Seismic microzonation of Mashhad city, northeast Iran

Abstract

Mashhad city has a substantial risk of earthquakes due to the potential of the underlying soft soils to amplify ground motions. It is therefore imperative to conduct detailed seismic hazard assessment of the area (seismic microzonation). Seismic microzonation of big cities, like Mashhad, provides a basis for site-specific hazard analysis. In the present study, microtremor observations were carried out at 90 sites in the east part of Mashhad city. The predominant frequency and spectral ratio amplitude of the ground were determined by the Nakamura technique, and a microzonation map was developed for Mashhad on the basis of the variations of the predominant frequency and spectral ratio amplitude of the ground. Soil texture and N-value standard-penetration-test (N-SPT) maps and sections were prepared for different depths below the city, by analysis and interpolation of data collected in the study area from 169 boreholes. Then the predominant frequency variations were compared with soil texture and N-SPT variations for four sections in different directions in the study area. This comparison shows that with decreasing soil softening and N-SPT values, the soil predominant frequency also decreases.

Keywords

• Site effects, Microtremor, Nakamura method, Seismic microzonation, SPT N-value.