Seismite

Abstract: SUMMARY Fault grading is expressed by a regular structural sequence of (from top) liquified zone, rubble zone and step-faulted zone, with gradational contacts between these zones and the bottom, but with a sharp boundary at the top. It is interpreted as an effect of strong earthquakes on gradationally compacted muds in quiet water basins. Fault-graded beds are geopetal and paleoslope criteria. If interpreted as seismites they may also be used as paleo-seismograms.

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 represents more than 10 times the 4000 years of historical earthquake records. It is 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 structures) associated with slip events on syndepositional faults. The seismites are layers consisting of mixtures of fragmented and pulverized laminae. 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 M L ≥5.5 earthquake. The 230 Th- 234 U ages of a columnar section, obtained by thermal ionization mass spectrometry, give a mean recurrence time of ∼1600 years of M L ≥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.