EQ I
EQ I is the youngest paleoseismic deformation
event identified in the Taba Sabkha trench. 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 active fault trace
lies along the base of a linear pressure ridge
at the southeastern margin of the sabkha and
continues northward beneath the sabkha without
obvious surface expression. The trench geometry
was chosen to intersect active faulting within
this complex
left-lateral strike-slip fault zone
(Allison,
2013:149-151).
EQ I is expressed on the far eastern end of the
south trench wall, where three upward-terminating
faults cut completely through unit TS-5 and are
capped by unit TS-3, an interbedded fine-grained
light brown clayey silt and silty sand. The EQ I
fault terminations are buried approximately
50 cm below the present ground surface, beneath
younger units TS-3, TS-2, TS-1, and TS-0
(Allison,
2013:151-159).
The principal paleoseismic evidence for EQ I is
therefore the cluster of upward fault
terminations at a consistent stratigraphic level.
Unlike EQ IV, EQ I is not associated with a
documented
fissure fill, major
subsidence basin, or thick
syntectonic sedimentary package. Its
expression is also more limited than EQ II and
EQ III, since the EQ I faults were observed only
on the far eastern end of the south wall and not
clearly in the north wall. This limited exposure
may reflect the
en-echelon geometry of the Wadi ‘Arabah
fault zone, where individual fault strands can
step laterally and appear only locally within a
single trench exposure.
The tectonic expression of EQ I is consistent
with localized deformation within a distributed
strike-slip
fault zone. Because the Wadi ‘Arabah fault is a
sinistral transform system, the EQ I
faults are best understood as part of a
left-lateral fault architecture. However, the
available description does not indicate a large
vertical
dip-slip component, major
subsidence, or clear creation of
accommodation space. EQ I is therefore
best interpreted as a relatively small,
localized rupture or near-surface deformation
event compared with the more strongly expressed
EQ IV (Allison,
2013:162-169). Allison also noted that EQ I
could represent
sympathetic slip, meaning a smaller fault
movement triggered by stress changes from a
larger event on another fault at a different
location.
In dating EQ I, a reliable
terminus post
quem was identified, but no
reliable
terminus ante
quem could be established,
leading to the conclusion that EQ I occurred
sometime after 1521 CE.
The
terminus post
quem comes from
charcoal sample
TSSW-20, collected from unit TS-5 in the south
wall, which yielded a radiocarbon age of
280 ± 20
yr BP,
calibrated
to 1521–1661 CE
(
2σ). Because all EQ I faults cut through
TS-5, this sample provides the principal
terminus post
quem for EQ I
(Allison,
2013:159-162).
Finding a reliable
terminus ante
quem, however, proved impossible.
Charcoal fragment TSSW-28, collected from the
uppermost portion of TS-5, yielded a radiocarbon
age of 12230 ± 140 yr BP, calibrated to
12870–11808 BCE, and was discarded as
remobilized inherited charcoal
(Allison
2013:159-162). Allison also attempted to
date the unit capping EQ I by analyzing organic
material from a
bulk sediment sample collected from the
upper part of unit TS-3, above the EQ I fault
terminations. This sample yielded a radiocarbon
age of 1960 ± 110 yr BP, calibrated to
348 BCE–331 CE, which was also far too old for
its stratigraphic position and was therefore
rejected.
Accordingly, the defensible chronological
constraint for EQ I is limited. The event must
postdate deposition of TS-5, dated by TSSW-20 to
1521–1661 CE, while no reliable capping date is
available because both TSSW-28 and the TS-3 bulk
sediment date were rejected as inherited or
reworked material. EQ I therefore postdates
1521–1661 CE, but its upper age limit remains
unconstrained by reliable radiocarbon evidence.
Figure 4.3
Figure 4.4