What is a reference site?

Abstract

Many methods for estimating site response compare ground motions at sites of interest to a nearby rock site that is considered a “reference” motion. The critical assumption in these methods is that the surface-rock-site record (reference) is equivalent to the input motion at the base of the soil layers. Data collected in this study show that surface-rock sites can have a site response of their own, which could lead to an underestimation of the seismic hazard when these sites are used as reference sites. Data were collected from local and regional earthquakes on digital recorders, both at the surface and in boreholes, at two rock sites and one basin site in the San Jacinto mountains, southern California. The two rock sites, Keenwild and Piñon Flat, are located on granitic bedrock of the southern California peninsular ranges batholith. The basin site, Garner Valley, is an ancestral lake bed with watersaturated sediments, on top of a section of decomposed granite, which overlies the competent bedrock. Ground motion is recorded simultaneously at the surface and in the bedrock at all three sites. When the surface-rock sites are used as the reference site, i.e., the surface-rock motion is used as the input to the basin, the computed amplification underestimates the actual amplification at the basin site for frequencies above 2 to 5 Hz. This underestimation, by a factor of 2 to 4 depending on frequency and site, results from the rock sites having a site response of their own above the 2-to 5-Hz frequencies. The near-surface weathering and cracking of the bedrock affects the recorded ground motions at frequencies of engineering interest, even at sites that appear to be located on competent crystalline rock. The bedrock borehole ground motion can be used as the reference motion, but the effect of the downgoing wave field and the resulting destructive interference must be considered. This destructive interference may produce pseudo-resonances in the spectral amplification estimates. If one is careful, the bedrock borehole ground motion can be considered a good reference site for seismic hazard analysis even at distances as large as 20 km from the soil site.

Multimethod Characterization of the French-Pyrenean Valley of Bagneres-de-Bigorre for Seismic-Hazard Evaluation: Observations and Models

“…Note that using a pillar at the reference station is equivalent to replace soft soil by a stiff structure. It is thus very different from using a reference station in a borehole, where downgoing waves perturb the reference signal, which must be corrected for this effect (Steidl et al, 1996;Assimaki et al, 2008).…”

Selection of an appropriate bedrock for site amplification studies

“…It was previously believed that such a ratio of surface to bedrock, where the rock site is directly below the surface site and much deeper than the local geological conditions to be studied, would be ideal. However, later studies (Liu et al 1992;Steidl et al 1996;Safak 2001) differ from this idea due to remarkably high amplification at particular frequencies in the transfer function estimated using Kagami's approach. Such high amplification values do not correctly compare the effect of site condition on seismic motion recorded at soft soil and rock sites.…”

“…The second assumption is that the reference station is selected on a rock outcrop to have minimum site effects. However, it is challenging to find any such site, which is free from any site effects, due to which it becomes tough to locate the reference site (Steidl et al 1996). The difficulty in finding the reference site is the major drawback of this methodology.…”

“…Most of the site amplification studies prefer to use soil layer (or site) having VS (or VS,30) equal to or greater than 760 m/s (NEHRP site class B sites) as bedrock (or reference value). Steidl et al (Steidl et al, 1996) explored the importance of correct bedrock for site amplification studies and underlying ambiguity in its selection. However, this ambiguity has not been cleared after the passing of two decades, and more research is required to answer the question raised by Steidl et al (Steidl et al 1996).…”

“…Steidl et al (Steidl et al, 1996) explored the importance of correct bedrock for site amplification studies and underlying ambiguity in its selection. However, this ambiguity has not been cleared after the passing of two decades, and more research is required to answer the question raised by Steidl et al (Steidl et al 1996). Hence, in this study, an attempt has been made to explore the problem's solution by combining the results of theoretical and experimental methods of site amplification estimation techniques.…”