Historical sand injections on the Mediterranean shore of Israel: evidence for liquefaction hazard

The abundant silt and sand along the coastal plain of Israel have long been considered susceptible to liquefaction, but previous searches have failed to find field evidence for it. We report the first finding of typical liquefaction features and silty sand injections in trenches that were excavated behind a fourth century Byzantine dam on the Taninim Creek, some 850 m inland of the Mediterranean shore. The trenches revealed a series of flame-shape injections of silty sand that penetrate the overlying clay-rich soil. The injections are largest and most frequent within several meters of the point where the dam is badly damaged on the seaward side, which we interpret as a possible result of a large wave. Three features make the sand injections special: (1) their lower extent is commonly asymmetric with dominant southeastward vergence, away from the breach in the dam, (2) zigzag shapes characterize the upper parts of many injections, and (3) the size and frequency of the injections diminish gradually with distance from the dam until they completely disappear some 100 m away from it. We suggest that the sand injections can be explained by overpressure that was induced either directly by earthquake shaking or by a tsunami wave that breached the dam, filled the reservoir behind the dam and increased the pressure on the water-saturated silt and sand layers and triggered liquefied sand injections. The movement of water sloshing back and forth in the lake accounts for the zigzag shape of the injections. The similarity to structures that were observed in Thailand after the great 2004 tsunami and other palaeotsunami observations lead us to prefer the tsunami origin of the liquefaction features. Based on the stratigraphic position, the archeological context, and the historical accounts, we suggest that an earthquake of November 25, 1759 is the most plausible trigger of the sand injections, either directly or via earthquake-induced tsunami. The observations demonstrate the vulnerability of the densely populated coastal plain to liquefaction.