Mean and modal ϵ in the deaggregation of probabilistic ground motion

Bulletin of the Seismological Society of America



An important element of probabilistic seismic-hazard analysis (PSHA) is the incorporation of ground-motion uncertainty from the earthquake sources. The standard normal variate ϵ measures the difference between any specified spectral-acceleration level, or SA0, and the estimated median spectral acceleration from each probabilistic source. In this article, mean and modal values of ϵ for a specified SA0 are defined and computed from all sources considered in the USGS 1996 PSHA maps. Contour maps of ϵ are presented for the conterminous United States for 1-, 0.3-, and 0.2-sec SA0 and for peak horizontal acceleration, PGA0 corresponding to a 2% probability of exceedance (PE) in 50 yr, or mean annual rate of exceedance, r, of 0.000404.

Mean and modal ϵ exhibit a wide variation geographically for any specified PE. Modal ϵ for the 2% in 50 yr PE exceeds 2 near the most active western California faults, is less than –1 near some less active faults of the western United States (principally in the Basin and Range), and may be less than 0 in areal fault zones of the central and eastern United States (CEUS). This geographic variation is useful for comparing probabilistic ground motions with ground motions from scenario earthquakes on dominating faults, often used in seismic-resistant provisions of building codes. An interactive seismic-hazard deaggregation menu item has been added to the USGS probabilistic seismic-hazard analysis Web site,, allowing visitors to compute mean and modal distance, magnitude, and ϵ corresponding to ground motions having mean return times from 250 to 5000 yr for any site in the United States.

Publication type Article
Publication Subtype Journal Article
Title Mean and modal ϵ in the deaggregation of probabilistic ground motion
Series title Bulletin of the Seismological Society of America
DOI 10.1785/0120000289
Volume 91
Issue 6
Year Published 2001
Language English
Publisher Seismological Society of America
Description 16 p.
First page 1537
Last page 1552
Online Only (Y/N) N
Additional Online Files (Y/N) N
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