i- segment of the convergence boundary between

i-     
Earthquake Hazard and susceptibility to Tsunami

Earthquakes
strike without warning and cause widespread damage to various structures and
systems. These can neither be predicted nor prevented in terms of their
magnitude, place, and time of occurrence. Globally, between 1950- 1999,
earthquakes constituted 29% of great natural catastrophes, with 47% of the
fatalities, 35% of economic losses and 18% of insured losses (Munich Re Group
2002). Egypt is located in the southeastern part of the Mediterranean Sea, and
represents a subordinate part of the Eastern Mediterranean region, which is a
small ocean basin known by its unusual tectonic complexity. It includes a short
segment of the convergence boundary between Africa and Eurasia creating
northward movement of the African Plate relative to the Eurasian Plate (Rabinowitz
and Ryan, 1970; Taymaz et al. 1990; Abou Elenean, 1993). Subduction in this
segment is along two very small arcs, the Hellenic and Cyprean arcs. The
shallower earthquakes’ activities in the Hellenic arc are higher frequency than
in the Cyprean arc, which extends from Albania in the west to southwestern
Turkey in the east (figure 3). These activities are mostly seaward that are
potential for creating Tsunami. Moreover the southern part of the Aegean Sea is
moving as a relatively rigid block compared with the surrounding zones
(McKenzie, 1978).

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 Applying the
Gets-Ord Gi analysis on the distribution of the earthquakes’ magnitudes, it optimized
hot spot areas that clearly show the highest number of earthquakes is located at
the Agean Sea plate figures (4). The majority of the hot spotted earthquakes
are located at the Agean sea plate while the cold spotted ones are at the
Anatolia Plate. Also, the earthquakes magnitudes’ clusters and outlier analysis
using Anselin Local Moran’s I statistics, shows that the high-high cluster
earthquakes are dominated at the Agean Sea plate while the high-low and low-low
cluster earthquakes are dominated at the Anatolia plate  figure (5).  

 

The
boundary between the African and the Anatolian-Aegean sub-plate is delineated
by the Hellenic arc, the Pliny-Strabo trench, the Flornce Rise and Cyprus in
the east (Aksu et al., 2005). The Cyprean arc is a part of the plate boundary
between Afro-Arabia and Eurasia in the Eastern Mediterranean. Seismic activity
and gravity anomalies indicate that, a northward subduction of oceanic material
related to the African Plate beneath the Turkish Plate is the mode of
convergence along the western segment of the Cyprean arc (Ben-Avraham et al.,
1988). Subduction is interrupted due to the collision of Eratosthenes seamount
at the central segment of the Cyprean arc, which forms a zone of intense
deformation (Giermann, 1969).

 Whereas, the
north most area of the African plate, from the Egyptian coast to the Agean
plate, is dissected by the Mediterranean Ridge, Strabo Trench and Pliny trench
proves two results. The first is that the majority of the hot spotted
earthquakes are located at the Agean Sea due to the named ridges/trenches which
make the frequency and severity of earthquakes than that of Anatolia plate. The
second is that the northward movement rate towards the Asia-Africa plate is a
bit lower due to named ridges/trenches making a resistance to the African plate
northward subduction than that to the eastern Mediterranean side where only the
Cyprus trench exists, figure (6). Therefore, the potentiality of tsunami
phenomenon might happen.

 

ii-   
Land subsidence – submergence

The Nile Delta
is geologically created as a deep graben that filled with thick column of clay
and fine sediments. Few researches recognized the difference in the land
subsidence along the Nile Delta coast founding that the eastern region is
higher than the west. They also anticipated that the land subsidence in the
Nile Delta is due to the compaction of sediments. The difference of the land
subsidence rates of the northern Nile delta is not only referred to the
sediments’ compaction that stated by Staley and Andrew (1993), Frihy (2003),
but also it may be varied due to the differential rates of the African plate
northward movements from the west to the east. This variation is previously
stated by (Stanley 1988 &1990) due to differential compaction and varying
thicknesses in the Holocene layers. Stanely and Clemente (2016) summarizes the
land subsidence rate of the Nile delta coastal zone as about 3.7 mm/yr in the
NW delta,  about 7.7 mm/yr in the N
delta, and  about 8.4 mm/yr in the NE
delta, based on compaction rates of Holocene sediments’ thicknesses which
decreases from the east to the west.

Further to the
east and based on age-dated sediment core sections, Stanley (1990) and Warne
and Stanley (1993) have estimated long-term average subsidence rates across the
Nile delta region. The processes of compaction and dewatering of the thick
accumulated deposits of fluvio-marine deltaic mud sequence formed in the
Holocene have induced higher rates of subsidence ranging from 1 to 5 mm/yr
(Stanley 1990; Chen et al. 1992). Subsidence has been considerably lower in a
westerly direction, ranging from 5 mm/yr at Port Said in the east to 1 mm/yr
further to the west of Alexandria region, Figure (7). Thickness of Holocene
strata beneath the modern delta plain is a direct function of subsidence, which
ranges from 50 m at Port Said and tends to decrease or be nearly absent
westward, below the Alexandria coastal plain, Warne and Stanley (1993).

 The variability
of emergence and subsidence of Alexandria land (ranging between –5 mm and +7 mm
per year) as calculated by Warne and Stanley (1993) may be attributed to the
impact of tectonic activities. Tectonic activities in Alexandria have been
recorded from the observations on submerged Roman and Greek ruins in the
Eastern harbor and Abu Quir Bay, as old as 2,500 years had submerged from 2 to
5.5 m, (El Sayed 1988; Warne and Stanley 1993). The Hellenistic city of Canopus
(500 BC) in Abu Quir Bay was originally located 3 m above sea level, which may
imply that it has submerged of 8 m during the past 2,500 years.

  This research
founds that the driving forces influence the subsidence of the Nile delta
coastal zone is varied from the west, where the geo-tectonic-controlled, to the
east, where the delta sediments compaction. We propose that land subsidence is not
only due to the compaction of the Nile delta grapen’s sediments (if this is the
case the rate of the subsidence should be similar along the Nile Delta), it is
driven by geo-tectonics. The differential movement of the African plate is also
generates the differential sea waves and tidal gauges as well. In this case,
the sea level rise potentiality is a result of the subduction movements of the
African, Agean and Arabian tectonic plates. This align with Zaghloul et al.
(1999) and Garziglia et al. (2008), where the structure framework controlling
the vertical motion due to the geodynamic setting including earthquakes
epicenters and major active fault trends were detected at the north western
Nile delta. The structural pattern results from a complex interplay of fault
trends of the N-S faults (Abu Quir), NE faults (Rosetta), NW
Suez-Cairo-Alexandria line, and NE-SW (Qattara-Eratosthenes line). These
structural trends indicate that the main cause of subsidence in the western
Nile delta is ongoing faulting, as well as down warping, of the underlying 3000
m of Late Miocene to Quaternary sequences.

 iii- 
Expected Damage and Loss Assessment

To map and determine the areas
probably exposed to natural hazards in the coastal zone of Egypt, the spatial
distribution of natural hazard information and the population were overlaid and
analyzed using geographic information system (GIS). The assessment of
earthquakes hazard was carried out base on the records of the last 60 years,
however tsunami hazard was projected based on the modeling of the seismic
amplification and using GIS. The marine submersion can be defined as the
temporary flooding of coastal region by sea during severe meteorological
(strong depressions and sea wind) and tidal conditions causing storm surges. The
submersions may occur due to very high waves (tsunamis) provoked by under water
land slides or earthquakes. The variations of the water level at the origin of
marine submersion are a combination of several phenomena: astronomical tide,
meteorological factors (wind, atmospheric pressure), hydrodynamic factors
(set-up, surf-beat, etc.). Based on a critical analysis of the IPCC projections
and the latest references in the literature on the subject, a global rise in
sea level of about 20cm by 2030 is assumed.

The total storm
surge considered in the case of an exceptional combination of events (50 years
return period) is therefore estimated as +1.57m LAT (Low Astronomical Tide) at
Alexandria by 2011 and is projected to +1.77m LAT by 2030, (World Bank, 2011).
In this case, the calculated change rate of Astronomical tide is about +0.01
m/yr. this means that, the Astronomical Tide will reach about +1.97m, by 2050.
Therefore, the vulnerability of the Egyptian coastal zone, especially, the
low-lying areas of the Nile delta will be threatened by a certain degree of
susceptibilities to erosion and thereby sea incursion.

The dominant
land use/land cover activities are the croplands/vegetation that cover about
45% of the studied area (more than 800,000 Acres), 10% urban, 11% water bodies
and wetlands and more than 30% bare, sand dunes and deserts, table (2). The
croplands varied from dense and healthy agricultural land, shrubs and reclaimed
land. In addition, there are industrial centers, ports, infrastructures
(transportation, power network, telephone, etc.), archeological sites, the drainage
network and the recreational.

To estimate the
losses from a disaster for identifying the post disasters needs, there is a well-known
worldwide methodology that analyze disaster effects and disaster impact and
identify recovery needs including human, socio-cultural, economic, and
environmental perspectives. This methodology called “The Post Disaster
Needs Assessment (PDNA)” or Damage and Loss Assessment “DaLA”, The
World Bank (2009). However, the assessment methodology focuses on three main elements:
assessment of disaster effects, assessment of disaster impact, and recovery
strategy and needs, in this research the focus is to assess the disaster impact.
This includes the quantification of damage and losses. Damage to infrastructure
and physical assets is the quantification of public and private sector
infrastructure and assets destroyed in the disaster either total or partial.

In this context, this methodology is adopted to estimate the
anticipated losses in the coastal zone of Egypt. The amount of losses is also
depending on the type and the degree of the hazard it may hit this area. We
propose three different scenarios possibly create damage and losses:

a-     
Tsunami
as a result of marine earthquake,

b-     
Earthquake
without tsunami, and

c-     
Tidal
gauges

 Rather than the
higher economic damages and losses that affecting the urban class, there are social
damage related to human life, jobs, homeless, and humanitarian consequence that
are difficult to be economically valued. The most vulnerable and sensitive
urbanizations are the informal or slummy setting up where it includes more than
one third of the population.  In
addition, they are characterized by high population density, poor building
conditions (not well structured), absence or bad conditions of infrastructure,
and high percentage of people living below poverty line. So, the damage and
losses is expected to be higher than at the well-set up building.

 

We proposed 2 variables that could estimate the total cost pf loss,
1) damage per meter (m/LE – Egyptian Pound) and 2) proposed cost of recovery to
this damage (m/LE – Egyptian Pound). This proposal differs from country to
other and based on the market prices. However, it presents an estimation of the
volume of losses. Applying this proposal to the first scenario of Tsunami
impact on the land use and land cover in the coastal zone of Egypt could
provide:

·        
if
only 10% of the urban area is affected, we will lose about 77,426
million LE as a damage estimate and need about 387,129 million LE as total
estimate of losses.

·        
The
croplands areas that are threaten by the marine submersion are calculated as
800,000 Acres, i.e. about 10% of the cultivated area in Egypt. Therefore, one
tenth of the national GDP of agriculture will be lost and it may recover
through 3-5 years. So, this may need 3-6 times of the required budget to fill
the food security gap until the full production taking into consideration the
food prices and the population increase.

 

The second scenario for a severe earthquake, the impact will
affecting all the urban activities like residential, industrial,
infrastructures, ports and harbors except the agricultural and the water bodies.
The estimate losses is summarized in table (2).

 

The third scenario for the tide gauges events that threaten the
low-laying areas, mainly 2 meters below m.s.l., including any land use
activities. In this scenario, the threatened area will be less than the study
area based on the topography, table (2). According to Frihy (2009), there are
about (700 km2) Low-lying vegetated lands down to -3 m below sea-level only
within Alexandria governorate. The Buildings and the infrastructures facing the
coastline likely to be affected by the coastal erosion and marine submersion
risks are port facilities, coastal roads, and dwelling houses on the coastline
directly exposed to these risks. The sanitation system is highly sensitive to
heavy rains, and in case of overflow, streets and tunnels may be flooded. The
reclaiming wetlands and other low-lying areas planned for construction put
future population at risk of damages from potential earthquake, land subsidence
or flooding.

 

5.     
Conclusion

However, the Egyptian coastal zone is known as medium to low
susceptible zone for Tsunami, it had been experienced two ancient destroying
tsunamis dated 365 and 1303 AD with wave height 1 and 2.9m respectively, and
the anticipated return period is 800 years, then there is potentiality of
tsunami might occur before the end of the 2100. Tsunami hazard might make severe
damage and losses of more than one third of the Nile delta coastal zone. The geomorphology
of the coastal zone is occupied by large stretches of sandy beaches easily exposed
to erosion and low topography lying areas together with land subsidence will
worsen flooding and marine submersion risks. The area could be categorized
according to the natural hazards as medium-high for flood and marine submersion,
low-medium for earthquakes and low for tsunami. The dense population and
infrastructures of the area maximizes the proposed damage and losses.

 

The magnitude of a marine submersion by storm surge or tsunami does
not depend only on the storm surge level, but also on the coastal morphology,
which minimizes or accentuates the phenomena impact. So, the low topography and
very low slope along the Egyptian coast zone, makes the hazard potentiality is
high especially on the most sensitive spots like ports and harbors of
Alexandria, Damietta and Port Said in addition to the highly sensitive
infrastructures and highly population as well. The natural resources and dense
socio-economic activities such as coastal lagoons (Bardawel, Manzala, Burullus,
Idko and Maryout), fish farms considered the main food supply of fish in Egypt
will be under major threat. Major sea ports are located on the Mediterranean,
which might extend the consequences of the damage to the global level. 

 

One of the mitigation mechanisms is to install protection concrete
structures that protect the coastal zone and minimize the impact of such
hazard.  Nearly 55% of the coastal zone
of Egypt is naturally protected by ridges and higher land that could act as a
protection wall. The Government of Egypt has installed protection engineering
constructions to the vulnerable areas, which covered nearly 15% of the coastal
zone. Currently, about 30% of the coastal zone is not protected which is under
full major threat. Figure (7) shows the three categories of the protection
zones within the Nile Delta. The western coastal west of Alexandria, is
partially naturally protected, however there is large touristic activities
directly to the sea with no protection. The other eastern part of northern Sinai
is not protected with sand dune and low laying topography.  It is recommended that a protection plan to be
adopted to install engineering structures that protect the highly economic
zones on the coastal zone.