The seismic episode affecting Granada and its metropolitan area, which had its maximum impact with the very shallow magnitude 4.8 earthquake that occurred on Saturday, August 15, has already recorded 348 tremors since Friday, most of low magnitude (less than 2) and all at very shallow depths (less than 10 kilometers). Of these, 37 have been felt by the population.
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The count made by the National Geographic Institute (IGN), dependent on the Ministry of Transport and Sustainable Mobility, since 00:00 hours on August 14 indicates that there were two earthquakes closer in magnitude to the main one, which was finally estimated at 4.8 and not 5 as initially believed. Thus, it indicated, there was one of 3.7 about six hours earlier and another of 3.6 on July 26 southwest of Armilla, both very shallow.
The main 4.8 earthquake has been felt in more than 500 localities, mainly in central and eastern Andalusia, and also in some localities in the Murcia region, southern Castilla La Mancha, and even the Community of Madrid. The maximum observed intensity is V-VI (EMS-98) in populations of Granada.
The IGN has received more than 12,000 questionnaires regarding the strong earthquake. The damage and effects caused by the earthquake are still being reviewed by the personnel of the Seismic Network, so the referred intensity will be updated as more information is gathered. The IGN also notes that some non-structural damage due to the earthquake and falling objects in several buildings within the epicentral zone have been reported.
The seismic moment tensor (SMT) provides information about the type of mechanism that produced the earthquake on the responsible fault, and a magnitude value, called moment magnitude (Mw), which is a more reliable measure of the earthquake size than the local magnitude.
The SMTs calculated by the IGN for the main earthquake and the previous one at 19:43 hours of 3.7 show normal fault type mechanisms consistent with the mapped active faults and the tectonics of the area. They specify that this series “is framed between the Dílar faults to the south, the Granada fault to the east, and the Belicena-Alhendín fault to the west.”
Control station network
For monitoring seismic activity in the vicinity of the epicentral zone (approximate radius of 50 km), the IGN has permanently installed 52 seismic sensors, 2 velocity sensors, 30 acceleration sensors, and 20 low-cost Silex accelerographs that complement the acceleration network.
The IGN’s permanent network stations closest to the epicenters are the acceleration stations of Armilla, Churriana de la Vega, and Dílar located about 3 and 5 kilometers away, respectively.
Additionally, in this region, the IGN has data shared with other institutions, specifically with seven seismic stations of the Andalusian Institute of Geophysics (IAG).
In order to strengthen monitoring capacity in the area, the IGN has deployed two portable seismic stations in the epicentral area. These installations allow for more information to improve the location and characterization of earthquakes.
Seismic episode
The National Geographic Institute indicates that this seismic series is framed in the Granada basin, in the central sector of the Eastern Betics, one of the areas with the highest seismic activity and hazard on the peninsula. In this region, several important destructive historical earthquakes have occurred, such as the one in 1884 in Arenas del Rey with intensity IX-X and the one in 1431 south of Granada with intensity VIII-IX.
In the vicinity of the current epicentral region, there are records of several seismic series that have reached intensities of VII and VIII (1806-1807 Pinos Puente, 1911 Santa Fe, 1918 Atarfe) and magnitude 5 earthquakes (1955 Armilla, 1956 Purchil).
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Recently, the series recorded between December 2020 and August 2021 stands out, in the vicinity of the localities of Santa Fe and Atarfe (Granada), located about 10 kilometers northwest of Alhendín. During that series, more than 3,000 shallow earthquakes were recorded, six of them with magnitudes between M4-M5, the maximum being M4.5. The maximum observed intensities were V-VI and the maximum acceleration was 0.195.
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