- Slides: 109
Geology 12 Presents
n n n Chp Chp Chp 10 Earth’s Interior & Isostacy 11 Ocean Floor 12 Plate Tectonics 9 Seismology EQ 13 Structure
Chp 9 Seismology
Not a safe place to be if an aftershock hits!!!
• Earthquake (EQ) is the vibration of the Earth caused by a sudden release of energy, usually the result of faulting • Aftershocks are small adjustment EQs that follow a major EQ – Smaller but still can cause considerable damage, especially in weakened structures – Can occur weeks afterwards
Fault Creep • Movement along fault lines occur smoothly & gradually • Causes cracks in sidewalks, offset fences • Very localized & slow
Elastic Rebound Theory 1. The plates are constantly moving, except along their edges. Due to friction between plates.
2. The plates are like large springs which deform (bend, fold) as the plates continue to move.
The plates (“springs”) wind up until…. 3. The rocks break along the fault, and the plates snap back to their original shape releasing the spring energy in the form of seismic waves = EQ.
• Seismology: study of EQs • Seismograph: instrument that detects, records, and measures various vibrations produced by EQs (+atomic blasts, explosions, meteorite impacts, landslides, and cavern collapses).
• Seismic Waves: radiate outward in all directions. Fa ult pla ne epicentre focus
• Benioff Zone: zone of EQ foci dipping into mantle away from trench, resulting from subduction. x x xx xx x
2 Types of Seismic Waves 1. Body waves: travel through Earth 2. Surface waves: travel only along ground surface (like ocean waves). Surface waves EQ Body waves
1. Body Waves: a) P-waves = primary waves • travel through everything (solids, liquids, gases) • fastest (1 st waves to arrive at seismograph) • Travel 5 km/sec b) S-waves = secondary waves • travel only through solids • Second wave to arrive at seismograph. • Travel at 3 km/sec
2. Surface Waves: several kinds but two most important are: a) L-waves = Love waves • Travel like an S-wave, but motion is horizontal = side to side • Very damaging to buildings and foundations b) Rayleigh waves • Slower than L-waves • Travel like water in waves • Particles travel in elliptical patterns
Rayleigh Wave Love Wave
n Hand out WS 9. 1 Note Helper
• Typical Seismogram (record from seismograph) Body waves Timing Arrival of PArrival of S -wave marks -wave Surface waves Know this!!! Background noise Arrival of Lwave amplitude P=S time interval time
n Handout WS 9. 15 note helper
Locating an EQ 1. Determine the distance to epicentre using P-S time interval (time may be given instead) a) Measure the P-S time interval/lag Arrival of P-wave S-wave 3: 00 5 6 7 8 9 10 11 12 minutes
b) use either: i) Rickter scale nonogram on WS 9. 1 side 2 ii) Time-distance graph
2. Using a circle compass: – a) open compass equal to distance to epicentre – b) put needle on seismogragh location and drawout circle 3. Repeat steps 1 & 2 for atleast 3 seismographs 4. Where circles cross = epicentre “triangle of error”
n Do Lab 9. 1 Epicentre Location, real soon (i. e. next class)
Measuring EQ Intensity and Magnitude • EQs are measured 2 ways: 1. Intensity: qualitative assessment of the kinds of damage done. • subjective: affected by: – a) energy released by EQ – b) distance to epicentre/focus (closer = more damage)
• Subjective: affected by a) Energy released from EQ b) Distance from EQ epicentre/focus (closer = more damage)
c) Local Geology (hard rock = less damage; loose sediment = more damage)
d) Water table height (high water table = more damage) - liquifaction.
Building collapse in Japan due to liquifaction.
e) Type/quality of building (wood sustain less damage than masonary) f) Population density
• Most common scale is the Modified Mercalli Intensity Scale I = not felt XII = total destruction
• After assessment of damage to area is done, Isoseismal Lines (lines of equal intensity) are drawn
Feb 28, 1925
2. Magnitude measures the amount of energy released • 2 Scales: – a) Richter Magnitude Scale: amount of energy released at fault (how much movement along fault) – RS Movement 1 0. 0001 mm 2 0. 001 mm 3 0. 01 mm 4 0. 1 mm
– RS 5 6 7 8 9 Movement 1 mm 10 mm = 1 cm 100 mm = 10 cm 100 cm = 1 m (devastation) 10 m (help, Mother!) *each number up (i. e. 4 to 5) = magnitude x 10 shaking *each number up (i. e. 4 to 5) = energy x 30 900 100 energy = x _______ 4 to 6: shaking x _____
• Maximum ampitude (P or S) determines Richter Magnitude
b) Seismic Moment Scale = modified Richter Scale that takes into account distance over which there is fault movement and amount of plate movement. • Is a more accurate measure of the energy released. 30 150 km Vs. km
Destructive Effects of Earthquakes 1. Ground shaking: • causes most damage and deaths • Structures on poorly consolidated sediments (earthfill, alluvium) are subjected to waves of longer duration and amplitude animation
• Liquifaction: when saturated soil turns into quick sand during an EQ and buildings, overpasses sink and tip over; gas mains and storage tanks surface.
• Building EQ resistance: – Very poor: adobe mud brick wall – Poor: unreinforced brick and concrete – Moderate: reinforced concrete – Best: steel and wood frame.
2. Fire: electrical, gas and water mains are broken from falling debris and shifting ground. – Most damage in San Francisco (1906), Tokyo (1923) & Kobe (1991) caused by fire. – SF and Vcr have put in back up salt water pumps & water lines to safe downtown core.
3. Tsunami = seismic sea wave (harbour wave) – Caused by EQs, underwater landslides and volcanic eruptions – In open ocean, waves travel at 750 km/h and are 0. 3 m high (can lose as little as 10% of energy crossing the ocean). – Can travel across Pacific in 12 hrs – At shore the wave slows and piles up into a giant waves
3. Tsunami movie clip
4. Ground Failure, landslide, or avalanche • unstable cliffs and hill sides are shook down burying towns.
EQ Prediction • Need: when, where and strength • Seismic risk maps have been constructed based on history, intensity and likelihood of past EQs re-occuring • Tend to be on fault boundaries
Precursors to EQs • Short and long-term changes within the Earth. 1. Seismic Gap = area that has had major EQs in past and is locked and not releasing energy.
• Ex: off BC coast Pacific Plate is moving 5. 5 cm/yr x 300 yrs = 16. 5 ___ m of stress build up. • Fault creep: slow, continual plate movement where plates are not locked = good thing
2. Land Elevation and Tilting: as stress builds up on the plates, they fold and push upward (or downward) • Measured with tiltmeters.
3. Earthquake Cycle: some EQs occur on regular cycles/schedules. Ex: Juan de Fuca: 3 times in 2, 000 yrs, last time 1700 Ex: San Francisco: every 100 -120 yrs, last time 1906
• And as stress builds and the rocks reach their breaking point, cracks open increasing pore space resulting in…. (precursors 4 -8).
4. Well Water Fluctuations: sudden, unexplained rise/fall in level of well water.
5. Fluctuations in Magnetic Field and Electrical Resistance: across fault: resistance drops
5. Fluctuations in Magnetic Field and Electrical Resistance: across fault: • Resistance Three hours before the earthquake there was a dramatic increase in the signal level. Power was lost for a couple of days after the quake, but when it was restored, the increased signal level continued for almost a month. The spikes in the data do not correlate to aftershocks
• 6. Radon Gas Emissions: increase due to more cracks
• 7. Seismic Wave Velocity decreases, then increases EQ Wave speed time
• 8. Bizarre Animal Behavior – Ex: ______ does his HW
• Dilantancy = increase in rock’s volume just before rupturing • Due to fault stress on rock along fault • Dilantancy model account for precursors 4 – 8. – 4. well water drops: water fills in new pore space – 5. magnetic field/electrical resistance changes as salt water fills in cracks decr’ resistance – 6. Radon emissions increase b/c of more cracks
– 7. Seismic waves slow b/c rock’s elasticity decreases, then become brittle just before rupture. – 8. Animals: maybe they can detect subtle changes in the magnetic field? ? ?
EQ Control • Impossible to prevent EQs but maybe we can dissipate the energy before it becomes destructive. • Idea is to have a series of small, managable EQs rather than one large one. • If planned the EQs, you can then evacuate people to safe areas.
• 2 tools: – 1. underground atomic bombs detonated along the fault (could H 2 so no radiation) – video – 2. inject water along fault which hydraulics plates apart and lubricates them so they slip pump Injection well f lt u a
• Not used b/c – 1. lack of political will – 2. litigation…I’ll sue…”
That’s Grad party damage!
• Read over Labs 9. 1 and 9. 2 • Do WS 9. 2 • Work on Unit 3 Review WS