Observations on Regional Variability in Ground-Motion Amplitude from Six Mw ~ 6.0 Earthquakes of the North–South Seismic Zone in China

Observed ground-motion intensity for six earthquakes of M w = 6.0–6.2 that occurred in China’s North–South Seismic Zone (NSSZ) was compared with predicted medians from the BSSA14 model. The resulting between-event ( δB e ) and within-event ( δW es ) residuals for peak ground acceleration and pseudo-spectral acceleration up to 5.0 s were used to investigate the impact of source effects and path propagation on the regional variability in observed ground-motion amplitude within the NSSZ. Although the magnitude and fault type were similar among the six earthquakes, the results showed that their source effects were significantly different, which contributed in part to the regional variability in observed ground-motion amplitude. Estimated values for stress drop were found to mirror the trend in variation of the δB e values for short-period ground motions in the six earthquakes. This suggests that stress drop is an important factor for accurate representation of source effects and should be considered in the functional form of ground-motion prediction equations. Anelastic attenuation of ground motion was found to be considerably different in local areas of the NSSZ, which may constitute the primary reason for the regional variability in the observed ground-motion amplitude. The variation in δW es values confirmed that regional adjustment of anelastic attenuation in the BSSA14 model is applicable to some local areas (i.e., around the Longmenshan fault) but not to the NSSZ in its entirety.