UNLV GEOL101 Lecture Outline Fall Week 11/7/11

Transcription

1 Week 11/7/11 I. Earthquakes A. vibration of the earth that results from a rapid release of energy B. sources of energy include 1. landslides 2. volcanoes 3. nuclear explosions 4. earth movement along faults a) tectonic activity b) rebound c) human activity (dams, deep well injection) C. most large quakes result from movement along faults 1. generally explained by the theory of plate tectonics 2. activity is concentrated on plate boundaries 3. some exceptions occur in continental interiors a) these are rare b) tend to be particularly devastating II. III. Basic mechanism for earthquakes along faults A. when rock is stressed, it deforms 1. stress is application of a force a) bending b) twisting c) compressing d) shear 2. deformation is a change in shape B. if deformation exceeds the rock strength, the rock ruptures 1. sometimes along an existing fault 2. sometimes creating a new fault C. elastic rebound theory 1. stress builds up during gradual deformation 2. eventually exceeds strength of the rock 3. the rupture releases the deformation stress 4. the two pieces ʻsnap backʼ 5. releases the energy stored during deformation 6. seismic waves travel outward from release point Focus and epicenter A. focus 1. point in the subsurface where energy is released 2. for faults, this may occur at several points over a length

2 3. nuclear weapons are truly point sources B. epicenter 1. spot on the surface directly above focus 2. identified by instrument 3. not all faults exhibit a surface rupture C. faults can be inclined 1. surface rupture may not occur at the focus IV. Foreshocks and aftershocks A. foreshocks 1. precursors to the main event 2. bend a pencil and it creaks before snapping 3. possible tool for earthquake prediction a) sometimes frequency increases before big one b) but not all quakes have foreshocks B. aftershocks 1. follow a major release of energy 2. it may take a while for the system to quiet down 3. intermediate ʻadjustmentsʼ lead to smaller quakes 4. remaining stress is redistributed about the region 5. although smaller than main quake, can be devastating a) psychologically b) structures damaged during the primary quake V. Seismic Waves A. energy waves spread out in all directions from the focus B. wave velocity depends on 1. type of wave 2. density and elasticity of the intervening materials a) increases in dense, elastic materials (basalt) b) decreases in light plastic materials (soil) 3. in a given material a) density generally increases with depth b) hence so does seismic velocity C. types of waves 1. body waves (P and S) a) pass through the body of the earth b) can travel very long distances c) P waves (1) P stands for primary, also pressure (a) fast (b) they arrive first (2) these are compressional waves (3) individual particles compress, then rebound

3 (4) travel through both solids and liquids d) S waves (1) S stands for secondary, also shear (a) slower than P waves (b) arrive second (2) these are shear waves (3) individual particles move in one axis (a) up and down (b) side to side (4) cannot travel through liquids 2. surface waves (also L for long wavelength) a) move along the earthʼs surface b) slower than S or P waves c) complex shear waves (demo) (1) vibration (a) both parallel and perpendicular (b) to the earthʼs surface (2) low frequency (large wavelength) (3) large magnitude d) donʼt travel very far e) can be very destructive (1) complex motion (2) large magnitude D. reflection/refraction 1. occurs at changes in material (layers in the earth) 2. some energy is reflected back 3. rest is transferred into the new material 4. path bends, just like light does in water VI. Seismograph A. device designed to record earth vibrations B. basics of how this works 1. a mass is decoupled from the earth a) e.g., hung from a thread 2. inertia of the mass resists motion a) it stays still b) provides a reference while the earth moves 3. we measure motion between the mass and earth C. the output from a seismograph is a seismogram 1. shows vibration as a function of time 2. gives magnitude and frequency D. usually set up in arrays of three 1. one each for N-S and E-W horizontal movement 2. a third for vertical movement

4 E. set up as close to bedrock as physically possible 1. sub-basement not the 10 th floor 2. interested in vibrations from the earth 3. not weather or anthropogenic effects a) wind b) trucks, trains, etc. VII. Finding the focus/epicenter A. earthquakes produce P, S, and surface waves B. at a given seismic station 1. P waves a) fast, high frequency, small amplitude waves b) arrive first 2. S waves a) considerable slower than P waves, b) lower frequency and greater amplitude 3. surface or L waves (long wavelength) a) donʼt travel far from the source b) all of the energy is expended quickly C. consider a seismograph a long distance from the epicenter 1. P waves arrive first, then S waves 2. time lag between first arrivals increases with distance 3. lag tells how far away the quake was 4. we can only get distance not direction 5. if we have 3 stations we can locate epicenter D. complications 1. velocity of seismic waves is dependent on the rock type a) changes with rigidity and elasticity 2. we need to know about the intervening rocks 3. in practice, we use data from hundreds of stations, not 3 4. faults release energy from a zone not a point 5. nuclear tests release from a point a) this is how we verify the test ban b) weapons have a different signature than the earth c) location is also important E. depth 1. focus may occur at depths ranging from km 2. 90% of all quakes occur at depths of less than 100 km 3. the strongest earthquakes recorded have been shallow F. type of movement on the fault 1. shape of the first wave indicates relative direction a) up is towards the seismograph b) down is away from the seismograph 2. 4 or more seismographs needed to characterize direction

5 a) can tell between strike and dip slip b) also get direction (1) left-lateral vs. right-lateral (2) normal vs. reverse VIII. Earthquake Intensity and Magnitude A. historically quakes have been measured two ways 1. intensity 2. magnitude B. intensity 1. empirical measure of the physical effects (destruction) a) observed on a local scale b) Modified Mercalli Scale is an example (1) ranks from problems with this approach a) degree of destruction varies from place to place (1) degree of urbanization (2) local soils (3) construction practices b) relies on human interpretation (1) if nobody sees it? (2) rating depends on judgment C. magnitude 1. numerical measure of the amount of energy released 2. purely objective 3. relies on instrumentation 4. is independent of human a) interpretation b) anthropogenic effects (1) death and destruction donʼt factor in D. Richter Scale 1. based on the largest amplitude seismic wave recorded 2. corrected to a distance of 100 km from the epicenter 3. logarithmic scale, increasing from 1 a) factor of 32 in total energy between each number b) 9 would release 32 times as much energy as an 8 E. does not work well for very large quakes (over 8) 1. appears to be a limit on wave amplitude 2. wavelength may change for very big quakes 3. not a lot of data on such quakes F. relative sizes 1. the largest earthquake recorded to date was an 8.6 a) Chili 1960 b) Alaska, 1964 also an 8.6, but actually smaller

6 (1) limitations on the Richter scale c) both were thrust faults d) Lisbon (1755) was estimated at 8.7, but no measurements 2. quakes below about 2.0 are not felt by humans 3. some examples a) train passing nearby b) 5.5 Amargosa valley, NV 1992 c) bomb at Hiroshima d) Northridge, 1994 e) Loma Prieta, 1989 f) 7.1 Winnemucca, NV 1915 g) Turkey 8/19/99 h) ~8.7 - Lisbon, Portugal 1755 IX. Global distribution of quakes A. ~30,000 per year that are strong enough to be felt by humans B. less than 100 of those are of large magnitude 1. still fewer occur in an urban area 2. unlikely to make the news unless death/destruction C. most earthquakes are associated with tectonic activity 1. boundaries of the Pacific Ocean 2. mid-oceanic ridges D. subduction zones 1. deepest quakes 2. continental plates adjacent to deep oceanic trenches E. oceanic ridges 1. quakes are shallow (5-70 km) 2. weak F. quakes do occur in the continental interiors 1. much less frequent 2. believed to be a readjustment in earth stress 3. occurs along a weak spot in the earthʼs crust a) inactive plate boundary b) old fault c) change in rock type 4. possible causes a) continental compression (1) tectonic activity on boundaries b) reservoir filling c) glacial rebound d) deep well injection/withdrawal (Rocky Flats Arsenal)

7 X. Destruction A. typical area of influence around epicenter 1. magnitude will be pretty constant within 20 to 50 km 2. beyond that, energy release decreases greatly 3. except in the continental interior a) the bedrock is more rigid b) transmits vibrations better c) area of destruction can be much larger B. location 1. most earthquakes occur in rural areas a year seem to hit a densely populated region C. most destruction is caused by L waves D. damage depends on: 1. intensity of the movement 2. duration of the movement 3. materials that the structure rests on a) soft sediments amplify vibrations b) some soils can liquefy (1) Japan, China, Mexico City c) bedrock lets them pass through 4. type of construction a) wood frame are flexible b) cast concrete or masonry are not c) height also makes a difference (1) buildings a have a natural period (2) can shake like a tuning fork E. death toll depends on 1. population density 2. degree of urbanization 3. collateral damage a) building collapse b) fire 4. time a) Loma Prieta hit during rush hour b) Tangshen hit at ~ 4 AM F. ancillary effects 1. seiche a) standing wave in a reservoir b) dam may be over topped c) water pressure can cause landslides 2. landslide 3. fire a) also disrupts services (rescue, water) b) 1906 San Francisco, more damage than quake

8 XI. XII. Tsunami A. seismic sea waves 1. incorrectly called tidal waves 2. have nothing to do with tides B. usually caused by undersea earthquakes 1. vertical displacement of the sea floor 2. can also result from a) volcanic eruptions b) undersea landslides (1) evidence of huge slides off coast of Hawaii (a) ancient (2) 100 ft high waves hit California C. characteristics 1. can move across the ocean at over 500 km per hour 2. in the open ocean a) wave crest is less than a meter high b) wavelength may be up to 1000 km long c) not the huge breaking wave of movies or TV D. behavior changes in shallow water 1. front of the wave hits bottom, slow down 2. back of the wave pushes forward 3. as a result the wave ʻstands upʼ E. impact 1. first water on shore recedes, then surges in 2. may travel 100ʼs of meters inland a) can go much further up rivers 3. there will be several surges a) never a solo wave F. we know very little about tsunamis 1. relatively rare occurrence 2. few survivor accounts Predicting earthquakes A. foreshocks B. dilation 1. changes in underground water level 2. tilting of the ground surface 3. expansion of the ground surface C. animal behavior D. China 1. Haicheng, 1975 (Richter magnitude 7.3) a) prediction based on (1) ground tilt (2) foreshocks

9 (3) magnetic fluctuations b) city was mostly evacuated (1) death toll was a few hundred 2. Tangshen, 1976 (Richter magnitude 7.8) a) 3:42 a.m. on July 28, 1976 XIII. San Andreas Fault A. transform fault 1. separates north American and pacific plates 2. runs through California and along NW US coast B. bends near Los Angles 1. binding at the bend creates a complex local fault structure 2. potential for very powerful quakes C. straight shot through Bay Area 1. lots of soft soils in the bay area 2. very susceptible to liquefaction XIV. New Madrid Fault A. Madrid Seismic zone lies within the central Mississippi valley 1. parts of KY, MO, AR, IL, TN 2. far from current plate boundaries 3. inactive boundary 4. probably "readjusts" to accommodate continental scale stresses B. earthquakes of big quakes in 3 months 2. all believed to be > felt in Maine and Florida 4. little damage or death because of low population 5. changed the course of the Mississippi River C. a big one here could be devastating 1. heavily populated 2. hard basement rocks transmit vibrations over large distances 3. building codes are not earthquake specific XV. Lisbon, Portugal, November 1, 1755 A. background 1. city of roughly 275, trade center a) about 8 miles upriver from the ocean b) excellent seaport 3. significant cultural center B. quake 1. believed to be ~8.7 on the Richter scale 2. epicenter was in the ocean ~ 200 km away

10 3. one of the 5 largest to hit during recorded history 4. the ground heaved 3 times 5. witness reported seeing streets undulating C. bad timing 1. struck at 9:40 AM, Saturday 2. All Saints day, churches were full D. masonry structures are vulnerable to ground motion E. Tsunamiʼs (3) hit 90 minutes later to 40 feet high 2. each lasted 20 to 30 minutes F. fires from oil lamps/stoves lasted 4 days 1. driven by high winds 2. may have been a blessing 3. stop spread of disease from the >60,000 dead G. prisons were breached H. looting rampant (34 executed) XVI. Probing Earthʼs Interior A. geologists believe that the earth has a layered structure 1. crust 2. mantle a) asthenosphere is a weak layer b) lithosphere rigid outer layer 3. outer core 4. inner core B. how can we surmise this? 1. oil and gas wells rarely exceed 7 kilometers in depth 2. deep research wells have gone to 13 km 3. but the earth has a radius of ~6370 km C. other sources of information 1. space exploration 2. interpretation of volcanic activity and crustal movements 3. gravitational studies 4. magnetic data 5. high-pressure/temperature laboratory experimentation 6. interpretation of seismic waves (the big one) D. seismic waves 1. vibrations transmitted through the earth 2. occur when energy is suddenly released into crust a) tectonic activity b) nuclear explosions 3. intervening materials change the wave properties a) speed b) direction

11 c) reflection d) refraction 4. by measuring waves at distances away from the source we can infer the properties of the intervening materials XVII. Seismic investigation of the earthʼs interior A. originally thought that the interior of the earth was homogeneous B. better measurements ended this hypothesis 1. improved seismographs 2. more seismographs 3. improved communication of data C. Moho Discontinuity 1. discovered by Mohororovicic in velocity of P waves increases below a depth of ~30 km 3. velocity change implies a material change at that depth 4. believed to separate crust from the underlying mantle 5. increasingly better equipment has refined this boundary D. shadow zones 1. found on the opposite side of the earth from a quake 2. zones are different for P and S waves a) S wave shadow zone is directly opposite quake b) P wave shadow zone is to the side 3. a solid earth would not have any shadow zones 4. shadow zones suggest a liquid core (outer) a) blocking of S waves b) refraction and velocity decrease for P waves E. inner core 1. refraction studies also suggested structure within the core 2. earthquakes are not the best investigative tools a) you donʼt know when, where, or how big in advance b) the point of energy release can only be estimated (1) energy is released across a zone (2) makes it difficult to pinpoint travel times 3. these problems were resolved by nuclear testing a) energy released from a point b) site of energy release could be pinpointed 4. found that P waves speed up in the inner core a) implies that the center of the earth is rigid F. test ban 1. seismicity is how we verify the nuclear test ban 2. test shots have a different seismic signature than quakes XVIII. Geothermal gradient A. earth gets hotter with depth

12 1. averages C per kilometer of depth 2. core is between 4500 and 6700 C a) hotter than surface of the sun B. heat source 1. radioactive decay of a) uranium b) thorium c) potassium 2. heat released through crystallization of iron a) core may have been all molten b) pressure forced crystallization c) changed from liquid to solid produces heat 3. kinetic energy from particle collisions (meteorites) a) dominant force early in earth history b) negligible now C. heat status 1. heat is being lost faster than it is produced 2. earth is cooling internally