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This paper re-evaluates the origin of some peculiar patterns of ground deformation in the Central Apennines, observed by space geodetic techniques during the two earthquakes of the Colfiorito seismic sequence on September 26th, 1997. The surface displacement field due to the fault dislocation, as modelled with the classic Okada elastic formulations, shows some areas with high residuals which cannot be attributed to non-simulated model complexities. The residuals were investigated using geomorphological analysis, recognising the geologic evidence of deep-seated gravitational slope deformations (DSGSD) of the block-slide type. The shape and direction of the co-seismic ground displacement observed in these areas are correlated with the expected pattern of movement produced by the reactivation of the identified DSGSD. At least a few centimetres of negative “Line of Sight” ground displacement was determined for the Costa Picchio, Mt. Pennino, and Mt. Prefoglio areas. A considerable horizontal component of movement in the Costa Picchio DSGSD is evident from a qualitative analysis of ascending and descending interferograms. The timing of the geodetic data indicates that the ground movement occurred during the seismic shaking, and that it did not progress appreciably during the following months. This work has verified the seismic triggering of DSGSD previously hypothesized by many researchers. A further implication is that in the assessment of DSGSD hazard seismic input needs to be considered as an important cause of accelerated deformation.  相似文献   
283.
This study reconstructed the weather and its impacts on society for the winter of 1827–1828, focusing on the eastern United States. Data comprise of daily and monthly instrumental records, diaries with both daily and seasonal resolution, newspapers, fur trapper accounts, and tree-rings. Temperature anomalies were calculated and mapped based on the means during the 1820–1840 period to account for different fixed observation times. Precipitation frequencies provided direct comparisons of the 1827–1828 weather with modern climatic normals. Daily plots of temperature also reveal weather variations at daily timeframes. Results indicate that the eastern United States experienced strong positive temperature anomalies that are among the most extreme known in the historical record, particularly its large spatial extent. In contrast, historical evidence reveals strong negative temperature anomalies over northwestern North America, and positive temperature anomalies are evident for coastal Alaska. These temperature anomaly patterns sharply contrast to what is normally experienced during a warm El Niño event. Furthermore, results clearly describe remarkable climatic impacts in the Southeast U.S., including widespread blossoming of fruit trees in mid-winter (false spring) that led to a widespread severe killing frost in early April of 1828. Widespread positive precipitation frequency anomalies are also evident for much of the Southeast U.S., which also played a prominent role on winter vegetation growth. Other weather events and impacts include unusual opening of river traffic in winter in New England, severe flooding in the Mississippi River Valley, and heavy snowfall in northwestern North America.  相似文献   
284.
We propose a model which describes the formation of a strong asperity on a fault. We consider a fault surface which differs slightly from a plane due to a jog-like topographic variation. The fault is placed in an elastic space and is subject to a uniform stress field. The orientation of the fault is such that the normal traction (which is compressive) is greater on the topographic variation, determining a higher static friction and hence an asperity. The value of friction on this asperity depends on the magnitude of shear stress. For times of seismological interest, the increase in shear stress, at rates typical of tectonic processes, does not produce a sensible increase in friction with respect to the adjacent fault segments. A considerable increase in friction and the formation of a strong asperity (or even a barrier) can occur due to repeated seismic-slip episodes on the fault. Slip results in an elastic medium deformation, causing an increase in normal traction on the asperity and hence in friction. This process is described with the aid of a tensile Somigliana dislocation. Regions with high friction undergo partial fracturing of the fault-face material, which can produce fault gouge. The tensile dislocation introduces a small non-double-couple component in the seismic moment of the seismic event, the magnitude of this component depending mainly on the relative size of the asperity.  相似文献   
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Multichannel seismic reflection profiles from the continental rise west of the Antarctic Peninsula between 63° and 69°S show the growth of eight very large mound-shaped sedimentary bodies. MCS profiles and long-range side-scan sonar (GLORIA) images show the sea floor between mounds is traversed by channels originating in a dendritic pattern near the base of the continental slope. The mounds are interpreted as sediment drifts, constructed mainly from the fine-grained components of turbidity currents originating on the continental slope, entrained in a nepheloid layer within the ambient southwesterly bottom currents and redeposited downcurrent.  相似文献   
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During three years preceding the 1976 Friuli earthquake, a continuous southward ground tilt was recorded by a tiltmeter placed near Tolmezzo, 15 km north-west of the epicentre of the impending earthquake. The cumulative ground tilt amounted to as much as 3 minutes of arc. Since the tiltmeter was placed in the proximity of an active fault, such a tilt can be explained if the fault slipped aseismically on its shallower section during the same three year period. Aseismic slip on the fault might have been caused by the same mechanism which concentrated stress in the region and eventually produced the 1976 earthquake.  相似文献   
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