Introduction
The COVID-19 pandemic has been impacting all aspects of our society, particularly public health and the economy. To reduce the spread of coronavirus, the COVID-19 measures such as working from home, self-isolation, and social distancing were implemented and resulted in a significant disruption to human activities. In the initial stage of the pandemic, the lockdown measures were adopted, resulting in fewer human activities; after the lifting of containment measures, community life and economy restarted, leading to the recovery of human activities. Quantifying the response of human activities to different COVID-19 measures may serve as a potential way to evaluate the effectiveness of the measures and optimize measures in the future (Gupta et al., 2020; Jarvis et al., 2020).
Seismologically, human activities generate vibration noise with the frequency above 1 Hz (anthropogenic noise) (Bonnefoy-Claudet et al., 2006). Several recent studies showed that a significant drop in high-frequency seismic noise levels (1–20 Hz) corresponded to fewer human activities after COVID-19 lockdown in the cities (Xiao et al., 2020; Poli et al., 2020; Lecocq et al., 2020; Yabe et al., 2020). These studies used a limited number of seismic stations in the cities to analyze the reduction of seismic noise attributed to the lockdown. Surprisingly, the recorded seismic data didn’t show much distinguishable difference when the level of human activity changed during early isolation before official restrictions were placed and the period after the relaxation of restrictions (Dias et al., 2020; Pulli and Kafka, 2020). One possible reason is that seismic stations have difficulty picking up high-frequency anthropic noise afar due to the city’s large spatial extent. During the COVID-19 pandemic, different sectors of the city might respond to the restrictions differently. This highly spatial-varying characteristic of anthropic noise brings the demand for dense seismic arrays in urban areas to provide high-resolution maps of noise variation.
Distributed acoustic sensing (DAS), a recent technology converting optic fibers to dense seismic sensor arrays, could provide high fidelity seismic strain/strain rate measurements at meter-scale spacing (Lindsey et al., 2017; Ajo-Franklin et al., 2019; Zhan et al., 2020; Lindsey and Martin, 2021). DAS has been used for seismic monitoring with tens of kilometers long telecommunication fiber cables (Martin et al., 2018; Lindsey et al., 2019). Recent studies reported new recordings of vehicles, footsteps, and music, highlighting the sensitivity of DAS-equipped fibers in the cities (e.g., Wang et al., 2020; Zhu et al., 2021). Lindsey et al. (2020) reported using DAS with telecommunication fiber to discriminate the traffic noise reduction in different areas (grocery store and hospital) during the COVID-19 pandemic.
Using an underground telecommunication fiber-optics DAS array in State College, PA, USA (Figure 1a), we show significant seismic noise variation from March to June 2020 responding to progressive COVID-19 measures: stay-at-home, Phase Yellow, and Phase Green.