GEOL-1021EL Lecture Notes - Lecture 3: Seismic Wave, Ground Vibrations, Plate Tectonics
16 Jun 2018
School
Department
Course
Professor
GEOL-1021
September 21, 2017
November 23, 2017
Earthquakes
•Can be understood in terms of the basic mechanisms of deformation
•most occur at plate boundaries (convergent, divergent, transform faults)
•cannot be reliably predicted, or mitigated
•Defined as a shaking or trembling of the earth that is volcanic or tectonic in origin
•Occur when rocks being stressed suddenly break along a new or pre-existing fault
•Seismic waves are ground vibrations caused by rocks slipping along opposite sides of a fault
•Factors involved:
•Stress (average force per unit area, resistance of a material to deformation)
•Strain (deformation; rigid body displacement)
•Strength (ability to resist stress without failure)
•Local build and release of stress
•Foreshock- before the main event
•Aftershock- after the main event (who would have thought)
Studying Earthquakes!
•Seismographs: machines that record the seismic waves emitted by earthquakes (both vertical
and horizontal movements)
•Seismic Waves- each wave hits the seismograph at different times, at different speeds.
• Seismic Wave Types:
•P waves (primary)- compressional waves (eg sound waves) that travel quickly through
rock. Travel as a series of contractions and expansions, pushing and pulling particles in
their direction of travel
•S waves (secondary)- Travel at about half the speed of P waves. Shear waves that push
material at right angles to their path of travel
•Surface Waves- Ripple across earth’s surface, where air above surface allows free
movement
