Temporal changes in anthropogenic seismic noise levels associated with economic and leisure activities during the COVID-19 pandemic

Seismic ambient noise with frequencies > 1 Hz includes noise related to human activities. A reduction in seismic noise during the COVID-19 pandemic has been observed worldwide, as restrictions were imposed to control outbreaks of the SARS-CoV-2 virus. In this context, we studied the effect of changes in anthropogenic activities during COVID-19 on the seismic noise levels in the Tokyo metropolitan area, Japan, considering time of day, day of the week, and seasonal changes. The results showed the largest reduction in noise levels during the first state of emergency under most conditions. After the first state of emergency was lifted, the daytime noise reverted to previous levels immediately on weekdays and gradually on Sundays. This was likely because economic activities instantly resumed, while non-essential outings on Sundays were still mostly avoided. Furthermore, the daytime noise level on Sundays was strongly reduced regardless of changes on weekdays after the second state of emergency, which restricted activities mainly at night. Sunday noise levels gradually increased from the middle of the second state of emergency, suggesting a gradual reduction in public concern about COVID-19 following a decrease in the number of infections. Our findings demonstrate that seismic noise can be used to monitor social activities.

Long-Chain Alkenones in the Shimosa Group Reveal Palaeotemperatures of the Pleistocene Interglacial Palaeo-Tokyo Bays

The Shimosa Group, middle- to late-Pleistocene sedimentary succession, has been the focus of stratigraphic attention because it is beneath the Tokyo metropolitan area of central Japan. It is also of palaeoclimatic significance because it contains important interglacial marine strata of the past 450,000 years. Since the marine strata of the Shimosa Group were formed in the fluvial, estuary, and shallow inner bay known as Palaeo-Tokyo Bay, few occurrences of marine microfossils, make it difficult to quantitatively reconstruct the palaeotemperatures. Here, we extracted long-chain alkenones from the core GS-UR-1 penetrating the Shimosa Group to Marine Isotope Stage (MIS) 11. We found that the alkenone unsaturation ratio appears to reflect the sea surface temperatures (SSTs) of Palaeo-Tokyo Bays formed during MIS 5e, 7e, 9, and 11, which might be recorded around the peak of each interglacial period. The palaeo-SSTs during each interglacial period were 2–3 ℃ higher than the palaeo-SSTs of Tokyo Bay in the pre-industrial era, seemed to reach the similar level as the Holocene thermal maximum. We suggest that the LCA-based proxy, which has not been utilized hitherto in studies on the Shimosa Group, demonstrates its potential to provide palaeoclimatic and stratigraphic information.