Faults: Key geological phenomenon associated with earthquakes

Definition: Fracture in rock along which movement has taken place

Range in scale from fracture in hand-specimen to plate boundary size

Joints: Fracture in rock along which no movement has taken place

(e.g., cooling fractures in lavas, fractures due to tectonic stresses)

Active vs. inactive faults

Practical definition: <10,000 years (since last glaciation) = 'seismogenic'

Fault creep: continuous movement along faults, not associated with major eqs

Controls:

• Rock composition
• Rigidity
• Friction

Classification based on (1) orientation of fault, (2) sense of movement

Definitions of Fault Geometry

Fault strike

Fault dip

Hanging Wall

Footwall

Types of fault movement

Strike-Slip

§ Right lateral (dextral)

§ Left lateral (sinistral)

Dip-Slip

§ Normal (hanging wall moves down)

§ Reverse (hanging wall moves up)

§ Thrust (= reverse fault, but low dip-angle)

Oblique-slip

Indicators of fault movement

• Displacement or offset of marker horizons.
• Fault scarp
• Slickensides
• Drag Folds
• Precise land surveying techniques
• Offset of 'cultural' features (roads, fences, etc.)
• Earthquake focal mechanisms

Initiation of rupture: fracture along fault plane

Hyocenter or Focus: Point of initial fracture

Epicenter: Point on Earth surface directly above hypocenter

Rupture may reach ground surface à Fault scarp