Francis Lemeille

Co-seismic ruptures and deformations recorded by speleothems in the epicentral zone of the Basel earthquake

The study of growth anomalies of speleothems in a karstic environment can provide potential evidence for palaeoearthquakes. These data are used to study the recurrence times of major earthquakes in areas where evidence for historic seismicity is lacking. A study has been carried out in the epicentral area of the 1356 Basel earthquake (epicentral intensity = VII–VIII, macroseismic magnitude = 6.2). The Battlerloch and Dieboldslochli caves, situated in the area of greatest damage, show growth anomalies of speleothems possibly related to a seismic event (several breaks of speleothems and offsets of the axis of the regrowths). The first U/Th disequilibrum measurements by alpha spectrometry show recent ages (less than several tens of thousands of years and probably historic). 14C dating by AMS of carbonate laminations taken on both sides of the anomalies confirm the evidence of a seismic event around 1300 AD. More accurate darings by U/Th TIMS are carried out in order to compare the information provided by the two different dating methods.

Les Ostracodes et le Quaternaire d’Aigion (golfe de Corinthe, Grèce)

Resume Le Pleistocene superieur et l’Holocene d’Aigion ont livre une microfaune abondante dont 35 especes d’Ostracodes. Le Pleistocene du forage d’Aigion contient generalement des Ostracodes de milieu oligohalin a Cyprideis torosa, Candona angulata et Tyrrhenocythere amnicola tandis que dans l’Holocene dominent les especes marines infralittorales a Cytheridea neapolitana, Carinocythereis whitei, Loxoconcha ovulata et Cytherois frequens. La sedimentation marine s’effectuait a des profondeurs de quelques metres a quelques dizaines de metres. Selon la coupe d’Aghios Constantinos, les niveaux, a caractere lagunaire, oligohalins, sont caracterises par la presence d’Euxinocythere schuldtae et d’une espece naine de Xestoleberis. Un changement radical d’environnement se produit donc a la limite Pleistocene–Holocene et pourrait concerner l’ensemble du golfe de Corinthe.

A Reappraisal of the 1894 Atalanti Earthquake Surface Ruptures, Central Greece

This article presents the results of new field and aerial photo surveys of the Atalanti fault and of the mesoseismal area of the 20 and 27 April 1894 earth- quakes. Coupled with a reanalysis of contemporary reports and previous investiga- tions, these are used to gain a better understanding of the faults responsible for these events and their seismic behavior. The first shock was smaller and probably located inshore or offshore the Malessina peninsula. No resolving field evidence has been found to locate the seismogenic structure responsible for this shock. On the basis of the limited information available, we suggest the Malessina escarpment, a 12-km- long, ENE-trending, NW-dipping fault as a possible structure responsible for this event. On the other hand, the second and largest shock is definitely related to the Atalanti fault sensu stricto, a main WNW-trending, N-dipping active fault extending between the Platirema valley (a few km NW of the town of Atalanti) and Larymna. The total length of the rupture recognized in the field is about 32 km, but it can be extended further SE up to 40 km. No evidence for a longer rupture extending some other 20 km to the NW, between the Karagiozis river and Ag. Kostantinos, is found. The complex geometry of the fault with bends and step overs appears to be controlled by preexisting transverse structures. Minimum coseismic vertical throws, measured in the field after more than a century elapsed from the earthquake, are 30-80 cm, thus consistent with contemporary reports indicating 1-m average. Slip rates are not well constrained. The available estimates fall in the range 0.1-0.5 mm/yr confirming the smaller amount of crustal extension taking place in this area with respect to other nearby regions such as the Corinth gulf. No new data are available to define the average recurrence interval typical of the Atalanti fault. However, a reconsideration of the existing information induced us to rule out the possibility that the famous 426 B.C. earthquake occurred on the Atalanti fault. On the basis of the extent and size of the rupture recognized in the field, a M 6.8 is estimated for the second and largest shock.

Paleoseismological Trenching across the Atalanti Fault (Central Greece): Evidence for the Ancestors of the 1894 Earthquake during the Middle Ages and Roman Times

The Atalanti fault bounds to the southwest the Evoikos Gulf, one of the major extensional basins of central Greece. This fault ruptured during the 1894 earth- quakes, producing at the surface a complex, ca. 30-km-long rupture. Paleoseismo- logical trenching performed at three sites along this fault provided the first insights on its seismogenic behavior. Unfavorable trench stratigraphy and scarcity of datable material made the identification and characterization of individual paleoearthquakes quite difficult. However, by integrating paleoseismological, geological, historical, and archaeoseismological data, we defined three surface-faulting earthquakes. The most recent event is the 1894 earthquake; the penultimate occurred during the Middle Ages between A.D. 770 and 1160, whereas the third event back occurred in Roman times between 50 B.C. and A.D. 230 and is interpreted to be the Opus earthquake of A.D. 105. These results suggest that 1894-type earthquakes repeat each 660-1120 yr. The average minimum slip per event and vertical slip rates are of the order of 45 cm and 0.4-1.6 mm/yr, respectively. These values are in agreement with other geological estimates and with geodetic measurements. Because of the short time elapsed since the 1894 earthquake, the Atalanti fault does not appear to contain an important seis- mogenic potential. On the other hand, these results may shed light on the potential of other seismogenic sources threatening the area.

Traces d'activité pléistocène de failles dans le Nord du fossé du Rhin supérieur (plaine d'Alsace, France)

Abstract In the Upper Rhine Graben, trenches excavated across a 100-km-long geomorphic lineament show that normal faults vertically offset Late Cromerian formations (400–450 ka) by 16 m and Saalian formations (150–300 ka) by 6 m. The re-interpretation of seismic profiles corroborates the existence of normal faults cutting through the sedimentary cover. However, the connection between superficial observations and deep structures remains unclear. This analysis suggests that displacements were coseismic. An average recurrence interval of 25 000 years is obtained for a M =6.0–6.5 magnitude earthquake (single metric displacement) with an inferred vertical deformation rate of 0.04 mm-yr −1 .