Site response modeling variability in "rupture-to-rafters" ground motion simulations

Domniki Asimaki, Michalis Fragiadakis, & Wei Li

Published October 2008, SCEC Contribution #1162

The quantification of site effects is of great significance in seismic hazard mitigation. Nonetheless, there currently exists a large degree of uncertainty concerning the mathematical model to be employed for the computationally efficient evaluation of these effects, and the site investigation program necessary to evaluate the input parameters and ensure cost-effective predictions. In this paper, we combine downhole observations and broadband ground motion synthetics for characteristic site conditions in the Los Angeles Basin, and investigate the variability in ground motion and structural performance estimation introduced by the site response assessment methodology. For this purpose, regional velocity and attenuation structures are initially compiled using near-surface geotechnical data and the crustal velocity structure at three downhole arrays in Southern California. Broadband ground motion simulations are next conducted for rupture scenaria of weak, medium and large magnitude events (M = 3.5 - 7.5), and three component seismograms are computed on a surface station grid at distances 1km-75km from the surface projection of the fault. Elastic, equivalent linear and nonlinear site response simulations are then evaluated, and the modeling site response uncertainty is reported by means of the COV of site amplification factors, defined as the ratio of the predicted peak ground acceleration (PGA) and spectral acceleration (SA) at short and long periods to the corresponding ground motion intensity measure on rock-outcrop (Vs30=760m/s). A frequency index is developed to identify the site and ground motion conditions where the high COV of free-field response implies that incremental nonlinear analyses should be employed in lieu of approximate methodologies based. Finally, bilinear single degree-of-freedom systems (SDOF's) with strain hardening and strain softening stress-strain response are subjected to the ensemble of ground motion predictions obtained via the alternative site response methodologies investigated, and the COV of selected intensity measures intended for probabilistic structural performance assessment is evaluated, thus illustrating the propagation of modeling uncertainty from the assessment of ground surface response to fundamental components of performance-based design for deterministic predictions of ground motion scenaria.

Asimaki, D., Fragiadakis, M., & Li, W. (2008, 10). Site response modeling variability in "rupture-to-rafters" ground motion simulations. Poster Presentation at 14th World Conference on Earthquake Engineering (14WCEE).