Analysis of Site Effects using Microtremors and Velocity Measurements

Analysis of Site Effects using Microtremors and Velocity Measurements

In order to develop a new microtremor technique, this study conducts microtremor measurements at free-field strong motion stations in Ilan, Taiwan. The Engineering Geological Database for TSMIP in this area, which contains 48 stations, is also used to analyze the properties of microtremors as well as site effects. In addition, we installed a downhole array of accelerometers at the National Center for Research on Earthquake Engineering(NCREE). The preparations for microtremor analysis, geological drilling, and PS-logging velocity measurements are introduced in this report. The dominant frequencies of the Horizontal-to-Vertical Spectral Ratio(HVSR) of microtremors are proportional to the average shear-wave velocity in the first 30 m of the subsoil(Vs30) in the Ilan area. The dominant frequency increases with the increase in Vs30. However, for stations belonging to classes D and E with a sediment thickness of more than 30 meters, the Vs30 value can indicate the difference in velocity between the two classes. HVSRs reflect the sedimentary depth, and therefore, it is not easy to define the difference between two classes. We found similar situations in Taipei, Kaohsiung, and Pintong. Most of the strong motion stations are located in plain areas and are classified as class D, showing no significant variations in the shear-wave velocity(Vs) with depths. Stations of class C are mainly distributed at the edge of the Ilan plain and show a larger variation in their Vs values with depth. Four class-B stations are distributed from the northeastern corner, along the north of the Ilan plain to the southwestern cornor. The shear-wave velocity of the bedrock is more than 1,000 m/s. There are only three class E stations in the area. The dominant frequencies are less than 2 Hz in the plain area and greater than 3 Hz in the mountainous area. According to drilling results of the borehole at NCREE, the sedimentary depth is 57.7 m, which covers weathering bedrock. A clear velocity increase occurs at a depth of 58 m from the velocity measurement. The estimated depth of the sediment layer is 50.1 m, which is underestimated by the microtremor technique.