The earthquake of magnitude 4.8 registered this Tuesday, August 18 at 10:37 AM has prolonged a seismic series that began four days earlier south of Granada. Since August 14, the National Geographic Institute (IGN) has located 432 earthquakes in the vicinity of Alhendín, Gójar, and La Zubia, all of them very shallow and concentrated between two and twelve kilometers south of the capital.
The episode this Tuesday has been the second earthquake of magnitude Mw 4.8 of the series. The first occurred during the night of August 14. After the new earthquake, several notable aftershocks have also been recorded, including movements of magnitude 3.9, 4.2, and 4.0.
The following maps and graphs allow us to see together where the earthquakes are occurring, which have been the most important movements, why they are perceived with such intensity, and how far the shaking has reached.
A series of 432 earthquakes in just five days
The accumulated figure allows us to gauge the activity that Granada has been recording since August 14. By 12:30 UTC this Tuesday, the IGN had located 432 seismic events, although the vast majority correspond to small magnitude movements.
Of all of them, 45 have been felt by the population and nine have reached or exceeded magnitude 3. The two most important have reached Mw 4.8: the first occurred on August 14 at 23:04 UTC and the second this Tuesday at 08:37 UTC, 10:37 in Spanish peninsular time.
The high number of earthquakes does not mean that all have the capacity to cause damage. A large part of the series consists of small movements that can only be recorded instrumentally.
Where the earthquakes are concentrating
The core of the activity is located south of the metropolitan area of Granada, mainly around Alhendín, Gójar, and La Zubia.
The map of the earthquakes recorded this Tuesday shows how the epicenters are also distributed around Armilla, Ogíjares, Otura, Churriana de la Vega, and Las Gabias, although there is a particularly marked concentration south of the capital.
The IGN places the entire sequence in the Granada basin, within the Eastern Betics, one of the areas with the highest seismic activity and danger in the Peninsula. The series develops between several known active faults: the Dílar fault to the south, the Granada fault to the east, and the Belicena-Alhendín fault to the west.
Two earthquakes of magnitude 4.8
The series currently presents two major references. On August 14, at 23:04 UTC, an earthquake of magnitude Mw 4.8 occurred with a maximum intensity observed of V-VI on the EMS-98 scale.
Four days later, this Tuesday the 18th, another earthquake reached exactly the same magnitude. It was recorded at 08:37 UTC and again reached a maximum intensity of V-VI.
After the movement this Tuesday, three particularly notable aftershocks were recorded: one of magnitude 3.9 at 08:53 UTC, another of 4.2 at 09:34, and a third of 4.0 at 10:39. Their epicenters were located respectively south of Churriana de la Vega, north of Alhendín, and northeast of Gójar.
The IGN also includes among the immediate background an earthquake of magnitude 3.7 recorded on August 14 about six hours before the first 4.8 and another of magnitude 3.6 that occurred on July 26 southwest of Armilla.
Why they are being felt so much
One of the keys to this series lies in its shallow depth. The IGN indicates that all earthquakes located since August 14 have occurred at less than ten kilometers deep. The two earthquakes of magnitude 4.8 have been considered very shallow.
When an earthquake occurs near the surface, the seismic waves travel less distance before reaching populated areas. This can cause moderate movements to be clearly perceived in municipalities close to the epicenter.
The recorded accelerations also reflect the strength of the shaking. The IGN has measured in both Mw 4.8 earthquakes a maximum acceleration of approximately 0.24 g at the Armilla station, located about three kilometers from the epicentral area.
More than 500 localities have felt the earthquakes
The movements have not been limited to the metropolitan area of Granada.
According to the IGN, the two earthquakes of magnitude 4.8 have been felt in more than 500 localities. Most are located in central and eastern Andalusia, but there are also reports of perception in points of Murcia, southern Castilla-La Mancha, and even the Community of Madrid.
Since the series began, the agency has also received more than 15,000 questionnaires from citizens through its website and mobile application. This information is used to determine how each earthquake has been felt in different municipalities and subsequently calculate its macrosismic intensity.
The maximum intensity observed in the two major earthquakes has been V-VI, with some non-structural damage and falling objects within the epicentral area.
Magnitude 4.8 and intensity V-VI: why they do not mean the same
The magnitude and the intensity measure different aspects of an earthquake. The Mw 4.8 magnitude represents the size of the earthquake based on the energy released and constitutes a single value for the entire quake.
The intensity, on the other hand, measures how the earthquake feels and what effects it generates in a specific place. That is why the same movement can reach intensity V-VI near the epicenter and lower values as the distance increases.
In this series, the two Mw 4.8 earthquakes have reached a maximum observed intensity of V-VI in municipalities of Granada, while the aftershocks of magnitudes 4.2 and 4.0 have reached intensity IV-V.
A region with significant seismic history
The current activity also does not occur in a territory foreign to earthquakes.
The IGN reminds that the Granada basin has historically suffered destructive movements. Among them is the Arenas del Rey earthquake of 1884, which reached intensity IX-X, and the one that occurred south of Granada in 1431, with intensity VIII-IX.
More recently, between December 2020 and August 2021, the area of Santa Fe and Atarfe recorded more than 3,000 shallow earthquakes, six of them with magnitudes between 4 and 5. The largest reached magnitude 4.5.
The difference is that the current seismic series is concentrated about ten kilometers further southeast, in the vicinity of Alhendín and Gójar.
More sensors to monitor the evolution of the series
The IGN maintains a particularly dense network around the affected area.
Within an approximate radius of 50 kilometers, there are 52 permanent seismic sensors from the IGN, in addition to seven stations belonging to the Andalusian Institute of Geophysics whose data are also used to analyze the activity.
After the series began, the agency has also deployed two portable seismic stations in the epicentral area. The goal is to improve the location and characterization of the movements while activity continues.
The map of this Tuesday confirms that the earthquakes continue to concentrate in a relatively small area south of Granada. What the data does allow to determine is where they are occurring, what magnitude they reach, and how the sequence evolves. What seismology cannot establish is when the next movement will occur nor what its magnitude will be.