Spatial distribution of noise variation during COVID-19
We first present the meter-scale spatial variation of seismic noise (RMS strain rate, calculation details in Text S1) across the 5-km DAS array (Figure 1b). Spatially, during the entire period, the seismic RMS noise data were impacted mostly on the main campus, exhibiting the slightest variation in agricultural/sports fields (AG area) and the intermediate variation in the western campus. A significant drop of seismic RMS noise was observed after the implementation of the stay-at-home measure. After Phase Yellow, seismic noise recovered somewhat but was maintained at a relatively low level.
To understand the spatial variation of seismic noise at a 2-meters spacing over the entire array, we calculated the RMS strain rate over 10 hours from 8 am to 6 pm. We repeated the calculation for data on March 5 (spring semester), April 16 (during the stay-at-home order) and June 4 (business reopening) (all Thursdays) and compared with the average daytime RMS strain rate during a week of the spring semester (February 3–7 2020).
Figure 2 shows the seismic noise spatial variation on March 5 (before the pandemic), April 16 (stay-at-home), and June 4 (Phase Green). By analyzing noise in four frequency bands (0.01–1 Hz, 1–10 Hz, 10–50 Hz, and 50–100 Hz), we could distinguish which frequency band of noise was affected by the COVID-19 measures most.
First, the biggest noise variation is detected on the main campus (Figure 2b). The peak noise reduction appears in all frequency bands on April 16 under the stay-at-home order. The largest reduction in the main campus could be up to 90% in the frequency band 1–10 Hz. With the gradual relaxation of the COVID-19 measure policies, the noise level on the main campus increases but remains relatively low (about 60% in 1–10 Hz). Exceptions can be found around Ch 1535–1580, where the noise level is higher during the stay-at-home order, possibly because cars were allowed to enter this area and generated stronger noise than previous pedestrian-only period (restricted before the school closure).
Both AG area and western campus area exhibited less university-related activity. Hence, the noise variation is relatively stable in all frequency bands. The largest noise reduction is in 10–50 Hz, which was likely caused by the decrease in traffic (e.g., school buses and commuter vehicles) due to the COVID-19 lockdown measures. On the western campus, significant noise variation near the end of the array is likely caused by the transition between shutdown (stay-at-home order) and opening (regular semester/Phase Green) of construction-related activities.
We also found that channels around the intersections could detect large noise variation in the frequency band below 50 Hz while noise levels of adjacent channels away from the road remained unchanged. Our fiber array could identify the exact places where traffic noise is dominant, which could help estimate the number of vehicles (Lindsey et al., 2020).