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. 2020 Jun 10;10:9404. doi: 10.1038/s41598-020-66368-0

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© The Author(s) 2020

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Spectrogram of ambient signal recorded on the vertical component of station MILN, December 1^st 2019-present (top); the same spectrogram, averaged (red lines) at frequencies from 1 to 5 Hz (middle) and from 10 to 15 Hz (bottom), compared with results obtained in the same way, from recordings made December 2018-April 2019 at the same station (blue). (The East-West and North-South components show very similar trends, albeit slightly less pronounced.) The 2018/2019 curve is slightly offset, in order for weekends to be “in phase” with those of 2019/20. To emphasize relatively-long term (weekly rather than daily) effects, curves are also smoothed over time (thicker lines). The dates when specific containment measures were first implemented are marked by a dotted line (lockdowns of eleven municipalities in the North), a dashed line (restriction of movement and closure of schools in the entire country) and a solid line (interruption of all non-essential industrial activities).