Fault Types and Tectonic Forces

Fault Types and Motion

• Normal faults feature the hanging wall moving downward relative to the footwall
• This is due to tensional forces
• Normal faults are common at divergent boundaries, like the East African Rift Valley
• Reverse faults feature the hanging wall moving upward relative to the footwall
• This is due to compressional forces
• Reverse faults are common at convergent boundaries
• Examples include the Himalayas and the Rocky Mountains
• Thrust faults are a type of reverse fault with a low-angle slope (less than 30 degrees)
• They result from strong compression
• They thicken the crust and form mountain belts, such as the Alps, Andes, and Himalayas
• Strike-Slip/Transform faults involve rocks sliding past each other horizontally due to shear forces
• There is little to no vertical movement
• These faults occur at transform plate boundaries, such as the San Andreas Fault in California and the North Anatolian Fault in Turkey

Tectonic Forces

• Compressional forces push rocks together, causing them to shorten and thicken
• This leads to folding and reverse faults
• Compressional forces form mountains at convergent plate boundaries, like the Himalayas
• Tensional/Extensional forces pull rocks apart, causing them to stretch and become thinner
• This results in normal faults and rift valleys, like the East African Rift
• Tensional forces occur at divergent plate boundaries, such as the Mid-Atlantic Ridge
• Shear forces cause rocks to slide past each other in opposite directions
• This leads to strike-slip faults, such as the San Andreas Fault in California
• Shear forces are the most common at transform plate boundaries

Earthquake Epicenter

• It takes 1 minute between the P and S wave

Earthquake Damage Factors

• The amount of structural damage from earthquake vibrations depends on factors like:
• The type of building
• The earthquake's duration

Earthquake Measurement Scales

• The Modified Mercalli Scale measures earthquake intensity based on observed effects
• The Richter Magnitude Scale measures the energy released by an earthquake

Earthquake Shadow Zone

• An earthquake shadow zone is where seismic waves are not detected on Earth's surface
• Seismic waves interact with the Earth's internal layers
• The P-wave shadow zone is between 103° and 142° from the epicenter
• The S-wave shadow zone begins at 103° and extends around the globe

Tsunami Formation

• Tsunamis typically form on the ocean floor at convergent boundaries involving an earthquake at a subduction zone
• A megathrust earthquake causes the overriding plate to rise suddenly
• This displaces a large amount of water and initiates the tsunami

Tsunami Wave Propagation

• Disturbance creates waves traveling outward at 500-800 km/h
• In deep water, waves have long wavelengths (100-200 km) and are a few meters high

Tsunami Approaching Shore

• As the tsunami reaches shallow water, the seafloor becomes shallower
• This causes the waves to slow down due to friction
• Wave energy compresses
• The wave height increases dramatically through wave shoaling
• Waves can grow 10-30+ meters tall depending on the coastline

Tsunami Impact

• The first sign is often a rapid withdrawal of water (drawback)
• The crest of the wave follows, causing immense flooding
• Tsunamis feature powerful surges that can last for minutes or hours
• Flooding destroys infrastructure and landscapes
• Water recedes, dragging debris and people out to sea
• Multiple waves and aftershocks may occur

Faults and Tsunamis

• Reverse (Thrust) faults trigger most tsunamis
• Megathrust earthquakes at subduction zones cause the overriding plate to shift upward, displacing water
• Normal faults create smaller tsunamis due to less water displacement
• Strike-Slip faults do not generally cause tsunamis
• Strike-Slip faults can trigger underwater landslides, which in turn generate local tsunamis

Tsunami Speed

• Tsunami speed depends on its water dpeth
• In deep ocean (5500 meters): 835 km/hr
• At 900 meters depth: 340 km/hr
• At 20 meters depth: 50 km/hr

Earthquake Definitions

• Earthquake: A sudden shaking from released energy in rocks, usually along faults
• Focus: The point inside the Earth where an earthquake originates (hypocenter)
• Epicenter: The point on the Earth's surface directly above the focus
• Fault: A fracture in the Earth's crust where movement occurs
• H.F. Reid: Geologist who developed the elastic rebound theory
• Elastic Rebound: Rocks deform elastically until they break
• Foreshock: A smaller earthquake before the main one
• Aftershock: A smaller earthquake after the main one
• Elastic Energy: Stored energy in rocks released during an earthquake
• Frictional Force: Resistance between surfaces preventing movement
• San Andreas Fault: A major strike-slip fault in California
• Fault Creep: Slow, gradual movement along a a fault
• Seismology: The scientific study of earthquakes and seismic waves
• Surface Waves: Seismic waves traveling along the Earth's surface
• Body Waves: Seismic waves traveling through the Earth's interior (P-waves and S-waves)
• Primary (P) Waves: Fast seismic waves traveling through solids, liquids, and gases
• Secondary (S) Waves: Slower seismic waves traveling through solids only
• Modified Mercalli Intensity Scale: Measures earthquake intensity based on observed effects (I to XII)
• Tsunami: A large sea wave generated by earthquakes, eruptions, or landslides
• Magnitude (Richter Scale): Measures energy released by an earthquake
• Wave Amplitude: Wave height determining seismic or tsunami waver strength
• Moment Magnitude Scale (Mw): Measures earthquake magnitude
• Shadow Zone: Area on Earth's surface without seismic waves