• from Allison (2013:133-134)

The goal of this study is to more thoroughly characterize the behavior and rupture pattern of the Dead Sea transform (DST) fault through time. Due to the difficulty of exposing the main fault within the city of Aqaba, another strand of the DST, the Wadi ‘Arabah (Evrona) fault, was investigated north of the city in the Taba (Yotvata) Sabkha as a part of this paleoseismic study. The Wadi ‘Arabah fault, exposed through trenching in the Taba Sabkha, was studied in order to find paleoseismic evidence for ground rupture of major earthquakes along the southern portion of the Dead Sea transform over the last two millennia.

The trench location in the Taba Sabkha provides a unique opportunity to conduct paleoseismic studies along the Wadi ‘Arabah fault segment of the DST fault zone in an area that is both uninhabited and geographically accessible. Proper military permission is required to work in this location, however, because of the sensitive position of the Taba Sabkha which straddles the international Jordanian-Israeli border. This international border, which roughly follows the Wadi ‘Arabah Valley northward, is likely one of the primary reasons this segment of the DST has not been studied in greater detail across the entire width of the valley. The focus of this research, therefore, is to determine the timing of ancient earthquakes occurring along this segment of the Dead Sea transform. To accomplish this goal, detailed mapping and radiocarbon dating of the subsurface stratigraphy as exposed in a trench that bisects the Wadi ‘Arabah fault was used in order to understand more thoroughly the seismic behavior of this transform plate boundary through time.

• from Allison (2013:134-139)

The trench site in this paleoseismic study is located 35 km north of the city of Aqaba, Jordan in the Taba (Yotvata) Sabkha, a 55 km2 continental sabkha situated in the southern portion of the Wadi ‘Arabah Valley (Figure 4.1). The term ‘sabkha’ is an Arabic word that means ‘salt flat,’ regardless of its geographic position relative to the sea (Abed, 1998). There are two types of sabkhas: 1) coastal or supratidal, and 2) inland or continental sabkhas. Evaporites formed within coastal sabkhas are related to sea water (e.g. Gavish, 1974; Butler et al., 1982; Kendall and Warren, 1988). In a continental sabkha like Taba, evaporites and sediments are related to meteoric waters only, or those waters derived directly from precipitation, and form well away from seas or oceans (e.g. Kinsman, 1969; Amiel and Friedman, 1971; Handford, 1988). More specifically, Kinsman (1969) describes coastal sabkhas as supratidal surfaces developed by depositional offlap of marine sediments in which evaporites precipitate from seawater-derived brines. Continental sabkhas, in contrast, are composed of continental or earlier cycle marine sediments in which the evaporites are precipitated from evaporated meteoric waters. Abed (2002) explains that this area is most properly referred to as a sabkha and not a playa, as may be expected, based on the surrounding geology. This is because the majority of the basin that does periodically flood only holds water for a few days unlike playas which tend to hold water for longer periods of time (Abed, 2002).

The Taba Sabkha is a deflation hollow in a sand-dominated landscape (Amiel and Friedman, 1971; Abed, 1998). Kinsman (1969) describes continental sabkhas as equilibrium deflation surfaces through to the local water table, where the capillary fringe above the water table marks the base-level of wind deflation. Sediments located stratigraphically above the capillary fringe, therefore, are subject to wind removal, which results in the formation of a relatively flat land surface that is related to the local groundwater table (Amiel and Friedman, 1971). Precipitation draining down mountains located to both the east and west of the sabkha and the subsequent alluvial fans created from such drainage, as well as flood waters draining down the Wadi ‘Arabah Valley itself, all contribute to the local groundwater table in the Taba Sabkha. In particular, an important amount of groundwater flow is postulated to originate from the large Wadi Darba fan located 3-6 km NNE of Taba (Abed, 1998). Abed (1998) also reports groundwater at a depth of only one meter at the toe of one of the eastern alluvial fans nearest the Taba Sabkha, with the groundwater table deepening westward toward the center of the sabkha. Historically, local Bedouin have used wells dug into this shallow water table on the eastern side of the sabkha to water their camels and goats, and this practice continues to the present-day. In winter months, the groundwater of this fan system actually emerges on the surface some 200 m west of these wells where a palm tree oasis is supported (Abed, 1998). In the concrete-lined well located near the trench site, the water table was approximately 3-3.5 m beneath the ground surface at the time of this study in February 2009, and was not encountered during trenching.

Abed (1998) excavated seventeen shallow pits up to 1.5 m deep and drilled 8 boreholes up to 17 m deep at various locations in the Taba Sabkha in order to study the sedimentology and mineralogy. The eastern side of the sabkha laterally interfingers with several alluvial fans where wadis have cut across the Precambrian basement rocks. Sand dominates the eastern, northern, and northeastern fringes of the Taba Sabkha, while massive clays that form the vast center of the sabkha are found at the distal part of the alluvial fans (e.g. Inci, 1991; Yagmurlu and Helvaci, 1994; Abed, 1998, 2002). The clay minerals present in the Taba sediments include kaolinite, smectite, illite, chlorite, and sepiolite; these minerals form up to 60% of the sediments in the center of the sabkha (Abed, 1998, 2002). Detrital minerals such as quartz, feldspars, and micas all decrease westward with distance away from the granitic source mountains located to the east. North of the sabkha, however, sand dunes are present for a distance of approximately 35 km, and as a result sand is often blown south into the sabkha by prevailing northerly winds (Abed, 1998, 2002).

Surface water that is periodically present in the sabkha precipitates halite, giving the ground a whitish appearance in places where a thin (about 1 cm thick) mineral crust has formed. These areas are often covered by desiccation cracks of various sizes, but the cracks involve only the upper 3 mm of the surface and do not penetrate deeper (Abed, 1998). Brownish areas within the Taba Sabkha are of a slightly higher (1-3 cm) elevation, and represent older desiccation cracks where holes several centimeters deep and 2-7 cm in diameter have formed in the halite-rich crust as a result of dissolution. Salt-rich “cakes” are found underlying these dissolution holes (Abed, 1998, 2002). Gypsum (and sometimes anhydrite), also an evaporitic mineral, dominates in the eastern portion of the sabkha and also contributes to the sometimes whitish appearance of the sabkha sediments. Calcite, much of it wind-blown from the Mesozoic limestone strata on the western side of the Wadi ‘Arabah, and authigenic dolomite, are also present in the Taba Sabkha, but to a lesser degree (Abed, 1998, 2002).

Amiel and Friedman (1971) also studied the Taba Sabkha stratigraphy and mineralogy, but focused primarily on the western, Israeli side of the sabkha. They excavated a total of 330 sample pits at 250 m intervals along traverse lines and collected samples every 30 cm down to the water table (Amiel and Friedman, 1971). Based on the sediment composition, distribution of vegetation, salinity, and depth to groundwater, these authors identified three distinct zones within the Taba Sabkha: (1) the central barren zone, (2) the transitional zone, and (3) the outer vegetated zone with halophytic vegetation.

The central barren zone, as its name implies, was found to be devoid of any vegetation and is composed of silty and sandy clay with strips of aeolian sand that cut across the zone, covering the sabkha to a depth of 30-100 cm in places. The halitic crust found throughout the Taba Sabkha is most pronounced in this central area (Amiel and Friedman, 1971). The transitional zone was the second zone identified in the study and was found to have sparse vegetation, primarily tamarisk. These shrubs act as a barrier to the movement of sand in the sabkha by trapping wind-blown particles that accumulate in small, irregular hillocks. According to Amiel and Friedman (1971), the sedimentation in this zone varies. Along the far western margin of the central barren zone, the sediment of the transitional zone is generally a silty sand. It becomes predominantly sandy silt northwest of the central barren zone. In both cases, the sediment is alluvial in origin and is derived from the wadis that drain the slopes of the mountains to the east and west (Amiel and Friedman, 1971). This zone differs mineralogically from the central barren zone in that gypsum is the dominant authigenic mineral in the sediment profile. Finally, the outer vegetated zone is the marginal zone of the sabkha that interfingers with the alluvial sediments of the fans. It is the largest zone in the Taba Sabkha, and sediments here consist primarily of silt and fine-grained sand, although sedimentation can also be coarse-grained and pebbly in places. Vegetation in this outer zone is mainly halophytic in nature, and thus capable of growing in salty soil (Amiel and Friedman, 1971).

Abed (2002) also identified vegetation zones within the Taba Sabkha, except he identified four zones present on the sabkha periphery, and found the vast area occupied by the sabkha center to be completely barren. The four zones of vegetation as determined by Abed (2002) include: 1) the Acacia zone on the alluvial fans, 2) the palm zone, 3) the Nitratia retusa and Tamarix sp. zone, and 4) the Cressa cretica cf. zone, although this zone is patchy.

• from Allison (2013:139-141)

The Wadi ‘Arabah fault, or Evrona fault as it is referred to on the Israeli side, trends up the Wadi ‘Arabah Valley at approximately N15˚E and is recognizable to about 35 km north of Aqaba. It terminates under sand dunes located just north of the Taba Sabkha. The Wadi ‘Arabah fault is complex and consists of several sub-parallel fault traces. In the south, the fault crosses unconsolidated recent alluvium and playa deposits in a belt approximately 1 km wide on the western side of the valley, and in the north, the Wadi ‘Arabah fault has displaced alluvial fans by en-echelon faulting (Garfunkel et al., 1981). The geometry of small, rhomb-shaped grabens indicates left-lateral slip along this north portion of the Wadi ‘Arabah fault. Active normal faulting is also present along the western mountains that flank the southern Wadi ‘Arabah (e.g. Zak and Freund, 1966; Garfunkel et al., 1981).

In all, there has been an estimated post-Eocene displacement of 107 km across the Dead Sea transform (e.g. Quennell, 1959; Freund, 1965; Freund et al., 1970; Bartov, 1974; Steinitz et al., 1978; Garfunkel and Ben-Avraham, 1996). This left-lateral movement occurred in at least two major stages as indicated by the palinspastic reconstruction of offset geological formations and structural features. According to earlier studies, 62 km of offset occurred along the DST by the early Miocene, with another 45 km of offset occurring since the late Miocene or Pliocene to present (e.g. Quennell, 1958, 1959; Garfunkel, 1981). Hatcher et al. (1981) also conducted geophysical studies that served to verify this displacement amount. The late Quaternary slip-rate of the DST has been estimated in multiple studies to be about 3-5 mm/yr. (e.g. Zak and Freund, 1966; Freund et al., 1968; Garfunkel et al., 1981; Jestin et al., 1994; Zhang, 1998; Klinger et al., 2000a; Niemi et al., 2001; Le Béon et al., 2010).

Garfunkel (1970) notes that north of the large, active fan of the Wadi Yutim there are older alluvial fans, as indicated by their dark patina-covered surface, which are truncated by what is probably a fault extending south to the northeastern shore of the Gulf of Aqaba. Strike-slip faults in the region tend to affect very young to recent sediments, while the marginal normal faulting events are apparent only in the older parts of the rift fill (Garfunkel et al., 1981). Le Béon et al. (2010) mapped offset Quaternary alluvial fans at the sites of Jebal al-Muhtadi and Hamrat al-Fidan, and Le Béon et al. (2012) mapped offset Quaternary alluvial fans at the sites of Al-Risha, Al-Dhawi, and Mazla, in order to study early Holocene and late Pleistocene slip-rates along the DST in Wadi ‘Arabah. Le Béon et al. (2012) calculated that the slip-rate along the Dead Sea transform for the last 300 ka (late Pleistocene) was between 5-12 mm/yr with preferred values of 5-7 mm/yr, very similar to the Holocene DST slip-rate. The concurrence between slip-rates across timescales (during the Pleistocene and Holocene epochs) supports a constant-over-time fault kinematics scenario (Le Béon et al., 2010) for the Dead Sea transform, at least for time periods longer than approximately 10 ka (Le Béon et al., 2012).

• from Allison (2013:141-143)

The subsurface stratigraphy in the Taba Sabkha was previously imaged using geophysical methods. Ground penetrating radar (GPR), a non-invasive ground survey technique that uses electromagnetic waves, offers a unique high-resolution image of subsurface soil and rock conditions down to a depth of several tens of meters (Basson, 2002). Abueladas (2005) conducted a ground penetrating radar survey across the Wadi ‘Arabah fault where it enters the south end of the Taba Sabkha along the margins of a pressure or shutter ridge. Four buried fault strands were identified across an area of 12 m in the GPR section within the sabkha. A change in reflector intensity (peak amplitude) and a mismatch of reflectors suggest that the faults identified in the Taba Sabkha are strike-slip faults. The increase in reflector mismatch with the depth also indicates repeat motion on the buried faults. Along the section of the Taba Sabkha surveyed by Abueladas (2005), the fault traces are buried by approximately 1 m of sediment which is an indication that these faults have not ruptured to the ground surface for some centuries.

Basson et al. (2002) also used ground penetrating radar imaging to map faults to a depth of 25 m in the Evrona playa approximately 20-25 km south of the Taba Sabkha. The study revealed a dense, inhomogeneous distribution of subsurface discontinuities along the Wadi ‘Arabah fault. The GPR images show dense sets of fractures and faults located at various depths within the ground (Basson et al., 2002). GPR observations for this study also indicated that the density of faults increases as a function of depth at the first 25 m, and the apparent dips of the faults also suggest that they merge at a depth of a few tens of meters (Basson et al., 2002). Shallow seismic reflection (SSR) data (Shtivelman et al., 1998) were also used to identify a parent fault below the group of faults detected by the shallower GPR profile data collected by Basson et al. (2002). A comparison of the SSR and GPR data confirmed that the faults are merging at depth. According to Basson et al. (2002), a typical single fault traced through the top hundred meters along this section of the Dead Sea transform abruptly changes its characteristics as it reaches a depth of approximately 35-20 m below the ground surface. As it approaches the surface, the fault tends to fan out in a series of splays through the soft sediment, and the tectonic displacement that accumulates along a plane of a parent fault is dispersed towards the surface as a result (Basson et al., 2002).

An analysis of the faults detected by this GPR study by Basson et al. (2002), coupled with the dating of samples collected from excavated trenches, enabled an evaluation of the relative level of tectonic activity of the Wadi ‘Arabah fault zone. The GPR record indicates that tectonic activity peaked somewhere between 18,000-27,000 yr BP. It was also found that the recent period (0-9000 yr BP) can be characterized by a relatively low level of seismicity. Basson et al. (2002) found that this level of activity is approximately only 70% of the average seismic activity seen throughout the last 45,000 years in the region, and only about 50% of the determined peak activity.

• from Allison (2013:143-149)

While the cumulative lateral displacement and slip-rate of the active Dead Sea transform have been well documented, the earthquake recurrence interval of the DST fault is poorly understood. Several studies in the last few decades have focused on the paleoseismicity of the Wadi ‘Arabah (Evrona) fault north of the Gulf of Aqaba (e.g. Gerson et al., 1993; Enzel et al., 1994, 1996; Amit et al., 1995, 1996, 1999, 2002; Porat et al., 1996, 2009; Zilberman et al., 2005). Previous paleoseismic work in the southern Wadi ‘Arabah has occurred in primarily two locations: along the normal faults of the Nahal Shehoret alluvial fan and strike-slip faults within the Evrona playa, both of which are located on the Israeli side of the valley (see Figure 4.1).

Twelve trenches up to 30 m long were excavated across normal fault scarps and other lineaments at the Shehoret site by Gerson et al. (1993). Located off of the main strike- slip fault of the Dead Sea transform, this site is situated 7 km north of Eilat, and approximately 28 km south of the Taba Sabkha. The presence of buried soils on the downthrown block at the fan site studied by Gerson et al. (1993) suggested a hiatus in the depositional record and was interpreted as a reliable indicator of periods of tectonic quiescence. In all, nine seismic events were identified as having occurred within the last 100,000 years along this section of the fault, with a recurrence interval of 1000-3000 years (Gerson et al., 1993). Working within the Nahal Shehoret fan site, Enzel et al. (1994) also calculated a 1000-3000 year recurrence interval by comparing the stratigraphy, sedimentology, and soils of colluvial and alluvial deposits on the fault scarps and near-by faulted terrace risers.

Infrared-stimulated luminescence ages of buried colluvial wedge deposits suggest that four surface faulting events occurred sometime between 35 ka and 14 ka with vertical displacements of 1.5 m (Amit et al., 1995, 1996; Porat et al., 1996). Smaller earthquakes are thought to have followed and continued until recent times. The mean recurrence interval for earthquakes of M ≥ 6.2 on this specific fault in the Shehoret fan was calculated as approximately 4000 years by Porat et al. (1996). Leonard et al. (1998) concluded that smaller, more frequent fault displacements in Holocene deposits within the Nahal Shehoret fan indicate a potential change in the seismic activity and behavior of the Wadi ‘Arabah fault around 14 ka. Enzel et al. (1996) suggested that at least one large seismic event (M > 6.5) occurred along the Wadi ‘Arabah fault within the last 1000-2000 years based on luminescence dating and fault scarp degradation modeling. Enzel et al. (1996) also point out that since so few fault scarps have been studied in the area, these scarps represent a minimum estimate for large (M > 6.5) earthquake events in the region.

A study by Porat et al. (2009) conducted within the Shehoret alluvial fan site dated single grains of quartz collected from colluvial wedges deposited shortly after each faulting event. Porat et al. (2009) determined that the most recent earthquake took place a short time before 500-1300 years ago, and that this chronological framework agrees with the geomorphic age of the fault scarp which was modeled to be less than 2000 years old.

A study by Amit et al. (2002) investigated both the Shehoret fan and the Evrona playa toward the center of the valley. Paleoseismic evidence shows that the southern Wadi ‘Arabah (Evrona) fault system has generated at least fifteen earthquakes of M > 6 during the Pleistocene and Holocene (Amit et al., 2002). Using the maximum displacement method for estimating paleoearthquake magnitudes, Amit et al. (2002) found that throughout the Pleistocene, earthquakes occurring along the Evrona fault within the Shehoret fan site had magnitudes of between M 6.7 and M 7. The recurrence interval was calculated as 2.8 +/- 0.7 ka. Holocene fault motion at the Shehoret fan site had both a higher frequency and higher recurrence rate of 1.2 +/- 0.3 ka as compared to activity during the Pleistocene (Amit et al., 2002). The vertical fault displacements during the Holocene were smaller (0.2 -1.3 m), and therefore the earthquake magnitudes were also smaller, M 5.9 - M 6.7. This study also suggests that these smaller earthquakes were likely not substantial enough to activate the marginal faults along the western side of the valley (Amit et al., 2002).

Paleoseismic studies that focused on the main strike-slip fault located in the southern Wadi ‘Arabah Valley at sites within the Evrona playa (Amit et al., 1999, 2002; Zilberman et al., 2005) indicate that at least six M > 6 seismic events have ruptured the fault within the last 14,000 years. Amit et al. (1999) have concluded that the last significant seismic event occurred within the last 1000 years based on the limiting age of the sequence studied, as well as the extent of soil development in the Evrona playa. This earthquake is understood to have been an event significant enough to change the morphology of the Evrona playa from a closed system with internal drainage to an open basin, which ultimately resulted in relief inversion of the playa sediments (Amit et al., 1999).

A study by Zilberman et al. (2005) presents archaeological evidence of a water irrigation system, known as a qanat, located in the western part of the Evrona playa that was deformed because of movement along the Evrona fault. The qanat system once irrigated an early Islamic farm that dates to the 11th C. and that likely supplied the city of Early Islamic Ayla with fresh agricultural products (Avner, 1993). The farm was irrigated by a network of water canals connected to a central reservoir. The water reservoir was fed by the qanat, an underground tunnel system connected to the surface by rows of vertical shafts, which collected groundwater from alluvial fans located along the western margin of the valley (Zilberman et al., 2005). Topographic profiles of the roofed water canal show that the inlet of the canal to the water reservoir is 1 m higher than the qanat outlet, and the canal itself is deformed at three separate points along its length. The uplift of the reservoir, therefore, resulted in an inversion in the gradient of the qanat system (Zilberman et al., 2005).

The destruction of this irrigation system caused the farm to be abandoned in the 11th C. at the same time the city of Early Islamic Ayla was destroyed by an earthquake (Avner, 1993). Zilberman et al. (2005) conclude that the faulting event that destroyed the qanat system at the Islamic farm site is likely the same seismic event described by Amit et al. (1999, 2002) that occurred in the last 1000 years, and most likely represents the catastrophic A.D. 1068 earthquake (e.g. Guidoboni and Comastri, 2005; Ambraseys, 2009). In all, by combining data from studies at both the Shehoret fan site and the Evrona playa, the recurrence interval for large M > 6 earthquakes during the Pleistocene and Holocene periods along the Wadi ‘Arabah fault is estimated to fall somewhere between 1200-2000 years (e.g. Gerson et al., 1993; Enzel et al., 1994, 1996; Amit et al., 1995, 1996, 1999, 2002; Porat et al., 1996, 2009; Zilberman et al., 2005).

Submarine paleoseismology has also been conducted along the Dead Sea transform fault within the Gulf of Aqaba in an effort to more thoroughly understand the behavior and pattern of rupture along the DST along this southern segment of the fault. In a study by Makovsky et al. (2008), submerged relict coastline features were identified in the Gulf by using high resolution sub-bottom profiles of the northwestern tip of the Gulf of Aqaba down to a water depth of approximately 120 m.

Along the northern shelf of the Gulf, Makovsky et al. (2008) identified a relict reef in 65 m of water that is vertically offset by 10 +/- 1 m and sinistrally offset by 30 +/- 10 m across the offshore Evrona fault. Assuming that the reef was formed when sea level was approximately 65 m lower than today, these authors suggest the reef developed at around 11 +/- 2 ka. Measured offset and age of the 65-m reef, therefore, were used to estimate the slip- rate along this section of the fault as 2.7 +/- 1.5 mm/yr (Makovsky et al., 2008).

Tibor et al. (2010) conducted a high-resolution marine geophysical study in the Gulf of Aqaba that produced the first multibeam imaging of the seafloor across the entire gulf head spanning both Jordanian and Israeli waters. Analyses of the seafloor morphology indicate that the Gulf of Aqaba transform basin is asymmetrical and can be divided into the Aqaba and Eilat sub-basins separated by the north-south trending Ayla high. They also found seafloor lineaments, which suggest that the Eilat Canyon and the boundaries of the Ayla high align along NNW-striking faults -- the Wadi ‘Arabah (Evrona) Fault zone to the west and the Ayla Fault zone to the east -- as suggested by slope gradient analyses (Tibor et al., 2010). The 100-m shelf-slope break in the Eilat sub-basin and the shallower 70-m shelf- break in the Aqaba sub-basin, which are correlated to the last glacial period approximately 21 ka, are offset by approximately 150 m along the eastern edge of the Ayla high, which Tibor et al. (2010) suggest may be the result of horizontal and vertical movements along the Ayla Fault on the east side of the structure. It also appears from this work that a lack of active fault morphology across fan deltas suggests that the Aqaba Fault has not been active recently or has been rapidly buried by sediment, and that the Eilat side of the Gulf is more tectonically active (Tibor et al., 2010).

Hartman (2012) studied the Quaternary evolution of the Gulf of Aqaba transform basin using high-resolution geophysical data, including seismic profiles, full multibeam bathymetric and acoustic backscatter, and sidescan imaging, which revealed six systems of relict reefs situated within transgressive depositional systems. Originally identified by Makovsky et al. (2008) and Tibor et al. (2010) as >1 km-long linear terraces, the two youngest reefs exposed along the northwest corner of the Gulf of Aqaba on the seafloor at depths of 15 m and 60 m were interpreted as being related to two decelerations in sea-level rise during the last two deglaciations, and portray a repeating pattern of stratigraphic reef development (Hartman, 2012). The results of this research were correlated with rates of sea- level change, climatic events, and reef generation phases to produce an age model for these reefs, which from high-resolution seismic profiles were also found to be offset by active fault strands (Hartman, 2012). The reefs and overlying sedimentary layers were deformed by a total of six NNE-SSW trending faults as well as one E-W trending transverse fault, and based on the measured offset and ages of the 15-m and 60-m reefs, 8.4-8 ka and 12.9-11.7 ka, respectively, slip-rates and fault activity were calculated (Hartman, 2012). Of these fault strands, the Wadi ‘Arabah (Evrona) fault was found to be the most active fault crossing the reef system, with an average slip-rate of 0.5 +/- 0.1 mm/yr through the late Quaternary and 4 +/- 2.3 mm/yr during the Holocene, acting to absorb the majority of the left-lateral slip within the Gulf of Aqaba transform basin (Hartman, 2012).