Transcript EARTHQUAKES
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. 1200 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 1910. 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 • http://redirect.conservation.ca.gov/cgs/rgh m/pshamap/pshamain.html