Earthquakes-Pt.2
•Earthquake Processes (mechanisms/causes)
•Effects of earthquakes (damage)
•Earthquake risk and prediction
•Responses to earthquake hazards
Earthquake Processes
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Earthquake Cycle: Elastic Rebound
Dilatancy-Diffusion Model
Fault-valve mechanism
Roles of fluid pressure
Earthquakes Cycle
Elastic Rebound Model
Four Stages
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Long period of inactivity (following a major earthquake)
increased seismicity: elastic strain accumulates, approaches,
locally exceeds rock strength
Foreshocks (hours or days before next large earthquake)
Major earthquake, aftershocks (few minutes, months, to a yr)
lastic
ebound
Model
Elastic Rebound
Dilatancy-Diffusion
Model/Fault Valve
Mechanism
Earthquakes Caused by Human
Activity
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Reservoir-induced seismicity (e.g., Hoover Dam)
Deep waste disposal (e.g., Rocky Mtn, arsenal)
Nuclear explosions
Shallow focus only
Effects of earthquakes
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Ground shaking and rupture
Liquifaction
Landslides
Fires
Tsunamis
Regional changes in land elevation
Earthquake Damage
• Buildings: Swaying, Pancaking
• Broken pipelines (gas, water) & electrical lines
• Fires & explosions (from pipelines & storage
tanks)
• Shearing & subsidence of sand fills
• Quicksand, sand boils, sand volcanoes
• Quickclays
• Landslides
Earthquake Damage:
San Francisco, 1989
Pancaked building:
Mexico City earthquake, 1985
Origins of Tsunamis
(seismic sea waves)
• Sudden vertical displacement of seafloor
(from dip-slip fault)
• Momentary drop in local sea level
• Water rushes into depression, but
overcorrects, locally raising the sea level
• Sea level locally oscillates before stabilizing
• Oscillations are transmitted as long, low
seismic sea waves
Sudden vertical displacement of
seafloor (from dip-slip fault)
Water rushes into depression, but
overcorrects, locally raising the sea level
Momentary drop in local sea level
Sea level locally oscillates before stabilizing
Oscillations are transmitted as long, low
seismic sea waves
Characteristics of Tsunamis
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Long wavelengths (up to 100 km)
Low wave height in open ocean (< 0.5 m)
Velocities up to 700 km/hr in deep water
As tsunami waves enter shallow coastal water:
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Speed decreases
Water withdraws from shore before tsunami hits
Water rises up, successive tsunami waves hit
First wave is not necessarily the largest
Waves sweep inland, like a flood front rather than an ocean wave
Tsunami waves range up to several tens of meters high
Most damaging are from nearby sources with little warning
Tsunami Warning System
Mercalli Intensity
(damage) map and peak
ground acceleration map
for the 1994 Northridge,
CA earthquake (M =6.7)
X- shows location of
epicenter
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Earthquake Risk and Prediction
• Short-term prediction
• Long-term prediction
• Estimation of seismic risk
– seismic hazard maps
– probability of events
• Conditional probabilities for future
earthquakes
Short-Term Prediction
• Pre-seismic uplift/subsidence
• Seismic Gaps
• Anomalous animal behavior
Longer-Term Prediction
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Idealized diagram
of an Earthquake
warning system
Relationships
between recurrence
interval, slip rate and
earthquake
magnitudes
Response/Prediction Options
Response to Earthquake Hazards
• Earthquake hazard-reduction programs
• Earthquakes and critical facilities
• Societal adjustments to earthquakes
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structural protection
land-use planning
increased insurance and relief measures
earthquake warning systems
perception of earthquake hazard
Learning objectives
• Understand the relationship of earthquakes to faulting
• Familiarization with earthquake wave (energy)
terminology
• Understand the concept of earthquake magnitude (and
its calculation)
• How seismic risk is estimated
• Familiarization with the major effects of earthquakes
• The prediction of earthquakes
• Mitigation of earthquake damage