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Quaternary transform kinematics constrained by sequence stratigraphy and submerged coastline features: The Gulf of Aqaba

Earth and Planetary Science Letters, Vol. 271, Issue: 1-4, July 15, 2008. pp. 109-122

Makovsky, Yizhaq; Wunch, Assaf; Ariely, Ronen; Shaked, Yonathan; Rivlin, Assaf; Shemesh, Aldo; Ben

The Dead Sea Fault System (DSFS) steps-over at the northern head of the Gulf of Aqaba (GOA), crossing the evolving continental shelf. We report detailed processing and interpretation of high resolution sub-bottom profiles of the north western tip of the GOA down to a depth of about 120 m. Our data reveal stepping seafloor morphology comprising a series of relict coastline features, primarily fossil reefs whose present depth results from the combined effect of sea level rise and tectonic…

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Depth Imaging of the Levant Basin, Offshore Israel Using 2D, Seismic Reflection Data

Trachtman P. 1, Gardosh M. 1, Ariely R. 1

1. Geophysical Institute of Israel, Lod, Israel

High-quality, regional seismic reflection lines were recently acquired in the Eastern Mediterranean, offshore Israel.  This project was aimed to create depth images from these seismic lines, previously processed in time domain.    

Four seismic lines, totaling 650 km, were selected for reprocessing. In an initial step a full, standard CMP-processing flow was applied.  This flow included pre-stack time migration procedure (Kirchhoff algorithm) that was used to create fully migrated but not imaged data.

In a second step we applied a pre-stack depth migration technique. PSDM is essential to seismic reflection imaging in areas of steeply dipping structures or extreme lateral velocity variations (salt layers). Both conditions are found in the subsurface of the Levant Basin, offshore. A 3D velocity model was built first, guided by existing seismic lines and well information. While these methods provide a sufficient velocity model to produce a preliminary image, a horizon velocity analysis, performed in several iterations was applied to improve the model and to produce the final depth images.

The resulted pre-stack depth migrated profiles provide more accurate images of the subsurface and can be directly used for the construction of geological and structural sections.

 

הדמיית עומק של אגן הלבנט בעזרת קווי רפלקציה סיסמית דו- מימדיים.

טרכטמן פ.(1), גרדוש  מ. (1),, אריאלי ר. (1),

1.  המכון הגיאופיזי לישראל, לוד, ישראל.

מטרת העבודה הייתה לקבל הדמית עומק של אגן הלבנט, מול חופי ישראל.  לצורך כך נעשה שימוש בקווים סיסמיים רגיונליים חדשים אשר עובדו בעבר במימד הזמן.  בעיבוד החדש אשר בוצע בשיטת PSDM  הוכנו ארבעה קוים במימד העומק אשר ניתן להשתמש בהם ישירות להכנת חתכים גיאולוגים וסטרוקטורליים

 

Evidence for large recent earthquakes in offset relic coast line features offshore Elat

Makovsky Y. 1, Agnon A. 2, Ben Avraham Z. 1, Ariely R.1, Shaked Y.2, Shemesh A. 3

1. Department of Geophysics & Planet. Sciences, Tel-Aviv University, Ramat Aviv, Tel-Aviv, Israel
2. Insititue of Earth Sciences, Hebrew University, Givat Ram, Jerusalem, Israel
3. Department of Environmental Sciences, Weizmann Institute, Rehovot, Israel

Abundant Paleoseismological evidence allows the reconstruction of recent large-earthquake record along the central and northern Dead Sea fault system (DSFS). However to the south the DSFS becomes primarily submarine, archeological record is scarce, and the paleoseismological reconstruction has so far been inhibited. In this study we exploit cross cutting relationship of the DSFS with relic coast line features to obtain a record of recent large earthquakes in the vicinity of Elat. To study the submarine extent of the DSFS we acquired a grid of acoustic Chirp single-channel high-resolution reflection profiles offshore Elat to a depth of 120 m. We then used these data to examine with high-resolution the seafloor morphologic patterns. The seafloor offshore Elat is stepping down in a series of gentle slopes and sharp drops, the most prominent step at a depth of about 100 m. We suggest that the stepping structure constitute of relic coastline features (e.g. reefs and alluvial fans) that reached their current depth due to eustatic sea level rise and tectonic slip. The stepping structure is crossed and offset by linear features we interpret to be faults of the DSFS. In particular a north-striking (ca. 2 m high) seafloor step is truncating and offsetting by about 10 m down to the east an east-striking elongate ridge, which we believe to be a fossil reef. The reflection profiles suggest that the seafloor step is the surface expression of a fault with about 10 m down to the west sense of shear. A set of sedimentary wedges imaged on the hanging wall suggest three events that offset the seafloor by about 2 m each. The relatively sharp seafloor expression of this fault and the presence of open cracks on the seafloor indicate recent activity on this fault. The undisturbed sedimentary cover is about the same thickness of the offset sedimentary wedges, which may suggest that the next event is near. Our data provides only relative dating of events and direct sampling of cores is required to provide absolute dating of fault activity.

Geophysical imaging of an active strand of the Carmel fault: A contribution to seismic hazard assessment

Kanari M.(1), Ariely R.(1), Harkavi A.(1), Meiler M.(1), Shapira S.(1), Yelin G.(1), Politi M.(2), Shaanan U.(2), Shaar R.(2), Steinberg J.(2,4), Bakun D.(3), Bar O.(3), Novitsky R.(3), Yagoda, G.(3), Wald, R.(4), Shtivelman V.(4), Goldman M.(4), Rybakov M.(4), Agnon A.(2), Feinstein S.(3), Marco S.(1)

1. Dept. of Geophysics and Planetary Sciences, Tel Aviv University, Tel Aviv 69978, Israel
2. The Institute of Earth Sciences, The Hebrew University, Givat Ram, Jerusalem 91904, Israel
3. Dept. of Geological and Environmental Sciences, Ben-Gurion University, P.O.B. 653, Beer-Sheva 84105, Israel
4. The Geophysical Institute of Israel, P.O.B. 182, Lod 71100, Israel

We performed a series of geophysical profiles in order to constrain the precise location of the active strand of the Carmel fault. This fault, which takes up a portion of the sinistral movement of the Dead Sea fault, is considered a major source of seismic risk for the populated and industrial area of Haifa bay. We have re-analyzed published seismic lines and surveyed geomorphic features in order to better define the fault trace at the surface. The geophysical surveys were carried across the suspected trace of the fault in the Jezre’el Valley, northeast of Kibbutz Mishmar-Ha’emek. Another goal of the research was to train MSc and PhD students from the universities of Jerusalem, Tel Aviv, and Beer Sheva, who performed the research under the guidance of senior researchers from the Geophysical Institute and these universities. Gravity, magnetic, and seismic refraction reveal the shallow sediment stratigraphy. The most informative survey is a high-resolution seismic reflection, which reveals several strands of faults, the southernmost of which is associated with a subtle topographic flexure. The fault appears to be active because it offsets very shallow reflectors. A very shallow topographical ledge, which coincides with the main fault location and continues further due northwest implies a recent normal slip component of about 1 m. A paleoseismic study that will be carried out in the next few months is expected to impose better constraints on the location and activity of this section of the Carmel fault.

The active tectonic-reef-sediment system in the head of Eilat Gulf.

Ariely r.1, Makovsky Y. 1, Agnon A. 2, Ben Avraham Z. 1, Reshef M. 1.

1. Department of Geophysics & planetary Sciences, Tel-Aviv University, Ramat Aviv, Tel-Aviv, Israel
2. Insititue of Earth Sciences, Hebrew University, Givat Ram, Jerusalem, Israel

            Our research is aimed to study the recent evolution of the seafloor at the head of the Gulf of Elat. We report here our up-to-date results based on detailed processing and interpretation of a grid of high resolution Chirp sub-bottom profiles acquired offshore Elat in 2002.

            A submerged band of reefs characterized by their distinctive seafloor reflectivity are mapped by us sub-parallel to Elat north beach at a depth range of 15 to 35 m. This band of reefs and similar sub bottom reflectivity patches to the east of it seem to belong to one stratigraphic level. We therefore suggest that they represent rejuvenated fossil reefs that evolved when the water level was 15 to 20 m lower than today. Sediments that flowed into the gulf subsequently where blocked by this band of reefs, flowed around it in the western part, and buried its eastern part where sediment supply is greater. Thus the sub-bottom profiles provide insight to the life cycle of a reef, and its causal relations with the eustatic sea-level, sediment supply, and tectonics. Correlation of discontinuities in the sedimentary section and seafloor lineaments reveal two recently-active fracture systems striking north-east, semi-parallel to the western coastline. The first is characterized bellow about 15 m depth by about 10 m down to the west offsets in the sedimentary sequence and a set of elongated mounds at the seafloor. A set of fractures are truncating the shallower sedimentary levels, close to the eastern marina of Elat, indicating recent activity along this system. The other fracture system is characterized by a minor bathymetric step; a set of diffractions suggesting open cracks at the seafloor; and a discontinuity in the sedimentary sequence. This system truncates and offsets the northern reef, and intersects with the northern coast of Elat west of the western marina. Both fracture systems project to areas of localized damage in the hotels area of Elat following the 1995 Dahab earthquake.

Velocity Model of the Levant Basin Using Seismic and Well Data

Ariely R. 1, Trachtman P. 1, Gardosh M. 1
 

1. Geophysical Institute of Israel, Habaal Shem Tov 6, Lod, Israel.

This project was aimed to estimate the acoustic velocities and to build a 3D velocity model of the Levant basin-fill, offshore Israel. Velocity information was obtained from stacking velocities of 2D, seismic reflection profiles and from check-shot surveys in offshore wells. The sources of information were combined, considering the accuracy of the data and its compatibility with the geological interpretation. The velocity model includes eight layers that correspond to the main stratigraphic units identified on seismic profiles within the basin-fill.

Building the velocity model involved the following steps: (a) Acquiring all available velocity information and estimate RMS, stacking velocities at the interpreted horizons, (b) Construction of 2D RMS, stacking velocity sections, (c) Building RMS velocity 3D cube models that were manipulated from the data using interpolation and extrapolation and, (d) Converting these 3D models to average and interval velocities, 3D cube models.  The results show an increase in the acoustic velocities from the margin in the east towards the basin in the west.  This effect is explained by the increased depth of burial and degree of compaction of the various sedimentary units.

The velocity model was used in two applications. First, as a guide function for horizon velocity analysis that was performed in pre-stack depth processing of four selected lines. Second, in depth conversion of regional, time-structure maps derived from the seismic interpretation.  The conversion incorporates the velocity variations within the sedimentary section and therefore produces a more accurate depth model of the subsurface of the Levant Basin.

Where is the main strand of the Eilat fault? Relict coastlines constrain faults slip in the Gulf of Eilat.

Yizhaq Makovsky1, Amotz Agnon2, Ronen Ariely3, Zvi Ben Avraham3, Aldo Shemesh4.

1. Ezra Hasofer 9/15, Tel Aviv, Israel; yizhaq@bezeqint.net.
2. Insititue of Earth Sciences, Hebrew University, Givat Ram, Jerusalem, Israel.
3. Department of Geophysics & Planet. Sciences, Tel-Aviv University, Ramat Aviv, Israel.
4. Environmental Sciences and Energy Research, Weizmann Institute of Science, P.O.Box 26, Rehovot, Israel.

We report here results of high resolution geophysical acoustic sub-bottom profiling of the shelf offshore Eilat down to about 120 m depth. On-lapping retrograde sedimentary layers observed beneath the seafloor of the northern slope down to at least 60 m depth suggest shelf evolution during sea level rise. A prominent sub-linear steep (>7°) face stretches between about 100 to 140 m sub-parallel to Eilat shoreline and outlines an internal basin. This face is fringed at a depth range of 60 to 85 m by c. 200 m wide gently (<3°) sloping terrace. We suggest following Reches et al. (1987) that this face-terrace complex is a relict coastline feature, and thus can serve as a marker to measure accumulated fault offsets. This face-terrace complex is modified in three places: 1. about 5 m down to the north offset of the terrace along a south-east striking lineament is observed in front of Eilat Marine Observatory; 2. horizontal truncation along at least 700 m stretch, and about 10 m down to the north-east vertical offset of the terrace is observed below the port of Eilat; 3. about 10 m down to the west and 150 m left lateral offset is measured across a north-east striking fracture system truncating the north slope. The offset in front of the Eilat Marine observatory is in agreement with late Holocene about 5 m drop of the Coral Beach reef discussed by Shaked et al. (2004). The age of the terrace-face complex should correspond to a substantial duration in which sea level was about 70 to 60 m below present. However, several such periods existed since early Pleistocene, and therefore the age of the terrace-face complex can not be constrained at this time. Reches et al. (1987) suggested that the face-terrace complex evolved during the inter-glacial sea level maximum about 50 Ka. They suggest that the major strand of Eilat fault offsets left laterally the terrace beneath the port of Eilat estimating an average slip rate >10 mm/y. Considering the c. 50 Ka for the face-terrace complex we estimate about 3 mm/y of left lateral faulting across the north slope fracture system. Noting that current relative plate motion estimates constrain the total slip rate of the Dead Sea fault system to about 5 mm/y we suggest that the north slope fracture system may be the major strand of the Eilat fault. The truncation of the terrace below the port of Eilat could be the result of recent land sliding. The ambiguity in the location and slip rate of the Eilat fault can only be resolved by directed dating of offshore Eilat sediments and relict coastal features.

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