The seismicity registered in 2011 in Portugal can be considered as moderate, with the majority of earthquakes having a local magnitude (ML) below 3.0, according to the Preliminary Report published today by the Meteorology Institute.
This year, 2424 earthquakes were recorded in the mainland and adjacent region, of which 1390 were located with a record in at least three stations of the national seismic network.
In addition to natural seismicity, IM's national seismic network recorded more than 6000 events of an artificial nature.
Most of the seismic activity had an epicentral origin in a submerged area, to the South and Southwest of mainland Portugal, in the zone of interaction between the Núbia and Eurasian tectonic plates, with some more important occurrences with an onshore epicenter, particularly in Alentejo and in the Algarve.
In the Azores archipelago, 1395 earthquakes were located nearby, which had an epicentral distribution along the alignment of the islands (Central and Eastern Groups) and also in a sector of the Mid-Atlantic Ridge.
In macro-seismic terms, 16 earthquakes were felt in the territory of Mainland Portugal, while in the Azores archipelago, 19 were felt, noting that none of them caused damage.
In the Madeira archipelago there were no earthquakes with macro-seismic effects.
On a global scale, the most significant seismic activity in 2011, in terms of large earthquakes, occurred in the interaction zones between the main tectonic plates, with the year being marked by the occurrence of some earthquakes with catastrophic effects, of which the from Christchurch, New Zealand (February 21), Tohoku, Japan (March 11) and Van, Turkey (October 23).
The Tohoku earthquake was clearly the most important, reaching a magnitude of 9.0Mw, corresponding to the fourth largest earthquake ever recorded instrumentally.
This event had a maximum seismic intensity of IX (MM56), having occurred several phenomena of liquefaction and landslides.
The earthquake caused a tsunami of great proportions, with waves that on average exceeded 10 meters in height in some areas of the coast and flooded places up to more than 9 km from the coastline.
According to official reports, more than 15000 people died. In total, the IM seismic network registered 738 earthquakes with magnitude equal to or greater than 5.
In comparative terms with the annual average values, based on observations since 1900 (USGS data), we can see that the world seismic activity detected in the national seismic network, in terms of magnitude, this year presents values slightly above the average values, and should this deviation is attributed to the sequence of aftershocks of the great earthquake in Japan.
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