In order to monitor a potential tsunamigenic structure outside the Southwest coast of Iberia, a Tsunami Early Warning System was developed and implemented within the NEAREST project. The prototype system was specifically connected to an abyssal observatory by the name GEOSTAR, as the tsunameter prototype was hosted by this multi-parameter, under-water observatory. A requirement for this information and observation system was the capability to detect near-field tsunami waves, and the existing seafloor observatory was therefore modified according to the project’s needs. A specific set of sensing equipment was installed on the observatory, including a seismometer, a hydrophone, an absolute pressure gauge, oceanographic and environmental sensors. The tsunamometer technology was based on a double control of the seismic signal and pressure, taking the movements of the bottom of the sea into account. The changes were measured, recorded and processed with the capability to recognize changes in pressure smaller than a centimeter in the water column. The study of the coupling between the motion of the sea bed and disturbance of the water column is one of the keys to understanding the generation of tsunamis after strong earthquakes.
The observatory deployment and recovery was carried out using a dedicated vehicle, which is able to install an abyssal station in deep water and recover it with a precision of few meters. It was installed at more than 3200 meters depth in a giant, 50 km wide geological structure, which can transfer large amounts of energy to the water column and thereby generate a tsunami.
As the project was to identify and characterize potential large tsunami sources in the Gulf of Cadiz and in Southwest Portugal, a one-year continuous Ocean Bottom Seismometer (OBS) survey was carried out in the area on top of the presumed tsunamigenic structures. This natural seismicity monitoring allowed the first detection of low-magnitude earthquake-events of the area. Also, the epicentres of these events could now be located very precisely and be related to the main active tectonic structures. The operation was accomplished using 25 observation points (24 OBS and the above-mentioned deep-sea observatory) from August 2007 until August 2008. All the 25 observation points were equipped with the same type of broad-band seismometer. During the one-year experiment the system observed and recorded the signal of 1354 earthquakes, among which 767 occurred in the Gulf of Cadiz. During the same interval of time, the seismic network of the Meteorological Institute of Portugal reported only 422 earthquakes in the same area from their land-based system. Also, when comparing results from the two measuring systems, there are clear indications that the epicentres are generally displaced to southwest of their original location and the focal depths (>40 km) are actually much deeper than computed by the land-based system. Thus, dense marine networks can provide are important components in obtaining more exact seismological information within the field of natural disaster management.
The system was required to communicate in near-real time by underwater acoustics to a surface buoy and by radio/satellite link from the buoy to shore stations. Two types of messages were considered necessary: Periodic messages (status and data) sent every 6 hours to check that all devices were operating well, and event messages sent in near-real-time to notify a pressure/seismic event.
NEAREST final publishable activity report, 2010