# Joint Multi-Scenario-Based Earthquake and Tsunami Hazard Assessment for Alexandria, Egypt
## 3 Multi-Scenario Physics-Based Seismic Hazard Analysis
### Data and Methods  
In this study, ground motion intensities were derived from broadband time histories computed at the urban scale for a set of selected sites located on a regular grid of approximately \(0.1^{\circ}\times 0.1^{\circ}\), as depicted in Figure 4. The NDSHA approach used in this study, which has been widely and flexibly used at both regional and urban scales [6; 8; 10; 41; 43; 45; 46; 47], is a physically based approach that simulates potential earthquake ground motions from multi-scenarios. Its suitability for Egypt's complex tectonic setting, limited seismicity record availability, ongoing infrastructure development, and redundancy of historical moments, makes it a valuable tool for sound seismic hazard assessments (see [43] for a full methodological description). As illustrated in Figure 5, this study incorporates two main components as input for hazard assessment: seismogenic sources (represented by zones and nodes) and crustal structural models for the bedrock. In this work, the NDSHA seismic scenario definition procedure was updated in terms of earthquake catalog, both in time (extended to 2020)  
Figure 4: Map of the sites of interest at the city scale, used as observation sites at which seismic hazard parameters are provided Each numbers represent the site number and the blue polygons represent the main districts in Alexandria. and space (the incorporation of far seismogenic scenarios, as shown in Section 3.3), and seismogenic sources from [PERSON] et al. [8]. In fact, the seismogenic zones model defined by [PERSON] et al. [8] was updated to take into account the recent seismic activity in the western region near Alexandria [39], adding seismogenic zone no. 22, as shown in Figure 6.  
Also, if the maximum recorded magnitude within the newly delineated seismogenic zone 22 was less than 5, a default magnitude of 5 was assigned, as for each zone, following the NDSHA gridding and smoothing process (see [43] for a complete description). This decision was based on the hypothesis that wherever a seismogenic zone is identified, potentially damaging earthquakes could occur. Conventionally, a magnitude of 5 is considered the lower bound for damaging earthquakes [48], and that is the value adopted as the minimum magnitude threshold for the updated catalog. Similarly to the work carried out by [PERSON] et al. [8], we selected the focal mechanism of the most significant event within a specific zone to represent that zone. However, in this study, we carefully examined and revised representative fault plane solutions based on the latest publications on seismic source characterization for Egypt, including recent works [19; 31]. Fault plane solutions for significant earthquakes in each seismogenic source were collected and cross-checked between different, previous works. Then, the focal mechanism for the largest magnitude was chosen to represent the present-day dominant tectonic regime of the corresponding seismogenic zone. Figure 5: Flow chart of the NDSHA approach adopted for the scale of our analysis. An average bedrock structural model representing the lithospheric properties of the study area was adapted from [6] and used in this study. This model is represented by several flat and anelastic layers with defined thickness, density, P- and S-wave velocities, and attenuation for bedrock conditions at the site of interest (Figure 6). [PERSON] et al. [6] originally derived the 1D-crustal model from deep seismic sounding and Bouguer anomaly profiles published by the Egyptian General Petroleum Company (GAPCO). These datasets are stored in the Atlas of Geology at Cornell University, USA [6]. Physics-based ground motion modeling is currently limited to 10 Hz. Therefore, all ground motion parameters in this study were computed for bedrock conditions for a grid of sites that covers Alexandria at a 10 Hz cut-off frequency.