 36 three-component stand-alone digital stations was deployed in the area of the Saronikos Gulf, in the vicinity of Athens (Greece), in the fall of 2001. In the present study, from an initial set of more than 1000 micro-earthquakes, 374 were selected and 6666 P- and S-wave arrivals were inverted, based on a 3D linearized tomography algorithm, in order to determine the 3D velocity structure of the region.
The resulting 3D velocity distribution, in agreement to the micro-seismicity distribution, reflects the Saronikos structure down to a depth of 12 km. So, the neotectonic basin of the Saronikos Gulf is divided in two parts by a central platform, which implies the existence of a NNE–SSW-trending rupture zone. This zone is probably the offshore extension of a large thrust belt dominating the adjacent onshore areas. Due to their different structure, the two basins are dominated by different velocity values in comparison to the central platform.
The western part is characterised by higher seismic activity than the eastern one. Furthermore, the western Saronikos Gulf is divided in a northern and a southern part by a well-defined rupture zone trending E–W. This seems to be the extension of the Corinthiakos Gulf fault zone. At the depth of 17 km, the velocity increases considerably and the crustal thickness is restricted down to 20 km. This ‘unexpected’ low thickness in the region of Saronikos Gulf seems to be the result of the extensional stress field, which dominates the region, as well as of the emergence of the mantle material along the volcanic arc, which clearly appears at the depth of 12 km. Yet the lack of deep events and, hence, the poor resolution below the depth of 17 km does not support a definite conclusion about the crust–mantle boundary in this region. 相似文献
Two new seismic refraction/wide-angle reflection profiles demonstrate that the crust beneath the southern Porcupine Seabight, out to water depths in excess of 4000 m, is of continental type. They also reveal the rifted margin of the Porcupine basin on its eastern side. Crustal thickness under the Seabight, inclusive of sediments which are up to 6 km thick, decreases from 23 km in the east to about 10 km at a sharp continent-ocean transition in the west. 相似文献
A seismic refraction/wide-angle reflection experiment was undertaken in the Levant Basin, eastern Mediterranean. Two roughly east–west profiles extend from the continental shelf of Israel toward the Levant Basin. The northern profile crosses the Eratosthenes Seamount and the southern profile crosses several distinct magnetic anomalies. The marine operation used 16 ocean bottom seismometers deployed along the profiles with an air gun array and explosive charges as energy sources. The results of this study strongly suggest the existence of oceanic crust under portions of the Levant Basin and continental crust under the Eratosthenes Seamount. The seismic refraction data also indicate a large sedimentary sequence, 10–14 km thick, in the Levant Basin and below the Levant continental margin. Assuming the crust is of Cretaceous age, this gives a fairly high sedimentation rate. The sequence can be divided into several units. A prominent unit is the 4.2 km/s layer, which is probably composed of the Messinian evaporites. Overlying the evaporitic layer are layers composed of Plio–Pleistocene sediments, whose velocity is 2.0 km/s. The refraction profiles and gravity and magnetic models indicate that a transition from a two layer continental to a single-layer oceanic crust takes place along the Levant margin. The transition in the structure along the southern profile is located beyond the continental margin and it is quite gradual. The northern profile, north of the Carmel structure, presents a different structure. The continental crust is much thinner there and the transition in the crustal structure is more rapid. The crustal thinning begins under western Galilee and terminates at the continental slope. The results of the present study indicate that the Levant Basin is composed of distinct crustal units and that the Levant continental margin is divided into at least two provinces of different crustal structure. 相似文献
A fracture mechanics model is developed for the initiation and propagation of a crack through a porous ice layer of finite thickness under gravitational overburden. It is found that surface cracks generated in response to a tidally induced stress field may penetrate through the entire outer brittle layer if a subsurface ocean is present on Europa. Such penetration is found to be very unlikely in the absence of an ocean. A cycloidal crack would then form as a sequence of near instantaneous discrete failures, each extending roughly the brittle layer thickness in range, linked with a much lower apparent propagation speed set by the moving tidal stress field. The implications of this porous ice fracture model for ice-penetrating radar scattering loss and seismic activity are quantified. 相似文献