Comptes Rendus de l'Académie des Sciences - Series IIA - Earth and Planetary Science最新文献

We report the discovery of a tribosphenic molar in the Purbeck Limestone Group, Early Cretaceous, southern England, which we attribute to a new taxon, Tribactonodon bonfieldi gen.n. sp.n. In addition to the fact that it represents the oldest unequivocal tribosphenic mammal known from Laurasia, this tooth shows characters which have an important bearing on the distinction between the two infraclasses of Holotheria (Australosphenida and Boreosphenida) as defined by Luo et al., and which lead us to question the timing and place of their emergence.

We propose to apply in this note a simple mathematical model for the sedimentation of gravity deposits in deep water. An inverse method has been developed in order to characterize the parameters associated with the submarine flow. This method also provides the initial conditions, which constitute a major uncertainty in geoscience modelling. Application of the inverse method to the 1979 Nice airport collapse (southern France), constrained by cable breaks and/or deposit thicknesses, leads, for example, to the estimation of the initial volume and/or the physical parameters of the flow (friction coefficient, modified turbulent Schmidt number, etc.). The application of the model to this real case shows that only the simultaneous inversion of dynamic (cable break times) and sedimentological (deposit) data allows a satisfactory reconstruction of the flow.

The Neanderthal skeleton from Saint-Césaire (France) shows a fragmentary right femora. We have decided to study the morphology and the metric proportions of this fragment. We have made a comparison with the other Neanderthal femora and with a sample of recent humans (N=181) from European collections. It seems that this fragment of femora is clearly identical to other Neanderthal femora.

The retreat of major ice sheets in North America and northern Europe caused a large-scale isostatic desequilibrium of the lithosphere–asthenosphere system. As a result, formerly glaciated areas started to uplift and lithospheric bending occurred, especially near ice margins. While it is well known that post-glacial uplift continues until present-day in formerly glaciated areas, so has the associated flexure of the lithosphere. This continues to alter the stress state in the areas surrounding the former ice sheet. We have addressed the question of the impact of deglaciation on intraplate stress and seismicity in two separate studies. In one study, we have mapped out and modeled the stress field in the Norwegian sector of the Northern North Sea, a prominent hydrocarbon region in the vicinity of the edge of ice sheet that covered Fennoscandia until $\text{∼20}\text{000}$ years ago. In the other study, we investigated the influence of lithospheric flexure as the possible cause of the anomalously high seismicity in the New Madrid Seismic Zone in the central United States. This was the site of three extremely large (M∼8) earthquakes in 1811–1812, following several previous large earthquakes in Holocene time. As there is no significant deformation of Tertiary sediments in the area, deglaciation appears to have been a trigger for recent seismicity.

Attenuation laws are widely used in order to estimate the peak ground acceleration that may occur at a given locality during an earthquake, for hazard evaluation purposes. However, these simplified laws should be regarded acceptable only in the first approximation, because numerous significant parameters at the local and regional scales are often ignored. We examined the relationship between distance and peak acceleration based on examples from the dense accelerometric network of Taiwan, specifically for the Chichi destructive earthquake. We thus observed significant discrepancies between the predicted and observed accelerations, resulting from (1) near-field saturation, (2) amplification in sedimentary basins, and (3) hanging wall effect. We mapped the residual accelerations (difference between observed and predicted peak ground accelerations). This highlights the role of the regional structure, independently revealed by the geological analysis, as a significant factor that controls the transmission of the seismic accelerations.

The destructive Chichi earthquake occurred in Taiwan on 21 September 1999. This earthquake had exceptional consequences in terms of both the seismic hazard concerns that it raised in the island and the new seismotectonic and geodynamic concepts that arose concerning the Taiwan collision. This earthquake is quite typical of the SE–NW plate convergence. It reactivated a thrust ramp at the front of the belt. The rupture trace shows typical tectonic patterns that control the distribution of the damage associated with fault slip. The importance of geological studies for earthquake mitigation purposes is thus highlighted. Based on two examples (Chelungpu fault for locking and earthquakes, Chihshang fault for creep), the need for a better definition of the seismic cycle in Taiwan is highlighted. The Chichi earthquake suggests that the average shortening velocities are of the same order at the belt front and in the suture zone in the back of the belt. The concept of double convergent boundary is consequently introduced in the lithospheric model of Taiwan.

Two major earthquakes (M_s=6.6) occurred on 17 and 21 June 2000 in the South Iceland Seismic Zone. This paper presents characteristic examples of surface traces of these seismic faults, called the Árnes and Hestfjall faults. The surface trace of the Árnes fault at Mykjunes shows a conjugate strike-slip pattern with a N30°E trending right-lateral fault and a N60°E trending left-lateral fault. Each of these faults is formed by ‘en-échelon’ arrays of fractures and push-ups. The surface trace of the Hestfjall fault near Bitra reveals a N50°–60°E trending left-lateral strike-slip fault including large open fractures, ‘en-échelon’ fractures and some push-ups. These observations show that the actual earthquake-related fault pattern is not restricted to the north–south-trending dextral strike-slip revealed by focal mechanisms and north–south alignment of aftershocks, but includes conjugate systems consistent with the structural pattern of the SISZ.

This paper presents the main characteristics of the historical seismicity in Iceland, as well as some examples of historical earthquakes in the South Iceland Seismic Zone. The geological aspects of the major earthquakes and their social impact are shown. The relationships with the volcanic phenomena and the presence of a hot spot beneath Iceland are discussed. Finally, the contribution of the knowledge of historical seismicity to the understanding of prevention and forecasting is highlighted.

The main shock of the Chichi earthquake took place along the north–south-trending Chelungpu fault between the coastal plain and western foothills of Taiwan. It consisted of five major sub-events with the rupture propagating from south to north. The largest seismic moment is located 30–35 km north of the initial rupture. Waveform inversion results indicate a low-angle thrust fault dipping to the east. The centroid depth is 11–12 km. The seismogenic patterns of aftershocks indicate that the Chichi earthquake sequence can be explained by westward block thrusting on the Chelungpu fault, with strike-slip motions at the northern and southern tips. A deeper seismogenic zone lies 15 km directly beneath the Chichi main fault plane, indicating that the deformation associated with the Chichi event involves deep crust and thus cannot be entirely explained by detachment at shallow crust levels. The result of stress tensor inversion is remarkably consistent with that derived from regional background seismic data before the Chichi sequence, and with the palaeostress pattern reconstructed from geological evidences. It is inferred that the stress regime corresponding to the regional collision in Taiwan is the same as that responsible for the occurrence of the Chichi earthquake sequence. Consequently, we propose to consider the Chichi earthquake sequence as a typical manifestation of the mountain-building process in Taiwan.