Abstract
A large amount of data about earthquake effects, supplied by citizens through a web-based questionnaire, enabled the analysis of the occurrence of many of the effects on humans and objects listed in macroseismic scales descriptions. Regarding the other diagnostic effects (rattling, moving, shifting, falling or overturning depending of the object type of doors, windows, china, glasses, small objects, pictures, vases, books, as well as frightened people and animal behaviour), data from more than 300,000 questionnaires about earthquakes felt in Italy from June 2007 to August 2017, were analysed by stacking them together as a function of hypocentral distance and magnitude. The comparison of the resulting percentages with the intensity prediction equation showed that almost all the chosen effects are good diagnostics for macroseismic intensity evaluation, as their percentages are well differentiated. We did not analyse the oscillations of hanging objects and liquids because the differences in effect attenuations, highlighted by the maps of the occurrence percentage, suggested to not consider them as diagnostic effect. This result allowed us to quantify the occurrence of each diagnostic effect for the intensity degrees from II to VI of the European macroseismic scale for the people who felt the earthquake. The application of the intensity assessment method to internet macroseismic data, based on the specifications herein proposed, should mitigate the problem of “not felt” undersampling in crowdsourced web data.
Highlights
Macroseismic scales are based on experimental observations that are categorised to obtain the estimation of intensity levels of ground shaking
This paper is focused on quantification of the occurrence of effects on humans and observed effects objects to reduce the subjective component in macroseismic intensity assessment
Many authors (Davison 1921; Ferrari and Guidoboni 2000; Musson et al 2010; Musson and Cecić 2012; Vannucci et al 2015) admit that one of the difficulties in assessing macroseismic intensity is the rough quantification of each diagnostic effect
Summary
Macroseismic scales are based on experimental observations that are categorised to obtain the estimation of intensity levels of ground shaking. As stated in the European macroseismic scale (EMS, Grünthal 1998), “The scale recognises the statistical nature of intensity, that is, that at any place a certain effect is likely to be observed in a proportion of cases only, and whether that proportion is small or large is itself something that tells one about the strength of the shaking.” These trends, based mainly on reports from the field, but even on common sense, have been studied systematically in large samples of data only a few times (Brazee 1979; Vannucci et al 2015) and the occurrence of each diagnostic for each intensity level has been poorly quantified using qualitative word such as few, many and most. Even if the data are scarce, we added the analysis of the loss of balance percentages (Figs. 1c, 2c, 3c) to the above-mentioned effects, because it is one of the diagnostic effects listed in the EMS characterising the medium–high degrees
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