- © 2009 by the Seismological Society of America
Shear-wave velocity structure is a crucial factor for site characterization and site effect assessment. There are several methods for estimating velocity structure (intrusive or nondestructive, with active or passive sources). We are not going to compare these different methods, but we will point out that all methods have certain limits: penetration depth, resolution, and cost are the main limiting factors. For instance, acquiring a complete and reliable velocity profile from surface to hard bedrock is often difficult and expensive. It is obvious that an alternative kind of parameter, instead of the complete velocity profile, should be used for site characterization.
In order to classify a site, the average shear wave velocity of the first 30 m, VS30, is widely used in most building codes, including the National Earthquake Hazards Reduction Program (NEHRP 2000) and Eurocode 8 (1998). This measure was originally chosen because it generally takes one day to realize a 30-m deep geotechnical testing for most of the American sites (P. Y. Bard, personal communication). So it was at first only a practical choice, but the VS30 value is nowadays widely used and the number of VS30 data is extremely high, so we cannot ignore this soil parameter. However, many measurements of shear wave velocity do not reach 30 m, as Boore (2004) has noticed. Thus, relationships between the different time-average velocity VSz (for z = 5, 10, 20 30 m) parameters are of great interest. This task has been done for California data (Boore 2004) and will be compared here with Japanese and some European data.
Following the 1995 Hyogo-ken Nanbu (Kobe) earthquake, the Japanese government installed networks consisting of a large number of strong-motion stations, including Kik-net (Kiban-Kyoshin), to improve the prevention of future earthquake disasters. The Kik-net (http://www.kik.bosai.go.jp …