EARTHQUAKES

An earthquake is the sudden movement of the earth's crust caused by the release of stress. This stress is built up along where two plates are moving past, away, or into one another. Earthquakes can also occur within a plate along cracks called faults, but most earthquakes occur along plate boundaries.

When Seismicity Occurs Earthquakes Occur along faults Sudden change in the arrangement of the minerals of rocks. Magma pushing and causing earth’s crust to break. Volcano explosions. Giant landslides. Meteorite impacts. Underground nuclear bomb test.

Elastic Rebound Any place where movement occurs along a crack in the earth’s crust, this is called a fault. Faults can build up energy, until the energy stored is greater than the strength of the rock itself, and the rock “snaps” to a rest position. This is known as elastic rebound, and this is how earthquakes occur.

Energy!!! Magnitude 4.0 = Appx tons of TNT Magnitude 7.0 = 32million tons of TNT Magnitude 9.0 = 99 million tons of TNT Note – Hiroshima Bomb = appx. 19,000 tons of TNT

Why Is Energy Stored? Diff. Heating Convection Plate Motion

How is energy released? Energy is transformed from Potential Energy to –Thermal –Seismic Wave –Kinetic Energy along the fault

Frequency Per month Approxim ately 80,000 Per day Approximately 2,600 Per minute Approximately 2

Deformation of the Crust Ductile deformation – bending without breaking. Brittle deformation – shattering of the earth’s crust. Fault creep – motion along a fault line where no seismic energy is generated.

Seismic Waves Body Waves –P- waves –S-waves Surface Waves Rayleigh – up and down. Love – side to side

Seismic Waves 1. Earthquakes generate waves that travel through the earth

Most Earthquakes Occur Along Plate Boundaries

Earthquakes Occur Along Faults

Faults Con’t Active Inactive Trace – fault that intersects the earth’s surface Fault scarp – normal, or reverse faults can create a small step. Blind faults – faults that exist below the surface.

Epicenter/Focus

Foreshock/Aftershock Foreshock- development of minor cracks before crust fails may lead to smaller tremors. Aftershock – main quake may activate nearby faults. These quakes are generally smaller.

Shadow Zone The shadow zone is the area of the earth from angular distances of 104 to 140 degrees that, for a given earthquake, does not receive any direct P waves. The shadow zone results from S waves being stopped entirely by the liquid core and P waves being bent (refracted) by the liquid core.

Shadow Zone Con’t Through measuring how P and S waves travel through the earth and out the other side, a seismic wave shadow zone was discovered in about From the lack of S waves and a great slowing of the P wave velocity (by about 40%) it was deduced that the outer core is made of liquid. The shadow zone also defined the diameter of the core.

Recording Earthquakes Seismograph - An instrument used to record earthquakes. Seismogram - The record of an earthquake from a seismograph.

Measuring Earthquakes Mercalli Intensity Scale Richter Scale Seismic Moment Scale – amount of slippage x length of rupture x depth of rupture x rock strength

Moment Magnitude

Locating An Earthquake 3 seismic stations are needed. Each station must determine the s-p interval and then based on average s and p wave travel times, determine the distance from each station. Triangulation is used to pinpoint the epicenter.

Determining Magnitude Richter magnitude uses the amplitude of the largest wave on the seismogram. This information along with logarithmic calculation, and the a distance factor can be used to provide a magnitude of the earthquake. A nomogram can be used, with the s-p lag time to more easily identify the richter magnitude.

Tsunami NOT A TIDAL WAVE!!! Different than a Mega-Tsunami which is created by mass movements into a body of water.

Deep Focus Earthquakes Wadati-Benioff Zone – at subduction zones, e.q.’s occur in a pattern that would represent the subducting plate.

Earthquake Destruction Earthquake destruction depends upon the following factors: –Intensity –Duration of vibration –Type of material –Structure design –Location –Depth

Earthquake Predictions Short Term Long Term

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