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 Model Four 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 stabilizing Oscillations 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