Earthquakes-Pt.2

1. Earthquakes-Pt.2 Earthquake Processes (mechanisms/causes) Effects of earthquakes (damage) Earthquake risk and prediction Responses to earthquake hazards

2. Earthquake Processes • Earthquake Cycle: Elastic Rebound • Dilatancy-Diffusion Model • Fault-valve mechanism • Roles of fluid pressure

3. 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)

4. Elastic Rebound Model

5. Elastic Rebound

6. Dilatancy-Diffusion Model/Fault Valve Mechanism

7. 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

9. Effects of earthquakes • Ground shaking and rupture • Liquifaction • Landslides • Fires • Tsunamis • Regional changes in land elevation

10. 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

11. Earthquake Damage: San Francisco, 1989

12. Pancaked building: Mexico City earthquake, 1985

14. 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

15. 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

16. 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

17. Tsunami Warning System

21. Mercalli Intensity (damage) map and peak ground acceleration map for the 1994 Northridge, CA earthquake (M =6.7) X- shows location of epicenter X

22. 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

23. Short-Term Prediction • Pre-seismic uplift/subsidence • Seismic Gaps • Anomalous animal behavior

24. Longer-Term Prediction • ???

25. Idealized diagram of an Earthquake warning system

27. Relationships between recurrence interval, slip rate and earthquake magnitudes

28. Response/Prediction Options

29. 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

30. 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