# Joint Multi-Scenario-Based Earthquake and Tsunami Hazard Assessment for Alexandria, Egypt
###### Abstract
However, it is noteworthy that large offshore earthquakes can be accompanied by tsunamis, as evidenced by historical reports ([5; 13] and references therein), geomorphologic studies, and investigations of paleo-tsunami deposits (e.g., [14]), which can have cascading effects. In this current work, we aim to provide a step forward in jointly assessing earthquake and tsunami hazards and risks for Alexandria. The inadequate understanding of seismic hazards for a given area under urban development may lead to the growth of megacities in seismically active regions with unsafely constructed buildings and infrastructure. Large and even medium-sized earthquakes can cause significant damage in such regions. Seismic hazard studies aim to assess the potential impact of earthquakes, either directly or indirectly, to provide a knowledge base suitable for seismic building codes to guide the design and retrofitting of engineering structures. The first objective of this work is to conduct a neo-deterministic seismic hazard analysis (NDSHA) at selected sites in Alexandria. The NDSHA methodology (outlined in Section 3.1) incorporates various geological, geophysical, and seismological data, such as earthquake catalogs, focal mechanisms, seismic nodes, seismogenic zones, and subsurface structural models. This study utilizes these data to update the seismic hazard maps for Alexandria and create a dataset of synthetic broadband accelerograms/seismograms. These synthesized records can be directly employed as seismic input for engineering analyses and offer a comprehensive investigation of seismic zoning on an urban scale. A second, complementary objective is to produce detailed tsunami inundation maps for the Alexandria coastal area. To achieve this, this work was divided into different steps: (a) identifying and extracting from the work of [PERSON] et al. [3], which contains the last updated maps at the national scale, significant pre-computed earthquake-induced tsunami scenarios in the Eastern Mediterranean that may cause maximum wave amplitude along the Alexandria shoreline; (b) adopting various Manning's Roughness Coefficients (i.e., 0.015, 0.03, 0.05, and 0.08); (c) drawing computed maps with an inundation depth of a 30 m resolution, based on an empirical model that utilizes the maps of average and maximum tsunami wave amplitude provided by [PERSON] et al. [3], along the Alexandria coastline using QGIS 3.34.11 [15] for spatial analysis and visualization; and (d) producing aggregate maps of the average and maximum tsunami inundation zone. Thus, this current work aims to provide a city-scale joint seismic and tsunami hazard analysis, updating and providing detail to some prior studies (i.e., [2; 3]). In terms of seismic hazard maps, significant differences arise from the updated earthquake catalog used in this study, which extends to 2020. By contrast, the catalog utilized by [PERSON] et al. [9] only extends to 2015. Also, the impacts of distant earthquake scenarios, not considered by [PERSON] et al. [9], on Alexandria are addressed in Section 3.3. The seismotectonic model employed in this study, differs from the models used in the aforementioned works. Furthermore, the computation in this study was performed at a frequency of 10 Hz, which complements the approach utilized by [PERSON] et al. [9], with significant variation in the spatial scale of interest and in the methodology employed. In terms of seismic input, this study provides comprehensive information such as peak ground acceleration (PGA), peak ground velocity (PGV), peak ground displacement (PGD), and response spectra for each site in the grid, which has not been previously achieved at this level of resolution [PERSON] et al. [2] developed tsunami hazard maps for Alexandria based on worst-case scenarios, employing a single scenario for each tsunamigenic source. They also developed an inundation map for Alexandria using a simplified hydrostatic model that did not consider water resistance.