No abstract

Ground structure characteristic is an important parameter in seismic site effect evaluation for earthquake structural design. Many geophysical methods can be used to estimate ground structure. The best field measurement option that requires least field effort and yet producing accurate estimation is the main concern of this study. This study applies microtremor array observation method to investigate the accuracy of dispersion curve estimated using Centerless Circular Array (CCA) methods of 5m radius. The dispersion curve observed was assessed by comparing to theoretical dispersion curve generated from proposed shear wave velocity (VS) profile of sites. The observed dispersion curve shows good agreement with theoretical dispersion curve over a wide frequency range, which is ranging from 3 -10 Hz. This segment of dispersion curve can be utilized to estimate ground structure by means of inversion of dispersion curve. CCA method is capable to be used to estimate ground structure in term of VS profile.

Microzonation in low to moderate seismicity regions is not extensively used due to a lack of attention given. Accordingly, the structures are not designed for earthquake loading. However, these countries are susceptible to earthquake threat caused by local and far field earthquakes. The far field effect of earthquakes from neighbouring countries can be felt in Penang Island, Malaysia, which is contrary to the perception that Malaysia is seismic free. This hazard serves as a reminder of the importance of seismic study, particularly in the light of the local geological conditions. This work presents the ground structure of Penang Island for seismic microzonation map development using a simpler and affordable method for low to moderate seismicity regions. Moreover, this study applies microtremor single point and array observation methods to estimate the ground structure of Penang Island. The estimated ground structures are validated using the available borelog data and the results show good agreement. The developed microzonation maps show that Penang Island has a ground structure with a predominant frequency ranging from 1.45 Hz to 7.4 Hz and Vs,30 in the range of 168 m/s to 468 m/s. The northeast (Georgetown) and western (Balik Pulau) regions of Penang Island show a low frequency and Vs,30 value, indicating that these areas have a soft ground structure. The study demonstrates a simple and affordable method that can be adopted by low to moderate seismicity countries to develop seismic microzonation maps.

Determination of ground structure is important for seismic hazard analysis and earthquake design of structures. Different types of ground structure at site, depending on its profile of shear-wave velocity, will amplify seismic wave at different extents. Therefore, structures should be designed accordingly to withstand the expected surface ground motion considering seismic site amplification. Microtremor observations were conducted in this study due to its simplicity, less field effort and economy. Field measurement was carried out at two sites at Universiti Sains Malaysia Engineering Campus where the actual soil profiles were known from the existing borelog data. Two sizes of circular array formed by three three-component velocity sensors along the circumference of circle and a three-component velocity sensor at the center for each array were performed. The recorded data were analysed using Spatial Autocorrelation (SPAC) method. Shear wave velocity profile for the site was then estimated through inversion of phase velocity. To produce convincing estimation of ground structure, the horizontal to vertical (H/V) spectra at the center of the circular array was compared with the computed ellipticity of the fundamental mode of Rayleigh wave. The obtained results were also compared with the borelog data. The accuracy for the estimation of ground structure based on various microtremor array sizes and analysis techniques was assessed.