Recurrence pattern of Holocene earthquakes along the Dead Sea transform revealed by varve-counting and radiocarbon dating of lacustrine sediments
Introduction
The Dead Sea Transform (DST), which separates the Arabian and Sinai plates [1], [2] (Fig. 1A), has been the locus of tectonic and seismic activity over timescales of several million years to historical periods [2], [3], [4]. A long-standing problem in the tectonic reconstruction of the DST is the apparent gap between the long-term rate of plate movement along the major faults and the seismic moment release [5]. This gap possibly indicates that the seismic activity is not uniform on a historical time scale, with alternating periods of activity and quiescence [6], [7]. More recent estimates of the long-term rate [2], [8] and geodetic measurements of the current rate [1], [9] confirm the gap with modern estimates of the seismic moment release [10]. The temporal alternation between activity and quiescence may be associated with spatial migration of activity between adjacent plate boundaries, as shown for the North and East Anatolian Fault systems [3]. An assessment of this notion requires detailed knowledge on the temporal occurrence of the earthquakes, with constraints on rupture area and magnitudes.
In the present study we examine the spatial and temporal distribution of earthquakes that occurred along the Dead Sea Transform (DST) during the Holocene period. The human development in the Dead Sea basin and the Jordan Valley reflects to a large extent the climatic and tectonic histories. As this region was the locus of human settlement since the early Pleistocene [11], rich historical documentation of earthquake activity is available for the past 2800 years [12], [13], [14], [15], allowing for comparison with the geological evidence of paleo-earthquakes. This evidence appears as disturbances in geological sections of lacustrine sediments that were deposited in the Dead Sea basin during the Holocene. The sedimentary section at the Ze'elim gully Fig. 1, Fig. 2 exposes disturbed sedimentary structures that were correlated with the historical earthquakes of the region [16]. Nevertheless, the exposed Ze'elim section reveals only parts of the Holocene paleo-seismic record because it is located on a terrace, elevated relatively to the level of the Dead Sea during much of the late Holocene [17]. The location of the Ze'elim terrace is sensitive to lake level fluctuations that induced hiatuses during low lake stands. In the Ze'elim record several of the missing major earthquakes lie indeed in periods of low lake stands and sedimentary hiatuses [16], [17].
In the present study, we extracted sediment cores at different sites of recently emerged shorelines (Fig. 1B). These cores recovered sedimentary sections that represent the deeper lacustrine environment of the Holocene Dead Sea. The Ein Gedi site is less sensitive to lake level changes and therefore should contain a continuous depositional and seismite sequence. We anticipated finding in the Ein Gedi core the “missing seismites” from earthquakes that correspond to hiatuses in the Ze'elim section, thus completing the entire historical record. This would provide a crucial test for the assessment of the disturbed sedimentary structures as seismites. The Ein Gedi core penetrated 21 m beneath the 1997 surface of the Dead Sea shore (at 413 m below mean sea level) reaching at its bottom a thick salt layer (Fig. 2), which marks the base of the Holocene at several sites in the region [18]. Thus, the Ein Gedi core comprises the entire Holocene period. Another core representing the lacustrine environment was recovered in the Ein Feshkha site on the north-western side of the Dead Sea (Fig. 1B) and its record is used for comparison with the Ein Gedi core. In addition, we recovered a core next to the exposed section of the Ze'elim gully, which allows us to compare the sedimentary record in a lake versus a nearshore environment. Fig. 1, Fig. 2.
Section snippets
The lithology and chronology of the cores
Textural and mineralogical properties of the cores were examined in thin sections under microscopic binocular. The thin sections were further used for laminae counting.
The Ein Gedi core typically comprises laminated clay-sized clastic sediments and authigenic aragonite and gypsum. The laminae are 0.2–2 mm in thickness, and the petrographic examination reveals clastic laminae, often alternating in couplets with aragonite, or appear as triplets of clastic detritus, aragonite and gypsum. The
Development of a seismite chronology in the Dead Sea sediments
The first earthquake records in the Dead Sea region, using a sedimentary inventory [7], [22], were established from sediments of late Pleistocene Lake Lisan. The Lisan Formation comprises sequences of alternating laminae of authigenic aragonite and silty detritus deposited during enhanced freshwater input to the lake and sequences of sands and silts deposited during low lake stands [16], [23], [24]. This sedimentary pattern is punctuated by sequences with disturbed sedimentary structures that
Temporal distribution of the Holocene seismites
We can use only the deformed sequences of Types I and II for analyzing the recurrence pattern of seismites along most of the core, because the identification of Type-III events is restricted to the counted interval. Within the span of the last 1000 years, ten disturbed sequences could be identified (additionally 3 of Type-III), representing a mean recurrence interval of 100 years. For the 1st Millennium (A.D. 0–1000) only a single seismite of Type-I is identified, so the recurrence interval
Comparison between seismic activity along the DST and the Anatolian faults
The core data provide a means to evaluate the timing of rupture events along the major plate boundaries in the region; the chronology of events can be further applied for understanding of the broad scale elastic coupling. The information from the Dead Sea sediments can be combined with historical catalogues and excavations of fault traces for recovering the actual energy and seismic moment release, which are currently in progress [8], [28], [30]. As discussed by Marco et al. [7], clustering of
Acknowledgements
F0inancial support by the German-Israeli Foundation for Scientific Research and Development (GIF) is gratefully acknowledged. [VC]
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