Every day, beyond tectonic events, seismic networks detect several non-natural quakes: among them, quarry and mine blasts are the most numerous anthropogenic recorded events. Such events are often not identified and thus collected together with tectonic events. Unluckily, most statistical seismologists ignore or underestimate the presence of explosions and quarry blasts in seismic catalogues.
In their latest research article, RISE members Laura Gulia and Paolo Gasperini use the example of the Italian Seismological Instrumental and Parametric Database (ISIDe) to show what happens when artificial seismicity is mixed with natural.
Having low magnitudes, the artificial events enrich the number of small earthquakes in a catalogue, contaminating the natural signals and seismicity datasets, adulterating the relative portion of micro-seismicity with respect to the higher magnitudes. Blasts misclassified as natural earthquakes, indeed, may artificially alter the seismicity rates and then the b-value of the Gutenberg and Richter relationship, an essential ingredient of several forecasting models.
To identify the quarry blasts, the authors mapped the ratio between the daytime to nighttime events (D/N), introduced by Wiemer and Baer in 2000. Since quarry blasts are performed during the day, the presence of non-natural events increases this ratio. An indicative value of anomalous D/N is about 1.5. As shown in the figure on the left, Gulia and Gasperini illustrate in maps for Italy this difference between using a whole data set (natural and non-natural earthquakes) and selecting the earthquake event type only. Their analysis reveals the presence of numerous quarry blasts in ISIDe from 16 April 2005 to 30 April 2012, misclassified as earthquakes. After 1 May 2012, there is a general improvement in identifying the event type. However, many quarry blasts are still not correctly classified.
The two researchers conclude that using a contaminated dataset reduces the statistical significance of the results and can lead to erroneous conclusions. Removing such non-natural events should therefore be the first step for a data analyst.
The research article "Contamination of Frequency–Magnitude Slope (b‐Value) by Quarry Blasts: An Example for Italy" is published here: https://doi.org/10.1785/0220210080.
Stefan Wiemer, Manfred Baer; Mapping and Removing Quarry Blast Events from Seismicity Catalogs. Bulletin of the Seismological Society of America 2000; 90 (2): 525–530. doi: https://doi.org/10.1785/0119990104