The 4.8 magnitude earthquake recorded this Tuesday in Granada, later accompanied by several lower intensity movements, has prolonged the seismic activity that has been affecting the Granada metropolitan area for days. The tremor, located by the National Geographic Institute (IGN) northwest of Gójar, occurred at 10:37 hours and reached a maximum intensity of V-VI.
This has not been an isolated episode. On August 15, numerous movements had already been recorded in the same area, and on the 14th another earthquake also reached a magnitude of 4.8 near Alhendín.
The succession raises a question that also arises when major earthquakes coincide in different countries: Are more earthquakes occurring than before?
Are there now more earthquakes than a few decades ago?
The United States Geological Survey (USGS) estimates that, since the beginning of the 20th century, the global average is around 16 large magnitude earthquakes each year: approximately 15 of magnitude 7 and one of magnitude 8 or higher.
This figure changes considerably from year to year. In 2010, for example, 23 earthquakes of magnitude 7 or higher were recorded, while in 1989 there were only six. The fact that several large earthquakes concentrate in a few months does not imply, therefore, that there is an increasing trend.
The USGS itself also points out that its systems currently locate about 20,000 earthquakes per year worldwide, around 55 daily. Part of the perception that more are occurring now is related to a much greater detection capability and the speed with which information reaches the population.
Can earthquakes occurring in different countries be related?
That two earthquakes occur within a few hours or days of each other does not automatically mean that there is a connection between them.
Earthquakes originate when the energy accumulated in a fault is released suddenly. This rupture can modify the stresses of nearby areas and cause aftershocks or new nearby movements, but this physical relationship cannot simply be transferred to earthquakes separated by hundreds or thousands of kilometers.
Therefore, the temporal coincidence of large earthquakes in different regions of the planet does not prove that they are part of the same global sequence.
In the case of Granada, the IGN reflects a clear concentration of movements in a very nearby area, with numerous episodes recorded around Gójar, Ogíjares, Armilla, Alhendín, or La Zubia during the last days.
Can an earthquake be predicted?
Currently, it is not possible to know precisely when, where, and with what magnitude an earthquake will occur.
The USGS points out that no scientific institution today has a method capable of making that type of prediction. What can be done is to calculate long-term probabilities and develop hazard maps based on known faults and the seismic history of a particular region.
This allows for the identification of more exposed territories and the preparation of preventive measures, but not to anticipate that a specific earthquake will occur on a certain day.
Early warning systems also do not predict earthquakes. They detect the movement once it has begun and, under certain circumstances, can provide a few seconds of margin before the most intense waves reach areas far from the epicenter.
What seismic risk does Spain have?
Spain is officially considered a country of moderate seismic activity, although certain areas have experienced significant earthquakes throughout history. This is noted in the State Civil Protection Plan against Seismic Risk.
Activity is particularly concentrated in the south and southeast of the peninsula, a circumstance linked to the contact between the African and Eurasian plates.
Granada is precisely part of one of the Spanish areas with the highest activity. The IGN catalog, which gathers earthquakes from the year 1370 to the present, records numerous historical episodes in the province.
Among them is the earthquake of Arenas del Rey in 1884, one of the most destructive recorded in Spain, in addition to numerous historical movements in the Granada area.
The damage does not solely depend on the magnitude
A higher magnitude earthquake does not necessarily have to cause more damage than another slightly lower one.
The consequences also depend on the depth of the focus, the distance to populated areas, the type of soil, the vulnerability of buildings, and the population density.
The magnitude measures the energy released by the earthquake, while the intensity describes its effects in a specific place. Therefore, the same earthquake can be felt very differently in two municipalities located at different distances from the epicenter.
In the earthquake of this Tuesday, the IGN has set a maximum intensity of V-VI, within the scale used to measure the perceived effects and possible damages.
Spain has a specific state plan for major earthquakes
The State has had a State Civil Protection Plan against Seismic Risk since 2010, whose objective is to coordinate public resources when an earthquake may cause an emergency of special severity.
The plan includes seismic information systems, coordination between administrations, mobilization of resources, and support mechanisms for the autonomous communities. It also establishes that the National Seismic Network of the IGN is responsible for observing and calculating the main parameters of earthquakes.
The regulations recognize that the probability of an earthquake with catastrophic consequences in Spain is relatively low, but warn that, if it were to occur, the damages could be significant in human lives, infrastructure, and essential services.
The earthquake in Granada does not demonstrate, therefore, that the planet is going through an exceptional stage. It does remind that Spain is a seismically active territory and that certain areas, especially eastern Andalusia and the southeastern peninsula, concentrate a good part of the risk.