Kazuyoshi Kudo

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.

Assessment of Site Effects on Seismic Motion in Ashigara Valley, Japan

We compared site amplifications at rock sites and sediment sites of Ashigara Valley, Japan, using ground-motion data from five remote (>700 km) large (> M 7) events. The use of remote large events is advantageous to estimating site factors because the source and path effects are considered to be common with a sufficient accuracy and the ground motions will cover a wide-frequency band. Ground motions at both sediment and rock sites were coherent in frequencies lower than 0.1 Hz. This means that the wavelength in these frequencies is longer than the size of the valley (12 km long and 5 km wide). Site amplification factors were determined by taking spectral ratios with reference to one rock outcrop site. The amplification factors of sediment sites deviated 2–10 times with respect to the rock site in the frequency range higher than 0.1 Hz, in which significant peaks at about 1–2 Hz were found at most sites. These dominant amplifications in sedimentary basin are most essential for assessing earthquake hazard in the region. For sediment sites, the peak frequencies of spectral ratios to the rock sites were stable for different events and coincided with those of horizontal to vertical spectral ratios for the S -wave portion and those of relative site factors estimated separately by the generalized inversion method using local small-events data in the frequency range higher than 2 Hz. Although spectral ratios for frequencies lower than about 1 Hz should be affected by 3D basin structure, 1D S -wave responses represent the amplification of ground motion in the sediment sites for frequencies higher than 2 Hz.