Ground‐Motion Attenuation Equations for Earthquakes on the Cascadia Subduction Zone

An extensive ground‐motion data base was compiled for earthquakes occurring in subduction zones considered representative of the Cascadia subduction zone in the Pacific Northwest. The attenuation characteristics of horizontal peak ground accelerations (PGA) and 5 percent damped pseudovelocity (PSV) were studied for various subsets of the total data base. These data suggested that the PGA tend to saturate at small source‐to‐site distances and large magnitudes. When unprocessed data were added to the data base, the attenuation of PGA with distance was found to be greater than the attenuation observed for the processed data only, a result which was attributed to the selection of only the stronger motion records for processing. The results of the data analysis were used to establish the proper form of regression equations for estimating PGA and PSV at firm‐soil sites in the Pacific Northwest. A total of 697 PGA components and 235 PSV components were selected for the regressions. The resulting equation for estimating PGA in gals was ln (PGA) = 6.36 + 1.76M − 2.73 ln (R + 1.58 exp (0.608M) + 0.00916h, σ=0.773 where M is moment magnitude, R is center‐of‐energy‐release distance in km, h is focal depth in km, and σ is the standard error of ln (PGA). Although σ was relatively large, the residuals from the regressions appeared to decrease with increasing M and R. The results of the PSV regressions showed that the M coefficient and the coefficient of the f(R, M) attenuation term generally increased with period, which is consistent with regression results reported by others. The regression equations were reasonably accurate in predicting the response spectra of accelerograms recorded at Olympia and Seattle, Washington during the 1949 and 1965 Puget Sound earthquakes, but overestimated the spectra of the weaker motions recorded at Tacoma and Portland during the latter event. The median response spectra predicted by these equations for a Washington Coastal Ranges site were similar to the spectra computed by Heaton and Hartzell based on their simulations of ground motions from hypothetical giant earthquakes (M = 9.0 and 9.5) in the Pacific Northwest.