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.
Highlights
During earthquakes, variations in ground motion according to geological site conditions make it necessary to perform detailed seismic hazard assessments for large cities
There is the possibility for amplification of ground motion, as studies of important earthquake events around the World have shown that alluvium characteristics can have strong effects on intensity and distribution of earthquake damage
Variations in the predominant frequency in the study area are shown in Figure 5, which shows that the predominant frequency in the city center was below 0.8 Hz, and that it increases in the S-SW and N-NE directions of the city
Summary
Variations in ground motion according to geological site conditions make it necessary to perform detailed seismic hazard assessments for large cities. To investigate the relationships between the alluvium characteristics and the frequency obtained in the study area, variations in the ground vibration predominant frequency and variations in the soil texture and blow counts (N-values) of the standard penetration test (NSPT) were investigated along four sections. The soil texture at the surface of Mashhad city in the study area is 70% fine grain, 25% medium grain, and 5% coarse grain.
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