Why locate earthquakes

An earthquake has one magnitude. The magnitude does not depend on where the measurement is made. Often, several slightly different magnitudes are reported for an earthquake. This happens because the relation between the seismic measurements and the How do you determine the magnitude for an earthquake that occurred prior to the creation of the magnitude scale?

For earthquakes that occurred between about when modern seismographs came into use and when Charles Richter developed the magnitude scale, people went back to the old records and compared the seismograms from those days with similar records for later earthquakes. For earthquakes prior to about , magnitudes have been estimated by Filter Total Items: 6.

Hayes, Gavin P. View Citation. Hayes, G. M, , Seismicity of the Earth — U. Lamb, Rynn M. Year Published: Taking the Earth's pulse During the past 35 years, scientists have developed a vast network of seismometers that record earthquakes, volcanic eruptions, and nuclear explosions throughout the world.

Woodward, Robert L. Filter Total Items: 3. Date published: May 25, Date published: January 23, Date published: September 24, Filter Total Items: 9. List Grid. January 25, Alerts could save lives and properties but several challenges remain.

With millions at risk, why isn't full public alerting happening yet? December 31, Map of historic seismicity, major faults, and paleoseismic summary of San Andreas Fault system. Attribution: Earthquake Hazards Program. December 17, USGS map of the magnitude 7. August 31, March 17, June 13, October 17, Attribution: Natural Hazards. Many people believe that earthquakes are more common in certain kinds of weather. In fact, no correlation with weather has been found.

Earthquakes begin many kilometers below the region affected by surface weather. People tend to notice earthquakes that fit the pattern and forget the ones that don't. Also, every region of the world has a story about earthquake weather, but the type of weather is whatever they had for their most memorable earthquake.

Earthquakes are recorded by a seismic network. Each seismic station in the network measures the movement of the ground at that site. In an earthquake, the slip of a block of rock over another releases energy that makes the ground vibrate. That vibration pushes the adjoining piece of ground, causing it to vibrate, and thus the energy travels out from the earthquake in a wave.

As the wave passes by a seismic station, that piece of ground vibrates and this vibration is recorded. Earthquakes produce two main types of waves - the P-wave a compressional wave , and the S-wave a shear wave. The S-wave is slower but larger than the P-wave and does most of the damage.

Scientists have used knowledge of the differences between these and other seismic waves to learn a great deal about the interior of the earth. S Shear Secondary Transverse Alternating transverse motions perpendicular to the direction of propagation. S-waves travel slower than P-waves in a solid and, therefore, arrive after the P-wave.

They are largest at the surface and decrease in amplitude with depth. Love waves are dispersive, that is, the wave velocity is dependent on frequency, with low frequencies normally propagating at higher velocity.

Depth of penetration of the Love waves is also dependent on frequency, with lower frequencies penetrating to greater depth. R Rayleigh Surface waves Motion is both in the direction of propagation and perpendicular in a vertical plane. S-wave animation Note the deformation of the black rectangle as the wave propagates through it. People at rest feel a swaying or light trembling. A few people are awakened. The level of vibration is not frightening. Windows, doors and dishes rattle.

Hanging objects swing. Many sleeping people awake. A few run outdoors. Buildings tremble throughout. Hanging objects swing considerably. China and glasses clatter together.

The vibration is strong. Top-heavy objects topple over. Doors and windows swing open or shut. Many people in buildings are frightened and run outdoors.

Small objects fall. Slight damage to many ordinary buildings, e. Furniture is shifted and objects fall from shelves in large numbers. Many ordinary buildings suffer moderate damage: small cracks in walls; partial collapse of chimneys.

Many ordinary buildings suffer damage: chimneys fall; large cracks appear in walls and a few buildings may partially collapse. Many ordinary buildings partially collapse and a few collapse completely. Richter magnitude local magnitude ML Originally earthquake magnitudes were based on the amplitude of ground motion displacement as measured by a standard seismograph. Surface wave magnitude For shallow earthquakes i. Table of seismic energy Magnitude Energy in joules Notes You may also be interested in.

Discovering Geology Discovering Geology introduces a range of geoscience topics to school-age students and learners of all ages. Earth hazards The Earth beneath our feet is constantly shifting and moving, and violently with catastrophic and immediate results. Earthquakes Earthquakes are among the most deadly natural hazards. What causes earthquakes? Where do earthquakes occur?

Where earthquakes occur around the world and in the UK. The focus is always at some depth below the ground surface in the crust, and not at the surface.

From the focus, the displacement propagates up, down, and laterally along the fault plane. The displacement produces shock waves, creates seismic waves.

The larger the displacement and the further it propagates, the more significant the seismic waves and ground shaking. More shaking is usually the result of more seismic energy released.

The epicenter is also the location that most news reports give because it is the center of the area where people are affected. The focus is the point along the fault plane from which the seismic waves spread outward. Seismic waves are an expression of the energy released after an earthquake in the form of body waves and surface waves.

Body waves include primary waves P waves and secondary waves S waves. Primary waves are the fastest seismic waves. They move through the rock via compression, very much like sound waves move through the air. Particles of rock move forward and back during the passage of the P waves. Primary waves can travel through both fluids and solids. Secondary waves travel slower and follow primary waves, propagating as shear waves.

Particles of rock move from side to side during the passage of S waves. Because of this, secondary waves cannot travel through liquids, plasma, or gas. When an earthquake occurs at a location in the earth, the body waves radiate outward, passing through the earth and into the rock of the mantle. A point on this spreading wave-front travels along a specific path that reaches a seismograph located at one of the thousands of seismic stations scattered over the earth.

That specific travel path is a line called a seismic ray. Since the density and seismic velocity of the mantle increases with depth, a process called refraction causes earthquake rays to curve away from the vertical and bend back toward the surface, passing through bodies of rock along the way. Surface waves travel more slowly than body waves.