Abstract
The Western Quebec seismic zone has moderate seismic activity with few historical damaging earthquakes. Nevertheless, recent risk analyses have shown that the combination of a high level of urbanization with soft soil deposits in the metropolitan area of Montreal could lead to significant damage and economic losses. Over the two decades, several projects have been completed to develop a seismic microzonation to identify zones where seismic waves could be amplified. During the same period, Natural Resources Canada developed an internet application to collect reports from the population after an earthquake and to convert them to the Modified Mercalli Intensity scale (MMI). This paper presents a first comparison of the MMI data compiled after eight recent earthquakes felt in Montreal area with the existing zonation in terms of soil classes. It shows that the MMI from individual reports increases when the observer is located in a soft soil zone. Statistics on average MMI over a regular grid confirms this trend. The numerous reports collected through the internet application, and future applications based on data collected from social media, could become a very useful source of information to complement seismic field measurements when developing and validating seismic microzonation maps.
Highlights
Macroseismic mapping based on felt reports was first used to estimate the extent of fault rupture and the distribution of damage after large earthquakes worldwide
The percentage of data grouped by soil classes as given in the microzonation map is calculated by level of Modified Mercalli Intensity (MMI)
MMI data derived from DYFI reports after eight earthquakes largely felt in the region of Montreal are analyzed
Summary
Macroseismic mapping based on felt reports was first used to estimate the extent of fault rupture and the distribution of damage after large earthquakes worldwide. The DYFI questionnaire, is permanently available online in both English and French (English and French links) allowing the public to submit felt information prior to the posting of a report for a significant earthquake and for smaller earthquakes that may receive less attention. The maps have been used to obtain Shakemap scenarios (Ghofrani, Atkinson, Chouinard, Rosset, & Tiampo, 2015) but has never been validated since no significantly large and close earthquake occurred in recent times and because the seismic network in this region is very limited in number and quality of stations. After 23 years of DYFI data collection, the number of available reports for the urban area of Montreal is large enough for a comparison and correlation with the site microzonation. This article presents the spatial distribution of data collected over this period and a statistical comparison between the spatial distribution of the MMI data and soil classes from the microzonation
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