Tina M. Niemi

Late Pleistocene and Holocene slip rate of the Northern Wadi Araba fault, Dead Sea Transform, Jordan

The Wadi Araba Valley is a morphotectonic depression along part of theDead Sea Transform (DST) plate boundary that separates the Arabian plateon the east from the Sinai subplate on the west. The Wadi Araba fault(WAF) is the main strike-slip faults one of between the Gulf of Aqaba and the E-Wtrending Khunayzira (Amatzayahu) fault that bounds the southern end ofthe Dead Sea. Just south of the Dead Sea, the WAF cuts across severalgenerations of alluvial fans that formed on tributaries to the Wadi Dahalafter the regression of Late Pleistocene Lake Lisan ca. 15 ka. Geomorphicand stratigraphic evidence of active faulting, including left-laterally offsetstream channels and alluvial-fan surfaces, yielded fault slip-rate data for thenorthern segment of WAF. Typical cumulative displacements of 54 m,39 m, and 22.5 m of stream channels and alluvial-fan surfaces acrossthe fault were measured from detailed geologic and topographic mapping.The 54 m offset of the oldest alluvial-fan surface (Qf1) occurredafter the final lowering of Lake Lisan (16–15 ka) and before 11 ka yieldinga slip-rate range of 3.4 mm/yr to 4.9 mm/yr. Based on radiocarbonages of charcoal and landsnail shell samples from the buried Qf2alluvial-fan deposits exposed in trenches excavated across the fault, the39 m and 22.5 m offsets occurred after 9 ka and 5.8 ka, respectively. These data yield a slip-rate range between 3.9 mm/yr and 6.0 mm/yr.The small variability in these slip-rate estimates for different time periodssuggests that the northern Wadi Araba fault has maintained a relativelyconstant slip rate in the past 15 ka. We calculate an average slip rate of 4.7± 1.3 mm/yr since 15 ka based on the three separate displacementsand age estimates. Five separate offsets of 3 m were measured from gullybends and the offset of small fault-scarp alluvial fans. These displacementdata suggest a coseismic slip of 3 m in the last earthquake, or acumulative slip of 3 m in the past few earthquakes. A maximum slip of3 m correspond to a Mw 7 earthquake that ruptures about 49 km offault length. Using an average slip rate of 4.7 ± 1.3 mm/yr togetherwith a 3-m slip-per-event suggests a maximum earthquake recurrence intervalof this fault segment of 500 to 885 years.

Ground-penetrating radar investigation of active faults along the Dead Sea Transform and implications for seismic hazards within the city of Aqaba, Jordan

Abstract Ground-penetrating radar (GPR) was used in an effort to locate a major active fault that traverses Aqaba City, Jordan. Measurements over an exposed (trenched) cross fault outside of the city identify a radar signature consisting of linear events and horizontal offset/flexured reflectors both showing a geometric correlation with two known faults at a control site. The asymmetric linear events are consistent with dipping planar reflectors matching the known direction of dip of the faults. However, other observations regarding this radar signature render the mechanism generating these events more complex and uncertain. GPR measurements in Aqaba City were limited to vacant lots. Seven GPR profiles were conducted approximately perpendicular to the assumed strike of the fault zone, based on regional geological evidence. A radar response very similar to that obtained over the cross fault was observed on five of the profiles in Aqaba City, although the response is weaker than that obtained at the control site. The positions of the identified responses form a near straight line with a strike of 45°. Although subsurface verification of the fault by trenching within the city is needed, the geophysical evidence for fault zone location is strong. The location of the interpreted fault zone relative to emergency services, military bases, commercial properties, and residential areas is defined to within a few meters. This study has significant implications for seismic hazard analysis in this tectonically active and heavily populated region.

Post-Eocene seismic stratigraphy of the deep ocean basin adjacent to the southeast African continental margin: a record of geostrophic bottom current systems

Abstract A high-resolution seismic-reflection survey of the Transkei Basin and Natal Valley permits the first recognition of three major reflectors that mark basin-wide unconformities across the continental rise and deep abyssal plain off the southeast African continental margin. Reflector O marks a change in acoustic reflectivity, coincident with a change in sedimentary bedforms from generally parallel bedding below to large-scale lenticular and clinoform shapes above. Reflector O probably marks the onset of cold, abyssal current circulation around the Eocene–Oligocene boundary. The overlying O sequence records deposition of a contourite drift (Oribi Drift) by northeast flowing abyssal currents at ∼4000 m water depths along the continental rise of the northeastern Agulhas Fracture Zone. This water depth is shallower than present-day Antarctic Bottom Water (AABW). The M reflector unconformity (possibly lower Middle Miocene) marks seafloor erosion in 4500 m water depth in the Transkei Basin and the cessation of drift construction along the continental rise. Above reflector M in the abyssal plain, a contourite drift (M-Drift) records deposition from an east-flowing bottom current in a location similar to, but slightly shallower than present-day AABW. The stagnation of bottom current activity in the northern Natal Valley and/or a rapid influx of sediment accumulation is marked by M sequence turbidite sediments (the Mzimkulu apron) deposited against and burying the Oribi Drift on the continental rise. Reworking of M sequence sediment along the continental rise to form low mounds (M4) and sediment waves in the northern Natal Valley indicate that a shallow, bottom current flowed at depths of 3800 to 3600 m. The coeval current-molding of the slope and abyssal plain indicates a two-layered structure of the bottom water may have commenced in the Miocene. Reflector P is the most pronounced unconformity in the deep abyssal plain, where it truncates M Sequence reflectors, and marks the base the Agulhas Drift which stands approximately 200 m above the surrounding seafloor. The P Sequence sedimentation is estimated to have begun in the Pliocene prior to or concurrent with an expansion of Southern and Northern polar ice-caps. Major slumping of the continental slope in the Natal Valley also began at this time, probably triggered by a combination of onland neotectonic activity and erosion of the base of the slope by vigorous bottom currents (possibly North Atlantic Deep Water, NADW).

The Southeast Araba Archaeological Survey: A Preliminary Report of the 1994 Season

Despite several past explorations of Wadi Araba, the southeast sector of the valley has remained largely terra incognita. This lack of exploration has created a significant gap in general knowledge of the archaeological history of the valley. Hence, any discussions of the historical geography of Wadi Araba are necessarily limited, particularly in a regional context. The first season of the Roman Aqaba Projects Southeast Araba Archaeological Survey (SAAS) finally bridged this gap by recording 162 sites in the eastern valley north of Aqaba. The results of this investigation provide important new data on several periods of human occupation within Wadi Araba. The evidence for the Chalcolithic/Early Bronze Age, Early Roman/Nabataean, and Byzantine periods is especially noteworthy. Moreover, ancient sites documented within the hinterland of classical Aila allow for a better understanding of the evolution of its economy within a broader regional framework.

Initial results of the southeastern Wadi Araba, Jordan Geoarchaeological Study: Implications for shifts in late quaternary aridity

Wadi Araba is a linear valley that follows the northeast-trending Dead Sea-Jordan transform (DST) fault zone ca. 165 km north from the Gulf of Aqaba to the escarpment overlooking the Southern Ghor of the Dead Sea. An archaeological survey and geologic investigation was conducted along the southeast 75 km of Wadi Araba during three field seasons. The geoarchaeological study focused on analyzing sediments and soil development at sites and air photo interpretation of fluvial systems draining into Wadi Araba. The region is sparsely vegetated with a mean annual precipitation of < 50 mm. The topography of the area is characterized mountains of granitic basement rock that rise steeply above the valley floor along DST escarpment. The valley floor is covered with Holocene to Pleistocene alluvial fan sediments emanating from the range front, mudflat (sabkha) sediments in localized valley axis depressions, and sand dune fields. Three dominant phases of valley floor alluviation occurred during the latest Pleistocene, middle Holocene, and historical times. Preliminary results of the Southeast Araba Archaeological Survey show a large number of Chalcolithic–Early Bronze and Nabataean/Roman–Byzantine age sites and a marked paucity in sites of other ages. Chalcolithic–Early Bronze sites are concentrated on alluvial terraces approximately 3–5 m above the wadi bed tributaries to Wadi Araba and form tells (Tell Magass and Tell Ghuzlan) on an alluvial fan surface north of Aqaba. Other major cultural expansions in the southeastern side of the valley include Nabataean/Roman to Byzantine sites that are situated predominately on the valley floor, and less frequently on low alluvial terraces. The lack of continuous occupation at sites in the southeast Araba valley suggests that the extremely arid climatic conditions lessened during parts of the Holocene. The extensive distribution of archaeological sites of Chalcolithic–Early Bronze Age and Nabataean/Roman–Byzantine age and the associated development of soils indicate that these periods were marked by wetter or cooler climates. These data agree with other climatic proxy data from the Negev Desert and the Dead Sea regions. Although the evidence from the southeast Wadi Araba is not conclusive, it does suggest a southern shift of precipitation belts at approximately 6–5 ka.

Holocene stratigraphy of the Dead Sea: Correlation of high-resolution seismic reflection profiles to sediment cores

The northern basin of the Dead Sea is occupied by a ∼300-m-deep lake. A series of cores in the deep-water part of the lake provide information about the top 365 cm of the sediments. The cores were correlated with high-resolution 3.5-kHz seismic profiles from this area and provide lithologic and age constraints for the high-resolution seismic reflection data. Visual comparison of the two data sets shows that strong surface and shallow subsurface reflectors (A and B) correlate to the massive salt at the seafloor surface and the indurated salt at the base of the cores, respectively. Calculations of an average seismic velocity based on the interval between these reflectors and the corresponding sedimentary thickness yield an average 3500 m/s velocity. This agrees closely with velocities determined from direct measurements of compressional velocities for sediment samples. Ultrasonic wave velocity measurements of salt samples from the cores and dry rock salt cores from the southern basin of the Dead Sea indicate that wave velocities are independent of the burial depth at shallow depths; however, velocities show strong dependence on porosity. At low hydrostatic pressure a reduction in porosity as well as closure of microcracks in the crystals cause an increase in the velocities. This increase disappears at higher stress levels. Synthetic seismograms of the upper 3 ms and the entire 25 ms penetrated by the seismic profiles reinforce the lithologic and seismic stratigraphic correlation and confirm that prominent reflectors in the basin represent the top boundary of halite layers which are separated by laminated sequences of evaporites and elastics. The salt in the upper salt sequence is deposited at a very fast rate of more than 20 mm/yr. However, at shallow depths, considerable compaction takes place. Variations in appearance and velocities of the upper salt sequence and middle salt sequence indicate that the porous, granular, and fine-grained precipitates of the surface salts are diagenetically altered to a coarse and compact crystalline aggregate by re-solution and crystallization with burial. The sedimentary sequences recovered in the cores suggest that significant lake level fluctuations took place in the past in response to climatic changes. The detailed correlation of the cores and seismic profiles makes it possible to extrapolate climatic data from earlier periods beneath the maximum core penetration by analyzing the seismic stratigraphic sequences of the seismic reflection data.

A GIS-Based Assessment of Liquefaction Potential of the City of Aqaba, Jordan

The city of Aqaba, Jordan, located along the northern shore of the Gulf of Aqaba, is built over the seismically active Dead Sea Transform plate boundary fault system. The subsurface stratigraphic sequence underlying the city is composed of alluvial fan deposits containing varying amounts of clay, silt, sand, and gravel interbedded within the coastal areas with aeolian and beach sand. Groundwater levels along the coastal areas are very shallow. The groundwater levels increase to the north and northeast to depths greater than 17 m about 5 km from the shore. The liquefaction potential of the city of Aqaba soils was analyzed using the Simplified Procedure of Youd and Idriss and the modified Chinese criteria of Wang. Maps of the liquefaction susceptibility of areas in the city of Aqaba were displayed using the Geographic Information System (GIS). Application of the modified Chinese criteria indicates that the cohesive subsurface layers are predominantly nonliquefiable. The types of sediments most susceptible to liquefaction are saturated silt and sand deposits. Two classification schemes were developed in this liquefaction analysis. Our results indicate that the coastal areas of Aqaba have a high potential to liquefy, whereas the eastern parts of the city lie predominantly within a nonliquefaction zone. The critical facilities were overlaid with the liquefaction hazard map of the city and show that only the hotel and the commercial districts lie within a zone of high susceptibility to liquefaction. The areas that have historically experienced liquefaction were found to exist within the high susceptibility zone as well, supporting the findings of this study.

Investigation of Microearthquakes, Macroseismic Data, and Liquefaction Associated with the 1867 Wamego Earthquake in Eastern Kansas

Historical felt earthquakes and instrumentally recorded microseismicity in eastern Kansas and western Missouri have been attributed to the movement of the Nemaha Ridge and Humboldt fault zone (nrhf). Our investigations of the nrhf have concentrated on relocation of microearthquakes in the Kansas catalog, re-evaluation of the 1867 earthquake felt reports, and field studies to determine the presence or absence of sedimentary evidence of earthquake-induced liquefaction. Microearthquakes in the Kansas catalog have been relocated in a joint inversion with 3D seismic velocity variations. This improved set of hypocenters affirms the loose association of seismicity with the basement nrhf structure, even though seismicity does not seem to follow a single, well-defined fault feature. Overall, the association of microearthquakes with the nrhf suggests that it remains a potential source of large earthquakes. Our field investigations confirm that sedimentary deposits with moderate liquefaction susceptibility are present in the vicinity of Wamego and Wabaunsee, Kansas. Soft-sediment deformation features, including flame and dish structures, are present in the late Holocene floodplain deposits of the Kansas River. These features may have formed by liquefaction or by processes unrelated to seismic activity, such as sediment dewatering. We found several clastic dikes that may be attributed to seismically induced liquefaction. Our initial results suggest that liquefaction features are present but may not be pervasive in this region. These data imply that the 5.2 magnitude of the 1867 Wamego earthquake may characterize the seismic source in this location. However, additional field investigations are still necessary to complete the assessment for liquefaction features.

Quaternary tectonic evolution of the Northern Gulf of Elat/Aqaba along the Dead Sea Transform

The northern Gulf of Elat/Aqaba is located in the transition between the deep marine basins of the gulf and the shallow onland basins of the Arava Valley. Interpretation of 500 km of high-resolution seismic reflection data collected across the northern shelf reveals the tectonic structure and evolution of this transition. Six NNE-trending faults and one E-W trending transverse fault are mapped. Slip rates are calculated based on measured offsets and age determination based on a radiocarbon-calibrated sedimentation rate and a Quaternary age model. The most active fault is the Evrona Fault that absorbs most of the left lateral slip within the basin with an average sinistral slip rate of 0.7 ± 0.3 mm/yr through the Late Pleistocene and 2.3–3.4 mm/yr during the Holocene. Two intrabasin faults east of the Evrona Fault that have been inactive for the last several tens of thousands of years were mapped, and motion from these faults has likely transferred to the Evrona Fault. The basin is flanked on the west by the Elat Fault and on the east by the Aqaba Fault. Both faults are marked by large bathymetric escarpments. Based on displaced seismic reflectors, we calculate a Holocene vertical slip rate of 1.0 ± 0.2 and 0.4 ± 0.1 mm/yr for the Elat and Aqaba Faults, respectively. The geometry, slip rates, and slip history of the northern Gulf of Elat/Aqaba faults show that during the Late Pleistocene several intrabasin faults became dominant across the basin but that during the Holocene the Evrona Fault accommodates most of the strike slip.

The 1906 Earthquake Fault Rupture and Paleoseismic Investigation of the Northern San Andreas Fault at the Dogtown Site, Marin County, California

Our research describes historical observations of the 1906 earthquake rupture trace in Marin County, documents the style of subsurface deformation, and summarizes our interpretation of the number and timing of pre-1906 ground- rupturing earthquakes exposed in 18 trenches excavated across the northern San Andreas fault (NSAF) at the Dogtown site. Dogtown is located immediately southeast of what was known as the Strain Ranch in 1906. The trenches reveal two stratigraphic sections of late Holocene age separated by a well-constrained depositional hiatus. The upper section, deposited in the past 200-250 yr, is separated from the lower section, which was deposited between A.D. 250 and 650, by a hiatus during the interval A.D. 650-1700. We see evidence for two events in the upper section—Event I (1906) and Event II, a likely penultimate earthquake, plus a minimum of two events in the lower section (Event(s) A and B). In the lower stratigraphic section, the oldest exposed potential surface-faulting evidence (Event B) is constrained to the interval between A.D. 330 and 580. While there is sound evidence for ground-rupturing earthquakes in the 1100 yr long interval between Event B and Event II, pedogenic processes have obscured their number and timing. Faulting evidence at the Dogtown site suggests the penultimate earthquake occurred between A.D. 1695 and 1776. Data from the Dogtown site, although equivocal, suggest that the time interval (130-210 yr) between the 1906 earthquake and the penultimate earthquake on the San Andreas Fault north of San Francisco may be shorter than previously documented.