A Risk-Targeted Alternative To Deterministic Capping of Maximum Considered Earthquake Ground Motion Maps

Abstract eng:
Since their inception nearly 20 years ago, the so-called Maximum Considered Earthquake ground motion maps in building codes of the United States of America (USA) have represented the lesser of probabilistic and deterministic values. Before the 2009 NEHRP (National Earthquake Hazards Reduction Program) Recommended Seismic Provisions for Seismic Regulations for New Buildings and Other Structures and the 2012 International Building Code, the probabilistic ground motions were uniform-hazard values corresponding to 2% probability of exceedance in 50 years. Since then, the probabilistic ground motions have been so-called risk-targeted values that are expected to result in building designs with uniform risk of collapse, namely a theoretical 1% probability of collapse in 50 years that is based on nonlinear response history analysis simulations. In either case, in arriving at the mapped ground motions, the probabilistic values have been capped at some locations by smaller deterministic ground motions from characteristic earthquakes on known active faults. Generally speaking, the locations where the deterministic ground motions cap their probabilistic counterparts are highhazard areas near very active faults (e.g. the San Andreas Fault in California). A consequence of deterministic capping of risk-targeted ground motions is unspecified increases in collapse risk in those high-hazard areas. In the 2015 NEHRP Recommended Seismic Provisions (which are in the process of being adopted into the 2018 International Building Code), deterministic capping can increase the collapse risk by factors of up to 9.5. In lieu of the unspecified increases in collapse risk that result from deterministic capping, this paper explores an alternative that specifies a higher target risk of collapse in high-hazard areas, up to only three times the theoretical 1% in 50 years. Since many high-hazard areas are also highly populated, changing deterministic capping can have significant societal impacts. Fortunately, the alternative Maximum Considered Earthquake ground motions are within ±20% of those mapped in the 2015 NEHRP Provisions, except near the New Madrid seismic zone (NMSZ) in central USA (where the current mapped values are the largest in the USA), in coastal northern California and southern Oregon (where the current ground motions are almost as large as those near the NMSZ), and near limited portions of the San Andreas Fault (where the current ground motions have return periods of less than 500 years).

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