Valentia Observatory

Seismology

Seismology is the study of seismic waves (shock waves) produced by earthquakes or explosions. Instruments called seismometers detect, amplify and record the movement of seismic waves. Seismic waves have been recorded at the Observatory since 1962. From these records the location and intensity of earthquakes is determined. The current seismological monitoring programme is run in close co-operation with the Dublin Institute for Advanced Studies.

Seismometer
The seismometer used at Valentia is a CMG-40T Broadband Seismometer. The system consists of three sensors, to measure north-south, east-west and vertical ground motions. The strength of an earthquake is measured on a scale of numbers called the Richter Scale.

Earthquakes
Large earthquakes cause violent motions of the earth's surface. Sometimes they cause huge sea waves, called tsunami that sweep up on land and add to the general destruction.

Much of the energy released in an earthquake travels away from its epicentre (the point on the surface above the focus) in waves called seismic waves. Near the epicentre, vibrations of seismic waves can be very destructive but as they spread out they diminish in amplitude. These waves are propagated to all parts of the earth following paths through the body of the earth itself and around its surface.

Seismic Waves
There are four elementary types of seismic waves, each with its own characteristics as regards velocity and type of motion. These characteristics depend on the path traversed i.e. whether the waves come through the body of the earth or around its surface and whether they acquire rigidity in the transmitting medium or not. Two wave types, compressional and transverse, penetrate the earth's interior. Love and Raleigh waves are two wave types restricted to surface propagation only.

Three characteristics of seismic waves deserve special attention, the velocity of the wave, the motion of the earth's particle and the appearance of the wave types. The velocities or their equivalent travel times are important because they form the basis of seismological tables from which the distance between the epicentre and the seismometer is determined. The motion of the earth's particle is used to compute the azimuth of the epicentre from the seismometer and also furnishes a means of identifying wave types. The appearance of various wave types and their period and amplitude, are used to identify the wave type.

Data Distribution
Seismic data are supplied by the Observatory to research bodies on request.

Phase data are forwarded on a regular basis to the International Seismological Centre, Newbury, England, where they are made available to seismologists world-wide and are included in their publications