Manly Hydraulics Laboratory

NSW Public Works

Manly Hydraulics Laboratory

Tsunami of May 1995

An earthquake of magnitude 7.6 on the Richter Scale occurred on 16 May 1995, 2013 UTC (17 May 1995 0613 Australian EST) centred south of New Caledonia in the South West Pacific Ocean. The resulting tsunami was recorded by the NSW Ocean Tide Network, the earthquake induced anomaly was approximately 0.5m on the NSW central coast. South of Sydney the anomaly was significantly less.

At 0949 EST on 17 May 1995, 3 hours and 36 minutes after the earthquake, the tsunami reached the NSW coastline at Tweed Heads. Typical periods of the seismic generated waves were 9 to 10 minutes, which is consistent with or similar to other worldwide recorded historical events. The average speed of propagation of the waves derived from the distance between the region of seismic activity and Tweed Heads, and the time taken to travel the distance is approximately 430 km/hr. Seismic waves propagating across deepwater with periods of 9 to 10 minutes have wave lengths which are long in comparison to water depth.

For a water depth of 1000m the wave speed is 360 km/hr and for a water depth of 2000m the wave speed is 570 km/hr. The average measured speed of 430 km/hr is reasonably consistent with mapped water depths for the region of interest, if wave theory is used a mean water depth of 1450m is derived.

Given a wave period of 9 to 10 minutes the data presented needs to be interpreted with care since the data recording equipment samples the water twice a second then digitally filters the data with a 10 minute cutoff.

Below are some plots of the recorded data.