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

We study the physics of rupture propagation on a set of active faults using observations of the 28 June 1992 Landers earthquake in California. This very well recorded event provides a wealth of information about the details of rupture propagation on earthquake faults that modify in a fundamental way the simple naive models of earthquake rupture that have been so popular in Earth Sciences. Different methods to invert seismic and geodetic data for the details of the rupture process are discussed and put to work in order to make a model of the Landers earthquake that explains essentially all available data down to a wavelength of about 3 km. It emerges from our studies that earthquakes propagate very rapidly but in a very complex and tortuous way. The kinematic description of the rupture history is used to constrain the parameters of friction on the fault. The numerical simulation completely reproduces the source properties, including wave radiation. A consequence of friction weakening is the existence of a phase of initiation prior to rupture propagation. This phase is associated with specific length and time characteristics. We show that the apparent friction on the fault during large earthquakes is the result of complex interactions between the fault segments. The weakening rate is therefore a scale-dependant property depending on the geometrical properties of the fault system at different scales.

In recent years, observational and theoretical descriptions of spatio-temporal patterns of seismicity have focused on two fundamental (and controversial) observations: static stress (Coulomb) interactions between earthquakes and accelerating seismic moment release before large earthquakes. While there have been several documented examples of static stress changes influencing the space-time pattern of seismicity following great earthquakes (main shocks and aftershocks), there have been few attempts to link this method to the evolution of seismicity before great earthquakes (precursory seismicity and foreshocks). In this paper, we describe a simple physical model that links static stress modeling to accelerating moment release before a large event. For practical reasons, it is not straightforward to apply this technique as a method of forecasting future large earthquakes. However, after the large event has occurred, the region of stress accumulation can be calculated with precision based on the known source parameters of the earthquake. This region can then be examined for seismic moment rate changes prior to the event. As examples, we have examined all M⩾6.5 earthquakes in California since 1950 in regions defined by their pre-event stress fields, and find a period of accelerating moment release before all of these events. While we illustrate the model using seismicity in California, the technique is general and can be applied to any tectonically active region. Where sufficient knowledge of the regional tectonics exists, this method can be used to augment current techniques for seismic hazard estimation.

At 00:02 GMT (03:02 local time) on 17 August, 1999 a magnitude 7.4 (M_s) earthquake occurred 100 km east of Istanbul causing extensive destruction. The event was expected and several scientists have published and attempted to publicize the danger. A paper on stress interactions for NW Turkey (J. Geophys. Res. 103 (1998) 24466–24469) concluded that “by combining the stress change map with the map of active faulting, likely locations for the occurrence of future earthquakes can be refined; faults in the Izmit Bay area, the western part of Biga Peninsula, the Saroz Gulf and a part of western Sea of Marmara must be regarded as posing a specific hazard”. An extension of that study is described here. It is shown that the Izmit (1999) earthquake loaded faults both to the east and west of the Izmit rupture. About three months after the Izmit event an M 7.2 earthquake occurred with an epicenter at Duzce extending the Izmit rupture to the east. In the Marmara Sea, west of Izmit, faults have been loaded by between 1 and 5 bar; 5 to 30 % of typical earthquake stress drops in the region suggesting the likelihood of a future event. The risk of a major event on a fault depends not just on stress increases associated with an individual earthquake, but also on the longer-term earthquake history and on tectonic loading. The roles of both are examined over two time periods from 1900 to 1999 and 1700 to 1999. Whatever interpretation we place on the data we conclude that one or two events as great or greater than the recent one is likely to occur within the next few decades near to the northern coast of the Marmara Sea.

The identification of seismogenic faults in regions with low level seismicity is a major problem in earth sciences. Recent developments of tectonic studies in earthquake-prone areas point out critical questions such as (1) what is the probability of occurrence of a large earthquake (M>6.5) in intraplate Europe? and (2) how much the morphology may record and preserve successive coseismic movements in regions with fault slip rates lower than 1 mm·yr⁻¹? In palaeoseismology, the identification of coseismic movements and related active fault parameters results from integrated field investigations that include geomorphic analysis of fault scarps, geophysical prospecting and trench investigations. Methodologies developed in regions with high seismic strain rates (fault slip rates higher than 1 mm·yr⁻¹, interplate regions) are of primary importance for the fault identification in regions with low level of seismicity (intraplate domains). Examples of active faults illustrate their physical properties and contribute to a better understanding of the faulting behaviour and related seismic hazard.

The destructive ‘Lambesc’ earthquake that struck southeastern France on 11 June 1909 reached maximum MSK intensity of IX. Critical reassessment of macroseismic observations shows that the VIII and VII isoseismal contours surround the Trévaresse ridge, a 15 km long, post-Miocene anticline. A rather steep escarpment, several tens of meters high, marks the southern flank of this fold. A locally steeper cumulative scarp, with, at one place, a south-facing free-faced scarplet, probably marks the emergence of successive surface ruptures, perhaps the last one in 1909. The bulk of the evidence suggests that the 1909 Lambesc earthquake activated a north-dipping thrust ramp below the growing Trévaresse anticline.

New U/Pb analyses obtained with a high-resolution ion microprobe (SHRIMP) fix an age of 608 ±7 Ma for spilites of the Erquy series, in Cadomian rocks of the Armorican massif, France. This Neoproterozoic age re-integrates this unit into the Brioverian, the age it was initially assigned to. A Rb/Sr whole-rock dating in the 1970s had undermined the regional Cadomian model, by suggesting an Ordovician age for these rocks; this was apparently further supported by the discovery of organic remains, interpreted as Palaeozoic microfossils. The reassessment of this palaeontological attribution and the new isotope dating are a final confirmation of the age of this series.

X-ray diffraction experiments were carried out on large ice crystals from accreted ice above Lake Vostok, a subglacial lake lying beneath the East Antarctic ice sheet. Results indicate a surprisingly very low lattice distortion. This crystalline quality does not seem to be affected by impurities. Abnormal grain growth should occur and could explain both the large grain size and the low lattice distortion. Accreted ice is therefore supposed to be non-plastically deforming. These results should be taken into account for further studies of the permeability of accreted ice to drilling fluid present in the borehole.

Between the Calata and Turea marine series, some Priabonian continental intercalations with charophytes and mammals were found. The presence of Cricetids (Rodentia, Muroidea), one of the migrants representative of the Grande Coupure event, lead to the conclusion that in this region, like in Southern Germany, the migration of the Asian elements is earlier than in western Europe.

Hydrogeomorphological, archaeological and geohistorical integrated studies of the very urbanised Nı̂mes area attest of piedmont slopes and torrential alluvial fan existence from the beginning of digging of the Vistre valley since the end of the ancient Pleistocene. The urbanisation is situated on top of a major superior Pleistocene–Holocene alluvial fan. This fan existed during the old Antiquity, and has been widely truncated by the anthropisation during the High Empire. This fan is connected to other little ones, and becomes a colluvial–alluvial fan landform connected to the present flood plain of the Vistre River. It is still functioning now, as during the recent flash flood disaster of 3 October 1988.

We present natural examples and experimental models of volcanic cones located above brittle substratum undergoing regional compressive deformation. The volcanic loading induces a strain partitioning involving deflection and flattening of regional compressive structures. The main control is the topographic load. Anticlinal thrust ridges, observed around many volcanoes, have generally been interpreted as being due to gravitational spreading; however, this study shows that this is not necessarily the case, as they can also be a symptom of regional compression.