Abstract

Paleoseismic investigations of the Oued Fodda fault, which underwent surface-fault rupture during the Algeria earthquake of October 10, 1980 (Ms 7.3), indicate that the average recurrence interval based on the long-term geologic slip rate (past 100 ka) is at least an order of magnitude longer than the intervals between recent surface-faulting events, which are determined by radiocarbon dating of faulted colluvial deposits exposed in trenches across the fault. Evidence from trenches excavated across the 1980 surface rupture indicates that there have been at least three and possibly four surface-faulting events on the Oued Fodda fault during the past 1.5 ka (including the 1980 earthquake). The ages of the three most recent events are well constrained. The most recent event occurred in 1980; a surface-faulting event occurred before 320 ±220 C-14 yr B.P. (radiocarbon years before 1950) and after 875 ±240 yr B.P., and another event occurred just prior to deposition of a layer containing charcoal dated at 875 ±250 yr B.P. (i.e., about 900 yr ago). The average interval between the three most recent surface-faulting events on the Oued Fodda fault was about 450 yr. The cumulative slip on the Oued Fodda fault was produced by relatively short episodes characterized by frequent displacements (surface-faulting events every few hundred years) separated by long periods of quiescence lasting several thousands to tens of thousands of years.

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