Themes of the Day:

• Faults (continued from Monday)
• Earthquake Basics
• Earthquake Effects
• Seismic Waves
• Measuring Quakes

Faults

• Brittle deformation - more common shallow in the crust or as a result of high strain rates
• Joints are fractures that have no offset - "Joints Relieve Stress"
• Fault Types:
  • Normal Faults - dipping fault planes, dip-slip movement, hanging wall moves down with respect to the footwall - thinning and extension of the crust - associated with divergent plate boundaries - Rooks County Normal Fault GigaPan - detachment faults or horst & graben structure - A horst is a horst, of course of course!
  • Reverse Faults (Thrust Faults)- dipping fault planes, dip-slip movement, hanging wall moves up with respect to the footwall - thickening and compression of the crust - Thrust Faults have shallow dips (< 30^o) - associated with convergent plate boundaries - Example: Keystone Thrust, NV
  • Strike-slip (transform) faults - vertical fault planes, strike-slip movement, right-lateral and left-lateral senses of movement - example: San Andreas Fault, CA
  • Oblique-slip - some combination of dip-slip and strike-slip - resulting from transtension or transpression - associated with transform plate boundaries

Fault & Earthquake Terminology

• Focus (or hypocenter) - point of initial breakage - seismic body waves emanate from the focus
• Epicenter - point at surface directly above focus - seismic surface waves emanate from the epicenter
• Fault Scarp - surface breakage on a fault zone

Earthquake Causes

• Earthquakes occur when rocks under stress undergo brittle failure and slide past each other along a fault plane
• Elastic Rebound Theory
• Foreshocks & Aftershocks
• Fault Types and Plate Boundaries
  • Normal Faults - Divergent Plate Boundaries
  • Reverse Faults - Convergent Plate Boundaries
  • Strike-slip Faults - Transform Plate Boundaries
• 90% of earthquakes occur at or near plate boundaries

Seismic Waves

• Measured on a Seismograph - printed record is a seismogram
• Body vs. Surface waves - travel paths, speed
• Body Waves - Slinky demonstration
  • Primary (P-wave) - compressional - particle motion is parallel to propogation direction - like sound waves
  • Secondary (S-wave) - shear - particle motion is transverse (perpendicular) to propogation direction
• Surface Waves
  • Love waves - "L-waves knock the "L" out of you." or "Love waves make the bed rock."
  • Rayleigh waves

Measuring Quakes

• Locating Earthquakes by Triangulation
  • Difference in travel time determines distance to quake
  • Triangulation determines a unique epicenter
• Intensity Scales
  • Modified Mercalli Scale I-XII - local effects
  • Richter Scale - logarithmic, based on maximum amplitude - rapid magnitude determination
  • Moment-Magnitude Scale - most quantitative - best scale for very large quakes
• First Motion Mechanisms - the seismic beach ball explained

Some Earthquake-Related Websites Worth Surfing

• USGS Earthquake Hazards Program
• USGS National Earthquake Information Center
• USGS Earthquake Hazards Program - Northern California
• Southern California Earthquake Data Center