# Shakemap PGA And PGV Station Corrections

These station corrections for peak ground acceleration (PGA) and peak ground velocity (PGV) were calculated using the Northern California Shakemap data set. Shakemaps have been created for all Northern California earthquakes with magnitude greater than or equal to 3.5 starting in August, 1999. Several large historical earthquakes, starting with the Coyote Lake earthquake of 1979, have been added to the set of Shakemap events.

The station corrections on the map show the relative amplification of earthquake waves, calculated for both velocity and acceleration. The values shown are the logarithm (base 10) of the actual deviation from the predicted ground motion. Thus, a station correction of +0.3 reflects ground motion that is approximately twice the expected value. A station correction of -0.3 represents ground motion that is half the expected value.

Station corrections are computed relative to an average NEHRP C soil-type. We expect station corrections to correlate with the local geology. On the map, a perfect correspondence between station correction and geology would result in the same color for the station symbols (filled circles) as the background (representing the underlying soil-type). Some discrepancies are undoubtedly due to the broad brush with which the map was painted as well as the lack of information about soil depth.

It should be noted that NEHRP soil types A and B have been combined. Occurrences of A are rare in the bay area. In addition, the boundary between NEHRP types D and E was modified to Vs = 200.

Plot of the distribution of all PGA station corrections, smoothed over an interval of 0.1, plotted for each NEHRP soil-type.

Plot of the distribution of all PGV station corrections, smoothed over an interval of 0.1, plotted for each NEHRP soil-type.

Table of all stations for which PGA and/or PGV were calculated. The table includes the number of observations that went into the map and the plots, as well as the standard deviations for both PGA and PGV. There is also a column labeled His that indicates the station is no longer in operation (Historic).