Overview
This lecture covers the fundamentals of earthquakes, the types and causes of faults, seismic waves, how earthquakes are measured, and the factors affecting earthquake damage.
Earthquake Basics
- An earthquake is ground shaking from the rapid release of energy, usually along faults.
- Earthquakes produce seismic waves that radiate outward in all directions from the focus (hypocenter).
- Most earthquakes are caused by tectonic plate movement; over a million detectable earthquakes occur each year, but most are too small to feel.
Causes and Locations of Earthquakes
- Main cause: sudden slip along a fault (a planar break in rocks).
- Other causes: volcanic activity, landslides, mineral structure changes, meteorite impacts, and man-made explosions.
- Earthquakes are most common near plate boundaries but can occur within plates.
Key Earthquake Terms
- Focus (Hypocenter): The starting point of an earthquake underground.
- Epicenter: The point on the Earth's surface directly above the focus.
- Fault scarp: The exposed surface of a fault after movement.
Fault Types
- Normal Fault: Hanging wall moves down relative to the footwall; caused by tension at divergent boundaries.
- Reverse Fault: Hanging wall moves up relative to the footwall; caused by compression at convergent boundaries.
- Thrust Fault: Low-angle reverse fault, often large-scale.
- Strike-Slip Fault: Blocks move horizontally past each other; caused by shear at transform boundaries.
- Active vs. Inactive Fault: Active faults have ongoing movement; inactive faults do not.
Fault-Related Structures
- Joint: A fracture in rock with no movement.
- Displacement (Offset): The amount rocks move during faulting, measurable and cumulative.
Earthquake Mechanics
- Rocks deform elastically until stress exceeds the yield point and the rock breaks, releasing energy.
- Elastic rebound model: rocks snap back to original shape after breaking, causing earthquakes.
- Aftershocks and foreshocks are smaller quakes before or after the main event.
- Earthquake prediction uses probability models and geophones, but exact prediction is not possible.
Seismic Waves
- Body Waves: Travel through Earth's interior.
- P-waves (Primary): Fastest, compression/extension, travel through solids, liquids, gases.
- S-waves (Secondary): Slower, up/down motion, only travel through solids.
- Surface Waves: Travel along Earth's surface.
- Love Waves (L-waves): Side-to-side motion, highly destructive.
- Rayleigh Waves (R-waves): Rolling, up/down/forward motion, most destructive.
Measuring Earthquakes
- Seismographs detect and record seismic waves.
- Triangulation using P- and S-wave arrival times locates epicenter.
- Intensity (Modified Mercalli Scale): Measures observed effects and damage (subjective).
- Magnitude:
- Richter Scale: Measures amplitude of the largest wave (logarithmic).
- Moment Magnitude Scale: Measures total energy released, most accurate for large quakes.
Earthquake Damage Factors
- Damage depends on quake magnitude, distance from epicenter, local geology, duration, and construction practices.
- Softer sediments amplify shaking; solid bedrock transmits waves farther.
- Engineering techniques (e.g., building on springs, flexible materials) reduce damage.
Plate Boundaries and Earthquake Distribution
- Most earthquakes occur at plate boundaries: convergent (biggest), divergent, and transform.
- Deep earthquakes happen in subduction zones (Wadati-Benioff zones); shallow at ridges and transform faults.
Key Terms & Definitions
- Seismic Wave — energy wave from an earthquake.
- Fault — planar fracture along which movement occurs.
- Focus/Hypocenter — underground origin of the earthquake.
- Epicenter — surface point above the focus.
- Normal fault — fault with hanging wall moving down.
- Reverse fault — fault with hanging wall moving up.
- Strike-slip fault — horizontal movement along fault.
- Elastic rebound — rocks return to shape after breaking.
- Aftershock — a smaller earthquake following the main shock.
- Modified Mercalli Intensity Scale — subjective scale of earthquake effects.
- Richter Scale — logarithmic measure of quake magnitude.
- Moment Magnitude Scale — scale of total energy released.
Action Items / Next Steps
- Watch assigned lecture and check the related lab video on Canvas.
- Complete homework assignment and submit lab for review.
- Post on the discussion board for attendance/participation points.
- Prepare for next week’s topics: earthquake hazards and topographic maps.
- Review all notes and consider making a study guide for extra credit.
- Study for the upcoming test covering all material so far.
- Email the instructor with any questions, especially about the lab.