Relationships between felt intensity and instrumental ground motion in the Central United States and California

In this study, we develop empirical relationships between instrumental ground-motion parameters and observed Modified Mercalli Intensity (MMI) by using data from felt moderate earthquakes in the central United States (CUS) that were also recorded on broadband seismographic networks and strong-motion recorders in the CUS region. The data are calibrated and supplemented at higher intensities based on observations in California. MMI for ShakeMap applications in the CUS region, and in California, can be predicted from recorded peak ground velocity (PGV), in cm/sec, with a standard deviation of 0.8 MMI units, using the following equation: NMI = 4.37 + 1.32(log PGV) log PGV ≤ 0.48 MMI = 3.54 + 3.03(log PGV) log PGV ≥ 0.48 There are weak-magnitude and distance-dependent trends in the residuals for this relationship. These trends, if not removed, may lead to apparent regional dependencies in MMI versus ground-motion amplitude relationships. Refined relationships that include magnitude and distance as predictive variables that are applicable throughout North America are defined.