Characteristics of long period microtremor and validation of microtremor array measurements in inland areas of China

To study the characteristics of long period microtremor and applicability of microtremor survey, we have made microtremor observations using long period seismometers of the China’s mainland from coastal cities like Shanghai and Tianjin through Beijing, Xi’an, to the far inland cities of Lanzhou and Tianshui. The observation shows that the level of power spectrum of long period microtremors reduced rapidly from the coast to the inland area. However, the energy of long period microtremors in Beijing, Xi’an, Lanzhou and Tianshui area are only approximately ten-thousandth to thousandth of that in Shanghai. Aiming at the complexity of the underground structure in the far inland, a series of underground structure models with different distributions were assessed using three-dimensional, dynamic finite element method (FEM) analyses. The results were used to evaluate microtremor survey methods and their limitations with regard to aggregate variability and thickness determinations. Multiple-wave reflections between layers with the change of underground structure distribution occurred, which have significant effect on the performance of the different field approaches. Information over a broad spectrum from which velocity-depth profiles were produced via inversion approaches. Neither the thickness nor the shear wave Velocity V of the subsurface layer inversion results appeared over a large evaluation with increasing slope angle. In particular, when the angle of the model reached 45°, it could not be accurately evaluated using one-dimensional inversion methods. Finally, the array microtremor survey (AMS) was carried out in Shanghai City, China. Although AMS techniques do not have the layer sensitivity or accuracy (velocity and layer thickness) of borehole techniques, the obtained shear wave velocity structure is especially useful for earthquake disaster prevention and seismic analysis.