Prehistoric Seismic Basin Effects in the Dead Sea Pull-apart

Abstract Site effect is the specific response to earthquakes that is characteristic of the attributes of a site. The two- and three-dimensional shape of sedimentary basins may constitute an important factor of site effects. In sediment-filled basins, in which a lens of soft sediments overlies rocks with higher seismic velocities, two-dimensional resonance patterns may prolong the duration of shaking, and induce a large amplification, much larger than the one predicted from the corresponding one-dimensional analysis. The main source of these phenomena is the development within the basins of surface waves, including the vertically and elliptically polarized Rayleigh waves, and horizontally polarized Love waves. The Dead Sea Fault is an active left-lateral transform, posing a major seismic threat to the population in its surroundings, and hence an earthquake-mitigating Building Code was adopted in Israel. However, the present Code does not deal with basin effects. The importance of these is here exemplified in the Dead Sea pull-apart, that generated a destructive M6.2 earthquake in 1927. Within it, seismites dated to 60-16 kyBP represent strong earthquakes, and their thickness is significantly different in three sites, 20 km apart: one near Massada and two in the ‘Amiaz basin. The median thickness is 3, 6 and 12 cm, respectively, presumably reflecting differences in both the duration and amplification of earthquakes. These differences are plausibly explained by assuming that they are caused by basin effects, due to the differences in both the shape of the basins in which the seismites are located (the basin being deeper at the ‘Amiaz sites than at the Massada site), as well as difference in location of the seismites, relative to the midline of the ‘Amiaz basin. Hence, these seismites may constitute the longest and most detailed known record of prehistoric site response.