Site amplification in the Lijiang Valley, Yunnan Province

The site amplifications for three stations, Libin (LIBI), Baisha (BASH) and Yulong (YULG) situated respectivelyin the southern, middle and northern parts of Lijiang Valley, are obtained by analyzing the S-wave soil/bedrockand microtremor horizontal/vertical spectral ratios. The data are digital recordings for the aftershocks of the Lijiang Ms=7.0 earthquake on February 3, 1996. In the frequency range of 1-4 Hi, the S-wave soil/bedrock spectralratio of E-W component for LIBI is the largest and amounts to 4.5. The microtremor soil N-S/vertical (V) spectralratio is approximate to l, E-W/V is about 4.5 and the same with above soil/bedrock spectral ratio. It is shown thatthe vertical and N-S components of microtremor have not been amplified by the soil and the Nakamura's method isapplicable. With this method, the microtremor spectral ratios for BASH and YULG are further evaluated. Theyhave similar characteristics with that of LIBI. In above frequency range, both N-S/V ratios are approximate to 1,while the E-W/V ratio is about 6 for BASH, 4.5 for YULG. Ljiang Valley is characterized by the trans-valleydirectional site response.