How Observer Conditions Impact Earthquake Perception

Abstract

Online Material: Figures of earthquake magnitude distribution; list of events; questionnaires and analysis; felt percentages analysis, Chi‐squared tests. 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 scales (Wood and Neumann, 1931) describe intensity II as a vibration felt only by a few people, extremely susceptible, almost always on the upper floors of buildings. Another example is the European Macroseismic Scale (EMS) (Grunthal, 1998), which 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 (1) people’s physical situation (what were you doing?), here categorized as “sleeping,” “at rest,” or “in motion,” and (2) 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 use an experimental method to study 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 quantity of data, gathered through an online macroseismic questionnaire, associated with …