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Earthquakes-Pt.2. Earthquake Processes (mechanisms/causes) Effects of earthquakes (damage) Earthquake risk and prediction Responses to earthquake hazards. Earthquake Processes. Earthquake Cycle: Elastic Rebound Dilatancy-Diffusion Model Fault-valve mechanism Roles of fluid pressure. - PowerPoint PPT Presentation
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Earthquakes-Pt.2
•Earthquake Processes (mechanisms/causes)
•Effects of earthquakes (damage)
•Earthquake risk and prediction
•Responses to earthquake hazards
Earthquake Processes
• Earthquake Cycle: Elastic Rebound
• Dilatancy-Diffusion Model
• Fault-valve mechanism
• Roles of fluid pressure
Earthquakes Cycle
Elastic Rebound ModelFour Stages
• 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)
Elastic Rebound Model
Elastic Rebound
Dilatancy-Diffusion Model/Fault Valve
Mechanism
Earthquakes Caused by Human Activity
• Reservoir-induced seismicity (e.g., Hoover Dam)
• Deep waste disposal (e.g., Rocky Mtn, arsenal)
• Nuclear explosions
• Shallow focus only
Effects of earthquakes• 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)
Momentary drop in local sea level
Water rushes into depression, but overcorrects, locally raising the sea level
Sea level locally oscillates before stabilizingOscillations are transmitted as long, low seismic sea waves
Characteristics of Tsunamis• 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:
– 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
X
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
• ???
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
– 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