EQ III
EQ III is a clearly expressed
paleoseismic event identified in the Taba Sabkha
trench, younger than EQ IV but apparently older than EQ II
and EQ I. The trench was excavated on the far
eastern side of the
Taba Sabkha,
a
continental sabkha, approximately 35 km north
of Aqaba, and was oriented N80°W so as to
bisect the
en-echelon fault
pattern of the
Wadi ‘Arabah fault, the southern
continuation of the
Dead Sea
Transform. The active fault trace lies
along the base of a linear pressure ridge at the
southeastern margin of the sabkha before
continuing northward beneath the sabkha without
obvious surface expression. This trench geometry
was specifically selected to intersect active
faulting within this complex
left-lateral strike-slip fault zone
(Allison,
2013:149-151).
The paleoseismic expression of EQ III is subtler
than that of EQ IV but remains clearly
recognizable in the trench stratigraphy. EQ III
faulted the upper portion of the thick,
interbedded reddish-brown clay, silty clay, and
clayey silt package of unit TS-12 as well as the
overlying compact sandy silt unit TS-10,
depending on location within the trench
(Allison,
2013:151-159). Some upward fault
terminations associated with EQ III terminate
within the upper portion of TS-12 and do not
propagate into TS-10, whereas other fault traces
extend upward into TS-10.
Paleoseismic evidence for EQ III includes
upward-terminating faults, vertically offset
strata,
mismatched unit thicknesses across fault
planes, and reactivation of older fault strands
first formed during EQ IV. Vertical displacement
associated with EQ III is modest, on the order
of approximately 2–3 cm, but stratigraphic
offset is clear. Several faults mapped as
EQ III were later reactivated during EQ II,
demonstrating repeated use of the same fault
architecture through successive seismic episodes
(Allison,
2013:151-159). Unlike EQ IV, there is no
documented fissure fill specifically attributable
to EQ III, and no evidence that this event
generated a major new surface depression.
The tectonic expression of EQ III indicates
predominantly
strike-slip
deformation with only a minor vertical
component. The Wadi ‘Arabah fault is a
sinistral transform system, regionally
expressed by
en-echelon faulting,
rhomb-shaped grabens, pressure ridges, and
localized deformation
(Allison,
2013:139-141). The mismatched thicknesses of
TS-10 and TS-12 across EQ III fault traces are
best explained by
lateral shear displacement,
while the minor vertical separation suggests a
small dip-slip component superimposed on the
transform motion. Unlike EQ IV, which produced
clear localized subsidence and accommodation
space, EQ III appears to reflect primarily
strike-slip rupture with only limited localized
extension or vertical adjustment.
Dating of EQ III relies on radiocarbon
constraints from charcoal samples collected from
units stratigraphically below and above the EQ
III event horizon. Because EQ III faults the
upper portion of TS-12 and, in places, TS-10,
the youngest reliably dated unit demonstrably
deformed by this event is TS-12.
Charcoal sample
TSSW-5, collected from TS-12 in the south wall,
yielded a radiocarbon age of 865 ± 35
yr BP,
calibrated
to 1045–1256 CE
(
2σ), providing the strongest
terminus post
quem for EQ III
(Allison
2013:159-162). Additional charcoal samples from TS-12 include sample TSSW-1 (1170 ± 20 yr BP;
774–943 CE) and sample TSSW-17 (1535 ± 25 yr BP;
428–591 CE), both of which are stratigraphically
consistent but provide weaker lower constraints
than sample TSSW-5.
An additional charcoal sample, sample TSSW-22, was
collected from TS-10, stratigraphically above
the possible EQ III terminations, and yielded a
radiocarbon age of 1055 ± 40 yr BP
(calibrated 893–1030 CE). In principle, this
sample could have provided an upper constraint
for EQ III, since TS-10 overlies parts of the EQ
III deformation horizon. However, because sample
TSSW-22 is older than sample TSSW-5 from the
underlying TS-12, Allison (2013) interpreted it
as remobilized inherited charcoal redeposited
into TS-10 prior to later erosion and fluvial
reworking. Accordingly, sample 22 was rejected
as chronologically unreliable
(Allison
2013:159-162).
The principal upper chronological constraint for
EQ III is charcoal sample TSSW-20, collected from
unit TS-5, which yielded a radiocarbon age of
280 ± 20 yr BP, calibrated to 1521–1661 CE.
Because TS-5 postdates EQ III and caps the
faulted sequence, this provides a usable
terminus ante
quem. Accordingly, EQ III is best bracketed between
1045 and 1661 CE, based on the youngest reliable
sample from a deformed unit (TSSW-5) and the
oldest reliable sample (TSSW-20) overlying the
capping unit TS-10. EQ II is constrained to the
same 1045–1661 CE interval, and Allison (2013)
suggested that EQ II and EQ III may represent the
same earthquake event.
Figure 4.3
Figure 4.4