Seismicity in Granada Basin
The Granada basin is one of the areas with the greatest seismicity in the whole of the Iberian Peninsula, where major earthquakes have occurred throughout history. The oldest known earthquake occurred in 1431 to the south of Granada, where it had a macro-seismic intensity of VIII-IX, producing great damage in the Alhambra, and the most virulent was that of 1884 in Arenas del Rey, which reached an intensity of IX-X, producing 839 deaths. Also noteworthy with an intensity of VIII are those of 1806 in Pinos Puente, 1911 in Santa Fe and the last in 1956 in Purchil. Since then, a multitude of moderate earthquakes have occurred without exceeding magnitude 5.0, such as the 1984 Lentegí earthquake with M5.0, as well as swarms of earthquakes of lower magnitudes.
The normal focal mechanisms of these earthquakes occurring in the Earth's crust indicate a predominantly extensive tectonic regime in the ENE-WSW direction compatible with the main active faults in the area with azimuth in the NNW-SSE direction.
Geology and tectonics of Granada Basin
The Granada Basin is the second largest intramountain Neogene - Quaternary basin in the Betic Mountain Range, after the Guadix-Baza basin, which is located in the contact between the internal zone of the mountain range, which limits it by the E and S, and the external zone which limits it by the N and W. It has an elongated shape in the E-W direction, increasing the thickness of sediments in the central zone (under the Genil River) and progressively decreasing towards the northern and southern edges. The basin was formed as a result of the extensional process that affected the mountain range 13 Ma ago. During this period of time, a short interval of less than a million years, the basin was flooded by the sea, then by a salt water lake and finally by small scattered fresh water lakes. The Granada Basin is bounded on its northern and eastern edges by a set of normal faults. This tectonic activity, currently active, is corroborated by the high seismicity.
Neogene and quaternary materials of powerful thickness and frequent lateral variations of facies were deposited in this basin. These materials are affected by a distensive tectonic regime which, in many sectors, conditions the palaeogeography. The materials are observed softly folded in general lines.
In this way, the basin was constituted as a closed depression that ended up taking shape at the end of the Miocene (with marine calcarenites and marl in the most distant facies), which sealed the contact between the Internal and External Zones of the mountain range. The last materials are gypsum and salt, which indicate the passage from marine to continental conditions, produced at the beginning of the Pleistocene. During the Middle Pleistocene, an important subsident area was developed to the southwest of Sierra Elvira, which was filled in by the contributions of the River Genil and other tributary rivers.
The accommodation space for the Plio-Quaternary sediments in the basin is generated thanks to the operation of several normal fault systems in the eastern and northern limits of the basin. Most of these faults have been active during the Pleistocene and have associated seismicity.
The alluvial filling of the River Genil has an outcrop area (about 200 km2) and a power of more than 250 m in some points.
Among the important faults, the Cádiz - Alicante fault should be highlighted, which consists of a transverse fault in the N60o70oE direction, located on the northern edge of the basin. The right lateral fault of Alhama de Granada-Alpujarra, with an E-W direction, showing accumulated kilometre-long displacements in the southern zone of the basin's edge. This fault was responsible for a highly destructive earthquake in Alhama de Granada in 1884. The Sierra Elvira-Padul-Durcal system, an impressive system of NW-directed normal faults, with an accumulated vertical jump of 1.5 km. It is located in the eastern part of the basin.