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G.G.R. Buchbinder 《Physics of the Earth and Planetary Interiors》1976,11(4):P13-P17
PcP and PmKP travel times are computed for three simple or parametric Earth models, based on free-oscillation and travel-time data B1, PEM-A and HB1 and compared with PcP and PmKP travel times from different sources. This comparison is made only for the region above and below the core-mantle boundary and is of interest because of the current search for a standard Earth model. The comparison shows that only model B1 does not need a correction for its PcP travel times. For the PmKP travel times for the three models, corrections of the form Δt = a + bm were obtained. The models need the following corrections for b: ?1.3 for B1, 2.8 for HB1 and 0.6 for PEM-A. The corrections a are shown to be equal to the observed corrections for PcP at large epicentral distances. The inversions of free-oscillation data to obtain Earth models are most successful when body-wave phases that interact with the core are included. 相似文献
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A significant drop in seismic travel times of up to 1.0% occurred in the Charlevoix region between 1979 and 1980, possibly related to the M = 5.0 (Aug., 1979) earthquake in the vicinity. A travel-time drop of this magnitude could have been produced either by the closing of dry or saturated cracks in the upper crustal material or by the saturation of dry or partly saturated cracks. However, the anisotropy of travel-time changes in this area supports the view that this travel-time drop was caused by the closing of water-saturated aligned vertical cracks in the crustal material. Three different crack directions with respect to north were resolved: 0 ° or 90 ° in the Precambrian rocks underneath the St. Lawrence River, −18° or 72° in the shallow rocks (< 5 km) of the Charlevoix crater, region, and +35° or 125° in the Paleozoic cover rocks. Crack closure would require a decrease in the pore volume of the rocks which would be expected to produce subsidence in the Charlevoix area. Since repeated levellings restrict the vertical crustal motion during this time interval to less than 2 cm, we conclude that either the effective aspect-ratio of cracks is less than 0.0001 or the process of crack closure occurred in a number of unconnected regions. More specifically the crack deformation would have to occur in isolated inclusions less than 1 km in diameter and no deeper than about 6 km. The process of crack closure may have been triggered by the passage of seismic waves from the M = 5.0 earthquake. 相似文献
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The Cenomanian–Turonian carbonate-dominated lithofacies of Israel reflect a complex interplay between tectonics, sea-level change, and palaeoecology. Improved correlation based on revision of the bio- and chronostratigraphic framework has enabled the establishment of a sequence-stratigraphic model comprising five sequences delineated by four sequence boundaries, in the Late Cenomanian–Early Coniacian interval. The Late Cenomanian–Turonian succession begins with prograding, highstand, carbonate-platform deposits of the first sequence. Interruption of progradation and drowning of this platform took place within the Late Cenomanian guerangeri Zone (=the vibrayeanus Zone in Israel), resulting in a drowning unconformity which is regarded as a Type 3 sequence boundary (labelled CeUp). The drowning is attributed in part to extinctions in the rudist-dominated biofacies (e.g., Caprinidae), which led to reduced carbonate production and enhanced the impact of the sea-level rise. Similar drowning of Tethyan platforms around the C/T boundary has been linked to the establishment of coastal upwelling and consequent eutrophication. Outer ramp hemipelagic facies (Derorim and the Lower Ora formations) replaced the platform carbonates, thickening substantially southwards in the Eshet-Zenifim Basin of southern Israel. Along the ancient continental slope (Mediterranean coastal plain) evidence of this drowning is obscured by submarine erosion, while in central and northern Israel the drowned section is represented by condensation or a hiatus, reflecting an elevated, sediment-starved sea-floor. A carbonate platform dominated by rudistid shoals (‘Meleke’ Member; Shivta Formation) was re-established in the Judean hills and northern Negev during the middle part of the Turonian coloradoense Zone (local zone T4). Later, during kallesi Zone times (T7), the platform facies prograded southwards towards the Eshet-Zenifim intra-shelf basin. The drowning succession and overlying resurrected carbonate platform are topped in central and southern Israel by a pronounced Type 1 sequence boundary (Tu1) between the kallesi (T7) and ornatissimum (T8) zones (Middle Turonian). In central Israel and northern Negev the sequence boundary is overlain by lowstand deposits of the ‘Clastic Unit’ and by the transgressive and highstand inner to mid-ramp deposits of the Nezer and Upper Bina formations. In the southern Negev the sequence boundary is overlain by lowstand and transgressive systems tracts of mixed carbonates, siliciclastics, and localized evaporites (Upper Ora Formation), and then by mid to inner ramp carbonates of the Gerofit Formation. The latter represents a very high rate of accumulation, indicating rapid, continued subsidence balanced by platform growth. The Tu2 sequence boundary of the Late Turonian is expressed in the southern Negev by a shift from inner ramp carbonates of the Gerofit Formation to outer ramp chalky limestones of the Zihor Formation, indicating localized drowning. The succeeding Co1 sequence boundary again indicates localized drowning of the prograding highstand deposits of the Zihor Formation (‘Transition Zone’) overlain by Lower Coniacian transgressive deposits of the upper part of the Zihor Formation. All of these third-order sequences are expressed in southern Israel, where the rate of subsidence was in balance with sea-level fluctuations. In contrast, the Judean Hills and eastern Galilee areas have a more incomplete succession, characterized by hiatuses and condensation, because of reduced subsidence. More distal areas of continuous deep-water deposition in western Galilee and the coastal plain failed to record the Middle Turonian lowstand, while a longer term, second-order sequence spanning the entire Late Cenomanian–Early Coniacian interval, is present in the Carmel and Yirka Basin areas. 相似文献
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