Earthquake ground motions vary depending on the subsurface soil conditions and surface topography. However, ground motion can only be recorded where there are seismic stations, so it is difficult to measure variations in ground motion changes within a small area.
To study more localized variability, Johnson et al.  used about one thousand geophones spaced 10 to 20 meters apart atop the San Jacinto Fault in Southern California. This is a much denser array of apparatus than usual seismic observations. Their observations of Peak Ground Velocity (PGV) within the 0.36 km2 study area over a period of one month revealed variations in ground motion of up to 22% across the array. They also observed that the recorded measurements of PGV depends not only on the site locations, but also on the directivity of seismic waves. Variations in amplification are much larger if the earthquake occurs within the San Jacinto fault zone.
The variability of site characteristics is important for the development of nonergodic ground-motion prediction equations (GMPE) and has thus implications for probabilistic seismic hazard analysis. This study should be useful for a range of scientists from engineering seismology to earthquake physics.
Citation: Johnson, C. W., Kilb, D., Baltay, A., & Vernon, F. . Peak ground velocity spatial variability revealed by dense seismic array in Southern California. Journal of Geophysical Research: Solid Earth, 125, e2019JB019157. https://doi.org/10.1029/2019JB019157
—Masumi Yamada, Associate Editor, JGR: Solid Earth