The Cavascope (CALedoniaVAnuatuSCOPE) Network

The broadband Cavascope network set up between New Caledonia and Vanuatu operated for over ten years until 2000 (the PVC Geoscope station is still operating) and we have now made the data available to the Scientific Community. The Cavascope network has an outstanding vantage point over an area with particularly rapid subduction in the middle of a very seismically active region. Data access is by common interface.

Geodynamic Background. The New Hebrides Active Margin, which spans 1500 km from the Santa Cruz Islands in the Solomons to the North down to 23° south, crossing the entire Vanuatu group, is linked to the subduction of the Australian Plate beneath the North Fiji Basin (NFB). The seismic zone that is the Benioff Plane has a 60-70° slope and is 300 km deep. The convergence direction is ENE, perpendicular to the trench and the convergence rate is at a minimum (9 cm/year) in the margin’s central part, reaching 12 cm/year in the South and 16 cm/year in the North (Pelletier et al (1998)). These high convergence rates give rise to high seismic activity and each year 10 quakes with an Mw (moment magnitude) of at least 6 are recorded on average.

New Caledonia and Vanuatu : a seismological desert. The table below provides the number of seismological stations in terms of seismic activity for regions of 150 square degrees in California, France and Vanuatu. The stations are listed by international organisations, ie NEIC and ISC. For example, the Vanuatu region’s 20 stations are a remnant from the Orstom-Cornell operation in 1978-1988. The number of earthquakes exceeding a given magnitude was provided by NEIC from 1973 to the present day :

Region	#stations	Mw>8	Mw>7	Mw>6	Mw>5
Vanuatu	20	1	48	327	2861
California	1497	0	3	37	331
France	510	0	0	0	2

Earthquakes classified in terms of magnitude, NEIC, 1973 to date

Brief Historical Overview of Seismic Networks in New Caledonia and Vanuatu. The International Geophysical Year from July 1957 to December 1958 led to the first seismic stations’ being set up in the New Hebrides (an Anglo-French Condominium that became the independent Republic of Vanuatu on 30 July 1980) and New Caledonia. Researchers from French institutes, particularly ORSTOM (overseas scientific and technical research agency) built the first seismic caves and set up seismographs in Noumea and Koumac, New Caledonia and Port Vila (Efate Island) and Luganville (Espiritu Santo Island), Vanuatu (Dubois (1965); Dubois and Pascal (1973)).
In 1978, New York’s Cornell University set up a network of analogical short-term seismic stations in Vanuatu equipped with 4.5-Hz vertical geophones that transmitted by radio link to the capital Port Vila on Efate Island. The network had up to 19 stations distributed between 15 and 20 degrees south operating with ORSTOM logistical and manpower support and operated for many years (Smalley et al (1987), Roecker et al (1988) and Chatelain et al (1992)). From 1990 onwards, the network gradually disappeared, though at the present time, two stations on Efate taken over by the Department of Geology Mines and Water Resources (DGMWR) are still using the original transmission instruments.

The Cavascope seismology network was set up in Vanuatu and New Caledonia in 1993 by Daniel Rouland, EOST, ORSTOM and DGMWR in view of establishing a continuously recording broadband network. It included Mont Dzumac Station (DZM, New Caledonia) belonging to DASE-CEA (Atomic Energy Commission’s Analysis, Monitoring and Environment Department). Four other stations were set up in 1993 at Koumac (KOU, New Caledonia) and on Lifou (LIF, Loyalty Islands, New Caledonia), Santo and Tanna (SAN and TAN, Vanuatu). Later, a station was set up as close as possible to the subduction at Southwest Bay (SWB, Malakula Island, Vanuatu), but only operated for 3 years from October 1998 to October 2001, as access to the Southwest Bay site was too difficult.

The sensors. In 1993, the Koumac (KOU) and Lifou (LIF, Loyalty Islands) stations in New Caledonia were originally equipped with SL-210 and SL-220 seismometers (vertical and horizontal respectively). The Vanuatu Stations on Santo (SAN) and Tanna (TAN) were equipped with Streckeisen STS-2 seismometers on loan from EOST, Strasburg. The Noumea (NOUC) and Port Vila (PVC) Geoscope stations were equipped with STS-IV and STS-1H Streckeisen seismometers in VBB (very broad band) mode.
Later, both STS-2 (SAN and TAN) were replaced by Gürlap CMG3-ESP seismometers. These sensors were also fitted at Southwest Bay (SWB, Malakula Island, Vanuatu) and finally at the Lifou (LIF, Loyalty Islands) station.
Digital Acquisition In 1993, digital acquisition developed at EOST, Strasburg, (Pillet et al, 1990) was fitted in the Cavascope network and recorded data by digitising all three transmission modes with a 20-bit dynamic range and 20-Hz sampling frequency recording continuously. The Tanna, Santo, Koumac, Lifou and Port Vila stations were equipped with digital data acquisition. The Port Vila equipment was still operating smoothly in 2007. The stations managed by DASE-CEA (DZM, Dzumac) and Geoscope (NOUC, Noumea) were fitted with acquisition modes specific to each programme, ie 24 bits, 20Hz for DZM and 2 X 16 bits, 2OHz for NOUC.