ALBERT

Description: We successfully reproduced important features of long-period (7 to 8 sec), large-amplitude ground motions observed in the Yufutsu basin, Hokkaido, during the M (sub w) 8.0 2003 Tokachi-oki, Japan, earthquake through numerical simulation of wave propagation. Recordings from the dense nationwide strong ground motion networks in Japan demonstrated significant spatial variation of long-period strong ground motions within the Yufutsu basin and revealed that the long-period shaking in the Tomakomai western port was the strongest of all the sites within the basin. In interpreting or predicting long-period strong ground motions in sedimentary basins, the importance of deep basin structures, such as the depth of bedrock with an S-wave velocity over 3 km/sec has been emphasized. However, the characteristics of the underground structure model of the Yufutsu basin that explain the observations indicate that thickness of near-surface (〈1 km) soft sediments rather than bedrock depth governs the spatial variation of amplitudes of the long-period (7 to 8 sec) shaking in the basin. These findings suggest the importance of the detailed models of soft near-surface deposits as well as deeper basin structures such as bedrock depth for a better understanding of long-period strong ground motions in deep sedimentary basins.

Description: 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 (sub s) approximately 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 Izmit Bay area in and around Golcuk is also indicated by a comparison of aftershock records.