Investigation of site fundamental frequency using H/V method guided by geomorphic features derived from old aerial photos

Abstract

The fundamental frequency of soil (non-lithified cover on the bedrock) is one of the most important factors in seismic hazard evaluations. On the other hand, this frequency expresses several factors such as the clast size, sorting, and the degree of compaction and cementation of soil, which is in close relationship with both the geomorphic setting and processes responsible for the soil formation. We present a rapid and cost-effective multi-disciplinary approach to study site effect in developing urban areas using the spatial correlation between geomorphic setting of soil units and their fundamental frequencies. As a case study, the city of Zanjan was selected because like many other cities of developing countries has been expanded drastically during the last half-century. We use old aerial photos (produced in 1966) of Zanjan and its surroundings to reconstruct a unified orthophoto-mosaic of the study area. This orthophoto-mosaic exhibits pristine geomorphic landforms and features, which predate the current urban coverage of the city, and provides an excellent possibility for mapping Quaternary geological units with distinct geomorphic settings and soil characteristics. Besides, the site effect is measured through the H/V spectral ratio method at 91 points along the profiles pre-designed based on our geomorphic mapping. Subsequently, the spatial pattern described in the geomorphic map was compared with the site effect values. This comparison shows a general correlation between the geomorphic setting of alluvia and the corresponding fundamental frequencies of the soil such that the closer to the mountain front, the higher the fundamental frequency. The results indicate that the peak frequency is observed in the northern regions of the city where a relatively thin layer of unsorted coarse-grained alluvium overlies the bedrock. In the southern geomorphic depressions close to the Zanjanrood river and behind the East-dipping North Zanjan reverse fault (east of the city), where a pile of fine-grained deposits is found, the fundamental frequency reaches its lowest values, namely less than 1 Hz. A comparison of the soil frequency map with the map of shear-wave velocities provided by geotechnical borehole data also indicates a meaningful correlation. Our approach provides a twofold benefit by (1) reducing both the time consumption and expenses in site effect studies through an optimal design of H/V measurement points and borehole drillings based on a precise mapping of distinct geomorphic units, and (2) proposing a unique possibility to produce a reliable site effect map in the urban areas developed during the last half-century.