Detecting Earthquakes

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The Elastic Rebound Theory

two parts of a body of rock are under stress due to opposing forces acting on the rock

the opposing forces are often due to tectonic plates moving in opposite directions

the body of rock is slowly deformed and put under strain

the energy applied to the deformed rock is stored as potential or strain energy within the rock

the deformation continues until the stress overcomes the strength of the rock and it fractures

the two parts of the rock move relative to each other and there is displacement along the fracture or fault

the strain energy which had been stored in the rock released, causing the ground to vibrate as an earthquake

Weak or plastic (incompetent) rocks such as mudstone, shale and hot metamorphic rocks steadily deform under stress by bending of moving by plastic flow and therefore do not fracture.

Detecting Waves - the Seismometer

A seismometer detects and records the ground motion. It is made of two parts. One is attachted to a large mass and does not vibrate with earth movements, while the other is allowed to move freely with the vibrations. The relative movemennt between the two is recorded. The recording is called a seismogram. Modern seismometers can measure movements smaller than one nm. They use the same principles but are based on the relative movement between a coil of wire and a magnet, which induces a current.

The data from seismometers can be collected automatically and analysed. A seismometer is a sensor placed in the ground to detect vibrations of the Earth. The seismometer together with the unit recording the signal is called a seismograph. The seismometer senses the ground vibration and converts this to a signal that can be recorded. A seismic station will normally have an array of seismometer arranged to pick up vibrations in the vertical and two principal directions.

The Seismogram

One earthquake shock will produce three vibrations on the seismogram, the P, S and L waves. Plotting the arrival times for these waves at seismographs which are at increasing distances from the epicentre, gives a time-distance curve, from which the speed can be calculated.

The time gap between the arrival of the P and S waves increases with distance from the epicentre. If you know the time gap, you can read off the graph the distance of your seismometer from the epicentre. You now know how far away the epicentre is, but it could be in any direction. Using distances from at least three seismometers, you can determine the location of the epicentre.

Now that you know the distance between the epicentre and the seismometer you can calculate the magnitude of the earthquake. The magnitude depends on the amount of energy released at the focus, whereas amplitude on the seismogram depends on the distance away from the epicentre. To calculate the magnitude, the amplitudes shown on seismograms at successive distances from the epicentre are compared.

The Shadow Zone

From 103 to 142 degrees from the epicentre, the seismograph receives no vibrations. Beyond 142 degrees the seismogram shows P but not S waves. The seismogram shows that P waves are late arriving because they are slowed down by the liquid outer core. However, they do not arrive as late as would be expected if the whole of the core were liquid, suggesting there is a solid inner core, due to the immense pressure at those depths.

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Shadow Zones