Why does PSHA overpredict historically observed shaking data?

Leah Salditch, U.S. Geological Survey, Geologic Hazards Science Center, Golden, United States, Molly Margaret Gallahue, Northwestern University, Department of Earth & Planetary Sciences, Evanston, IL, United States, James Neely, Northwestern University, Evanston, United States, Seth Stein, Northwestern University, Earth & Planetary Sciences, Evanston, IL, United States, Susan Elizabeth Hough, U.S. Geological Survey, Earthquake Science Center, Pasadena, United States and Norman Abrahamson, University of California Berkeley, Civil and Environmental Engineering, Berkeley, CA, United States