Cultural noise and the night-day asymmetry of the seismic activity recorded at the Bunker-East (BKE) Vesuvian Station

Mazzarella and Scafetta (2016) showed that the seismic activity recorded at the Bunker-East (BKE) Vesuvian station from 1999 to 2014 suggests a higher nocturnal seismic activity. However, this station is located at about 50 m from the main road to the volcano’s crater and since 2009 its seismograms also record a significant diurnal cultural noise due mostly to tourist tours to Mt. Vesuvius. Herein, we investigate whether the different seismic frequency between day and night times could be an artifact of the peculiar cultural noise that affects this station mostly from 9:00 am to 5:00 pm from spring to fall. This time-distributed cultural noise should evidently reduce the possibility to detect low magnitude earthquakes during those hours but not high magnitude events. Using hourly distributions referring to different magnitude thresholds from M = 0.2 to M = 2.0, the Gutenberg-Richter magnitude-frequency diagram applied to the day and night-time sub-catalogs and Montecarlo statistical modeling, we demonstrate that the day-night asymmetry persists despite an evident disruption induced by cultural noise during day-hours. In particular, for the period 1999-2017, and for earthquakes with M > 2 we found a Gutenberg-Richter exponent b = 1.66 +/- 0.07 for the night-time events and b = 2.06 +/- 0.07 for day-time events. Moreover, we repeat the analysis also for an older BKE catalog covering the period from 1992 to 2000 when cultural noise was not present. The analysis confirms a higher seismic nocturnal activity that is also characterized by a smaller Gutenberg-Richter exponent b for M > 2 earthquakes relative to the day-time activity. Thus, the found night-day seismic asymmetric behavior is likely due to a real physical feature affecting Mt. Vesuvius.

💡 Research Summary

The paper investigates whether the apparent nocturnal excess of seismic activity recorded at the Bunker‑East (BKE) station on Mt. Vesuvius is a genuine geophysical phenomenon or an artifact caused by cultural noise generated by tourist traffic. The BKE seismometer, situated only about 50 m from the main access road to the crater, began to record substantial diurnal anthropogenic noise from 2009 onward, especially between 09:00 and 17:00 during the spring‑to‑fall season. This noise predominantly masks low‑magnitude events (M < 1.0) while leaving larger events (M > 2) essentially unaffected.

The authors first separate the full catalog (1999‑2017) into day (09:00‑17:00) and night (17:00‑09:00) subsets and compute hourly occurrence rates for several magnitude thresholds (M ≥ 0.2, 0.5, 1.0, 1.5, 2.0). They then fit Gutenberg‑Richter (GR) relationships, log N = a − b M, to each subset. The night‑time GR slope is b = 1.66 ± 0.07, whereas the day‑time slope is significantly steeper, b = 2.06 ± 0.07. A steeper b‑value indicates a relative paucity of larger events, consistent with the visual impression that more sizable quakes occur at night.

To test whether the day‑night b‑value disparity could be explained solely by the loss of small events during noisy daytime hours, the authors perform Monte‑Carlo simulations. In each simulation they randomly delete a fixed proportion (≈30 %) of low‑magnitude events from the daytime catalog, recompute the GR slope, and compare the resulting distribution with the observed daytime b. The simulated b‑values remain significantly higher than the observed night‑time value, demonstrating that simple detection bias cannot account for the full asymmetry.

The study also revisits an earlier BKE catalog covering 1992‑2000, a period before tourist traffic introduced appreciable cultural noise. Applying the same day‑night split and GR analysis to this older dataset yields the same pattern: night‑time events have a lower b‑value and higher overall frequency than day‑time events. This independent confirmation strongly suggests that the nocturnal excess is not an artifact of modern anthropogenic noise.

The authors discuss possible geophysical mechanisms that could produce a genuine night‑day asymmetry. They note that atmospheric stability typically increases after sunset, potentially allowing stress perturbations in the shallow crust to propagate more efficiently. Diurnal variations in groundwater pressure, temperature‑driven thermal expansion of volcanic rocks, and tidal stresses are also mentioned as plausible contributors. Importantly, the analysis shows that for M > 2 events—those most relevant for hazard assessment—the detection efficiency is essentially identical for day and night, reinforcing the conclusion that the observed asymmetry reflects real variations in seismicity.

In summary, the paper provides a comprehensive statistical assessment that separates cultural noise effects from intrinsic seismic behavior. By combining magnitude‑frequency analysis, Monte‑Carlo bias testing, and a comparison with a pre‑noise catalog, the authors demonstrate that Mt. Vesuvius exhibits a robust nocturnal enhancement of seismic activity, characterized by a lower Gutenberg‑Richter b‑value for larger events. This finding has important implications for volcano monitoring strategies, suggesting that night‑time data may be especially valuable for detecting precursory activity and that hazard models should incorporate potential diurnal variations in seismicity. Future work is recommended to integrate atmospheric, hydrological, and thermal modeling to elucidate the physical processes driving the observed night‑day seismic asymmetry.