A Possible Submarine Landslide and Associated Tsunami at the Northwest Nile Delta , Mediterranean Sea

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POTENTIAL FOR SUBMARINE L ANDSLIDES OFFSHORE THE NILE DELTA
Several researchers have studied submarine mass transports and slope failures on the Nile River delta.Using an extensive bathymetric survey and seismic profiling, Loncke et al. (2002) showed that the NDSTS is extremely vulnerable to mass-wasting events and has had several slope failures in the past; the failures were possibly triggered by seismic activity, gas seep instabilities, mud volcanoes, and salt-related faults.Ducassou et al. (2009) identified six deep-sea fans (DSF) offshore the Nile Delta (Figure 1), each of which includes a large canyon with walls that are susceptible to failure, and they reported several large-scale submarine slope failures over the last 200,000 years.Garziglia et al. (2008) identified seven mass-transport deposits in the western part of the NDSTS with thicknesses ranging from 10 m to 70 m and volumes between 3 km 3 and 500 km 3 (Figure 1).
The volumes of slides labeled 2, 4, 6, and 7 in Figure 1 were 500 km 3 , 63 km 3 , 14 km 3 , and 2.5 km 3 , respectively.Garziglia et al. (2008) speculated that the main trigger for these mass failures might have been large remote or even moderate local earthquakes.Loncke et al. (2009) showed that the NDSTS is the locus of

METHODOLOGY AND DATA
We numerically model tsunami waves by assuming realistic landslide scenarios.
We use two different models to simulate tsunami generation, propagation, and runup: TWO LAYER (Imamura and Imteaz, 1995;Yalciner et al., 2002) for

INTRODUCTION
Large hazards posed by landslidegenerated waves have attracted the attention of the scientific community in recent years (e.g., Synolakis et al., 2002, Yalciner et al., 2003;Bardet et al., 2003).
Unlike tectonic tsunamis, which often follow noticeable shaking caused by moderate/large earthquakes, landslidegenerated waves may occur without any advance alarm, making them a major hazard to coastal communities (Heidarzadeh et al., 2014).Thus, it is important to identify coastal communities that may be affected by submarine landslides and study the associated risks.
Here, we investigate the hazards from submarine landslide-generated tsunamis offshore of the Nile River delta (Figure 1) through numerical modeling of a possible landslide source.As the world's longest river, the Nile carries a large sediment load, and the area offshore the Nile Delta in the Eastern Mediterranean Sea where these sediments are deposited is known as the Nile deep-sea turbidite system (NDSTS; Heidarzadeh et al., 2014).The dimensions of the sedimented area is estimated to be 600 km × 300 km (Ducassou et al., 2009), indicating its potential as a source of large-scale submarine mass failure.In addition, the Eastern Mediterranean Sea is a complex tectonic zone that includes the edges of three plates, the African, the Arabian, and the Anatolian (Figure 1).Hence, the region is subject to many earthquakes (El-Sayed et al., 2004).The combination of high sedimentation rates and high seismicity makes the area susceptible to underwater landslides that could potentially generate tsunamis.By simulating realistic landslide volumes as possible tsunami sources, we produce tsunami runup heights along the region's coast.it was common to apply a static dipole wave as the initial condition for tsunami simulations (not only for the slidegenerated waves but also for solid block models; Harbitz, 1992;Bondevik et al., 2005a,b).This kind of approximation is acceptable for tectonic tsunamis with a high source speed (Heidarzadeh et al., 2014).However, the kinematic characteristics of mass flows on the seafloor due to landslides or slumps necessitate use of different types of initial wave profiles that take into account the slow generation velocity of seafloor landslides.(Insel, 2009).

SIMUL ATION RESULTS
Figures 3-6 and Table 1 show the results of the tsunami simulations.Snapshots of tsunami propagation show that the tsunami waves reach Eastern Mediterranean coasts about one hour after the landslide (Figure 3).They demonstrate that     The runup heights along northern, southern, and eastern coasts are in the range of 1-12 m, 1-6.5 m, and 0.5-3 m, respectively.In the simulation, computed maximum current velocities reached 2-5 m s -1 along the coast.As the region's coastal areas are heavily populated, such large runup heights along with high current velocities of ~ 5 m s -1 can be catastrophic.

DISCUSSION
Although submarine landslides with volumes of up to 500 km 3 are identified in the Nile Delta (Garziglia et al., 2008),   In some coastal sites, the largest waves occur in the second wave train, indicating that wave reflection is responsible.
A network of deepwater pressure gauges may be useful for detection and early warning of possible landslide tsunamis for the Turkish and Greek coasts.

Figure
Figure 1.General tectonic setting of the study area and locations of some submarine landslides in the Nile deep-sea turbidite system (NDSTS) identified by Garziglia et al. (2008).Ducassou et al. (2009) describe the deep-sea fans (DSFs) noted.
modeling landslide generation and NAMI DANCE for modeling tsunami propagation and runup.Until recently, Therefore, it is important to incorporate the slow movement at the ocean floor into the mathematical modeling.TWO LAYER(Imamura and Imteaz, 1995) solves nonlinear long-wave equations using the finite difference technique and following the leap-frog solution procedure within two interfacing layers with appropriate kinematic and dynamic boundary conditions at the seabed, interface, and water surface.The two interfacing layers are the water body and the mass moving at the bottom.When TWO LAYER tests the volume/size of the slide mass and the bottom slope in a regular-shaped basin, it is observed that after a certain time period, the landslide movement becomes very slow and then stops.Water surface elevation and discharge fluxes are stored in files generated by TWO LAYER when the landslide stops (approximately five to six minutes after its start).These files are then used as initial conditions in simulations that compute wave propagation and coastal amplification.NAMI DANCE is used in the simulations.NAMI DANCE is a numerical tool that solves the nonlinear form of shallow-water equations using the necessary initial conditions: water elevations and discharge fluxes computed by the TWO LAYER model

Figure 2 .
Figure 2. The study domain and locations of selected numerical gauge points as well as the landslide area (red polygon).

Figure
Figure 3. Snapshots of tsunami simulations at different times.The red rectangle in panel a shows the borders of a smaller domain used in panels b-d.
the entire tsunami simulation time (color map).In fact, it presents a deterministic tsunami hazard assessment for the region based on the landslide scenario we used.The color map in Figure 4 clearly indicates that the largest tsunami waves are funneled along specific paths in the region.These paths are usually forced by two factors: (1) the orientation of the landslide, and(2) morphological features in the region (e.g.,Satake 1988).According to the results displayed in Figure4, maximum tsunami waves are directed along about eight preferential paths that result in Cyprus acts as a shield protecting the Syrian and southern Turkish coasts, which receive relatively smaller tsunami waves compared to Cyprus (Figure4).

Figure 4
Figure 4 also shows the distribution of maximum wave heights along different coastlines in the region (bar plots) as well as maximum tsunami

Figure 5
Figure 5 plots the oscillations of tsunami waves at selected coastal sites in the region.This figure shows that tsunami waves are composed of several wave trains that result from multiple wave reflections because the Mediterranean is a semi-enclosed basin.Some of these late wave trains contain high energy and can be hazardous.For example, at two stations, Anamur and Antalya Bay, Figure 5 shows that the largest wave occurs in the second wave train.Table 1 we modeled tsunami generation by using a reasonable but significant size landslide with a volume of 41 km 3 .The simulated runup heights were up to ~ 12 m along the nearest coastal areas.Such large runup heights have the potential to produce significant damage and yield high death tolls; hence, tsunami mitigation measures are necessary.However, options for landslide tsunami countermeasures are limited in comparison to tectonic (earthquake-generated) tsunamis for several reasons.First, usually there are no precursors (e.g., an earthquake) warning of the possible arrival of landslide tsunamis unless they are caused by instabilities resulting from earthquake shaking.Second, landslide tsunamis usually produce larger wave heights on adjacent coastlines than tectonic tsunamis, which generate smaller events locally.Third, tectonic tsunamis usually occur along subduction zones whereas landslide tsunamis can occur in other specific circumstances such as high sedimentation rates.

Figure 4 .
Figure 4. Distribution of maximum (+) wave amplitudes at each computational grid point during the entire tsunami simulation time (color map) along with maximum runup amplitudes (bar plots) along different coasts in the Eastern Mediterranean.
, and 108Y227 project by TUBITAK and DPT 2011K140210 project and TUBITAK 2215 Grant for PhD Fellowship for Foreign Citizens.

Figure 5 .
Figure 5.Time histories of water surface fluctuations at different numerical gauge points.
Mohammad Heidarzadeh is Research Associate, Earthquake Research Institute, University of Tokyo, Tokyo, Japan.Rozita Kian is Research/Project Assistant, Ocean Engineering Research Center, Department of Civil Engineering, METU, Ankara, Turkey.Fumihiko Imamura is Professor of Tsunami Engineering, International Research Institute of Disaster Science, Tohoku University, Sendai, Japan.