They are calculated after determining the fault throw (distance the fault moved), the area broken on the fault (depth and length), and the rigidity of the fault rocks (the springiness of the rocks). Moment magnitude: Moment magnitudes measure the amount of stress energy released in an earthquake. For example, amagnitude 5 earthquake produces ten times greater ground motion thana magnitude 4 earthquake. The Richter magnitude isrecorded on a logarithmic scale on which an increase of onerepresents a ten times increase in amplitude. The Richter magnitude of an earthquakecan be easily determined from the S-P interval combined with themaximum motion recorded on the seismogram. Becausethe amplitude recorded on seismograms will decrease with increasingdistance from the earthquake the distance must first be determined sothat it can be corrected for. Richter magnitude: The Richtermagnitude is a measure of the amplitude of ground movement. Theintersection of three circles uniquely identifies the earthquakeepicenter. Circleswith the appropriate radius are then drawn around each station. Triangulation: To determine thelocation of an earthquake the distance of the earthquake must bedetermined from at least three seismic recording stations. The distance of a seismic stationfrom an earthquake is easily determined from the S-P interval, thetime difference between the time of arrival of the first P wave andthe first S wave. As the P and S waves travel out from an earthquakethe P waves get progressively farther ahead of the S waves.Therefore, the farther a seismic recording station is from theearthquake epicenter the greater will be the difference in time ofarrival between the P and S wave. S waves are slower than P wavesby a known amount. If properly suspended, the mass forms a stationary reference point from which up-down, north-south, or east-west ground motions can be measured, for example with a stylus attached to the mass.ĭistance: P and S waves travel atknown velocities through the Earth. Seismometers work on the principal of a heavy suspended inertial mass that doesn't move as the ground moves. This is similar to a p wave but with the extra up-down motion.  The ground rolls forward and up and then down and backwards. Rayleigh waves displace the ground like rolling ocean waves. Love waves shake the ground side-to-side like an S wave. Surface Waves - travel along theEarth's surface. S waves shear the rock that they pass through. They cause matter to oscillate side-to-side, perpendicular to the motion of the wavefront. S, Secondary, or Shear waves are somewhat slower (~3.5 km/sec in the upper crust). P waves push (compress) and pull (dilate) the rock that they pass through. They cause the matter to oscillate forward and backward, parallel to the motion of the seismic wavefront. It lies directly over the focus.īody Waves - travel through theinterior of the Earth P, Primary, or Compressional waves travel the fastest (~6 km/sec in the upper crust). The epicenter is the map position of the Earthquake. The focus or hypocenter is the exact position on the fault, including the depth, where the slippage began. Seismologists can determine the point on a fault where the slippage began, the area (length and depth) of the fault that slipped, the amount of slippage or fault throw (how far the crust moved), and the time it took for the slippage to occur. The whole fault doesn't move at one time only the part of the fault around which the stress exceeded the strength. Seismic waves travel outward from the portion of the faultthat broke, like expanding ripples from a pebble dropped in stillwater. The ground shakes asthe stress energy is released and the rocks lurch to their newposition in a matter of seconds. Then,sudden slippage of rock along a fault occurs. Those stresses (compression,tension, shear) build up in the crust until the stress exceeds the strength of the rockor the friction along a preexisting fault. As the lithospheric plates of the Earthcontinue their slow motions, stresses build up in the crust,especially near the plate boundaries.

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