EQ IV
EQ IV is the oldest and most strongly expressed
paleoseismic 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, 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 and continues northward
beneath the sabkha without obvious surface
expression. This trench geometry was chosen
specifically to intersect active faulting
associated with this complex
left-lateral strike-slip fault zone
(Allison,
2013:149-151).
The paleoseismic expression of EQ IV is
considerably more pronounced than that of the
younger events. EQ IV deformed the deepest
exposed stratigraphic sequence, including the
basal interbedded silty and sandy units TS-18,
TS-17, TS-16, TS-15, TS-14, and the overlying
sandy silt unit TS-13, which stratigraphic
mapping indicates was the ground surface at the
time of rupture
(Allison, 2013:151-159). A conspicuous
fissure, visible in both the north and
south trench walls, formed during this event and
was subsequently filled with material derived
from unit TS-13. This
fissure fill is one of the clearest
indicators that this was a surface-rupturing earthquake. One mapped EQ IV fault
trends N12°E, approximately parallel to regional
faulting. Some of the EQ IV fault strands were later
reactivated during younger seismic episodes,
indicating repeated use of the same fault
architecture through time
(Allison,
2013:151-159).
The tectonic character of EQ IV indicates
predominantly
strike-slip
deformation with a significant
dip-slip component. The Wadi ‘Arabah fault
is fundamentally a
sinistral transform system expressed
regionally by
en-echelon faulting,
rhomb-shaped grabens, and
localized extension
(Allison, 2013:139-141). In the trench, EQ IV
produced down-to-the-east displacement of the
sabkha sediments, demonstrating localized
extension superimposed upon the transform
setting. This dip-slip component generated
subsidence on the eastern side of the
trench, producing
accommodation space that was later filled
by younger sediments. The geometry is best
interpreted as localized
transtension.
Following EQ IV rupture, the newly created
depression trapped standing or periodically
ponded runoff, leading to deposition of the
approximately 1 m thick TS-12 unit composed of
finely laminated reddish-brown clay, silty
clay, and clayey silt
(Allison,
2013:151-159). The thickening of these
deposits toward the trench center and their
lateral pinch-out are consistent with
infill of a localized
subsidence basin generated by the earthquake.
This provides strong indirect evidence for
substantial
coseismic deformation during EQ IV.
Dating of EQ IV relies on radiocarbon constraints
from units stratigraphically above the event
horizon. The highest stratigraphic unit cut
directly by EQ IV is TS-13.
Charcoal sample TSSW-1, collected from
unit TS-12 immediately above and near the EQ IV
fissure fill in the south trench wall, yielded a
radiocarbon age of 1170 ± 20
yr BP,
calibrated to 774–943 CE
(
2σ), providing a potential
terminus ante
quem for the earthquake
(Allison
2013:159-162).
Another sample, charcoal sample TSSW-18, was
collected from above the EQ IV horizon in TS-13
on the north wall and yielded a radiocarbon age
of 2110 ± 35 yr BP (calibrated age 345–43 BCE).
This age was rejected as unreliable by Alison (2013) because was deemd inconsistent with TSSW-1.
Allison
(2013:159-162) interpreted TSSW-18 as
remobilized inherited charcoal
redeposited into younger sediment, rendering it
unsuitable for constraining the age of the
earthquake.
Although TSSW-18 is older than the
other dated samples, it is not out of
chronological sequence and may record a valid
date. While Allison (2013)
argued that sample TSSW-1 provides a defensible terminus ante
quem for EQ IV of 774–943 CE, a
broader view of the calibrated radiocarbon ages
from samples TSSW-5 (1045–1256 CE), TSSW-1
(774–943 CE), TSSW-17 (428–591 CE), and TSSW-18
(345–43 BCE) suggests that the chronological
constraints may permit a substantially older
event horizon.
Accordingly, although Allison (2013) favored correlation of EQ IV with one of the
mid-8th century CE earthquakes, the available radiocarbon
evidence does not require this attribution. In particular, the absence of a secure
terminus post quem
means that earlier earthquakes remain plausible candidates.
That said, estimated vertical displacement associated with EQ IV,
approximately
60–85 cm, compares favorably with the 65 cm of vertical displacement measured for
Event E4 in the nearby
Qatar Trench.
Klinger et al. (2015) dated Event E4 to a 671–845 CE
window using a
Bayesian model
of
radiocarbon dates
from
detrital charcoal,
and associated Event E4 with the
mid-8th century CE earthquakes.
Figure 4.3
Figure 4.3
Figure 4.4