Estimation of shallow and deep soil shear wave velocity structures in Padang, Indonesia, using microtremor explorations

• Main Author: Han, Xiaohan.
• Other Authors: School of Civil and Environmental Engineering
• Format: Theses and Dissertations
• Language: English
• Published: 2013
• Subjects: DRNTU::Engineering::Civil engineering
• Online Access: http://hdl.handle.net/10356/54937
• Institution: Nanyang Technological University

The city of Padang is at great risk from a large earthquake, which could potentially occur along the Sumatran megathrust, 150km off the coast. As intensities and frequencies of ground motions depend very much on the near-surface site conditions, it is necessary to construct a 3D soil shear-wave velocity structure beneath the city to assist in the assessment of its seismic hazard. In this research, microtremor survey methods (MSM) are used to understand the shallow and deep soil shear wave velocity structure beneath Padang. Microtremor array measurements were conducted in the city from 16 – 26 August 2010 to determine the phase velocity of Rayleigh waves. Surface waves recorded by small arrays were analyzed using the spatial autocorrelation (SPAC) method and those recorded by medium to large arrays were analyzed using the frequency-wavenumber (F-k) method. Following the analysis, the phase velocity spectrum that is produced, is used to estimate the subsurface shear wave velocity structure. The results show that the soil in the Western sediment plain has shear-wave velocity values ranging from 140m/s to 230m/s. The soil in East Padang has higher shear-wave velocity values of over 300m/s. The layer of soft soil which has a shear wave velocity of less than 400 m/s is approximately 25m to 80m thick under the city. The variations of shear wave velocity at various depths are interpolated in several cross-sections to show the spatial variations. The new map, produced as a result of this research, indicates that the site classifications based on previous values that suggested soil type D, are to be downgraded to soil type E in West Padang. Site Class D soil may amplify the PGA by a factor of 2.4, when the incoming seismic waves have periods between 0.5s to 2s with an acceleration of 0.2g at the bedrock, which is the PGA estimated by Indonesian seismic design code for a return period of 500 years. Site Class E soil may amplify the PGA by a factor of 2.8, when the same condition is imposed. The derived PGAs of 0.48g and 0.56g at both the soft and medium soil surface exceeds the design value of PGA proposed in the Indonesian Seismic Standards (0.33g and 0.36g, respectively) and could impose a great challenge to the structural performance of buildings, especially concrete and masonry houses. Large array results were then used to estimate the depth to engineering bedrock and hard rock. In Padang, the engineering bedrock depth lies at a depth between 50m to 100m and the depth to hard rock is between 170m to 300m. In Pariaman, an adjacent town of interest, thick sediment of 400m is observed. The deep soil structures are interpolated to obtain the shape of the alluvium basin, and provide information on impedance relating to different soil layer properties. For further investigation, more detailed site amplifications could be simulated numerically using the established soil structures obtained in this study.