Relationships between Peak Ground Acceleration, Peak Ground Velocity, and Modified Mercalli Intensity in California

We have developed regression relationships between Modified Mercalli Intensity ( I mm ) and peak ground acceleration (PGA) and velocity (PGV) by comparing horizontal peak ground motions to observed intensities for eight significant California earthquakes. For the limited range of Modified Mercalli intensities ( I mm ), we find that for peak acceleration with V ≤ I mm ≤ VIII, I mm = 3.66 log( PGA) − 1.66, and for peak velocity with V ≤ I mm ≤ IX, I mm = 3.47 log( PGV) + 2.35. From comparison with observed intensity maps, we find that a combined regression based on peak velocity for intensity > VII and on peak acceleration for intensity < VII is most suitable for reproducing observed I mm patterns, consistent with high intensities being related to damage (proportional to ground velocity) and with lower intensities determined by felt accounts (most sensitive to higher-frequency ground acceleration). These new I mm relationships are significantly different from the Trifunac and Brady (1975) correlations, which have been used extensively in loss estimation.