Spatial Changes in Earthquake-Generated Ground Motion Observations: An Examination of Seven Small Aperture Arrays in Southern California

Ground motions recorded at one location are often extrapolated to nearby regions within a given radius. We explore the appropriateness of spatial extrapolation using data from seven small aperture (diameters < 3km) seismic networks in southern California that were operational for a year or more. Five of the seven arrays traverse the San Jacinto fault, and two locate off-fault. From our base catalog (4038 M2.5+ earthquakes; September 2010 - June 2023), we remove temporally overprinted waveforms from nearby events (≤ 5 km) in quick succession (≤ 5 minutes) and data with non-viable waveforms. These 200 samples per second data are filtered at 0.5-25 Hz and must have signal-to-noise ratios 2.5+. Peak ground acceleration and ground velocity are derived individually from the maximum absolute values of each of the three-component waveforms. We compare ground motion observations with empirical estimates from Abrahamson et al. 2014. On average, arrays deployed across fault zones consistently record ground motions above empirical expectations, whereas off-fault arrays record ground motions at or slightly below empirical expectations. For all seven networks, the coefficient of variation shows large standard deviations at 26-38% of the mean, suggesting extrapolation should be done with caution even within small regions (< 3km).