Site-Response Spectra for POLARIS Station Sites in Southern Ontario and Quebec

Abstract

Local geologic and soil conditions can greatly increase the intensity of earthquake ground shaking (Kramer 1996). These local site effects are particularly significant at sites with deep, soft soil deposits, although hard rock sites may also exhibit amplification (Siddiqqi and Atkinson 2002). Estimation of this localized site response can be accomplished using theoretical or empirical methods. Theoretical modeling requires extensive characterization of a site's near-surface geology, as well as the use of sophisticated computing methods. In contrast, empirical approaches require only ground-motion data records from the site. The relative ease of application has popularized the use of empirical methods in investigating site response. Ground-motion records have become readily available at a number of Canadian sites, owing to the recent deployment and subsequent expansion of the Portable Observatories for Lithospheric Analysis and Research Investigating Seismicity (POLARIS) seismograph network (Eaton et al. 2005). In this study, empirical site-response spectra were produced for 11 stations of the POLARIS Ontario (POLO) array of POLARIS (Figure 1 and Table 1). A variety of near-surface conditions exist at these sites. While most are located on competent Precambrian bedrock, others rest on fractured Paleozoic bedrock, soil, or a combination of soil and manmade fill. An additional, temporary, soil station was also studied (Figure 1 and Table 1). Installed near a cement quarry in St. Marys, Ontario, this station collected data over a weeklong period. View this table: TABLE 1 Seismograph Stations Used for This Study The primary objective of this study was to report the site-response spectra for 11 POLO stations that have not been previously studied. Spectra produced in this study supplement those previously reported by Murphy and Eaton (2005) for 18 other POLO stations. Two popular empirical methods were used to obtain the site-response spectra, allowing for a comparison of the results. The empirical methods employed were Nakamura's (1989) microtremor …