Primary-level Site Effect Zoning in Developing Urban Areas Through the Geomorphic Interpretation of Landforms

The site effect has a great impact on seismic hazard assessment in urban and industrial regions.
For instance, a layer of soft soil with a thickness of several meters amplifies seismic waves from
1.5 to 6 times relative to the underlying bedrock. Therefore, investigating the main characteristics
of Quaternary deposits such as the granulometry and mechanical layering is crucial in site effect
studies. These parameters are directly related to the local geologic/geomorphic setting and genesis
processes of the Quaternary deposits. Nevertheless, large cities in development countries have 
rapidly been enlarged covering Quaternary terrains before being evaluated for the site effect. This
rather rapid growth in urbanization interested us to take advantage of ancient aerial photographs
reprocessed with new image processing techniques in order to provide 3D terrain models from
such kind of areas before the recent urbanization. It helped us in the geomorphic terrain
classification and the detection of regions with different site effects originally caused by the
geomorphic setting and genesis of the Quaternary terrains. For example, site effect in a river flood
plain will be different from surrounding areas underlined by alluvial conglomerates or bedrock.
The main target of this study is investigating the primary-level site effect in Urmia city using 3D
geomorphic models derived from ancient aerial photos taken in 1955. Urmia in NW Iran is one of
the populated high-risk areas according to the standard regulations of earthquake in Iran, and
covers a wide region from mountainous areas to the ancient coast of Lake Urmia, with the Shahr
Chai River as the axial drainage. We created the 3D terrain model through the Structure from
Motion (SfM) algorithm. We have provided a detailed geomorphic map of Plio-Quaternary terrains
using the 3D Anaglyph view, Digital Elevation Model (DEM), and orthophoto-mosaic of the
region. It was complemented by granulometry and mechanical layering information from the
available geotechnical boreholes to reconstruct a shallow soil structure model for the area. It
allowed us establishing a primary-level site effect zoning for Urmia. Our results reveal the
presence of five distinct geomorphic zones, with different genesis processes and soil characteristics
from piedmont to coastal zones, which represent different soil structures and probable site effects.
This zoning paves the way for performing complementary site effect investigations with lower
time consummation and cost. The developed method, proposes a sophisticated tool to evaluate
primary site effect in areas covered by urbanization subjected to future natural hazards like
earthquake, landslide and flood before designing geophysical networks for the measurement of
quantitative site effect parameters such as Nakamura microtremor H/V and Multichannel Analysis
of Surface Waves.
Key words: Earthquake hazard, Site effect, Image Processing, Aerial photos, Quaternary geology, Structure from
Motion