Last night, at 4:05 (Italian time) on April 20, 2024, an earthquake of magnitude 3.5 in Southern Calabria, in the province of Reggio. The closest municipality is Cittanova, a municipality of 10,000 inhabitants. The earthquake was felt in the epicentral area, but did not cause any damage. The magnitude was well below the threshold for damaging earthquakes.
We use this event to remember how our hypocentral localization system works and its magnitude calculation.
In the case of tonight's earthquake, the automatic recognition system identified the event immediately after its occurrence and within two minutes it provided the first reliable estimate of the parameters (figure on the left, indicated with AUTO 1). As you can see, the earthquake was localized with data from many of the Calabrian seismic stations of the INGV National Seismic Network, some from north-eastern Sicily and also some stations from the Aeolian Islands. The results are also shown in the first row of the table below. At 4.07 am Italian time this preliminary information on the event was sent to the Department of Civil Protection and made known to the public through the social channels of INGVterremoti, Twitter/X and Facebook, the INGVterremoti App (IOS and Android) and the portal of earthquakes in real time terremoti.ingv.it, indicating an estimated magnitude between 3.3 and 3.8.
In the following two minutes, the seismic waves reached other areas further away from the epicenter and the new preliminary parameters were automatically recalculated with more data (map in the center and second row of the table below, indicated with AUTO 2*). In such a case, when the Seismic Network covers the epicentral area well, the differences between its preliminary solutions are always contained within a few kilometers.
In the meantime, the seismologist shift workers present in the INGV's Seismic Surveillance and Tsunami Alert Room reviewed the seismograms of the various stations that had recorded the earthquake, perfecting the identification of the first arrival (the “picking”) of the P waves done by the automatic localization system and above all by “reading” the S phases. These, especially those detected near the epicenter, are important for constraining the hypocentral depth. In the two automatic solutions, in fact, the picking occurs only on the P phases and this leads to greater indeterminacy in the estimate of the depth parameter.
As seen in the figure above (right panel) the Seismologist Shift Workers have reviewed the data from over thirty seismic stations in Calabria, Sicily and as far as Cilento, to the north. The table below shows the result of the revised solution: the epicentral parameters remained almost unchanged, as did the magnitude (3.5), while the depth was identified with accuracy in 14 km. The time it took to publish the revised solution was approximately 14 minutes.
| Solution | Origin time (Italian time) | Latitude | Longitude | Depth (km) |
ML | Calculation time | Time after the event |
| Car1 | 04:05:18 | 38:32 | 16.08 | 20 | 3.6 | 04:07 | ~2' |
| Car2 | 04:05:18 | 38:35 | 16.06 | 10 | 3.5 | 04:09 | ~4' |
| Magazine | 04:05:18 | 38:32 | 16.07 | 14 | 3.5 | 04:19 | ~14' |
As mentioned above, such a case is easy to manage, because the affected area is well covered by the Seismic Network. In other cases (epicenters at sea or in areas where the Seismic Network does not have optimal coverage (fortunately there are not many cases of this type in Italy), the work of the Seismologist Shift Workers is more complex and the time to obtain a revised solution can be greater.
All revised locations have an information page on the real-time earthquake portal terremoti.ingv.it.