New approaches and recent mehodologies in Paleoseismology

Conventional techniques in paleoseismology.

Different techniques of research in paleoseismology are used to identify primary and secondary evidence of paleoearthquakes. The primary evidence consists of the deformation directly related to the coseismic displacement along the fault (tectonic features). The secondary evidence corresponds to the deformational and earthquake-induced structures associated with the land-shaking (liquefaction, landslide, flooding, ..., etc.).

During the last decades, the paleoseismological investigations were based on the geomorphic studies along a fault zone and trenching across a fault scarp. These two techniques have provided a large amount of high-quality data and outstanding results on the faulting episodes as related to earthquakes.

A - Active faulting and geomorphology.

The geomorphic studies are related to surface deformation along an active fault and to the ratio between tectonic and erosional processes for a given active zone. When associated with successive past earthquakes, the tectonic signature depends on the rate of deformation and it is generally expressed as a landform and scarplet with proeminent topographic relief (a few meters to several hundred meters).

The coupled geological and geomorphological analysis is an important step for characterizing the degree of activity along a fault and for an appropriate selection between seismogenic faults and nonseismically active faults. The site selection for a paleoseismic study depends on the seismotectonic background information and detailed local geological and geomorphological investigations. For instance, the identification of a recent fault scarp, with a few meters of vertical offset preserved along a fault, constitute unequivocal signs for the occurrence of recent large earthquakes in the past.

Most major fault scarps with prominent vertical offsets result from a succession of smaller scarps (multiple or composite scarps) which correspond to repeated faulting and coseismic displacements affecting superficial sedimentary units (soft sediments). These scarps will progressively degrade over a time range as a result of the climatic effects (and undergo erosional processes). Knowing the rate of erosional transport across a scarp, the modelling of the fault scarp degradation using a diffusion equation may provide a good estimation of the fault scarp age (Andrews and Hanks, 1985).

B - Trench-exposures.

The trenching consists on an excavation of a fault exposure [with a depth of » 4 m and a length ranging from 10 to 100 m] which permits a detailed logging of the trench-walls (using 1 m² wire grid placed on the wall) and a precise reconstruction of the successive coseismic displacements in the past. The interaction between sedimentary processes (erosional/depositional) and tectonic deformation can be described, with a particular attention given to primary as well as secondary evidences for the paleoseismic activity. A collection of samples from each significant sedimentary unit can be analysed for both the mechanical behaviour tests (laboratory tests) and radiometric dating (¹⁴C dating being the most commonly used in the case of samples with rich carbon content). This technique is presently utilized worldwide in the active zones.

C - Secondary evidence can be of primary importance.

Sometimes, when active faults do not appear at the surface, various evidence of earthquake-induced effects can be used to determine the occurrence of damaging earthquakes. This evidence can also be correlated to the successive coseismic displacements observed along visible surface faulting and in trenches.

• Liquefaction, flooding and landslides. An extended literature exists concerning the 1811-1812 large earthquake sequence of New Madrid, in the continental interior of the American plate (central U.S.A.). The largest magnitude attributed to the earthquake (M ( 7.8 - 8.3 ) is partly based on the detailed study of liquefaction features which correspond to an upward-directed hydraulic force suddenly applied to wet and saturated soft sediments (McCalpin, 1996). Paleoearthquake-induced flooding comparable to the one related to the Ms 7.3 El Asnam earthquake (Atlas mountains of Algeria) were also retrieved in trench-excavations by means of successive flood deposits; these deposits were also warped and faulted during the Holocene (Meghraoui and Doumaz, 1996). In the absence of any direct evidence of faulting, these effects that may also include landslides are extensively described in several seismogenic zones and they constitute a powerful tool for the retrieval of past earthquakes.
• Speleothems. Deformation (warping), ruptures and differential growth of stalagmites and stalactites in caves and buried ancient constructions can be a remarkable source of information on the occurrence of historic and prehistoric earthquakes. Postpichl et al. (1991) developped a methodology using the dating of different generations of tilted and collapsed speleothems attributed to paleoearthquakes of central Italy. Cave deposits in Europe typically range in age from present-day to over 10 My; precise Uranium/thorium series dating of speleothem calcite allows to place in absolute time scale events that occurred up to 500,000 years Before Present, which commonly are not preserved at the surface. Meso- and microstructural characteristics, morphology, textural and sedimentological features of cave deposits enable us to recognize and characterise any post-depositional disturbances. When correlated with historical seismic catalogues and paleoseismological analyses, the study of speleothems provides substantial data on the size, extension and return period of large earthquakes in a region.
• Archaeoseismology. This method is applied in regions of rich archeological buildings, monument and man-made objects around the Mediterranean and can also be applied in some other regions of Europe. It consists on the evaluation of archeologic damage or faulted archeologic sites as demonstrated in deformed ancient graves in Greece (Stiros and Jones, 1996), and the detailed study of the Nimes Roman aquaduct in France (Levret et al., 1996). Although some controversies exist on the seismic versus nonseismic causes of damaged ancient constructions, the study of archeological sites represents an important source of information for the identification of paleoearthquakes.