Recent Tectonic Acti Vity Analysis of Mallorca Island
Abstract
During Neogene times Mallorca Island was subjected to an extensive regime characterized by NE-SW normal faults. On the other hand, geologic evidences indicate that this stress regime changed in Pliocene times. Seismic profiles crossing the island and field observations indicate that Pliocene and Quaternary materials are slightly folded in two main directions (NE-SW and NW-SE), however, NW-SE folds are more evident. In a smaller scale compressive structures affecting Neogene and Quaternary sediments can be observed in the field, nevertheless extensive and strike slip ones also can be seen. The recent tectonic activity of Mallorca must be considered small, nevertheless there are thermal anomalies, geomorphological evidences and low seismicity that can be related to the activity of the former Neogene Faults. The thermal anomalies, which range between 30ºC and 50ºC, are located along the NE-SW main faults. The most evident geomorphological evidence is anomalous drainage network that is clearly controlled by main faults. The Instituto Geografico Nacional of Spain seismic catalogue account three I>VI earthquakes since 1660, but only two can be considered tectonic earthquakes. The first, felt with an intensity VII in the southern part of the island, took place in 1660 and was followed by three I=III-IV aftershocks. The second one, that accomplish an intensity VIII at Palma de Mallorca city and the surrounding localities, took place in 1851 and was followed by 18 aftershocks, ranging between I=III and I=V, an d extending for more than one year. The remainder felt earthquakes are concentrated at the central part of the Island. The seismicity of the XXth century was low as only one I=V earthquake was felt, and no M>3.5 earthquakes were registered inside the island. On the other hand, two M~4.5 seismic events took place at the southern continental platform (1925 and 1978). Unfortunately no focal mechanisms have been obtained near Mallorca. The nearest ones are located north of Minorca Island and at the continental platform of the Iberian Peninsula. All of them are characterized by a strike slip regime associated with near vertical NW-SE or NE-SW faults. All that geological and seismological evidences demonstrate that the Miocene extensive regime has finished and that Plio-Quaternary and present day tectonic regime is closer to a strike slip regime. That stress field can explain the presence of coetaneous extensive and compressive structures that can be seen in Mallorca Island. Due to the small longitude (<15 km) of the active structures that can be seen in the field and taking into account the empirical relationships between fault length and earthquake magnitude, we should not expect an M>5.5 earthquake in the Island.
- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27.4526G