Seismotectonic description

GEOLOGICAL CONTEXT AND TECTONIC FRAMEWORK

The Betic-Balearic domain extends from the most eastern edge of the Betic Cordillera to the Balearic continental block, which correspond to the NE prolongation of the Betic System, and it is attached to the eastern margin of the Iberian Peninsula by its southwestern sector.

This NE-SW Balearic promontory is a broad plateau 400 km long and 100 km wide approximately. It is bounded by three extensional marine basins, to the NW and to the SE on continental crust, the Valencia trough and the Liguro-Provençal basin respectively, and to the SE on oceanic crust, the Algero-Balearic basin.

Geologically speaking and at regional scale, the Balearic Islands are mainly formed by mesozoic and cenozoic carbonatic rocks (limestones and marls) with siliciclastic layers interbedded. These formations correlate mainly with the Outer Zone unit present in the Betic cordillera. It is also worth mentioning the paleozoic rocks outcrops in Menorca, and very locally in Mallorca, that probably correspond to fragments of the Iberian Massif.

Tectonically, this region is very complex and both compressive and extensional processes were involved in its evolution. It can be distinguished a first pre-orogenic phase characterized by an extensive stress field (Mesozoic), a second compressive phase during the Alpine orogeny (Cenozoic), and a third post-orogeny phase of tectonic quiescence (Late Miocene – Today).

During the second phase (alpine compression) in which significant ENE-WSW thrust faults and folds were formed, the extensional marine basins surrounding the Balearic promontory opened and developed. Besides, the promontory was split into three sectors because of the development of NW-SE transcurrent faults. Each sector is governed by one of the main highs of the promontory emerging as the principal islands in the archipelago, from SW to NE, Ibiza, Mallorca and Menorca.

As a result of this tectonic evolution, the Mallorca Island is the most rugged island in the archipelago, and it is characterized by several successive NE-SW horst and grabens. The most western horst is the most relevant one as it shapes the Tramontana Mountain Range. This structure frame in Mallorca is linked to the development of a series of NW-verging thrusts whose detachment level is the triassic evaporites.

Currently, the stress field is very quiet or slightly compressive, and there is almost no seismic activity in the archipelago. It is only worth mentioning one active fault with ground expression, the Sencelles fault located in Mallorca Island. It is a 20 km long NE-SW left-lateral strike-slip fault aligned with the big compressive structures in the island, and a slip rate below 1 mm/yr.

HISTORICAL AND INSTRUMENTAL SEISMICITY

The seismicity in the Betic-Balearic domain is mainly clustered at its southeastern edge, in the Eastern Betic Cordillera which is characterized by a shallow crustal seismicity (<30 km depth) of moderate magnitude, with the occurrence of some large destructive earthquakes. This seismicity is related to the shear active fault systems that cross the crust, with predominance of strike-slip focal mechanisms. On the contrary, seismicity in the Balearic block is very scarce and dispersed and of low magnitude (mostly M <4).

Nevertheless, the Balearic archipelago presents some tsunami hazard, its coasts have been affected by tsunamis generated by earthquakes offshore the Algerian north coast. In fact, in the IGN catalog there is evidence of two tsunamis that have affected the Balearic Islands. One historical, due to an earthquake in 1856 of estimated magnitude 6.6 that caused the sea level to rise and damage to the port of Mahón. And the most recent, in 2003, generated by another earthquake off the coast of Algeria of Mw 6.8, which took about 45-50 min to reach the islands of Ibiza and Palma de Mallorca and caused material damage to boats in several ports of the islands, being the maximum wave height recorded of 2 m.

In the historical period, the highest estimated magnitudes and maximum historical intensities recorded in Balearic sector have been M5.4 and intensity VII for two earthquakes located in the Island of Mallorca, one occurred in 1721 in Selva and other in 1851 in La Cabaneta. It is also worth mentioning several earthquakes felt with a maximum intensity of VI, one in 1660 in Ibiza and another in 1912 in Ciudadela (Menorca) with an estimated magnitude of 4.9.

Recent seismicity in the Balearic block is scarce, of low magnitude (M<3) and scattered, mainly located in the Mediterranean Sea between Mallorca and Ibiza Islands at depths between 1-30 km. However, sporadically, some earthquakes of magnitudes between 3 and 4 also occur, being 4.3 the maximum magnitude recorded in the instrumental period. The maximum intensities observed generally range between II and III, being a value of IV the maximum observed intensity to date.