The New Caledonia RAP

Introduction

In 2003, Geoazur Mixed Research Unit applied to GIS-RAP to set up five accelerometric stations, namely three in New Caledonia and two on Futuna Island. All five stations were set up as from mid-April 2006, .starting with New Caledonia. The New Caledonian RAP is currently a regional network within the national RAP network.

New Caledonian and Southwest Pacific Geodynamic Background

New Hebrides Arc Historical Seismicity

New Caledonia in the Southwest Pacific is located in one of the globe’s most active areas on the edge of the Australian Plate, which is subducting beneath the Pacific Plate. Convergence velocities along the active zone (New Hebrides Trench) vary from 5 cm/year in the North (Santo) to 12 cm/year in the South (Tanna). Earthquake characteristics and recurrence: as displayed by the region’s historical seismicity map, earthquakes occurring ahead of the arc are relatively superficial (> 70 km) and it is believed that an earthquake of magnitude ≥ 7.0 occurs once or twice a year.

New Caledonia’s Local Context

Along with the subduction seismicity, there is a poorly understood local seismicity around the Greater Noumea area and the southern mainland :

Southern New Caledonia’s seismic activity. The superficial earthquakes were detected near Dzumac and Port Laguerre stations.

There is significant seismic actvity, as was demonstrated by the recent 2005 seismic crisis. There are currently only two seismic stations around Noumea. One of them, located at the Port Laguerre livestock research centre, has two separate arrays. One is a very-long-period array belonging to the Geoscope international network and provides a global view of seismic activity while the other, short-period array, is particularly well suited to recording local (southern Mainland) and regional (Vanuatu) seismic activity. The Monts Dzumac station managed by DASE/CEA belongs to CTBTO and IRD receives free recordings in real time from this equipment. The broadband seismometer located at Monts Dzumac provides a view of all seismic activity, whether local or global. Unequivocal epicentre determination requires at least three seismic stations and epicentres located outside the network will be inaccurately determined. Following the January 2005 seismic crisis in the area surrounding Noumea, 18 quakes were determined using both the Port Laguerre (PLG) and Monts Dzumac (DZM) stations plus macroseismic information provided to us by the public. Three quarters of the quakes occurred in Plum, on the eastern edge of Mont Dore and the remainder towards Tontouta and further north, although the seismic zone at Plum does not appear in IRD’s previous work.

Two types of seismic activity have therefore been observed: a regional one that can be recorded and determined by worldwide networks such as USGS and a local one that cannot, because of its low magnitudes (3.5 at the most). Coverage of the Mainland by velocimetric stations is currently inadequate for determining such seismic activity locally. At any rate, as the Greater Noumea area is located in the Mainland’s south, it was decided to deploy the five RAP stations in the Noumea urban area and the neighbouring communes to the East :

Location of the Five New Caledonian RAP Stations

The five locations are as follows:

ONTR: Ouen Toro seismic cave in southern Noumea – this is a location belonging to IRD and operated from 1955 to 1984;
MVNO (Noumea City Centre): this station is located in the former Noumea Town Hall seismic cave
NORM (northern Noumea) station located in the Normandy observatory (permanent GPS station and DORIS beacon);
LASL (Saint Louis Agricultural School) station located to the west of the Mont Dore massif; and
PLUM (Plum Junior High School) station located to the east of the Mont-Dore massif, at the nearest point to the active zone during the January 2005 crisis.

Initial Results

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

The seismograms indicating the event were automatically generated, eg for NORM station:

Data Availability

RAP New Caledonia data is only available at the RAP central site:

1995-2006 RAP data

2007 RAP data

Conclusion and Future Prospects

Most USGS-determined events occurred in Vanuatu, ie at least 600 km away. Although it is still too early to generalise, it can nevertheless be stated that New Caledonian network detections follow the general trend described in the RAP newsletter’s fourth edition (Lettre d’Informaton du RAP):

Distribution of accelerometric data contained in the RAP database, based on magnitude and epicentral distance

The RAP stations operate on detection, ie the first data transfer stage consists of collecting the file index from a given station and then uploading only the recordings known to be generated by a given event that has been recorded by a regional or global catalogue. National RAP networks operate in this manner. Also, station density in the region is low, ie a quake in Vanuatu will only be detected by the global network (USGS) if its magnitude is sufficiently high (at least 5). Global network stations are too far away from the zone to detect any lower magnitude quakes. That is why we use IRD network velocimetric stations and are creating a local catalogue which will act as memory dump driver for RAP stations. Thus we not only collect earthquakes detected by the world network, but also quakes too small to be picked up by it. This applies particularly to all local seismic activity in southern New Caledonia.