Sedimentary impacts of recent moderate earthquakes from the shelves to the basin floor in the western Gulf of Corinth
Résumé
In seismically active areas, long term records of large earthquakes are indispensable to constrain reccurence patterns of large earthquakes. In the western Corinth Rift, one of the most active areas in Europe in terms of seismicity, data about ancient earthquakes are still insufficient, despite historical records covering the last two millenia and several studies in onshore paleoseimology. In this paper, we test the use of offshore sediments from the Gulf of Corinth to identify sediment failures and tsunamis that have been triggered by historical earthquakes Two shelves (40-100 m deep), one sub-basin (180 m) and the basin floor (330 m) have been sampled by short gravity cores. The cores were analysed in order to identify and characterize event deposits. The age control has been provided by 137Cs and 210Pb activity measurements showing that the cores represent 2 to 4 centuries of sedimentation. In each site, sandy event deposits are interbeded in the muddy, hemipelagic sedimentation. The age of event deposits has been compared to the record of historical earthquakes using new and published macroseismic data. This comparison shows temporal coincidence of some event deposits and documented earthquakes with a macroseismic intensity ≥ VII in the area, e.g. in A.D. 1861, 1888 and 1909. In nearshore shallow-water settings, the record of event deposits does not exactly fit with the historical record of large earthquakes because too few event deposits are present. This may be due to the absence of sediment failures or to a lower preservation of the deposits in such settings. In the deepest site, in the basin floor, the correspondence
is better: a sandy turbidite probably corresponds to each large earthquake since A.D. 1850, except one aseismic sediment density flow that occurred at the end of the 20th century. Surprisingly, the Ms=6.2, June 15, 1995 Aigion earthquake is only possibly recorded in one nearshore site on the Aigion Shelf, in the form of a tsunami back-wash flow deposit. This study showed that moderate earthquakes (M 5.8-6.5) can significantly impact marine sediments. Regarding the evaluation of seismic hazard in the area, the basin floor is proposed as a promising site for long term paleoseismology in the Gulf of Corinth, while shallower settings need to be considered more carefully.