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Towards a 3D Geotechnical Model of the Greater Beirut Area for Seismic Ground Motion Prediction

Description: 

Soil heterogeneity has a strong influence on the seismic motion variability. Local site conditions (lithological and topographical) can indeed generate amplification of ground motion at specific frequencies. Furthermore, small-scale heterogeneities of the lithological layers can redistribute the energy of the trapped waves, increasing the variability of the seismic movement recorded at the surface.
The Greater Beirut (GB-Lebanon) region is exposed to high seismic risk because of its location near major faults which can generate earthquakes of high magnitudes (>7). Moreover, the city is built upon a large variety of soils. The geology of GB (Dubertret 1944) shows that the area is divided into two categories: quaternary deposits and rocky outcrops. Quaternary zones are mainly composed of Embankments with Limestone Pebbles, Sand, Fixed Sand Dunes, and moving Sand Dunes. Rocky outcrops are made of Cenomanian, Senonian and Miocene.
As the area is not a classical sedimentary basin, mixing colluviums, wind- and fluviatile-deposits, site effects assessment is challenging. A 3D geotechnical model of the uppermost geology is thus established combining the scarce dataset of about 500 geotechnical boreholes and 700 geophysical measurements, randomly distributed over the area. First, combining H/V measurements and boreholes data allows us to define the average S-wave velocity propagation (Vs_mean). Then, knowing Vs_mean, we estimate the soft soil thickness at all of the measurement sites. These data helped us in reaching a primary simplified model of 2 layers, sediments over bedrock. The results show variability in the sedimentary thickness over the area.
This model will be used in numerical computations based on Finite Element Method to simulate the seismic wave propagation. To assess for the impact of small scale soil heterogeneities, a statistical study will be carried on aiming to estimate the vertical and horizontal correlation lengths of the soil layers mechanical properties from the borehole database.

Session: Site-specific Modeling of Seismic Ground Response: Are We Quantitative Enough to Predict? [Poster]

Type: Poster

Date: 4/19/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Marwa Safa

Student Presenter: Yes

Invited Presentation: 

Authors