Mulgor Logo Long Waves: GeoNet Sites


As a result of changes GeoNet is making to their data interrogation procedures, and because the calculations are done on a machine running Windows XP, this webpage is being replaced by: a new webpage.
This webpage will no longer be available after 31-Jul-2018.


The purpose of this webpage is to present the results of long-wave analysis of the sea-level data that were captured by the GeoNet tsunami gauge network. The data are downloaded from the GeoNet site at 12:10 NZST every day, then processed using the methods described here to extract the long waves. Prior to 14-Mar-2011, we downloaded the 1-minute data, but we found that they had been low-pass filtered so as to remove waves with periods 10 min or less, so nowadays we download 1 Hz data and take 1-minute means to emulate a tide gauge. The results for each site are posted on individual web pages that can be accessed by clicking on the stations in the map below.

Each of the sites has a characteristic long-wave Fourier spectrum and these are presented here, along with the topographic admittance funtions. Spectra are useful for finding the resonance or seiche periods. Topographic admittance functions tell by how much tsunami waves approaching the site will be amplified.

We acknowledge the New Zealand GeoNet project and its sponsors Earthquake Commission (EQC), GNS Science, and Land Information New Zealand (LINZ).

Long Waves at Individual Sites

To retrieve details of the long waves at a particular station, click on the name in the map below:

Long Waves

The long waves that can be extracted from 1-minute tide gauge data come in three categories:

Long waves of any type can also cause basins, embayments or harbours to oscillate at their natural frequency. This is called "seiche". For example, the main seiche period of Lyttelton Harbour is 11 minutes, so incoming long waves with this period are amplified within the harbour. Pegasus Bay has a seiche period of 3.4 hours and these waves can be seen in Lyttelton Harbour also, though this seiche is mainly driven by tides and storm surge.

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Enquiries: Derek Goring

Mulgor Consulting Ltd

24 Brockworth Place

Riccarton, Christchurch

New Zealand

Phone: +64 3 942 5452

Fax: +64 3 942 5461

Email: d.goring[at]