An ENA Empirical GMPE based on the NGA East Database and Intensity Observations

English Abstract:

We have developed an empirical GMPE for Eastern North America (ENA). Our GMPE is based on the two-stage regression approach of Joyner and Boore (1993, 1994). This approach provides both within event and between event variability along with event and site terms. Besides ground motion observations from the latest NGA East flatfile, we included ground motion observations from the 1976 M6.8 Gazli and 2001 M7.6 Bhuj earthquakes plus intensities converted to ground motion estimates for the M>6 ENA historical 1811-1812 New Madrid, 1886 Charleston SC, 1925 Charlevoix, and 1929 Grand Banks earthquakes. We fit linear regressions to median log₁₀ ground motions vs. Modified Mercali Intensity (MMI). Community Internet intensity (CII) observations (Did You Feel It?) have been corrected to MMI using the approach suggested by Hough (2014). We have applied a magnitude correction based on residuals, which corrects for the effect of shifting source spectral corner frequency with magnitude. The magnitude correction (an increase in estimated ground motion with increasing magnitude) is more pronounced at longer periods, as expected. Magnitudes for historical events were taken from Cramer and Boyd (2014) for the 1811-1812 and 1886 M7 earthquakes and from Bent (1992, 1995) for the 1925 and 1929 earthquakes. Our empirical GMPEs are for 23 periods, including PGA and PGV. Empirical observations were restricted to mid-continent crustal regions, avoiding Gulf Coast and western US Q regions for both earthquakes and recording stations. Site conditions were modeled with a Vs30 term using NGA East provided Vs30 estimates at each site. The reference Vs30 is 760 m/s. Geometrical spreading is modeled by a single term due to the limited observations at distances less than 50 km. We included a magnitude-dependent geometrical spreading term to help model magnitude saturation at large magnitudes. Alternative terms are also provided for focal mechanism type (reverse, strike-slip, and undefined).