Tsunami event summary - Friday, 11 March 2011

Details of earthquake:

Magnitude 9.0, centred off the east coast of northern Honshu, Japan at 4:46pm EDT on Friday 11 March 2011.

Australia's warning response:

The tsunami warning authority for Australia is the Joint Australian Tsunami Warning Centre (JATWC), which is operated by the Bureau of Meteorology and Geoscience Australia.

Friday, 11 March 2011

  • 4.55pm EDT: Geoscience Australia releases initial earthquake magnitude assessment of 7.5.
  • 5.02pm EDT: Geoscience Australia releases revised earthquake magnitude assessment of 7.9.
  • 5.11pm EDT: Geoscience Australia releases revised earthquake magnitude assessment of 8.5.
  • 5.16pm EDT: Bureau of Meteorology issues a No Tsunami Threat Bulletin for Australia.
  • 6.36pm EDT: Geoscience Australia releases revised earthquake magnitude assessment of 8.8.
  • 7.07pm EDT: Bureau of Meteorology issues an updated No Tsunami Threat Bulletin for Australia.

Tsunami Bulletins are distributed to media outlets, state emergency service authorities, Surf Life Saving Australia, port and harbour authorities and others. They are also published on the Bureau's website at www.bom.gov.au/tsunami.

During this event some users of the Bureau's web page did not see the "No Threat Bulletin" link. The Bureau is considering how to better highlight this link for future 'no threat' events.

Impacts on Australia

The JATWC tsunami modelling indicated that tsunami waves reaching Australia would not reach the threshold at which warnings would be required. However, smaller waves were expected to reach the Queensland and NSW coasts, and offshore islands from about 12 hours after the earthquake. The diagram below shows the model prediction of the maximum waves expected during the 24 hour period from the time of the earthquake.

Model of a tsunami on 11 March 2011.

The model predictions only apply to deep water (greater than about 20 metres depth). Wave heights build up in shallow water and are estimated by the JATWC using the model as guidance.

The tsunami was measured at several tide gauges at Australian locations on Saturday 12 March 2011:

  • Kingston, Norfolk Island - tsunami effects observed from 4:22am EDT, highest wave of 56 cm observed at 8:20am EDT.
  • Port Kembla, NSW - tsunami effects observed from 7:11am EDT, highest wave of 35 cm observed at 11:20am EDT.
  • Spring Bay, Tasmania - tsunami effects observed from 8:32am EDT, highest wave of 23 cm observed at 5:33pm EDT

The diagram below shows the output from the sea-level gauge at Port Kembla on Saturday 12 March. The red line shows the tsunami effect with normal tidal fluctuations removed. The blue line shows the normal tidal fluctuation.

Graph of sea level fluctuations at Port Kembla, NSW from a tsunami on 11 March 2011.

Anecdotal reports of tsunami effects

Report from Merimbula, Far South Coast NSW.

On Saturday 12th March 2011 at 2:00pm, coinciding with the high tide, a significant surge of water, 0.6 to 1 metre above normal, was experienced over a two minute period causing several swimmers to be washed into the lagoon 500 metres away. The current generated chop of approximately 30cm in height.

Report from the Tomaga River at Mossy Point.

"The tide pushed in faster than I have ever seen, then it got sucked out again 15 minutes later. This happened about six times, and it was still happening when we left. There was definitely something freaky going on."

Report from a fisherman, about 10km up the Clyde River, who noticed something unusual around midday.

"The tide rose by about a foot and then went out again, and this happened four times in about 15 minutes."

Reports from around Sydney Harbour.

At 8:35pm 12th March, a ferry reported a strong eddy of anticlockwise swirling waters north of Glebe Island bridge while entering Blackwattle Bay.

At 9:15am 13th March, a ferry reported strong currents into Blackwattle Bay up to 8 knots

The low water tide at 9:29am in Sydney was 10 centimetres below prediction.

Report from Port Kembla during Monday 14th March.

"Our pilots experienced unusual surge and current movements in and around the port area. Both in the harbour and in the approaches to the harbour over a prolonged period of time. This had significant effect on ship movements through the harbour which was managed by prudent tug use. This event is similar to but more drawn out and confused than the Chilean Tsunami of February last year where we experienced a surge approximately 2 days after the earthquake."

Report from Throsby Creek, in Newcastle, on Saturday 12th March at 6.15pm.

"I watched the wave of water come in over 10 minutes, about 0.9m (3 ft) of water had come past where I was standing. After 10 minutes it was on it's way back out.

I watched it again 30 minutes later and it was coming and going every 3-5 minutes. This occurred at a time when the tide was almost at its lowest."