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