What is an earthquake n An earthquake is
- Slides: 52
What is an earthquake? n 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 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 ((( Hands Frictional resistance holding the rocks together is overcome by Demo forces tectonic ((( ))))
§ 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 Demo: Rubber Band 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 Demo: Pencil
Relationship Between Stress and Strain under High Temps or Pressure Demo: gum
Strike and Dip Strike is long line, dip is short line Note the angle of dip given 45 o Strike intersection w horizontal, dip perpendicular, angle from horizontal down toward surface
Vertical Movement along Dip-Slip Faults Divergent Convergent
Horizontal Movement Along Strike-Slip Fault
Reverse Fault Quake - Japan DEMO – Types of faults Strike Slip Fault Quake - California Normal Fault Quake - Nevada
Fence offset by the 1906 San Francisco earthquake n San Andreas is the most studied transform fault system in the world discrete segments 100 to 200 kilometers long n § 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 Demo: Liquifaction
Seismology Seismometers - instruments that record seismic waves §Records the movement of Earth in relation to a stationary mass on a rotating drum or magnetic tape
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 – High amplitude and low velocity – Longest periods (interval between crests) – Termed long, or L waves
n 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 § Moves at right angles to their direction of travel § Travels only through solids
P and S waves Demo: P and S waves Smaller amplitude than surface (L) waves, but faster, P arrives first, then S, then L
Earthquake focus and epicenter
Note how much bigger the surface waves are
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
Earthquake Depth and Plate Tectonic Setting Subduction Zones discovered by Benioff
Earthquake in subduction zones
Earthquakes at Divergent Boundaries - Iceland Crust pulling apart – normal faults
Measuring the size of earthquakes n 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 § 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 § Richter magnitude - concept introduced by Charles Richter in 1935 § Richter scale – Based on amplitude of largest seismic wave recorded – LOG 10 SCALE Each unit of Richter magnitude corresponds to 10 X increase in wave amplitude and 32 X 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 § 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 n Long-range forecasts § Calculates probability of a certain magnitude earthquake occurring over a given time period n Short-range predictions § Ongoing research, presently not much success
Long Term Predictions Seismic Gaps
Seismic Gaps at the Aleutian Islands SUBDUCTION ZONE
Seismic Gap along Himalayas 2005
Short-Term Earthquake Prediction Dilatancy of Highly Stressed Rocks 45
Investigating Earth’s Interior n Seismology helps us understand Earth’s Interior Structure. We use: n Speed changes in different materials due changes rigidity, density, elasticity n Reflections from layers with different properties n Attenuation of Shear Waves in fluids n Direction changes (Refraction)
Investigating Earth’s Interior 47
Surface Components magnified !
Seismic-wave velocities are faster in the upper mantle Velocity increases w depth, waves bend back to surface. Waves that travel via mantle arrive sooner at far destinations Mohorovičić discontinuity
Wave Velocities Upper Mantle Fast Asthenosphere Slow Lower Mantle Fast
The S-Wave Shadow Zone http: //en. wikipedia. org/wiki/Richard_Dixon_Oldham 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.
The P-Wave Shadow Zone http: //www. amnh. org/education/resources/rfl/web/essaybooks/earth/p_lehmann. html 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. Inge Lehmann Behavior of waves through center reveal Earth’s Interior
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