[ SEISMICITY ]
Natural seismic activity is mainly due to movement and interaction of tectonic plates that the earth crust is split in.
About 90% of all earthquakes are generated within tectonic plates; these are called tectonic earthquakes; their sources (hypocenters) lie within the Earth crust at depths from hundred meters to 70 km below Earth surface.
Several percentage of earthquakes are caused by volcanic activity. The hypocenter of the volcanic earthquakes can be located even above Earth surface.
Australia is situated within the large Indian-Australian tectonic plate, which is being pushed north at approximately seven centimeters a year.
The Indian-Australian tectonic plate is being squeezed between other plates. The compressive stresses from the squeezing within the Australian continent lead to local earthquakes.
The earthquakes occurring within tectonic plates rather than plate boundary earthquakes are almost impossible to predict.
“More than 30 earthquakes can be recorded every day around the world.”
[ EARTHQUAKE DYNAMICS ]
Every earthquake produces seismic bulk waves that begin to propagate from hypocenter in every direction.
Geophysical studies identified two bulk waves propagating with different frequencies.
Eventually, the bulk waves reach the Earth surface, causing ground vibrations that result in appearing visible ground faults and damage of structures.