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The Earth is an active planet. Several hundreds of small earthquakes, that cannot even be felt, happen every day around the world. Large earthquakes are more rare, and major events (magnitude greater than 8) only occur about once a year. Seismometers measure the motions of the ground before, during and after an earthquake, allowing seismologists to locate and measure the size of events, and also to map the Earth's interior. AusPass plays a critical role in this process by disseminating the passive seismic data acquired in Australia since 1997 to the scientific community and the general public. This initiative empowers various applications with important implications for our society, such as the mitigation of earthquake hazards, the exploration of oil reservoirs and other mineral resources, the monitoring of nuclear explosions, and a better understanding of the Earth's structure and dynamics.
Earthquakes
Earthquakes occur when when two blocks of the earth suddenly slip one from another. The slip surface is usually a fault, or more generally a friction plane. The earthquake starts at the hypocentre, whose closest location at the Earth's surface is known as the epicentre. During an earthquake, energy is released in vibrations (seismic waves) travelling through the Earth and at its surface. The seismic waves shake the earth as they move through it, and when they reach the surface, they shake the ground and anything on it. The magnitude of an earthquake depends on the size of the fault and the amount of slip on the fault, but is determined in practice by the amplitude of motions measured on the ground with seismometers.
To learn more about earthquakes, visit the AuSIS website!
Passive seismology
As CT scans provide images of the human body with X-rays, seismologists explore the interior of the Earth using the seismic waves generated by earthquakes. Passive seismology uses natural sources, while active seismology uses man-made sources such as hydraulic vibrators and air guns. In this example, traces of ground motions (seismograms, shown on the right side of the image) are recorded at seismometers located around the globe after an earthquake of magnitude 6.9 occuring in California on January 17, 1994. Although seismic waves are generated together, they travel at different speeds. Compressive waves (P waves) are for example much faster than shear waves (S waves). The seismic waves reflected at major discontinuities arrive at different times at the seismometer, revealing four concentric layers inside the Earth: the inner core, outer core, mantle and crust. Seismic waves travel at different speeds depending on the composition, pressure and temperature of rocks. The observations collected at seismometers (usually arrival times) are combined together by seismologists to model the speed of seismic waves through the Earth, and then interpreted in terms of structure, temperature and composition.
To learn more about passive seismology, visit the IRIS website!
More information on "Exploring the Earth with seismology" on this poster.
