Empirical Site Response for POLARIS Stations in Southern Ontario, Canada

It is well known that local geology, particularly overburden, can greatly increase ground shaking during an earthquake; the best method for determining the degree of amplification remains controversial, however. The so-called site response is a localized resonant amplification of seismic wave motion that is largely controlled by the thickness, shear modulus, and viscosity of soft soil layers. Accurate knowledge of site-response spectra is important for understanding potentially destructive amplification that could occur during a large earthquake, as well as for calibration of seismic networks. In the absence of cosited measurements of ground motion in deep bedrock and at the surface, site response must be estimated either by numerical computation, based on detailed knowledge of the properties of near-surface layers, or by empirical means. A number of different empirical methods for determining site response are in common use, with various authors citing advantages of one technique over another. The objectives of this study are twofold. Our first purpose is to report empirical site-response spectra for 18 stations (Table 1) of the recently installed POLARIS seismograph network in southern Ontario, Canada (Atkinson et al. , 2003). This network is equipped with Guralp CMG-ESP three-component broadband seismometers and Nanometrics Libra digitizers. Data are digitally sampled at 100 Hz and transmitted in near real-time via satellite to two data-collection centers in London and Ottawa, Ontario. The Ontario POLARIS stations were deployed commencing in late 2001 and cover a roughly rectangular area within the lower Great Lakes region, with a station spacing of 50-100 km (Figure 1). Near-surface layers at these sites depend on local conditions and range from unweathered Precambrian bedrock to fractured Paleozoic limestone, to thick Quaternary till deposits. As a consequence of the variable near-surface conditions, we observe a wide range of site-response spectra. View this table: TABLE 1 POLARIS Stations in Southern Ontario Used for This …