ABSTRACT

Unmanned aerial systems (UAS) provide a framework for recording perishable surficial data or information. Open fractures exhibiting regular en-echelon patterns were captured by a 12-megapixel, FL-9 mm camera attached to a Phantom IV UAS over the epicenter of the magnitude (Mw) 5.8 earthquake of September 3, 2016, 15 months later. The Digital Surface Models (DSMs) and orthoimagery offered a spatial resolution (∼1 cm) sufficient to identify small-scale plastic deformations that appear to be controlled by en-echelon joint sets developed in the underlying formation. The fissure boundaries and intersections are remarkably linear and sharp. They appeared to have been recently formed, presumably by seismic swarms believed to have been associated with wastewater injection. The DSMs revealed a series of conjugate patterns suggestive of regional systematic joints with apparent subsidence of infilling up to 50 cm. The earthquakes emanated from the Precambrian metamorphic basement, with epicentral clusters at ∼5- and 8-km depths. Low energy release from depths >1.5 km appears to be locally attenuated by an unconsolidated “soil cap,” which likely formed an impedance contrast. The maximum deformation direction from the cumulative energy of earthquakes correlates with a wrench fault tectonics model that could conceivably produce the observed en-echelon joint sets observed in the orthoimagery and DSMs. These features were observed within 275 m of the reported Mw 5.8 epicenter. The remarkably linear repeating pattern of deformation appears to express fissures that preserve the wrench fault fractures generated by the Mw 5.8 earthquake emanating from discontinuity suites within marine sandstone, shale, and limestone of Pennsylvanian to Permian age.

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