Seismology can confirm that Granada is undergoing an active sequence, analyze how it evolves, and calculate general probabilities. What it cannot do is accurately anticipate whether there will be another earthquake, when it will occur, or what magnitude it will have.
Granada has once again suffered a tremor of magnitude 4.8, with an epicenter northwest of Gójar and a very shallow depth. The earthquake occurred at 10:37 a.m. and was followed by several movements in the metropolitan area, including tremors of magnitudes 3.9, 4.2, and 4.0, according to provisional data from the National Geographic Institute (IGN).
The episode comes four days after the moment magnitude 4.8 earthquake recorded at 23:04 UTC on August 14 -1:04 a.m. on Saturday, August 15, in mainland Spain-, with an epicenter northeast of Alhendín. Two earthquakes of similar size, separated by a few days and located in the same area, confirm that seismic activity continues. However, they do not allow for anticipating what the next movement will be.
More than 350 movements since August 14
The latest technical note from the IGN, updated before the earthquakes this Tuesday, counted 348 events in the series since 00:00 hours on August 14. The majority had magnitudes below 2 and all occurred at less than ten kilometers deep. A total of 37 had been felt by the population.
By adding the new movements incorporated this Tuesday into the seismic catalog of the IGN, the sequence exceeds 350 events. Not all should be automatically classified as aftershocks: the technical term depends on the spatial and temporal relationship between the tremors and which one ends up being identified as the largest earthquake in the series.
Until this Tuesday, the IGN considered the movement of Alhendín as the main one and placed its largest aftershock at magnitude 3.3. The appearance of another earthquake of 4.8 forces a review of the evolution of the sequence, but does not turn the new tremor into a precursor of a larger one.
Can a larger earthquake occur in Granada?
The National Geographic Institute explains that there is currently no method capable of simultaneously predicting the moment, place, and magnitude of an earthquake. The succession of movements confirms that the area remains active and that more tremors may be recorded, but it does not contain an unequivocal signal that a more powerful earthquake is approaching.
The majority of seismic sequences evolve with subsequent smaller earthquakes and with a frequency that tends to decrease over time. That statistical pattern also does not work as a guarantee: there can be days with spikes in activity or with clearly perceptible aftershocks before the series loses strength.
As a global reference, the United States Geological Survey estimates that around 5% of earthquakes are followed in the following week by another larger and nearby one. That percentage is a global average, not a specific forecast for Granada, so it should not be directly applied to the sequence of Alhendín and Gójar.
Does the previous earthquake now become a precursor?
Seismologists refer to a precursor as an earthquake that precedes another larger one in the same area. The problem is that no movement can be identified as a precursor in real time: it only receives that name if another of greater magnitude occurs afterwards.
If the next earthquake is smaller, it is normally integrated among the aftershocks. If a larger one appears, it retrospectively changes the classification of the previous ones. That is why the repetition of tremors does not constitute a prediction by itself.
The two main earthquakes known so far have a moment magnitude of 4.8. Talking about a "seismic doublet" would require studying in detail their location, depth, rupture mechanism, and relationship with the surrounding faults. The similarity of magnitudes and geographical proximity is not enough to establish it automatically.
Why they are being felt so strongly
The perception of the earthquakes in Granada is related to their shallow depth and the proximity of the epicenters to densely populated municipalities in the metropolitan area.
A shallow earthquake can cause an intense shaking near the epicenter even if its magnitude is moderate. The magnitude measures the energy released; intensity describes how it feels and what effects it produces in each place. They are distinct concepts.
The Alhendín earthquake was felt in more than 500 localities and reached a maximum intensity of V-VI on the European macroseismic scale. The IGN received more than 12,000 citizen questionnaires and confirmed non-structural damage and falls of objects in the epicentral area.
The most likely now: more movements, but not necessarily larger
The scientific conclusion is not that a larger earthquake is going to occur, but neither that the sequence has ended. It is reasonable to expect new movements while the activity readjusts, most of them of low magnitude and possibly imperceptible.
The immediate risk does not solely depend on the occurrence of a larger earthquake. A moderate aftershock can cause new detachments in facades, cornices, or previously damaged buildings. Hence, the authorities maintain vigilance and ask to avoid the marked areas, not to stay next to deteriorated elements, and to follow only the information from the IGN and emergency services.