Shmuel Marco

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

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.

Prehistoric earthquake deformations near Masada, Dead Sea graben

Earthquake-inducedfluidizations and suspensions of lake sediments, associated with syndepositionalfaults,formapaleoseismicrecordintheDeadSeagraben.Theassociation offluidized beds with surface faulting supports the recognition of mixed layers as reliable earthquake indicators and provides a tool for the study of very long term (>70 kar) seismicity along the Dead Sea transform. The faults compose a fault zone that offsets laminated sediments of the late Pleistocene Lake Lisan. They exhibit displacements of as much as 2 m. Layers of massive mixtures of laminated fragments are interpreted as disturbed beds, each formed by an earthquake. The undisturbed laminated layers between these mixed layers represent the interseismic interval. A typical vertical slip of about 0.5 m per event is separated by several hundred years of quiescence. The fault zone lies within the Dead Sea graben, 2 km east of Masada, where archaeology and historical accounts indicate repeated strong earthquake damage. The distribution of strikes in the fault zone resembles that of the faults exposed in and around the graben, including the seismogenic ones. The excellent exposures over hundreds of metres allow an unprecedented temporal and spatial resolution of slip events on faults.

Crusader castle torn apart by earthquake at dawn, 20 May 1202

The Crusader castle of Vadum Jacob, an outpost overlooking the Jordan River, was deformed during a destructive earthquake triggered by motion along the Dead Sea Transform. The M >7 earthquake occurred at dawn, 20 May 1202, and offset the castle walls by 1.6 m. This exceptional precision in dating and estimating displacement was achieved by combining accounts from primary historical sources, by excavating the Dead Sea Transform where it bisects the castle, and by dating faulted archaeological strata. The earthquakes of October 1759 and/or January 1837 may account for the remaining 0.5 m out of a total 2.1 m of offset. Our study exploits the potential embodied in interdisciplinary historical-archaeological-geological research and illustrates how detailed histories of seismogenic faults can be reconstructed.

817-Year-old walls offset sinistrally 2.1 m by the Dead Sea transform, Israel

Abstract Archeological excavations in the Crusader Ateret Fortress near the Jordan River exposed E-W trending walls displaced sinistrally up to 2.1 m by the Dead Sea transform fault. A water duct, probably of Crusader age, is also offset sinistrally across the fault by about 1–2 m, but newer water ducts parallel to the former one show no displacement. The maximum width of the fault zone is about 10 m. Post-Crusader structures show significantly less deformation, and together with the low seismic activity, suggest there has been negligible creep. It is therefore conceivable that in this particular fault segment, stress is occasionally relieved by strong destructive earthquakes associated with surface ruptures. Historical accounts include descriptions of post-Crusader earthquakes in the northern part of Israel in A.D. 1202, 1546, 1759, and 1837. These events caused destruction and casualties over large areas. We conclude that most of the displacement of the Ateret Fortress walls occurred during one of these strong earthquakes, probably that of 1202 A.D., and some additional offset occurred during subsequent events. The associated magnitude is estimated at 6.5–7.1. The Ateret site is extremely valuable for paleoseismic studies in general, and assessment of seismic hazard to nearby population centers in particular, as there is an abundance of well-dated man-made structures and a small number of candidate earthquakes.

Archaeology, history, and geology of the A.D. 749 earthquake, Dead Sea transform

Historical records of earthquakes can contribute significantly to understanding active faulting and seismic hazards. However, pre–twentieth century historians were unaware of the association of earthquakes and fault ruptures. Consequently, historical texts usually report the time and damage caused by earthquakes, but not the associated faults. Conversely, observed fault ruptures are often difficult to date. In order to overcome these difficulties, we have analyzed archaeological and sedimentological observations in recent excavations in the ancient city of Tiberias and have combined them with interpretation of historical accounts. Tiberias was founded in A.D. 19 by King Herod on the western shore of the Sea of Galilee (Kinneret). Herod9s stadium, exposed in these excavations for the first time, was damaged by boulder-bearing flash floods and by an earthquake. Later buildings, dated as late as the early eighth century, are all covered by alluvium and lake deposits. They are also damaged and offset by normal faults, whereas buildings from the late eighth century are intact. We therefore attribute the damage to the earthquake of 18 January 749. The paleoseismic observations are in good agreement with the distribution of damage on the basis of historical records. Both data sets indicate a 100-km-long rupture segment between the Kinneret and the Dead Sea pull-apart basins, demonstrating that it is capable of generating M > 7 earthquakes.

A large-scale radial pattern of seismogenic slumping towards the Dead Sea Basin

Although it has been tacitly assumed since the seminal work of Jones in the 1930s that slump folds bear a systematic and meaningful relationship to the slope upon which they were presumably created, there has in reality been very little attempt to objectively verify this association via the collection of regional slump data in a relatively controlled setting. The potential to walk around the intact Dead Sea Basin at c. 425 m below mean sea level provides a perhaps unparalleled opportunity to undertake such verification via the direct examination of slump fold relationships. The collection of slump data in this well-constrained environment, where the seismogenic trigger for slumping is established via earthquake records, and the palaeogeographical controls are also recognizable and clearly link to the present bathymetry and landscape, thereby permits an evaluation of the use of slump folds as indicators of palaeoslope. The Late Pleistocene Lisan Formation cropping out to the west of the Dead Sea contains superb examples of slump folds that systematically face (>95%) and verge (>90%) towards the east. This study employs and evaluates five statistical techniques, including a new mean axial-planar dip (MAD) method, to analyse relationships between the orientation of slump folds and palaeoslopes. We recognize for the first time that the direction of slumping inferred from slump folds and thrusts varies systematically along the entire c. 100 km length of the western Dead Sea Basin. SE-directed slumping is preserved in the north, easterly directed slumping in the central portion and NE-directed slumping at the southern end of the Dead Sea. They are interpreted to form part of a large-scale and newly recognized radial slump system directed towards the depocentre of the precursor to the Dead Sea, and to be triggered by earthquakes associated with seismicity along the Dead Sea Fault.

A 40,000 year unchanging seismic regime in the Dead Sea rift

We studied breccia beds in lacustrine sediments within the active Dead Sea basin. The beds were deformed by M >5.5 earthquakes during the past 60 k.y. Our new analysis considers both the thickness of breccia beds and the lithology of beds directly overlying them in order to identify 11 M >7 earthquakes that originated within the Dead Sea pull-apart between 54 and 16 ka. The resulting time series is a unique long record of earthquakes in a well-constrained segment of a fault system in which the time interval between consecutive earthquakes increased from hundreds of years to a background recurrence interval of ∼11 k.y. since ca. 40 ka. Since this recurrence interval is similar to the M ≥7.2 recurrence interval in the Dead Sea basin, as extrapolated from present seismicity, we suggest that the present seismic regime in the Dead Sea basin, as reflected in its magnitude-frequency relation as well as in its deficiency in seismic moment, has been stationary for the past ∼40 k.y. Since the increasing interval between consecutive earthquakes in the studied segment of the Dead Sea fault is time-logarithmic, it may be a result of healing of the brittle crust as well as a diminishing strain rate following the first strong earthquake in the sequence.

HIGH-RESOLUTION RECORD OF GEOMAGNETIC SECULAR VARIATION FROM LATE PLEISTOCENE LAKE LISAN SEDIMENTS (PALEO DEAD SEA)

We measured geomagnetic secular variation in Lake Lisan sediments (paleo Dead Sea). More than 1500 oriented samples were collected from a 27.3-m section of alternating aragonite and detritus laminae in the Dead Sea basin ranging in age from 67 to 32 ka. The natural remanent magnetization (NRM) is carried by titanomagnetite in the detrital laminae whereas the aragonite is diamagnetic. The NRM is very stable and was acquired several hundred years after deposition. The mean direction of 878 horizons is DD 005o, ID 45o (95D 1o;D 22). We observed three modes of directional geomagnetic variation as a function of (and by inference, time): very rapid inter-sample changes, slow variation in mean direction, and inclination shallowing of about 1o=m. The overall rate of change in direction is 0:57 0:57o=year, not significantly different from zero. For about 83% of the record the rate of change is less than 1o=year and comparable to historical values. High rates of change are observed more frequently in the Lisan than in historical records, and peak rates are up to ten times faster. A smoothed curve resulting in a maximum rate of change of 0.66o=year and a mean 0:10 0:10o=year may be a more realistic representation of the field behavior. No reverse NRMs were observed, but geomagnetic field excursions may be present where the VGPs deviate by more than 40o from the geographic north at about 52 and 41 ka; the latter may represent the Laschamp event.

Quantitative analysis of seismogenic shear-induced turbulence in lake sediments

Spectacular deformations observed in lake sediments in an earthquake prone region (Lisan Formation, pre–Dead Sea lake) appear in phases of laminar, moderate folds, billow-like asymmetric folds, coherent vortices, and turbulent chaotic structures. Power spectral analysis of the deformation indicates that the geometry robustly obeys a power-law of –1.89, similar to the measured value of Kelvin-Helmholtz (KH) turbulence in other environments. Numerical simulations are performed using properties of the layer materials based on measurements of the modern Dead Sea sediments, which are a reasonable analogue of Lake Lisan. The simulations show that for a given induced shear, the smaller the thickness of the layers, the greater is the turbulent deformation. This is due to the fact that although the effective viscosity increases (the Reynolds number decreases) the bulk Richardson number becomes smaller with decrease in the layer thickness. The latter represents the ratio between the gravitational potential energy of the stably stratified sediments and the shear energy generated by the earthquake. Therefore, for thin layers, the shear energy density is larger and the KH instability mechanism becomes more efficient. The peak ground acceleration (PGA) is related to the seismogenic shear established during the earthquake. Hence, a link is made between the observed thickness and geometry of a deformed layer with its causative earthquake9s PGA.

Precision of calibrated radiocarbon ages of historic earthquakes in the Dead Sea Basin

The precise determination of the age of historical and geological events by radiocarbon dating is often hampered by the long intersection ranges of the measured data with the calibration curve. In this study we examine the possibility of narrowing the calibrated range of the 14 C ages of earthquake-disturbed sediments (seismites) from the Late Holocene lacustrine section in the Dead Sea Basin. The calibrated ranges of samples collected from seismites were refined by applying stratigraphic constraints and tuning the calibrated ranges to known historical earthquakes. Most of the earthquakes fall well within the 1 σ error envelope of the 14 C age. This refinement demonstrates that the lag period due to transport and deposition of vegetation debris is very short in this arid environment, probably not more than a few decades. This assessment of seismite 14 C ages attests to the validity of 14 C ages in Holocene sediments of the arid area of the Dead Sea. Furthermore, it demonstrates our ability to achieve highly precise (correct to within several decades) 14 C ages.