Source and Site Characteristics of Earthquakes that have Caused Exceptional Ground Accelerations and Velocities

Speaker: John G. Anderson
This study investigates the characteristics of the freefield strong-motion records that have yielded the 100 largest peak accelerations and the 100 largest peak velocities available from any of several data sources through July, 2007. The peak is defined as the maximum zero-to-peak amplitude of the acceleration or velocity vector. This compilation found 35 records with peak acceleration greater than 1g (980 cm/s2), and 41 records with peak velocity greater than 100 cm/s. The results sample an estimated 150,000 instrument-years of strong-motion recordings. The geometric mean of the two horizontal components of acceleration or velocity, as used in many ground motion prediction equations, is typically 0.76 times the magnitude of this vector peak. Accelerations in the top 100 come from earthquakes as small as magnitude 4.8, while velocities in the top 100 all come from earthquakes with M 5.7. These records are dominated by crustal earthquakes with thrust, oblique-thrust, or strike-slip mechanisms. Normal faulting mechanisms in crustal earthquakes constitute under 5% of the records in the databases searched, and an even smaller percentage of the 100 largest acceleration or velocity records. All NEHRP site categories have contributed exceptional records, in proportions similar to the extent that they are represented in larger databases.