The detection and mechanism of earthquakes

Earthquakes are recorded by seismographs which measure movement of the ground. A mass is loosely coupled to the Earth by means of a pendulum or by suspending it from a spring. When the ground moves the mass tends to remain stationary because of its inertia, and the displacement of the Earth relative to it can be recorded. This motion is magnified electronically and modern instruments can detect ground displacements as small as 0.1 nanometre (nm).

This is more sensitivity than can be used since the ground is continually being disturbed by winds, waves and man-made noise. The signal is usually recorded on a sheet mounted on a drum, which rotates at a fixed rate and moves sideways as it rotates. A continuous record is obtained, which, when the sheet is removed from the drum, appears as a number of parallel lines (Figure 3.4).

3.4: Seismometers are designed to measure either vertical or horizontal around motions. A set of seismometers always includes one for recording vertical movements and two, at right angles to one another, for horizontal motions. One of each kind is shown here, with examples of the signals received from u single earthquake event. Successive arrivals (from left to right) of four different types of seismic wave are shown

A moving system such as that described will have a natural period of vibration. If it is carefully designed it may have about the same response over a considerable range of periods. However, seismic waves can have periods ranging from a few tenths of a second to many minutes, and no vibrating system can give an equal and adequate response over this range.

It is therefore necessary to design instruments for specific purposes, and a well-equipped seismograph station will have more than one set of instruments. It is common practice to have one set that will respond well to short-period waves with periods from 0.2 - 2 seconds and another set to respond to a long-period range of 15 - 100 seconds.

The travel time of a seismic wave from its source to any point on the Earth's surface is a direct measure of the distance between the two points. Seismologists have been able to determine by trial and error the average travel time of P and S waves for any given distance. The times have been printed in tables and graphs as a function of the distance.

These expected travel times can then be compared with actual measurements of the distance between an earthquake source and a seismic observatory, and the distance between the observatory and epicentre can be read from the tables. Alternatively the difference in travel times of P and S waves can be used to determine the distance graphically to the epicentre (Figure 3.5).

If arrival times at only one observatory are available then only the distance of the earth-quake source from that observatory can be determined and not its location. If arrival times at three observatories are available then the epicentre and time of occurrence of the earthquake can be determined by triangulation. In practice, readings from many observatories are used. A moderate-sized earthquake on the mid-Atlantic ridge, for example, might be located using readings from sixty or more seismic stations around the world. The calculation is carried out by high-speed computers.