Estimation of Site Characteristics in Sagaing City, Myanmar By Microtremor Survey

Abstract eng:
Local site effect is one of the most important aspects in the assessment of seismic hazard and many of the destructive earthquakes in the past have shown that local site conditions have major effects on ground shaking. Depending on the local site conditions, the characteristics of seismic waves generated and propagated through different geological formations during the earthquakes can be modified. The microtremor Horizontal-to-Vertical spectral ratios (MHVRs) become an important tool to estimate the fundamental frequencies and amplifications during the ground motion. Many of the former studies have proved that MHVR technique is effective when carried out at the sites with soft deposits by using the long duration ambient seismic noise. However, array measurements of microtremors to obtain phase velocity of Rayleigh wave would be quite effective to invert Swave velocity structures (e.g., Kawase et al., [1]; Satoh et al., [2]) until recently Sanchez-Sesma et al., [3] have proposed a new theory based on the diffuse field concept. Exploration based on the H/V spectral ratio using single-station microtremor records is simple, quick, and inexpensive, and so the MHVR technique with diffuse field theory will be applied to Sagaing City, Myanmar to estimate the site characteristics from the results of single measurement. At the same time the array measurement technique will also be applied to obtain starting velocity model for single station MHVR technique. To achieve this goal, we have conducted over 100 sites of single station and 5 sites of array measurements in and around Sagaing City. Additionally, we selected nearly 25 sites close to the Sagaing Fault including eastern and western sites to investigate the site characteristics across the fault. As a result, lower peak frequency values of (≤1Hz) were observed at western part of Sagaing City. Generally, most of the MHVRs for downtown area of Sagaing City show peaks in the low frequency range of (1Hz-3Hz). MHVRs of eastern and western sites of the Sagaing Fault show systematic difference in peaks in low frequency range. In some places around the Sagaing Fault, MHVRs do not show significant peak amplitude for the lower frequency range. As for the array measurement, we obtained stable Rayleigh wave phase velocity dispersion curve for each array site and the initial S-wave velocity models were created based on the phase velocity and the corresponding wavelength formula. After that, we adjust the theoretical dispersion curves with the observed one and we got good agreement. The results from current research will contribute to seismic microzonation and site amplification information in the Sagaing region. Our final objective of the research is to support in part the implementation of national seismic hazard mitigation program.

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