Earthquakes Parameters from Citizen Testimonies: A Retrospective Analysis of EMSC Database

We aim to compute macroseismic parameters (location and magnitude) using the BOXER
code for the first time on the citizen testimonies, that is, individual intensity data points
(IDPs) at the global scale collected and made available by the LastQuake system of the
European–Mediterranean Seismological Centre (EMSC). IDPs available for different earthquakes
are selected to eliminate those that are geographically inconsistent with most
data; then they are clustered spatially based on various methods. For each cluster with
at least three IDPs, a macroseismic data point (MDP), corresponding to an intensity value
assessed for given localities as in classical macroseismic studies, is computed by various
central tendency estimators (average, median, and trimmed averages). Finally, macroseismic
parameters are obtained by MDP distribution using two location methods of BOXER
code. For each earthquake, we used raw and corrected intensities and 132 different combinations
of grouping methods, estimators, and BOXER methods. We assigned a ranking
to the combinations that best reproduce instrumental parameters and used such a ranking
to select preferred combinations for each earthquake. We analyzed retrospectively
the reliability of the parameters as a function of time and space. The results are essentially
identical using original and corrected intensities and show higher reliability for BOXER’s
method 1 than for method 0; they are dependent on the geographical area, and generally
improve over time and with the number of IDPs collected. These findings are useful for
the future real-time analyses, and for evaluating the location and magnitude of earthquakes
whenever a sufficient number of IDPs are available and with a distribution such
that MDPs can be derived and the BOXER method applied.