As reported on the ABC, there was a massive magnitude 8.6 earthquake off the coast. This was followed a few hours later by another massive magnitude 8.2 aftershock. This earthquake happened near the same location as the 2004 Indian Ocean earthquake, though there was no major tsunami, and initial warnings were quickly called off.
Why was there no threat of tsunami? It all has to do with the way the
earth moved. The magnitude scale determines the energy released in the
earthquake (at magnitude 8.6, this earthquake was actually several times
smaller than the 2004 earthquake and the earthquake that struck Japan a
year ago). However, to produce a tsunami, there must be a vertical
displacement at the ocean floor to cause a massive wave. Yesterday’s
earthquake was caused by movement along a strike-slip fault. In this
configuration, the earth moves horizontally across the Earth’s surface.
Since there was no vertical motion on the ocean floor, there was little
threat for a devastating tsunami like the one that happened in 2004,
despite the large size of the earthquake.
For the scientific details on the earthquake, the USGS’s Earthquake
Hazards program has a detailed assessment on the earthquake (click here). Also, the Seismometers in Schools Facebook page has information and seismometer recordings of the earthquake from schools in Australia!