How Observer Conditions Impact Earthquake Perception

Intensity scales define the criteria used to determine different levels of shaking in relation to environmental effects. Objective evaluations of low intensity degrees based on transient effects may be difficult. In particular, estimations for the number of people feeling an earthquake are critical, and are qualitatively described by words such as “few”, “many”, and “most” for determining various intensity levels. In general, such qualitative amounts are converted into specific percentages for each macroseismic scale. Additionally, estimations of macroseismic intensity are influenced by variables that are mentioned in macroseismic scale degree descriptions. For example, the Mercalli-Cancani-Sieberg (MCS; Sieberg, 1930) and the Modified Mercalli Intensity (MMI) scales (Wood and Neumann, 1931) describe the intensity II as “Felt only by a few people, extremely susceptible, in perfectly quiet situations, almost always on the upper floors of buildings”. Another example is the European Macroseismic Scale (EMS) (Grunthal, 1998) that describes the intensity V as “felt indoors by most, outdoors by few. Many sleeping people awake”.
In this work, we focus on two variables referred to as people’s physical “situation” (what were you doing?), here categorized as “sleeping”, “at rest”, or “in motion”; and the observer’s “location”, here categorized as “higher floors”, “lower floors”, and “outdoors”. Both variables have a partial influence on intensity assessments because they condition vibration perception. However, it is important to study, using an experimental method, the weights of these variables in the quantification of felt effects. Musson (2005a) also recognized the influence of such conditions on the number of people feeling an earthquake, stating that the proportion of people in different conditions “are generally difficult to quantify in any case”. Today, we have a large amount of data available through the macroseismic web site “haisentitoilterremoto” associated with specific observer conditions. Using this data, a study of these effects is possible. For this analysis, we placed attention on transitory effects that, in the past, could not be easily studied due to the intrinsic difficulty in collecting this type of data. The aim of this work was to specifically analyze and quantify how the observer’s “situation” and “location” influence earthquake perception suggesting a new scale description that can be easily used for low intensity estimation.

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