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Research School of Earth Sciences 
Geodynamics Group
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The Geodynamics Group at the Research School of Earth Sciences have been conducting a tectonic monitoring program in Papua New Guinea since 1990. This has led to the development of tectonic models for the present-day motion in the region. In conjunction with the Rabaul Volcano Observatory, the National Mapping Bureau and the PNG University of Technology, field parties conduct repeat GPS observations at the geodetic marks of of an ever-increasing network of sites spanning the country. Site coordinates are estimated on a regular basis and temporal changes in coordinates are related to tectonic motion.

 

 

Duke of York Islands - are they sinking?

There have been reports from local villagers that the Duke of York Islands are sinking. Some stories claim that the rate of subsidence is as much as 30 cm/yr and it was reported in the Post Courier (April 28, 2000) that "the Duke of York Islands may submerge into the Weitin Valley". Is this true?

At this point, instrumental records do not support this. Measurements made at Nabual in 1995 and again in August 2000 show a change in height of -80 +/- 40 mm over 5 years. This amounts to a vertical rate of only -14+/- 8 mm/yr. If a subsidence of 300 mm/yr was happening in this area then there would have been about 1.5 m difference in the height of Nabual between 1995 and 2000. This is clearly not the case.
 


    Figure 1. Time series of height estimates at Nabual. The slope of the line is a measure of the subsidence rate and is -14 +/- 8 mm/yr. The red triangle shows the height after the recent Mw=8.0 earthquake. The black vertical line is the day on which the earthquake occurred (November 16, 2000).

However, the observations in 1995 were made over only 3 hours and are not very precise. With our current regime of 24 hour observations we can estimate the height accurate to about 10-15 mm in each occupation. Therefore, in as little as a few months we would be able to detect motion of 300 mm/yr if this were actually happening.

The very real encroachment by the sea in several low lying areas is therefore likely to be a combination of small scale subsidence and other factors like sea level rise, erosion etc, rather than a general large scale subsidence/sinking of the islands.

In fact, a likely cause of the concern at the Duke of York Islands is changes in sea level as a result of the El Nino event of 1997. Tidal records from the National Tidal Facility at Flinders University, South Australia show that the sea level in the Papua New Guinea region (at Manus) fell by about 0.3 m over 1996 levels (0.7 m average) then rose sharply from the beginning of 1998 to about 0.1 m above 1996 levels before returning to 1996 level in late 2000.
 


    Figure 2. Monthly mean sea level heights as recorded by the tide gauge at Manus. Data supplied by the National Tidal Facility, Flinders University, South Australia.

 


    Figure 3. Monthly mean sea level heights as recorded by the tide gauge at Rabaul. Data supplied by the University of Hawaii Sea Level Center.

While the sea level may have appeared to rise by nearly 0.4 m in just over one year between 1998 and 1999, this is actually just the natural response of the Earth's oceans to the El Nino cycle. The net result is that the sea level is essentially unchanged in the region from 1996 levels.

 

 

Effect of the November 16, 2000 Earthquake

The magnitude 8.0 earthquake which occurred north of the Duke of York Islands on 16 November 2000 caused an uplift of 30-50 mm at Nabual and Rakanda. It has also caused considerable horizontal movement of the region, with Tokua (at the eastern end of New Britain) having moved about 90 cm to the ESE as a result of the quake.

The Rabaul Volcano Observatory in collaboration with the Research School of Earth Sciences, The Australian National University plan to monitor the position of Nabual on a regular basis during 2001 to determine more accurately whether the islands are sinking and, if so, at what rate they are subsiding.

 
 

Related Papers

Related papers on present-day tectonic motion and the tectonic setting in Papua New Guinea are available at our publications list.

 
 

For further information please contact

Dr Paul Tregoning
Research School of Earth Sciences
The Australian National University
Tel: +61 2 6125 5510
Fax: +61 2 6125 5443
Email:   pault@rses.anu.edu.au


         
         

Steve Saunders and Ima Itikarai
Rabaul Volcano Observatory
Tunnel Hill Road
Rabaul
Tel: +675 982 1699
Email:   rvo@global.net.pg
 


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Last modified:     2000 November 30   pault@rses.anu.edu.au