What is an earthquake?
Date conversion 19.05.2018 Size 14,34 Kb.
What is an earthquake? An earthquake is the vibration of Earth produced by the rapid release of energy Energy radiates in all directions from its source, the focus Energy moves like waves Seismographs record the event Slinky, Rubber Band SEISMOGRAM Beaker, Wet Sand, Weight Cardboard Fault models Chewing Gum Wood meter stick or plastic ruler pencil Anatomy of Earthquakes Earthquakes are associated with faults Earthquakes are caused by sudden release of accumulated strain energy along Faults Rocks on sides of fault are deformed by tectonic forces Rocks bend and store elastic energy Frictional resistance holding the rocks together is overcome by tectonic forces Earthquake mechanism Slip starts at the weakest point (the focus) Earthquakes occur as the deformed rock “springs back” to its original shape (elastic rebound) The motion moves neighboring rocks And so on. DEMO – elastic rebound w/ ruler Relationship Between Stress and Strain Strain can be a change in shape (a deformation) due to an applied stress Relationship Between Stress and Strain at low Temps and Pressure or Sudden Stress Strike and Dip Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface Strike is long line, dip is short line Note the angle of dip given 45o Vertical Movement along Dip-Slip Faults Normal Fault Quake - Nevada Reverse Fault Quake - Japan Strike Slip Fault Quake - California Fence offset by the 1906 San Francisco earthquake San Andreas is the most studied transform fault system in the world discrete segments 100 to 200 kilometers long slip every 100-200 years producing large earthquakes Some portions exhibit slow, gradual displacement known as fault creep Fires caused by 1906 San Francisco Earthquake Gas mains break, fires shaken out of furnaces. Water mains break, cannot fight fires. Debris in streets, Fire department cannot reach fires. Landscape Shifting, Wallace Creek San Andreas Fault, a Transform Margin Liquefaction Seismology Seismometers - instruments that record seismic waves A seismograph designed to record vertical ground motion The heavy mass doesn’t move much The drum moves Lateral Movement Detector In reality, copper wire coils move around magnets, generating current which is recorded. Seismic Waves 1: Surface waves Complex motion, great destruction and low velocity High amplitude Longest periods (interval between crests) Termed long, or L waves Types of seismic waves (continued) Types of seismic waves (continued) Body waves Travel through Earth’s interior Two types based on mode of travel Primary (P) waves Push-pull motion Travel thru solids, liquids & gases Secondary (S) waves Smaller amplitude than surface (L) waves, but faster, P arrives first, then S, then L Earthquake focus and epicenter Graph to find distance to epicenter Locating Earthquake Epicenter Epicenter located using three seismographs 95% of energy released by earthquakes originates in narrow zones that wind around the Earth These zones mark of edges of tectonic plates Broad are subduction zone earthquakes, narrow are MOR. Lead to recognition of plates Subduction Zones discovered by Benioff Earthquake in subduction zones Earthquakes at Divergent Boundaries - Iceland Crust pulling apart – normal faults Measuring the size of earthquakes Two measurements describe the size of an earthquake Intensity – a measure of earthquake shaking at a given location based on amount of damage Magnitude – estimates the amount of energy released by the earthquake Intensity scales Intensity scales Modified Mercalli Intensity Scale was developed using California buildings as its standard Drawback is that destruction may not be true measure of earthquakes actual severity Magnitude scales Magnitude scales Richter magnitude - concept introduced by Charles Richter in 1935 Richter scale Based on amplitude of largest seismic wave recorded LOG10 SCALE Each unit of Richter magnitude corresponds to 10X increase in wave amplitude and 32X increase in Energy Magnitude scales Moment magnitude was developed because Richter magnitude does not closely estimate the size of very large earthquakes Derived from the amount of displacement that occurs along a fault and the area of the fault that slips Tsunamis, or seismic sea waves Tsunamis, or seismic sea waves Destructive waves called “tidal waves” Result from “push” of underwater fault or undersea landslide In open ocean height is > 1 meter In shallow coast water wave can be > 30 meters Very destructive Formation of a tsunami Tsunamis are actually huge, extending from the fault on the sea floor up to the surface, but they don’t stick up more than a meter or so in the deep ocean. However, when they reach shallow water they must rear up and slow down. Discussion: Kinetic vs. potential energy Honolulu officials know exactly how long it takes a Tsunami to reach them from anywhere Tsunami 1960, Hilo Hawaii Tsunami Model, Alaska Quake Earthquake prediction Long-range forecasts Calculates probability of a certain magnitude earthquake occurring over a given time period Short-range predictions Ongoing research, presently not much success Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE Dilatancy of Highly Stressed Rocks Short-Term Earthquake Prediction Investigating Earth’s Interior Seismology helps us understand Earth’s Interior Structure. We use: Speed changes in different materials due changes rigidity, density, elasticity Reflections from layers with different properties Attenuation of Shear Waves in fluids Direction changes (Refraction) Investigating Earth’s Interior Surface Components magnified Seismic-wave velocities are faster in the upper mantle Waves that travel via mantle arrive sooner at far destinations Velocity increases w depth, waves bend back to surface. Wave Velocities The S-Wave Shadow Zone Since Shear (S) waves cannot travel through liquids, the liquid outer core casts a larger shadow for S waves covering everything past 103 degrees away from the source. http://en.wikipedia.org/wiki/Richard_Dixon_Oldham The P-Wave Shadow Zone Behavior of waves through center reveal Earth’s Interior P-waves through the liquid outer core bend, leaving a low intensity shadow zone 103 to 143 degrees away from the source, here shown as the north pole HOWEVER, P-waves traveling straight through the center continue, and because speeds in the solid inner core are faster, they arrive sooner than expected if the core was all liquid. http://www.amnh.org/education/resources/rfl/web/essaybooks/earth/p_lehmann.html
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