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GEOL 2390 Study Guide - Radon, Deep Foundation

Geological Sciences
Course Code
GEOL 2390
William M.Last

of 3
Earthquakes Continued...
lateral spread/flow failure: avoidance, zoning, excavation (if small)
général liquéfaction/bearing strength loss: engineering designs:
drainage, grouting, artificial compaction, deep foundation
small area only
Summary EQ Hazards Mitigation
1. community preparation/education
2. land use management
3. EG engineering/design -- retrofitting, reinforced concrete, etc.
4. EQ prediction/EQ control
1. Community Preparation
Mexico City 1985 : perception of EQ hazard very low: no preparation
2. Land use management
simplest, most direct way of reducing losses - simply don't live, build in eq prone ar-
options if aware of risk:
refusal of rebuilding/renovation permits construction of critical structures
classic example: Bodega Bay Nuclear Plant
3. EQ Engineering/design
most deaths caused by collapse of man-made structure
technologically possible construct building to withstand operate-strong (6-7) eq but
50% more expensive
result - most EQ prone areas under coded
San Fernando 1971: 90% of buildings collapsed exceeded building code
Caracas 1967: much new construction in previous 10 yr (i.e. modern build-
most rigid code ater Calif
total collapse 5 high rise
major structural damage 500 other 30 story buildings
biggest upgrading problem
30% of population lives/works in high risk zone
sufficient spacing
no ornamentation
rectangular vs. complex
steel reinforced concrete
4. EQ Prediction/Control
only within past decade
last 5 years major advances but still elusive
1. long-term analysis seismicity/recurrence rates
ID seismicity gaps
plot historical seismic activity on spatial basis
much used before 60s but with advent of plate tectonics
two possibilities
slow release of strain (creep)
strain accumulating (release in major quake)
i.e. gap may be the most hazardous
gaps: nothing about how large, how often statistical analysis of historical records:
develop relationship between # eq and magnitude
logN = a -bM
N = number of quakes
a, b = constants
M = magnitude
In US/Can
49,000/yr 3-4 M (area affected ~2000km2)
120/yr 6-7M (area ~160,000 km2)
problem: relies totally on length of data
large data = good relationship
small database = poor relationship
2. short term approach
study of precursors
key 'event' that started modern investigation
Garm, Siberia
quake 1946 area extensive oil drilling and instrumentation
notice significant changes in velocity of seismic waves prior to ma-
jor quake
several months ~yr prior
Vp/Vs decrease (mostly Vp
then just before EQ camp back to normal value
quantitative aspect:
duration of Vp/Vs anomaly (rather than magnitude of anomaly) -- eq intensity
as magnitude increases, anomaly time increases exponentially
i.e. Log T = 0.8 - 1.92
Deliatancy-Diffusion Theory
1. strain in bedrock near focus eq
microporosity (small cracks/holes)
rock becomes less rigid
more compressive Vp increases
2. GW inflow slowly filled not porosity
resaturates rock
increase pore pressure
decreases compressibility
Vp/Vs returns to normal
BUT rock is now weaker
3. larger volume of rock undergoing strain
increase time to re-saturaate -- increase anomaly time
Electrical resistivity changes
resistivity function amount of water in rock
when micropores develops more water
resistivity decreases
water in wells
development of microporosity
Ground water level may decrease before an EQ
increase of radon (200%)
ground level deformation
stain involves vertical as well as horizontal
bulging before quake
best studied: Palmdale Ca
84,000 km
uplift started in 50s to 45 cm by mid 70s but subsided
problematic interpretation
Animal Behaviour
Summary EQ prediction
true prediction must provide
must be
few false alarms
not presently possible