Geos 218: Earthquakes & Faults

• Geos 218 is a course covering Geological Disasters & Society.
• Unit 4b focuses on Earthquakes.
• Topics include Seismology, P-waves, S-waves, Surface waves, Locating Earthquakes and Magnitude vs Intensity.

Deforming Earth's Crust

• There are 3 types of stress: extensional, compressional, and shear.
• This results in 3 kinds of faults: normal, thrust/reverse, and strike-slip.
• Earthquakes occur on faults.
• Undeformed strata undergo horizontal compressional stress, causing them to shorten horizontally and thicken vertically.
• Horizontal tensional stress causes rock bodies to lengthen horizontally and thin vertically.
• Shear stress causes displacements along fault zones or through ductile flow.
• Faults are complex zones of breakage with irregular surfaces, often miles wide and long.
• Stress accumulates until rupture at a weak point, propagating along the fault surface.
• The point where rupture first occurs is the hypocenter, also known as the focus.
• The epicenter is the point directly above the hypocenter on the surface.

Types of Faults and Movement

• Normal faults result from extensional stress.
• Thrust or reverse faults result from compressional stress.
• Strike-slip faults result from shear stress.
• Strike-slip faults are dominated by horizontal movement.
• Looking across a right-lateral fault, the far side appears to have moved to the right.
• Looking across a left-lateral fault, the far side appears to have moved to the left.

Development of Seismology

• Seismology is the study of earthquakes.
• The earliest known earthquake detection device was invented in China in 132 B.C.
• Seismometers detect earthquake waves.
• Seismographs record earthquake waves.
• Earth movement is captured in three components: north-south, east-west, and vertical.
• A heavy mass stays stationary due to inertia while the Earth vibrates around it.
• Differences between the frame's position and the mass are recorded digitally.
• Amplitude is displacement.
• Wavelength is the distance between successive waves.
• Period is the time between waves (equals 1 divided by frequency).
• Frequency is the number of waves in one second.

Seismic Waves

• Waves include Surface waves and Body waves
• Body waves travel inside the Earth.
• Surface waves travel along the Earth's surface.
• Wave velocity and direction change as they encounter different layers of different density.

Types of Seismic Waves

• P waves are fast.
• S waves are intermediate in speed.
• Surface Waves are slow.
• A single seismic station records the arrival of compressive P-waves, shearing S-waves, and rolling surface waves from distant earthquakes.

Seismographs

• A seismograph records ground motion produced by an earthquake.
• Seismometers measure vibrations in 3 directions: N/S, E/W, and Up/Down.
• Waves from large earthquakes can pass through the entire Earth.
• Waves do not follow straight paths and change in velocity and direction.

Locating Earthquakes

• P waves travel about 1.7 times faster than S waves.
• The farther from the hypocenter, the greater the time lag of the S wave behind the P wave (S-P interval).
• The S-P interval depends on the distance to the earthquake.
• Difference in the arrival times of P & S waves increases as the distance increases
• (S-P) time indicates how far away earthquake was from station but in what direction?
• The distance of the earthquake is determined by the S-P time from three seismic stations.
• The intersection of circles from each station determines the epicenter location.

On-Line Lecture Topics

• Earthquake basics
• Faults (3 kinds)
• Seismic Waves
• Locating Earthquakes
• Magnitude vs Intensity

This lesson covers earthquakes, different types of stress (extensional, compressional, and shear), and the resulting faults (normal, thrust/reverse, and strike-slip). It discusses how stress accumulation leads to rupture at a hypocenter, with the epicenter being the point on the surface above it.