Long-term earthquake clustering: A 50,000-year paleoseismic record in the Dead Sea Graben

Partners:

Amotz Agnon and   Hagai Ron, Institute of Earth Sciences, Hebrew University, Jerusalem

Mordechai Stein, The Geological Survey of Israel

Publication: J. Geophys. Res. 1996 V101(B3): 6179-6192 (pdf file)

Abstract

The temporal distribution of earthquakes in the Dead Sea Graben is studied through a 50,000-year paleoseismic record recovered in laminated sediments of the Late Pleistocene Lake Lisan (paleo-Dead Sea).  The Lisan Formation contains the longest and most complete paleoseismic record along the Dead Sea Transform and possibly the longest continuous record on Earth.  It includes unique exposures of seismite beds (earthquake-induced sedimentary structures) associated with slip events on syndepositional faults.  The seismites are layers consisting of mixtures of brecciated and pulverized laminae, which we call "breccia layers".  The places where the seismites abut syndepositional faults are interpreted as evidence for their formation at the sediment-water interface during slip events on these faults.  Thicker sediment accumulation above the seismites in the downthrown blocks indicates that a seismite formed at the water-sediment interface on both sides of the fault scarps.  Modern analogs and the association with surface ruptures suggest that each seismite formed during a ML≥5.5 earthquake.  The 230Th-234U ages of a columnar section, obtained by thermal ionization mass spectrometry, give a mean recurrence time of ~1600 years of ML>5.5 earthquakes in the Dead Sea Graben.  The earthquakes cluster in ~10,000-year periods separated by quiet periods of similar length.  This distribution implies that a long-term behavior of the Dead Sea Transform should be represented by a mean recurrence of at least 20,000 year record.  This observation has ramifications for seismic hazard assessment based on shorter records.

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Typical view of Lisan Formation below the cliff of Masada. Vertical exposures are about 40 m.

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View of the Peratzim Creek at the PZ site.

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Interpretation:

Top: A typical breccia layer, formerly called "mixed layers", overlying laminated layers. The breccia shows graded deformation from folded strata at the bottom, through fragment-supported texture, to matrix-supported texture at the top.  Undisturbed, post-seismic, layers overlie the breccia layer. The event horizon (marked EH) is the sediment-water interface at the time of the earthquake.

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Amotz is showing a syndepositional normal fault in the Lisan Formation near Masada. The fault is overlain by undisturbed layers. Breccia layers (formerly called "mixed layers") terminate at the fault in every block. The lower breccia layer in the downthrown block here contains a large vertically-laminated chunk, which probably fell off the subaqueous fault scarp. Because of such scarps, sediment accumulation above the breccia layers in the downthrown block is thicker than in the footwall.

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Interpretation of the fault-breccia layer association. (A) Laminated sediments are deposited at the bottom of Lake Lisan. (B) A fault ruptures the surface creating a subaqueous scarp. The coseismic movements deform, fluidize, and resuspend the sediment. A breccia layer forms on both sides of the fault scarp when the suspended sediments resettle. The breccia layer in the downthrown block is thicker. (C) Another slip event on the same plane offsets the sequence again, forming a new fault scarp.

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A group of three breccia layers, part of a cluster.

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The distribution of breccia layers (i.e., earthquakes) along section PZ1; open diamonds show the individual layers. Left: Yellow squares and red circles are the number of earthquakes per 5 kyr sliding window, shifted by 2-kyr increments leaving 3-kyr overlap. The crosses show the distribution when the ages of the breccia layers are calculated by a single linear regression, whereas the circles show the distribution when the ages are calculated in three segments. The distribution shows two clusters of frequent events. A cycle about 20-kyrs-long includes a cluster period and a quiet intercluster period. The pattern of clustering is independent of our choice of calculated sedimentation rates. Right: The distribution of individual breccia layers along sections PZ1, PZ2, and M1. The top of the Lisan is used as a datum. All three sections show clusters of breccia layers separated by quiescent intervals.

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