The Science of Seismology and Earthquake Engineering
Introduction
Seismology is the scientific study of earthquakes and the propagation of elastic waves through the Earth. The field also includes studies of earthquake environmental effects, such as tsunamis as well as diverse seismic sources such as volcanic, tectonic, oceanic, atmospheric, and artificial processes (such as explosions). A related field that uses geology to infer information regarding past earthquakes is paleoseismology. A recording of earth motion as a function of time is called a seismogram. A seismologist is a scientist who does research in seismology.
History of Seismology
The field of seismology was first developed in response to the need to understand the destructive earthquakes that occur on our planet. The earliest recorded earthquake observations date back to ancient times in China and the Mediterranean region. However, the scientific study of earthquakes began in the early 18th century with the work of John Michell and others. The development of seismographs in the 19th century allowed for the recording of seismic waves and the precise locating of earthquake epicenters. In the 20th century, the study of seismic waves (seismometry) led to the discovery of the earth's core and the mapping of the earth's interior. Seismology has also provided important contributions to the theory of plate tectonics.
Seismic Waves
Seismic waves are the waves of energy caused by the sudden breaking of rock within the earth or an explosion. They are the energy that travels through the earth and is recorded on seismographs. There are several different kinds of seismic waves, and they all move in different ways. The two main types of waves are body waves (often called P waves and S waves) and surface waves (Love waves and Rayleigh waves). P waves are the fastest seismic waves and can travel through solid, liquid, or gas. S waves are slower than P waves and can only move through solid material. Both body and surface waves can cause damage during earthquakes.
Earthquake Engineering
Earthquake engineering is the scientific field concerned with protecting society, the natural environment, and the man-made environment from earthquakes by limiting the seismic risk to socio-economically acceptable levels. This is accomplished by designing buildings, bridges, and other structures to withstand the potential destructive forces from earthquakes. While the main focus of earthquake engineering is the design and construction of structures, it also involves research in various areas such as seismology, geotechnical engineering, and structural engineering.
Seismometers and Seismographs
A seismometer is an instrument that responds to ground motions, such as caused by earthquakes, volcanic eruptions, and explosions. Seismometers are usually combined with a timing device and a recording device to form a seismograph. The output of such a device — formerly recorded on paper (see picture) or film, now recorded and processed digitally — is a seismogram. Such data is used to locate and characterize earthquakes, and to study the earth's internal structure.
Earthquake Prediction and Forecasting
Earthquake prediction is a branch of the science of seismology concerned with the specification of the time, location, and magnitude of future earthquakes within stated limits. Many methods have been developed for predicting the time and place in which earthquakes will occur. Despite considerable research efforts by seismologists, scientifically reproducible predictions cannot yet be made to a specific day or month. Instead, most earthquake prediction efforts currently focus on providing probabilistic seismic hazard assessments (PSHA) which outline the earthquake hazard in a given area (a map of expected ground shaking intensity) for the public and for engineers to use in designing buildings.