Can we quickly calculate reliable earthquake parameters from citizen testimonies?

In a recent work, we tested the ability to compute macroseismic parameters (location and magnitude) using citizen testimonies collected by the European-Mediterranean Seismological Centre (EMSC). Each intensity estimated by individual non-professional users of the LastQuake smartphone application is indicated as an individual data point (IDP). Each IDP is archived by EMSC with a time stamp, allowing the calculation of the time delay from the earthquake origin time. To use IDPs as classic intensities, i.e. macroseismic data points (MDPs), identifying damage at the scale of towns or cities, they must be grouped into spatial clusters, which are then processed by the BOXER code to locate and size earthquakes. A retrospective analysis on a dataset of more than 15000 events collected over the past 10 years shows that the procedure can provide reliable parameters and that the results depend on the geographical area and improve over time and as the number of available IDPs/MDPs increases.The key question is whether early IDPs/MDPs can quickly provide reliable parameters (location and magnitude) for users and stakeholders (e.g. the civil protection agencies). Using clustering methods that statistically provide, on average, the best agreement with instrumental data, we tested some predefined time intervals within which to group the available IDPs into MDPs. We then applied the BOXER code to these MDPs, evaluating the agreement with the final instrumental parameters. Results confirm that reliability increases with the number and distribution of MDPs, strictly dependent on the number and distribution of available IDPs. This retrospective analysis demonstrates the effectiveness of the approach and its potential to quickly provide parameters for future real-time applications. The method may offer a reliable and rapid tool to support emergency response, improving as more IDPs/MDPs are collected.