Sadanori Higashi

Site-Specific Issues for Strong Ground Motions during the Kocaeli, Turkey, Earthquake of 17 August 1999, as Inferred from Array Observations of Microtremors and Aftershocks

Array observations of microtremors and aftershocks were carried out near the permanent strong-motion observation sites and the damaged areas, after the Kocaeli, Turkey, earthquake of 17 August 1999. The major objectives were to determine S -wave velocity structures at the sites and to understand the site effects on strong motion or damage. Array observation of microtremors is a useful method for determining the S -wave velocity structures in a sedimentary basin, with less practical restriction than the other geotechnical methods. The spatial autocorrelation method (SPAC) was applied to array data of microtremors for determining S -wave velocity structures. The SPAC method generally provides an equivalent result with that of the frequency-wavenumber method, using fewer array sites and a smaller array size. Most strong-motion sites near the fault are classified into stiff and/or very hard soils. The records cannot directly be used for interpreting damage to buildings in the sedimentary basin. Records of long durations of strong motion at ATS, near Avcilar, west of Istanbul, are closely related to the low velocity ( V s ∼ 200 m/sec) of the surface layers. The S -wave velocity structure at Avcilar, where there was severe damage during the mainshock, is similar to that of the lowland (ATS), and it differs significantly from that of CNA, located 4 km northeast of Avcilar, where the strong-motion record was obtained from the mainshock. The strong ground motion at Avcilar during the mainshock is estimated to be similar to that at ATS. Of the strong-motion sites, Sakarya (SKR) is located on very hard soil, whereas thick and soft sediments cover downtown Adapazari. It is plausible that strong ground motions during the mainshock in the damaged area, ADC, were significantly different from those of SKR. A large difference between the strong motions of a hillside and the lzmit Bay area in and around Golcuk is also indicated by a comparison of aftershock records.

S-Wave Velocity Structure of the Taichung Basin, Taiwan, Estimated from Array and Single-Station Records of Microtremors

The objective of this study is to estimate the S-wave velocity structure of the Taichung basin in a near-fault region, which is needed for strong-motion evaluation for the 1999 Chi-Chi earthquake. We have conducted array measurements of microtremors with a total of 12 arrays at four sites and single-station measurements of microtremors at 48 sites in and around the Taichung basin. Based on the Rayleigh- wave inversion technique using phase velocities estimated from array records of microtremors, we find that a thick layer (the thickness of about 1000 to 1400 m) with an S-wave velocity of VS 1100 m/sec exists in the east-central part of the Tai- chung basin. We estimate the thicknesses of sedimentary layers above the pre- Tertiary bedrock at 48 sites to fit calculated peak and trough frequencies of horizontal-to-vertical spectral ratios of Rayleigh waves to observed peak and trough frequencies, assuming the same S-wave velocities estimated using array records. The pre-Tertiary bedrock depth was estimated to be about 5 to 6 km in this region. The estimated thickness of the layer with VS 1100 m/sec is largest in the east-central part of the basin and rapidly decreases to less than 400 m in the northeastern and western parts inside the basin. The estimated S-wave velocity structures reasonably explain arrival time of initial P and S waves of aftershock records observed by Higashi et al. (2001).