LINZ’s Sea Level Gauges Monitor Yesterday’s Tsunami

1 October 2009

The tsunami generated by yesterday’s Samoan Islands earthquake was detected by LINZ’s sea level gauges as it approached the New Zealand coastline.

Differences in sea level of up to a metre were recorded by the gauges in some areas.

The figure below shows sea level plots taken during the hours before the earthquake to 10AM today superimposed on the normal twelve-hourly tidal cycles.

“The sampling is at one minute intervals, with the approximate time of the earthquake indicated by the red line. The dark part of the trace shows the average sea height and the grey part shows the maximum and minimum values due to the effect of wave action,” explains Glen Rowe, Senior Tidal Officer.

“You can see that the gauges at open coast sites such as Raoul Island detected greater wave action than harbour sites such as Wellington,” notes Glen. “Disturbance of the sea surface can still be seen at some sites almost thirty hours after the earthquake.”

The tsunami arrived at the two Raoul Island sites first, followed by North Cape, East Cape, Chatham Island, Gisborne and Napier.

How the gauges work

Each of LINZ’s sea level gauges consists of pressure sensors mounted on the sea floor that measure the height of the water column above them. Data from the gauges is recorded in real time and transmitted to GNS Science’s GeoNet office, where data is analysed to provide scientific advice to Civil Defence.

Establishing the gauges

Following the devastating tsunami caused by the Sumatran earthquake in 2004, LINZ developed a sea level gauge network around New Zealand’s coastline and off-shore islands as part of a tsunami warning system for New Zealand and the Pacific.

“As the events in Samoa have reinforced, tsunamis can be disastrous and having an effective warning system is vital,” says Graeme Blick, National Geodesist.

“Ten of the proposed seventeen sea level gauges have now been installed in partnership with GNS Science,” says Graeme. “The full network is expected to be operational by mid 2010.”

Find out more about sea level data.

Media enquiries: Brad Young, Land Information New Zealand, phone +64 4 460 0147, email media@linz.govt.nz