Understanding Earthquakes: Types, Causes, and Terminology, Slides of Geology

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Global

Seismology

Chapter 4

Faults: Red

• Earthquake: an event of ground shaking usually caused by the rupturing of a fault within the Earth.
• Who studies Earthquakes?
  • Seismologists
    • waves
  • Geophysicists
    • Mechanics
    • Geodesy
  • Geologists
    • Structures
    • Paleoseismology

Why Do Earthquakes Occur?

Earthquakes can occur due to:

• Sudden formation of a new fault (fracture on which sliding occurs)
• Sudden slip on an existing fault
• Movement of magma in a volcano / Explosion of a volcano
• Giant landslides (technically involves a fault)
• Meteorite impact
• Underground nuclear bomb tests

Earthquake Terminology

• Hypocenter (Focus): actual location of the earthquake at depth
• Epicenter: location on the surface of the Earth above the hypocenter
• Hanging Wall: top block of a fault (where a light would hang from)
• Footwall: bottom block of a fault (where you would stand)

Types of Faults

• In general, faults come in three different types: Normal, Reverse,

and Strike-Slip

• Shallow angle (< 30 ) reverse faults are called thrust faults
• Faults that have a mix of slip styles are called oblique slip faults

Fault animations

• Normal Faults : from stretching of or extending rock; points on

opposite sides of a fault are father apart after an earthquake

• Reverse Faults : from contracting or squishing rock; points on

opposite sides of the fault are closer together after an earthquake

• Strike-Slip : can form in either areas of stretching or squishing,

material slides laterally past each side of the fault.

• Described by sense of motion:
  • Right-lateral (Dextral)
  • Left-lateral (Sinistral)

Why are there different types of faults?

Quantifying Deformation: Stress & Strain

A simplistic view…

• Stress = force/area
  • So both force and area of contact are important
  • Stress [=] Pascals [=] Kg⋅m-1⋅s-
  • Types: tension, compression, shear
• Strain = Δl/l (^0)
  • Measures change in size/shape (i.e. deformation)
  • Dimensionless (i.e. it is a percent).
    • Some may say, e.g., 2.5 μstrains or 3.1 nstrains
  • Types: extension/dilatation, contraction, shear
• Most scientists agree that stress causes strain
  • Chicken and egg argument

Normal Stress / Normal Strain

• A normal stress acts

perpendicular to the

applied surface.

• A normal strain results

from a normal stress

• Deforms a square into

a rectangle

• Angles between sides

remain unchanged