Shallow seismic surveys across the Ulsan fault near a province boundary in Korea

Recent earthquakes near nuclear power plants in Korea have triggered public concems about possible seismicity of the Ulsan fault in the southeastern part of the Korean peninsula. To reveal and characterize subsurface structures of this fault, we conducted high-resolution seismic reflection and refraction surveys along two profiles of 835 m and 415 m long, respectively, across a fault valley. A 5 kg sledgehammer and twenty four 100-Hz geophones were used for seismic energy and receiver, respectively. Velocity tomograms from first arrivals of refraction data and 12-fold stacked sections obtained through conventional reflection processing show both seismic velocity structures and subsurface images of the fault zone. We interpret that the fault valley was initially formed by strike-slip motions, and then the near-vertical faults in basements were reactivated later by compressional stresses yielding reversed faults in the overburden.