Melbourne shook when a 4.3 magnitude earthquake at a depth of three kilometres at Rawson, east of the Victorian capital, took off in the early hours of Friday morning.
Within 25 minutes, the Joint Australian Tsunami Warning Centre (JATWC) – a partnership of Geosciences Australia and the Bureau of Meteorology – declared ‘no tsunami threat.’
Why?
The JATWC will usually conduct assessments for such events when an earthquake over magnitude 6.5 within 200km of territorial coastline – that includes small offshore islands like Christmas or Norfolk islands.
“Due to the size and location of this earthquake, there was no chance of a tsunami generating and therefore the no threat bulletin was issued to alleviate concerns for anyone near the coast who felt the earthquake,” a Bureau spokesperson tells Cosmos.
But a bulletin must still be issued, as a matter of procedure.
“We will issue a no threat tsunami bulletin for any earthquake that does not meet our standard issuing criteria (for example, too far inland or less than M6.5) but is widely felt by coastal communities,” the Bureau says.
“We aim to issue a tsunami bulletin (either no threat or threat) to an earthquake event within 30 minutes.”
The bulletins issued by the JATWC indicate threat levels for Australian waters, but not those of surrounding nations.
But since 2011, its monitoring has more broadly contributed to tsunami warning centres for other Indian Ocean nations.
Australia’s JATWC and the national monitoring services of India and Indonesia provide a primary source of tsunami warning for the region as part of the Indian Ocean Tsunami Warning and Mitigation System.
This system was established following the devastating Boxing Day tsunami of 2004, which claimed more than 250,000 lives across the region.