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results of theoretical modeling and field
measurements of sediment displaced from
various coastal and offshore shelf sectors
and their migration patterns through time
(UNDP/UNESCO, 1978; Frihy and
El-Sayed, 2013). Teams studying these
problems have included several interna-
tional organizations, the Coastal Research
Institute in Alexandria, and other Egyptian
research centers and universities. Shoreline Considerable information on seismicity
stabilization is now partially achieved, at
least locally where effective protection
measures have been implemented, mostly
since the past half-century following clo-
sure in 1965 of the Aswan High Dam.
The much-increased role of human
activity through time in this region needs
to be emphasized. In Predynastic time,
from ~5000 B.C. onward, hunters and
gatherers in small numbers migrated to the
delta from adjoining arid, sand-rich Sahara
terrains. The major cultural-ecological
shift to irrigation farming and urbanism on
the Nile’s delta plain, with its available
water and organic-rich soil, occurred dur-
ing the Early Dynastic to Dynastic at ca.
3000 B.C. This evolved to what Butzer
(1976) defined as an early hydraulic civili- less frequently (Korrat et al., 2005).
zation, a socio-economic system based on
deliberate flooding and draining by sluice
gates and use of water basins contained by
dikes. The major shift in population from
the Nile valley northward to the delta took
place during the Hellenistic period (323–
30 B.C.), a time when Egypt’s total popula-
tion increased to ~5 million (Butzer, 1976). Suez-Cairo-Alexandria trend and a NE to
At present, the country’s population has
reached ~90 million, with a growth rate
above 2.0%/yr. Of this number, ~45–50
million people live in the delta and its
proximity and, of these, roughly 20 mil-
lion are concentrated in and around Cairo
at the delta’s southern apex, with another
15–20 million occupying the delta’s south-
ern and central sectors. About 10 million
more people inhabit the northern delta’s
coastal region; this number includes the
4.5 million people living in Alexandria,
Egypt’s second largest city and its major
port and industrial center. Population den-
sities are extremely high (to 1000 or more/
km2) in much of the Nile delta and valley,
two sectors that comprise areas of 22,000
km2 and 13,000 km2, respectively, and that
together form only ~3.5% of Egypt’s total
area. Due to anthropogenic stresses, the
Nile delta has been modified to the extent
that it no longer functions as a natural
6 GSA Today | May 2017
fluvial-marine depocenter (Stanley and
Warne, 1998).
NEOTECTONIC EFFECTS
Neotectonics refers broadly to active
seismicity and tectonic movements that are
geologically recent, from the Tertiary
onward and continuing to the present.
and tectonics affecting the delta’s surface
and subsurface sequences has been pub-
lished in recent years, following active
exploration at the delta’s hydrocarbon
provinces. Data has been obtained from
2D and 3D seismic surveys, along with
well core analysis, in the northern delta
and offshore shelf. These and seismograph
monitoring stations have helped clarify the
relation between Holocene sediment
deposits and their displacement by earth-
quakes, faults, and delta surface linea-
ments, the latter mapped by Landsat.
Although identified as a passive margin,
northern Egypt is characterized by fre-
quent earthquake events of shallow origin
and small magnitude (M of 3 or less) and
of moderate quakes (M to 5) that occur
Earthquake and fault patterns are inter-
preted as results of ongoing interactions
among African, Arabian, and Eurasian
plates and the Sinai subplate.
Two active linear belts cross Egypt and
intersect in the delta: a SE to NW Gulf of
SW Eastern Mediterranean-Cairo-Fayum
trend (Fig. 3A), with slip faults associated times have exceeded those of eustatic fac-
with these megashears (Kebeasy, 1990;
Gamal, 2013). Earthquakes related to
major trends include those that recently
affected the Cairo area (in 1992, M = 5.9),
and the Alexandria to offshore sector (in
1998, M = 6.7); the latter initiated at a
depth of 28 km. Alexandria, in particular,
is a high earthquake risk zone that has
periodically experienced considerable
damage, such as the powerful quake and
associated tsunami recorded in 365 A.D.
that destroyed much of the city (Guidoboni
et al., 1994), and major quakes in 1303 and
1323 A.D. that damaged the famous
Alexandria lighthouse. There is evidence
of seismic damage and fault offset in
Alexandria that have disturbed ancient
construction and also recently tilted some
buildings. Modern earthquakes with M t5
occur about every 23 years. Although most
quakes are of lower seismicity, together
they constitute a source of seismic hazard
for the many who live in the delta and its
coastal margin (El-Ela et al., 2012).
An E-W–trending series of faults crosses
the northern delta and form a hingeline or
hinge zone (Fig. 3B) that separates a south-
ern delta block from a northern delta block.
It is defined by step faults and gravity-
induced displacements associated with
down-to-north rotated fault blocks that
extend from deep structural offsets to the
surface. Geophysical surveys show that
such features mapped in subsurface on
land in the northern delta block continue at
depth offshore (Sharaf et al., 2014). Ground
motion measurements indicate that these
can affect both surface deposits of the
northern delta and seafloor strata offshore
(Mohamed et al., 2015).
Tectonic activity associated with reacti-
vation of faults at depth can cause struc-
tural displacements that in some cases
extend upward to Late Pleistocene and
Holocene surface deposits (Fig. 3C).
Surveys have shown a close relation
between underlying structural patterns and
some delta plain surface lineaments, faults,
and earthquakes (Elmahdy and Mohamed,
2016). By offset and subsidence, in part
from isostatic loading and readjustments of
strata at depth, the northern delta–offshore
shelf sector has accumulated up to 3500 m
of underlying stratigraphic section since
the late Miocene (Fig. 3C); the depth to
basement rock is 9–10 km (Kellner et al.,
2009). Tectonic effects in this region at
tors in controlling coastline evolution
(Sarhan, 2015), such as at the NE delta
coast in the Manzala lagoon area. This
may be caused by pull-apart basin develop-
ment (Stanley, 1990) where a much thicker
Holocene sequence (to ~50 m) of mud-rich
deposits accumulated beneath the present
shallow, elongate lagoon (Fig. 2B).
Petrologic examination of well core sec-
tions records examples of post-depositional
faulting, slumping, and sediment deforma-
tion (Rizzini et al., 1978). Features such as
convolute bedding of mud- and silt-rich
deposits suggest processes associated with
sediment dewatering and liquefaction.
Likely triggers are earthquake tremors that
can generate high excess pore-water pres-
sure within unconsolidated subsurface and
surface deposits. Examples of recent earth-
quake-triggered liquefaction were recorded
at ground surface shortly following the 1992