MICROTREMOR MEASUREMENTS FOR THE ESTIMATION OF SEISMIC MOTION ALONG EXPRESSWAYS Yoshihisa MARUYAMA 1 , Fumio YAMAZAKI 2 and Tatsuya HAMADA 3 ABSTRACT In order to gather earthquake information at an early stage and to establish an efficient traffic control just after an earthquake, Japan Highway Public Corporation (JH) has been developing the new seismometer network along their expressways since the 1995 Kobe Earthquake. However variation of earthquake records was found, comparing with the records from the nearest stations of K-NET. In this study, the horizontal-to-vertical (H/V) Fourier spectrum ratios were calculated for microtremors observed at four JH seismic stations and four K-NET stations. The H/V ratios for the earthquake motions observed at these seismic stations were also calculated. It was observed that the H/V Fourier spectrum ratios for the microtremor and seismic motion are similar. Hence, using the H/V ratios for microtremor as quasi transfer functions, the earthquake records at the microtremor observation points can be predicted. This idea can be applied for the estimation of seismic motion along the expressway networks in Japan. Introduction Japan Highway Public Corporation (JH) owns expressway networks with a total length of 6,615 km (as of April, 2000). In order to gather earthquake information at an early stage and to establish an efficient traffic control just after an earthquake, JH had deployed 123 accelerometers along their expressways before the 1995 Kobe earthquake. Since the earthquake, JH has further deployed 202 new seismometers (Yamazaki et al., 2000). Currently, the average distance between adjacent seismometers is about 20 km together with the existing ones (Fig. 1). Using earthquake information from these instruments, JH closes their expressways if the peak ground acceleration (PGA) equal or larger than 80 cm/s 2 is recorded or reduces the maximum speed limit if the PGA equal or larger than 50 cm/s 2 is observed. However variation of earthquake records is often found in comparison with the records from neighboring seismic stations. Figure 2 shows the acceleration response spectra of an earthquake recorded at JH Utsunomiya, K-NET Utsunomiya, and JMA (Japan Meteorological Agency) Utsunomiya stations. Note that 1,000 K-NET stations were deployed throughout in Japan by National Research Institute for Earth Science and Disaster Prevention (NIED), Science and Technology Agency. Although these three stations are located within a distance of 10 km, the spectral shape and amplitude of JH Utsunomiya station are quite different from those of the others. Because of the variation of earthquake records, the efficiency of traffic control after an earthquake is 1 Graduate student, Civil Engineering, The University of Tokyo, Japan 2 Associate Professor, Institute of Industrial Science, The University of Tokyo, Japan 3 Expressway Research Institute, Japan Highway Public Corporation, Tokyo Japan