Changes in Holocene Paleoseismic activity in the Hula pull-apart basin, Dead Sea Rift, northern Israel

Paleoseismic analysis of the Azaz fault segment of the Dead Sea Fault (nort hern Israel), indicates that this region was subjected to intermittent peri ods of strong seismicity and quiescence during the latest Pleistocene and t he Holocene. The Azaz fault forms the eastern margin of the Hula Valley, a pull-apart basin between two left-stepping segments of the Dead Sea Fault. The fault is currently inactive, but it displaces a 25-30 ka travertine in the northeastern corner of the Hula Valley and near-surface marsh sediments inside the valley. Two trenches excavated across a 15-20 In high fault sca rp near Kefar Szold expose fluvial and colluvial sequences showing clear ev idence of recent tectonics. One trench exposes well sorted, bedded alluvial sediments comprising two upward-fining units, each capped by a weakly deve loped paleosol. The sediments were deposited on the down-faulted block and are bounded in the east by the fault scarp. The second trench uncovers fine colluvial sediments cross-cut by a wide fault-parallel fissure filled by c ollapsed sediments. In both trenches, the lower sedimentary unit is capped by a coarse colluvium containing boulders up to 1.5 m in size. The colluviu m shows no clear bedding and has a weakly developed paleosol on top. Optica lly stimulated luminescence dating of the lower fluvial sequence shows that its age ranges between 12 ka at the base to 6 ka at the top, while the mid dle part of the overlying colluvium is ca. 4.8 ka. The relation between the lower fluvial units and the fault indicates that at least two large-scale earthquakes occurred in the Early Holocene, each one resulting in a 1-1.5 m high fault scarp. During this seismically active period, no coarse colluvi al sediments were deposited along the fault trace and thus the region must have had low tectonic relief. The clear contact with the overlying coarse c olluvium reflects a Middle Holocene rapid change to the present high, steep relief. This change was achieved by frequent, strong seismic events, which triggered colluvial processes and prevented stratification and soil format ion on the slope. The soil on the present slope reflects a quiescent period that has lasted at least several hundred years. during which stress has ac cumulated and seismic risk increased. (C) 2000 Elsevier Science B.V. All ri ghts reserved.