Spatiotemporal Variation in Ambient Seismic Noise and Its Effect on the Microearthquake Monitoring Capability of the Pohang Community Seismograph Network (PCSN), South Korea

ABSTRACT Ambient seismic noise from human activities, site conditions, meteorology, and instrument self-noise limits earthquake monitoring. We characterize noise in the Pohang Community Seismograph Network (PCSN), deployed after the 2017 Mw 5.5 Pohang earthquake, using power spectral density (PSD) estimates. The PCSN stations were categorized into four groups based on their location and depth: rural, urban, coastal, and borehole array. We compared PSDs across different period ranges. At very short periods (<0.03 s), we could occasionally identify elevated noise levels caused by everyday weather conditions. The seismic signature of rainfall was clearly recorded at surface stations, but barely recorded at 500 m depth. At short periods (0.02–1.00 s), anthropogenic noise was predominant. The noise levels were highest in urban areas, followed by coastal areas, and lowest in rural areas. At long periods (1–10 s), we investigated the effect of typhoons and ocean waves. As typhoons Hinnamnor and Nanmadol approached the Korean Peninsula, the background noise level increased and reached a peak as it passed the Pohang region. The geological setting affected the ambient seismic noise: the PSDs of the horizontal component are higher than those of the vertical component in areas with thick sediments. The detection thresholds map in the Pohang region shows the smallest magnitude in the middle of the network in regions surrounded by stations with low-ambient noise levels. The recently compiled PCSN earthquake catalog demonstrates the effect of background noise on earthquake detection capability. The number of detected earthquakes under the magnitude of completeness (0.34) was 53 at night and 5 during the day. These findings underscore the importance of reducing ambient noise levels by relocating or installing seismic stations at depth on bedrock for effective microearthquake observations in a densely populated metropolitan area.