PS18, Tahiti, France
Thumbnail Profile: Tahiti
Tahiti, for many, is the equivalent of tropical paradise. Historically known as Otaheite, these islands were settled between 300 and 800 AD by Polynesians migrating from Tonga and Samoa. The fertile island soil and the tremendously lush rain forest, combined with fishing, provided ample food for the population. Today tourism is the main income generator.
Politically, Tahiti is part of French Polynesia, a semi-autonomous territory of France. After France became the fourth nation to detonate a nuclear device (in Algeria in 1960) it tested for thirty years (1966-1996) in the South Pacific (e.g. on the Mururoa and Fangataufa atolls, approximatively 1200 km southeast of Tahiti).
Since the signing of the Nuclear-Weapon-Free Zone Treaty of Rarotonga in 1985, with its protocol committing nuclear weapon States not to carry out nuclear tests within the application zone, French Polynesia has assumed an active non-nuclear weapons stance.
France closed its testing sites in the South Pacific in 1996, before signing the CTBT later on that same year.
Geography and Geology
The pearl of the Pacific, Tahiti is located more or less equidistant from Sydney, Australia, (6,900 km), Los Angeles, CA, USA (6,500 km) and Lima, Peru (7,100 km).
Tahiti is known as a “hotspot” not only with the jet set but also with geologists — in this context, a location on the Earth's surface that has experienced active volcanism for a long time.
In the 1960s J. Tuzo Wilson came up with the innovative notion that volcanic chains (e.g. the Hawaiian Islands) resulted from the slow movement of a tectonic plate across a "fixed" hot spot deep beneath the surface of the planet. These hotspots are thought to be caused by an intensely hot, narrow stream, called a mantle plume, bubbling up from the Earth's core-mantle boundary.
Geologists have identified 40–50 such places around the globe: Hawaii, Réunion, Yellowstone, Galápagos and Iceland overly those currently most active hotspots. Other locations are Tahiti, Cap Verde, Crozet Island, Easter Island, Juan Fernandez Islands, St Helena, Socorro Island and Tristan da Cunha. Most of these islands are home to seismic stations built to monitor the Earth's tectonic movements that were later integrated into the CTBT's International Monitoring System (IMS).
Most hotspot volcanoes are basaltic like Tahiti’s because they erupt through oceanic lithosphere—the solid outermost shell of a rocky planet, including the crust and uppermost mantle. As the continents and seafloor drift across the mantle plume, "hotspot" volcanoes generally leave unmistakable evidence of their passage through seafloor or continental crust (i.e. island chains). Thus, geologists use hotspots to track the movement of the Earth's plates.
In the case of Tahiti, its so-called Society Islands sit atop an undersea volcano some 6,000 m high. When this shield volcano erupted millions of years ago, the centre collapsed, forming a large caldera that is now filled in with dense tropical vegetation.
In addition to primary seismic station PS18, France hosts IMS facilities representing each of the four verification technologies: two auxiliary seismic station, two hydroacoustic stations, five infrasound stations, six radionuclide stations and one radionuclide laboratory. Most of these facilities are found in France’s overseas territories.
Tahiti hosts three IMS facilities: in addition to PS18, it is home also to radionuclide station RN18 and infrasound station IS24. This particular trilogy of IMS verification technologies is also found on Easter Island.
PS18 in Tahiti is one of 50 so-called primary seismic stations of the IMS network. These stations transmit data continuously to the International Date Centre (IDC) of Preparatory Commission of the Comprehensive Nuclear-Test-Ban-Treaty Organization (CTBTO) in Vienna. In the Pacific Ocean, the world’s largest body of water, the network of infrasound and hydroacoustic stations is comparatively dense, whereas seismic stations are few. In fact, PS18 is the only primary seismic station within the entire Pacific Ocean.
Originally built in the early 1960s, the Geophysical Observatory at Pamatai (located southeast of Papeete) was pegged as a priority candidate to host PS18 as early as the year 2000.
Enhanced with its current seismic vault and sensors, both installed in 1985, it was upgraded to meet the stringent specifications of the International Monitoring System in 1998.
PS18 has been established in one of the seismically most active places in the world. This is an inherent feature of its location on a small oceanic island in weathered volcanic rock. The land on which the station is located comprises thin layers of lava that have coalesced to become clays (mamu) like most in Tahiti.
At PS18, the seismic vault pier and floor are constructed on volcanic rock cleared of soil and connected to the Central Recording Facility (CRF) by a buried 400-meter long fiber optic uplink cable which relays the signals from the system. Computer facilities are located nearby in an air-conditioned, access-controlled room at the Geophysical Observatory.
Testing and Certification
Transmission of data via the French independent sub-network of the CTBTO Global Communications Infrastructure (GCI) began in 1999. PS18 was certified in September 2001 and has been running reliably since then, with annual data availability consistently above the operational target of 98%, ever since.
On 13 September 2001, PS18 became the sixth primary seismic station to be certified out of a total of 50 planned stations in the network.