Monitoring Earthquake Activity Offshore Iberia

Abstract

The Portuguese mainland territory is located in a tectonic setting responsible for significant neotectonic and seismic activities. The regional geodynamical evolution is dominated by the slow convergence (c. 2mm/year) of Eurasia and Africa in a NNW-SSE to NW-SE direction along the eastern segment of the Azores-Gibraltar plate boundary (AGPB). This tectonic framework explains the history of destructive earthquakes and tsunamis recorded along the Portuguese coast. Of these, the most significant event known was the Lisbon earthquake of 1 November 1755 with an assumed origin at the Goringe Bank and attaining an extreme magnitude (83/4). The tsunami generated by this event produced large destruction along the Iberian and Morocco coasts. The need to reduce the social and economic impact of the large historical events reported, led to the intensive research on the seismicity affecting Portugal Mainland, namely, long and medium term prediction, characterization of microseismicity through the improvement of the available seismic network and development of a Tsunami Warning System.

The long term prediction was produced assuming a Gutenberg-Richter law for the time occurrence of earthquakes but for medium term prediction of large earthquakes (M>5.8) for the western Iberia area it was used the Keilis-Borok methodology. The Time of Increased Probability (TIP) windows seems to be a useful technique for assessing the seismic hazard, as has been shown in the Pacific Area. For even shorter term predictions a multi-disciplinary and continuous geophysical survey of the source areas must be developed. One of the long established phenomena associated with the occurrence of strong earthquakes is the alteration of source seismicity and microseismicity. In order to address the question of “how well is our seismic network designed to characterize the small scale seismicity and their changes” the seismic network design was studied under the assumptions of Information Theory. In particular, the best location for an additional station is defined by the one that maximizes the net information transfer.

The installation of an Ocean Bottom Observatory including a broadband seismic station is of the utmost importance for the monitoring of the most important seismogenic and tsunamigenic area, the Goringe bank. As the travel-time for tsunamis generated in this area exceed 30 minutes for nearby coast line we were able to design and develop a pilot destructive earthquake and tsunami warning system (DETWS) that could give almost realtime warnings for the civil authorities.

The extensive experience in monitoring natural seismic sources can be very useful when designing a control system for strong artificial sources such as nuclear explosions. The lessons to be learned can be summarized as follows: study as thorough as possible all past experiences, design the best monitoring network using objective criteria and evaluate its performance, install when possible near source sensors and/or complementary systems in remote areas such as the ocean bottom.