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GEOL 2390 Study Guide - Rocky Mountain Arsenal, Granular Material, Mercalli Intensity Scale


Department
Geological Sciences
Course Code
GEOL 2390
Professor
William M.Last

Page:
of 5
EARTHQUAKES
Associated hazards
ground shaking
surface faulting
ground failure
tsunamis
In NA: relatively low $ loss/yr (relative to floods, subsidence etc.)
Key point: greatest threat (in terms of $ and life losses)
MODERATE earthquakes
Why? frequency
Calif = 1 Moderate (mag 6-8) eq/3 years -- >8 eq/150 years
most destructive in NA-- see handout
In the path of a killer volcano
dormant for 600 years
eruption april-july 1991
largest eruption of 20th century
20 x 10 ^6 tonnes SO2
lowered global mean temp by 1 degree
150 x larger than Mt. St. Helens
ash flow 200 m thick
200 m elevation difference
500, 000 evacuated
NB
1. prediction techniques used?
2. hazard types?
Earthquake (def'n)
motion/trembling of earth cause by sudden release of accumulated strain
motion is wave, seismic wave
quake severity function of:
rupture/fault characteristics
factors related to how wave propagated through earth materials
Point: impact not
some places separating, some places meeting, other places sliding past
plate velocity ~1mm/yr to several cm/yr
sometimes continuous/smooth
other times strain accumulates until threshold
when threshold is exceeded - earthquake
Physics of seismic waves
2 basic types - surface waves vs. body waves
surface waves - Rayleigh and Love
R= slowest "ocean" L = faster (back and forth)
strongest motions
greatest threat to structures
slowest velocity
Body Waves
Compressional (P waves, primary)
motion indirection of NRG pulse
highest velocity
Shear waves (S waves, secondary)
displaces material perpendicular to travel direction
slower than P waves
POINT:
different movement, different hazard impact
surface waves greater threat to structures
S waves used to suggest interior earth properties (solid vs liquid)
P fastest to Rayleigh slowest
P waves; 8-11km/s
S waves; 5-7
Love; 3.5-4.5
Rayleigh; 3-4
Richter Scale
best known and most often quoted by press
1-10
logarithmic
but NRG released is ~30 factor at each level of magnitude
Richter also showed - larger the NRG of EQ, the larger amount of ground motion at
a given distance from the EQ
need at least 3 seismographs to determine epicentre
measure P vs. S wave arrival times
greater the time lag, farther the waves have travelled
measure amplitude of S wave
greater the amplitude …?
only partly measures EQ 'intensity'
other ways to evaluate intensity and magnitude
Mercalli Intensity Scale
I - XII based on visible damage and people's assessment of movement
Evaluation of actual energy released by WQ
Moment magniftude (Mm)
real magnitude of EQ
function of rock properties, extent of fracture.ovement, amount of displacement,
etc.
point
slightly different # than Richter Mag
Causes of earthquakes
early thoughts
earth sat on the back of an animal, animal moved, Earth move
hot air escaping, widespread belief, Aristotle
God/retribution
Modern thought
majority associated with faulting
most faulting associated with plate margins
divergent, convergent, transform
POINT
global distribution of EQ not random
closely related to plate margins
Although oat EQ entirely natural process
humans can also induce EQ
POINT
our ability to initiate EQ increases significantly in the 20th century
AND increase populations in EQ prone zone
thus EQ risk significantly rises
Dams/Large Reservoirs
1st correlation 1930s --> SWUS Lake Mead/Hoover dam
since then, many other documented
but still 'rare' BUT overall the bigger the reservoir, the greater seisms reaction to
leaden gthe surface with water
increased EQ of 10% reservoirs >90m deep
increased EQ on 20% reservoirs >140m deep
Fluid Injection
pumping fluid into underground reservoirs
best known - Rocky Mountain Arsenal
early 1960s: disposal liquid wastes in 12,000 well -- north immediate
quakes in Denver
So, whats happening here?
both fluid injection and creating reservoirs generate EQ; WHY??
connection factor - fluid
what does fluid do??? increases pore pressure
reduces strength of roc, weaker rock, easier to break
Ground shaking
lurching/vibrations
cause of most spectacular damage partial/total collapse of structures
San Fernando 1971 ~75% of total damage due to buildings being shaken
SF 1906: then $1 x 10 ^6 now $24 x 10^9 .. damage mostly due to shaking
POINT
structures vibrate/shake at different frequencies
function of:
shape, size
substrate
energy applied/direction