Site response and liquefaction hazard analysis in Ramsar city, North of Iran

Abstract

Liquefaction hazard analysis is critical to the safety and cost-effectiveness of structures. Ramsar, the westernmost city of northern Iran's Mazandaran province, borders the Caspian Sea to the north. Since the soil strata of the Ramsar region are predominantly composed of low-grade sand and the groundwater is at low depths, it is highly vulnerable to liquefaction. Understanding how sedimentary basins react to seismic energy generated by earthquakes is a major concern for seismic hazard assessment and risk analysis. The primary purpose of this study is to determine the distribution of the natural frequency value, the amplification factor value and the soil vulnerability index. These were carried out as indicators for potential liquefaction sites in the city of Ramsar based on measurements of seismic ambient noise or microtremors. In this regard, ambient seismic noise was collected at 100 stations in the Ramsar region by Nanometrics Trillium40s intermediate sensors. Data are processed using the horizontal-to-vertical spectral ratio (HVSR) method provided by Nakamura (1989). Based on the results, the vulnerability index Kg is determined, which can be used as a parameter for calculating the region's liquefaction potential. Huang and Tseng (2002) suggested that the HVSR of microtremor data can be a good alternative indicator of the liquefaction potential of an area. It is revealed by previous researchers that improving the accuracy of geology and geomorphology-based liquefaction susceptibility map is accomplished by supplementing it with subsurface data (e.g., SPT, CPT, shear wave velocity data). The results of this study show that with the use of seismic ambient noise, the accuracy of the assessment of liquefaction susceptibility and zonation improves substantially.