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150 Cards in this Set
- Front
- Back
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What is an earthquake?
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A sudden movement of the ground that releases energy stored in the rocks and generates seismic waves
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Seismology
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study of earthquakes and wave propagation through earth
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Seismicity =seismic activity
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the occurrence of earthquakes in space and time
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Seismologist
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one who studies earthquakes
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Different types of seismologists
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1. study quakes at source, and how they are generated
2. use stats to determine the likelihood of occurence of a quake 3. use quake recording to image Earth's interior and understand how earth works [seismic tomography] |
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Why are quakes interesting?
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1. expensive, high economic impact
2. not uniform in space/time 3. basic scientific problem |
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SF Quake
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San Andreas Fault- offset was mostly horizontal, almost no vertical motion
Magnitude: 8 Quake felt from LA to Oregon |
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Typical CA quake depths are...
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4-6 miles
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Loma Prieta
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Named after mts in Santa Cruz.
Would have caused more damage, but because of world series, most people were at home watching TV |
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Northridge
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effected UCLA south campus [toxic spills]
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Sumatra Quake/ Tsunami
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2004
Magnitude: 9.2; second largest ever recorded! |
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Scientific Method
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Careful Observation > hypothesis > tests of competing hypotheses > refine/abandon and rebuild
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Making an earthquake...
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depends on friction resisting plate-tectonic forces.
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tectonic plates move at speeds of mm per year
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ie: 0.00000000003 m/s [.3X10^-11]
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10^10 equals
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1 followed by TEN zeros
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10^ -10 equals
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1 with TEN decimal places
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10^0 equals
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1
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1 km equals
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10^3 m [1000 m]
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Earth's radius
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6371 km
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Avg depth of oceans
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4 km
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Avg elevation of continents
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840 m above sea level
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transition from crust, mantle, and core refer to changes in...
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CHEMICAL Composition
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Hypsometry
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measurement of elevation relative to sea level
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Lithosphere
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Strong, UPPER ~100km
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Asthenosphere
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weak, easliy DEFORMED, but MOSTLY solid [~200km]
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Plate includes:
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"shell" on Earth's surface that is about 100-200 km thick and includes all of teh crust, and uppermost part of mantle
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Pangaea
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'super-continent' idea suggested by Alfred Wegner.. Continental Drift
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Evidence used in SUPPORT of continental drift
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1. Fit of Continent
2. Fossil Evidence 3. Rock type and Structural Similarities 4. Paleoclimate evidence [glacier deposit at equator; coral reefs in antartica] |
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arguments AGAINST continental drift
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1. lack of mechanism for moving continents
2. prevented acceptance by majority of scientists 3. Strong oppostion to the hypothesis from all areas of the scientific community |
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Arthur Holmes
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Proposed the idea of Convection- cold sinks, hot rises
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how old is Earth
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4.6 billion yrs
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oldest minerals are how old
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4.2 billlion yrs
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oldest oceanic seafloor today
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200 million yrs
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most forms of life started to appear..
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500 million yrs
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humans emergesd ________ and dinos became extinct _____
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1-4 Myr; 65 Myr
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What produces the heat for convectionn
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radioactive elements in EARTH's mantle
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How long does the process of Seafloor Spreading take
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10-25 myr
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Evidence for PLATE TECTONICS
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1. Mid ocean ridges
2. sea floor spreading 3. Magentic orientation |
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How much do tectonic plates move in one year
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0-20 cm/ yr... which is about 25 miles/million yr
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What are the NORTHERN and SOUTHERN parts of Pangea and whats the name of the sea which separated them
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GONDWANA [S] and LAURASIA [N]... seprated by Tethys Sea
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Examples of OTHER super continents
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Gondwanaland, Rodinia
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The Wilson Cycle
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Idea: Continental Drift is Cyclic... its based opening and closing of Atlantic
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Oceanic Crust
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thin [8km]
Basalt |
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Continental Crust
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Thick [30km]
Granite |
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Main continental plates
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North and South American, Eurasian, African, Indo-Australian and Antartic
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Main Oceanic Plates
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Pacific, Nazca and Cocos
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Three types of Boundaries
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Divergent [plates grow]
Convergent [plates shrink] Transform [plates stay same] |
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Divergent boundaries are MOST frequent where
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mid ocean ridges
ex: mid-atlantic ridge and East pacific rise |
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Divergent boundaries are LESS frequent where
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Rift valleys
ex: great rift valley of eAst africA [bt Ethiopia and Tanzania] |
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Convergent zone is the location for __________ and it is also where there are ________ and __________ quakes
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Subduction zone; numerous/large quakes
quakes occur from surface to depth of 700km- can only happen here and the zone is referred to as BENIOFF ZONE, WADALF-BENIOFF ZONE |
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Ocean-ocean convergent
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mid ocean ridges, island arcs [ex:western pacific arcs]
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ocean- continent convergent
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continental arc [produce mt. range- ie: the andes]
active volcanism |
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continent-continent convergent
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mts [ie:himalayas]
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Transform Boundaries
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LEAST common boundary
* frequent, SHALLOW earthquakes and little volcanism can also connect two ridge segments, two trenches or ridge and trench |
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What are the THREE main types of FAULTS associates with each plate boundary
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Normal- Divergent
Thrust- Convergent Strike Slip- Transform |
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Faults can be....
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active, inactive
seismic, aseismic |
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Reverse/ thrust fault
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hanging wall [ABOVE fault] slides UP and footwall slides DOWN
COMPRESSION |
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Normal faults
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hanging wall {ABOVE fault] slides down and Footwall slides UP
EXTENSIONAL |
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Dip slip faults
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associated with relative up/down movement
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Shear force
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action of coinciding and oppositely directed forces acting parallel to each other across a surface
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oblique-slip
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involves both STRIKE SLIP and DIP SLIP movement
ex: moves side/side and up/down |
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Thrust Fault- Compression
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Action of oppositely directed forces acting towards each other at the same time
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thrust fault trace
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- topographicaly irregular scarp
- low dip-angle fault plane - perched terraces - deeply incised canyons |
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Normal Fault- Tension
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action of coinciding and oppositely directed forces acting away from each other
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normal fault trace
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-typically steep dip-angle fault plane
-perched terraces -Alluvial fans [fan shaped deposit formed where a fast flowing stream falttens] - often large, clean fault surfaces |
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Most seismicity occurs at plate boundaries.. but not all. What are the exceptions?
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1. Blurring of the Plate Boundary
2. Broad Plate Boundary Zone 3. Intraplate Earthquakes 4. Hotpots |
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Example of an Hotspot on CONTINENT and in OCEAN
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yellowstone; hawaii
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With all the exceptions to PLATE TECTONICS, why do we still believe them?
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explains:
-matching geology/fossils -magnetic stripes -locations of most earthquakes -locations of most volcanoes |
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How are PLATE TECTONIC theories tested [ie: the fact the p[lates ACTUALLY do move]
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Via GPS, which allows scientist to measure ground position with mm accuracy > plate velocities
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what types of quakes at mid ocean ridges; subduction zones; and transform plate boundaries
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shallow/small; both shallow/deep [can be large]; shallow [can be large in continents]
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Blind Thrust Faults
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not visible at surface
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Subduction megathrust faults
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visible only deep under the ocean
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A LEFT bend in the fault results in local ___________
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compression
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A RIGHT bend in fault results in local ____________
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extension
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Ductility
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Think silly putty; the extent to which materials can be deformed plastically without fracture under applied force
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A material is PLASTIC if...
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DOES NOT return to its original state after stress is applied to it
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a material is ELASTIC if..
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DOES return to its original state after stress is applied to it... like a rubber band
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how often should we except LARGE earthquakes?
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T = slip/rate
25/0.077 = 325yr |
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why cant we predict the EXACT date of the next quake?
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- little quakes take up some slip
- aseismic creeps happen - quakes trigger each other to happen before they are "due" |
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What is stress
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its the force per unit area
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what are the types of FORCE
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Force is a vector [only direction and intensity]
-contact [physical contact] ie: kicking a ball, pushing a chair - field [NO physical contact] ie: gravity |
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What are the THREE types of forces causing deformation?
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1. COMPRESSIVE [push rock together; shortening & squeezing]
2. TENSIONAL [pull rock apart; strectching] 3. SHEARING [pull rock in oppposite directions] |
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what is the process of lowering the coefficient of friction called?
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lubrication; in earth WATER is the lubricant
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What are the CONSEQUENCES of elastic rebound
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- faults store energy slowly, but realease the energy RAPIDLY [in a quake!!!!!... BAM, BANG and other such sound effects :P]
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What complications make the DEFORMATION cycle difficult to predict?
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-variations in fault strength and structure
- fault interactions |
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InSAR
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Interferometric Synthetic Aperture Radar
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Which quakes release more energy: DEEP/ SHALLOW?
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Deep
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FOCUS
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point where the rupture started [can be deep]
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HYPOCENTER
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location and time of quake beginning
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EPICENTER
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Surface projection of Hypocenter
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the RUPTURE process
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1. rock breaks, and one side slides against the other side
2. Energy released by rock breaking sets vibrations which travel outward 3. vibrations are felt and cause damage at the surface of the earth 4. only SMALL amount of damage caused by fault breaking, majority caused by vibrations vibrations shaking the ground |
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Surface rupture
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the breaking of the rocks and sliding of one side of the fault against the other side
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Surface rupture
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the trace of the fault which broke at the surface
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the larger the quake...
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the bigger the rupture area and GENERALLY take a longer time to rupture, and have a greater slip
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Magnitude 8 quake has rupture size of ________
Magnitude 5 quake has rupture size of __________ Magnitude 2 quake has rupture size of __________ |
500km; 1.5km; 5m
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What is a wave
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a disturbance that travels through a medium
ie:ripples on a pond,'wave' effect at sporting events |
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radiation
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energy that is carried in the form of waves
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seismic waves
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vibrations of the ground. They are ELASTIC waves
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In what direction do raypaths travel relative to wavefronts
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perpendicular
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Amplitude
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the maximum value of the wave functions. The higher the amplitude, the louder the sound
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Period
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time required for one wavelength to pass a certain point. generally, a longer period indicates a lower pitch
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frequency
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the number of waves that pass a given point in a given amount of time
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Velocity =
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wavelength/period
wavelength(frequency) |
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What effects wave velocity?
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- weaker is slower [high pressure=stronger; high temp=weaker]
- denser is slower -type of wave |
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Free Oscillation
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Oscillations of the entire Earth
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what are the two types of surface waves?
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- LOVE [move side to side]
-RAYLEIGH [circular motion] |
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What are the two types of BODY waves
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Primary [P] and Shearing [S] waves
P travels faster and S can't travel through liquids |
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Typical velocities in crust
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5-7 km/sec
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P-waves
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longitudinal- material moves back and forth [vibrates] in same direction that wave travels, produces compression
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S waves
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AKA: Secondary Waves
typical velocities: 3-5 km/sec |
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Why don't S Waves travel through fluids?
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Because there is no restoring force for the perpendicular motions
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Surface Waves
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LARGEST amplitude [cause most damage]
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What behaviors are typical of rocks that are deeply buried?
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Ductile and plastic
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What is the average thickness of the mantle transition zone?
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260 km
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Which waves cause a particle motion that is back and forth in a direction
perpendicular to the wave motion? |
S-waves
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What are the cause and effect of movement along a reverse fault?
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Shortening of the crust due to compression
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The Mohorovicic discontinuity lies at the boundary of what two layers?
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crust and mantle
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What property distinguishes the earth's crust, mantle, and core?
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Composition
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Through which layer of the earth do P-waves travel fastest?
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lower mantle
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S Wave shadow Zone
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105-180 degrees
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P Wave Shadow Zone
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105-140 degrees
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the hypocenter is always on fault trace or plane?
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fault plane
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The Gutenberg-Richter relationship governs
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the number of earthquakes above a certain magnitude
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a micrometer is how many meters?
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10^ -6
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What part of the Earth’s interior is LIQUID iron
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outter core
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Seismic tomography
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measured using seismic waves and natural sources to determine varitions in velocity of seismic waves
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Bullen's layers
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broke down crust, mantle, core into more detailed, complex layers
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At what depth is the upper and lower mantle separated?
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670 km
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velocity increases/decreases with depth... along with velocity, density also increases/decreases
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increases
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stiffness is controlled by
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– Temperature
– Composition – Water content – Crystal structure |
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Stress if force per area
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Stress= F/A
Tension- Pull Compression- Push |
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Strain is deformation
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-Tensional stress: stretch
-compressional stress: shrink |
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Seismoscope
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first earthquake detector- CHINESE
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Seismometer
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measures the displacement of the mass relative to the ground.
AKA: seismograph [according to textbook] |
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Seimogram
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graph showing record of ground movement
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What is the name of the
phenomenon of amplification at a particular frequency |
resonance
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Seismic Networks
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• Regional short-period networks
– 50-400 instruments, vertical component only – emphasis on earthquake detection & location. • Regional broadband networks – fewer instruments (10-100), – emphasis on understanding bigger quakes • Global networks – Global detection and Earth interior studies – run by many countries |
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Southern California Integrated Geodetic Network
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• Has about 250 stations across southern
California • Mainly to watch – Displacement produced by earthquakes – continuous deformation |
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Some Measures of
the Size of an Earthquake |
• Magnitude (measures earthquake itself)
• Intensity (measures effect at a location) • Length of fault that breaks • Area of fault break • Displacement (average, or at a point) • Seismic Moment (essentially area times displacement) • Deaths or injuries • Number of homes destroyed • Damage (usually in $) |
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Earthquake Effects
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– Ground shaking => natural hazard
– Structural collapse – Falling objects – Ground settling, liquefaction (water saturated soil loses its resistance and becomes more like a dense liquid) – Landslides and avalanches – Fault offset – Tsunamis (=unusually large sea wave produced by a seaquake or undersea volcanic eruption) – Seiches (=occasional and sudden oscillation of the water) |
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measuring earthquakes
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1. Felt reports - Intensity
Not precise, but best data for old earthquakes 2. Seismic measurements 3. Mapping of rupture zone If surface break exists and is accessible 4. Geodetic measurements of ground shift 5. Geologic observation of past earthquakes (fault displacement from offset features) |
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intensity scales
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-Each developed for a given region
-Roman numerals to distinguish from magnitude -No fractional numbers -Limited range of numbers |
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scientific measurement
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• We want a measurement of quakes that is:
– Independent of distance from quake – Independent of local conditions (soil, buildings, etc.) – Same for all sizes of quakes – Can be made with an instrument |
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logarithmic scales are used to measure amplitude, intensity or magnitude
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magnitude
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local/ richter magnitude
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local magnitude = log (A)
where A is the maximum seismic amplitude [measured in micron 10 ^-6] |
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bigger magnitude is a result of...
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bigger amplitude and greater distance
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seismic moment
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[rigidity][amount of slip][area ruptured]
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Advantages of seismic moment
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– Physically related to the amount of energy
released during a quake. – Not sensitive to distance, instrumentation or local effects. |
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disadvantages of seismic moment
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– Harder to measure
– Harder to explain |
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moment magnitude
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(2/3)(logMo) - 6
Mo = seismic moment (Newton-meters) |
