An evaluation of near-surface sediments enhancing ground motions excited by shallow local earthquakes near Union City, Oklahoma

Abstract Hydraulic fracturing activities and the subsequent wastewater injection into the subsurface have increased the number of felt seismicity in Oklahoma. Some of the smaller magnitude (M < 3.5) earthquakes have been associated with reported damage to the built environment. We evaluate the subsurface structure of the near-surface sediments in Union City, Oklahoma to identify and delineate subsurface structures that amplify ground motion in the area. The study area is adjacent to the Canadian River, a major braided meandering river that traverses Oklahoma generally from west to east, that has significant alluvial depositions that are visible near roadways and from aerial imagery. We analyzed ambient noise data from 60 continuously-recording seismic nodes as well as approximately two kilometers of electrical resistivity tomography (ERT) and induced polarization (IP) data. We performed horizontal-vertical spectral ratio (HVSR) to deduce the dominant resonant frequencies and paired the resonant frequency information with the higher resolution ERT to resolve the sedimentary structures that produce the observed resonance. Our analysis indicates that the higher resonance frequencies are found further away from the riverbanks, and the inverted depth matches the depth and thickness of the mapped terrace deposits in the area. In the southern area, closer to the riverbank, the dominant resonant frequencies are controlled by thick clay layers that are deeper than the alluvial deposits from the Canadian river. The lower frequency resonance corresponds to the depths where the transition between Permian and Pennsylvanian deposits produce high impedance contrast at the strata boundaries resulting in amplified ground motion.