EQ II
EQ II is a younger paleoseismic event identified
in the Taba Sabkha trench, younger than EQ III
and EQ IV but older than 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. The trench geometry
was selected specifically to intersect active
faulting within this complex
left-lateral strike-slip fault zone
(Allison,
2013:149-151).
EQ II is expressed by a series of upward
terminating faults that rupture to the top of
unit TS-10, making this the defining event
horizon for the earthquake
(Allison,
2013:151-159). Paleoseismic evidence for EQ
II includes upward fault terminations, vertical
offset of stratigraphic units, offset channel
deposits,
mismatched unit thicknesses across fault
planes, and reactivation of older fault
strands first active during EQ IV and, in some
cases, EQ III. Vertical displacement associated
with EQ II is modest but measurable, generally
on the order of 3–4 cm.
One of the clearest indicators of seismic
deformation is displacement of narrow sand
channel deposit TS-11 near the top of TS-12.
Because TS-11 predates deposition of TS-10 but
is clearly offset by later movement, it provides
independent evidence that deformation was not
simply superficial disturbance but genuine
tectonic displacement. Additional evidence comes
from abrupt differences in the thickness of
faulted stratigraphic units across individual
fault planes, indicating lateral displacement
consistent with strike-slip faulting
(Allison,
2013:151-159).
Ground shaking associated with EQ II appears to
have substantially disturbed unit TS-10. Allison
noted that TS-10 became largely homogenized in
places, particularly in the northern trench
wall, where the unit is thicker. This
homogenization likely reflects seismic shaking
and soft-sediment disturbance rather than normal
depositional processes, providing additional
support for a significant seismic origin
(Allison,
2013:151-159).
Interpretation of EQ II is complicated by later
erosion. Following EQ II, fluvial incision and
deposition produced units TS-8, TS-7, and TS-6,
which scoured away portions of the upper TS-10
sequence, especially in the south trench wall
and toward the western end of the trench. As a
result, some upward terminations that may once
have been visible were removed. Allison noted
that some scoured-away faults might potentially
belong to EQ III rather than EQ II, introducing
some ambiguity into event separation. For this
reason, Allison considered the possibility that
EQ II and EQ III may represent the same
earthquake expressed differently through the
soft-sediment fault architecture, although they
were initially mapped separately
(Allison,
2013:162-168).
The tectonic expression of EQ II indicates
predominantly
strike-slip
deformation with a minor vertical
dip-slip component. The mismatched
thicknesses of TS-10 and displaced TS-11 channel
deposits indicate lateral shear, while the small
vertical offsets indicate limited vertical
movement. Unlike EQ IV, EQ II does not show
evidence for major new subsidence, fissure
formation, or significant creation of
accommodation space. The event is therefore best
interpreted as dominantly strike-slip rupture
within a distributed soft-sediment transform
setting, perhaps with minor localized
transtensional deformation.
Dating of EQ II relies on radiocarbon
constraints from units stratigraphically below
and above the EQ II event horizon. Because EQ II
faults TS-10, the youngest reliably dated unit
deformed by this event lies below the rupture
surface. 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 II
(Allison,
2013:159-162).
Charcoal sample TSSW-22, collected from TS-10,
yielded a radiocarbon age of 1055 ± 40 yr BP
(calibrated 893–1030 CE). Although this sample
was collected near the EQ II horizon, Allison
rejected it as chronologically unreliable
because its age is older than TSSW-5 despite
being stratigraphically higher. It was therefore
interpreted as
remobilized inherited charcoal and not
considered a valid chronological constraint
(Allison,
2013:159-162).
The principal upper chronological constraint is
provided by charcoal sample TSSW-20 from unit
TS-5, which yielded a radiocarbon age of
280 ± 20 yr BP, calibrated to 1521–1661 CE.
Because TS-5 overlies the EQ II rupture horizon,
this provides a defensible
terminus ante
quem.
Accordingly, the best-supported chronological
bracket for EQ II is between 1045 and
1661 CE, although this range overlaps that
of EQ III and reflects the uncertainty regarding
whether EQ II and EQ III represent separate
earthquakes or different expressions of the same
rupture within this distributed strike-slip fault
zone.
Figure 4.3
Figure 4.4