RAP network

Written by Pierre Lebellegard

Wednesday, 13 June 2007 09:08

New Caledonia permanent accelerometric network (RAP)

Introduction

In 2003, the UMR Géoazur deposited a request at the GIS-RAP for the installation of five accelerometer stations, three in New Caledonia, and two on the island of Futuna. The five stations were installed starting from mid-April 2006, initially only in New Caledonia. At present, the RAP of New Caledonia constitutes a regional network of RAP national network.

Southwest Pacific and New Caledonia geodynamic environment

New Hebrides arc historical seismicity

New Caledonia, in southwest Pacific, is in one of the most active zones of the world: it is on the edge of the Australian plate, which subducts under the Pacific plate. Convergence speeds along the active zone (New Hebrides trench) vary from 5 cm/yr in north (Santo) up to 12 cm/yr in the south (Tanna). Characteristics and recurrence of the earthquakes: as one can note it on the chart of the historical seismicity in the area, earthquakes occurring in front of the arc are relatively of shallow depth, and it is admitted that between one and twice a year earthquakes with magnitude higher than 7.0. occur.

New Caledonia's local environment

Aside subduction seismicity, there is an incompletely known local seismicity, in particular in the neighbourhoods of the agglomeration of Nouméa and in the south of the Grande Terre (main island):

New Caledonia's southern part seismicity. Detected earthquakes are close to the surface and close to Dzumac and Port-Laguerre stations.

This seismicity is not neglectible as recalled by the recent seismic crisis of January 2005; at the present time, there are only two seismological stations around. One located on the zootechnical center of Port-Laguerre is composed of two distinct equipment. One very long period belonging to international GEOSCOPE network has a global vision of the seismicity and the other short period is adapted perfectly to the recording of the local seismicity (southern of Grande Terre) and regional (Vanuatu seismicity). Mont Dzumac' station, managed by the DASE/CEA belongs to CTBTO network and the IRD gracefully receives in real time the recordings. The broad band seismometer installed at Mont Dzumac makes it possible to have a vision of all the seismicities, local as much as total. The unambiguous determination of the epicentres imposes three seismological stations at least, knowing that the epicentres located out of the network, with approximately a dimension characteristic of this network will be badly given. After seismic crisis of January 2005 in the neighbourhoods of Nouméa, 18 earthquakes could be given using the two stations of Port Laguerre (PLG) and the Mont Dzumac (DZM) together with macroseismic information that the population provided us. Three out of four of these earthquakes occured near Mont Dore, in Plum and the other part towards Tontouta and more northward. This seismic zone located at Plum does not appear in former work of the IRD.
We have thus two kinds of seismicity: one regional is likely to be recorded and determined by the world networks such as the USGS; the other kind, local seismicity is not detected because of too low magnitudes (not more than 3.5). At present, Grande Terre's coverage in velocimetric stations is too weak to locally allow the determination of this seismicity. Nevertheles, the agglomeration of Nouméa, being located in the south of Grande Terre, it has been decided to deploy the 5 stations of the RAP's five station in the agglomeration and the neighbouring cities eastward:

Location of RAP stations

Five stations' locations are following:

ONTR : Ouen Toro seismic cave (south of Nouméa): this site belongs to IRD and was operating between 1955 and 1984 ;

MVNO (downtown Nouméa) : station is located under former Nouméa's City Hall;

NORM (north of Nouméa) : station is located in Normandie's observatory, together with DORIS beacon and permanent GPS station;

LASL (Saint-Louis agricultural college) : station is located westward of Mont-Dore mountain;

PLUM (Plum college) : station is located eastward of Mont-Dore mountain, closest to January 2005 seismic crisis active zone.

First results

The five stations were gradually set up between mid-April and mid-September 2006 and at present the network is completely operational. Here an example of event located by USGS and recorded by RAP stations and the velocimetric station of Mont Dzumac:

Corresponding plots are automatically produced, for example for NORM station:

Distribution of data

Data can be obtained on national RAP server, or using our common interface.

Conclusion and prospects

The majority of the events determined by the USGS occur in the Vanuatu, i.e. at a distance of at least 600 kms. If it is still difficult to take retreat, one can however estimate that detections of the network of New Caledonia respect the general outline provided in RAP newsletter n° 4:
RAP stations are not recording data continuously, only on detection: the first stage of data transfer thus consists in recovering the file index present on a given station, and after downloading only recordings corresponding to a given event, present in a regional or world catalogue. This is the way RAP national network is operating. However stations' density in the area is low, i.e. an earthquake occurring in Vanuatu will be detected by the world network (USGS) only if its magnitude is sufficiently high (5 at least). Indeed, global network stations are too distant from our area to detect weaker earthquakes. We thus use the velocimetric stations of IRD network to build a local catalogue, which will control RAP stations data download. We detect not only earthquake detected by the world network, but also the earthquakes thar are too weak to be detected by this network. Specially, we detect south of New Caledonia's local seismicity.

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