What is an Earthquake?
An earthquake is a sudden trembling of the ground caused by the release of energy stored in Earth's crust.
Seismologists estimate that over one million earthquakes occur each year—about one every second!
Causes of Earthquakes
- Faulting: The primary cause of earthquakes is the sudden movement of rock along planes of weakness, called faults, in Earth's crust.
- Volcanic Activity: Some earthquakes are triggered by volcanic eruptions.
- Human Activities: Activities like nuclear testing, geothermal energy extraction, and hydraulic fracturing (fracking) can induce small earthquakes.
The elastic rebound theory explains how faulting causes earthquakes. Under stress, rocks bend elastically until they reach their elastic limit. When this limit is exceeded, the rocks snap and rebound, releasing energy as seismic waves.
Key Terms
- Focus: The point within Earth where the rock breaks and energy is released.
- Epicenter: The point on Earth's surface directly above the focus.
Think of a wooden ruler bending under pressure. It can bend only so far before snapping back to its original shape. This snapping action is similar to how rocks behave during an earthquake.
Earthquake Waves
Wave Behavior
- Wave Motion: Waves transport energy through a medium by causing deformations.
- Reflection and Refraction: Waves can reflect off boundaries or refract (bend) when crossing materials of different densities.
When a wave travels from a less dense to a more dense medium, part of the wave is transmitted, and part is reflected. This behavior helps scientists infer Earth's layered structure.
Types of Waves
- Transverse Waves: Particles move perpendicular to the wave's direction (e.g., waves on a rope).
- Longitudinal Waves: Particles move parallel to the wave's direction (e.g., compressions in a spring).
Don't confuse P-waves and S-waves. P-waves are compressional and travel through all states of matter, while S-waves are shear waves and cannot travel through liquids or gases.
Analyzing Seismograms
Determining Distance to the Epicenter
- Record Arrival Times: Note when P-waves and S-waves arrive.
- Calculate Time Difference: Subtract the P-wave arrival time from the S-wave arrival time.
- Use a Travel Time Graph: Match the time difference to the distance on a graph.
Always use data from at least three seismograph stations to pinpoint the epicenter. Two stations will narrow it down to two possible points, but a third station is needed to confirm the exact location.
Measuring Earthquakes
Richter Magnitude Scale
- Measures: Energy released by an earthquake.
- Scale: Logarithmic; each increase of 1 unit represents a tenfold increase in wave amplitude and a hundredfold increase in energy.
A magnitude-6 earthquake releases 100 times more energy than a magnitude-4 earthquake.
Modified Mercalli Intensity Scale
- Measures: Observed effects and damage.
- Scale: Ranges from I (not felt) to XII (total destruction).
A magnitude-5.5 earthquake may cause different damage levels depending on the local geology. Bedrock absorbs energy better than loose sediments, which amplify shaking.
Effects of Earthquakes
- Ground Shaking: Stronger in loose, water-saturated soils than in solid bedrock.
- Surface Faulting: Movement along faults can displace surface features.
- Tsunamis: Large sea waves caused by undersea earthquakes or landslides.
- Liquefaction: Water-saturated soils lose strength and behave like a liquid during shaking.
Imagine tapping a bowl of Jello. The solid bowl vibrates slightly, but the Jello inside jiggles dramatically. Similarly, loose soils amplify earthquake shaking compared to solid bedrock.
Earthquake Zones
- Earthquakes occur in narrow zones, such as:
- Mid-ocean ridges
- The Ring of Fire (around the Pacific Ocean)
- The Mediterranean Belt
These zones correspond to tectonic plate boundaries, where most seismic activity occurs.
The Inferred Structure of Earth's Interior
Seismic Waves and Earth's Layers
- Reflection and Refraction: Seismic waves reflect and refract at boundaries between layers of different densities.
- Curved Paths: Waves bend due to increasing density with depth.
The inner core is solid, while the outer core is liquid. This explains why S-waves cannot travel through the outer core.
Composition of Earth's Core
- Iron-Nickel Alloy: Inferred from seismic data and the composition of iron-nickel meteorites.
- Magnetic Field: Convection currents in the liquid outer core generate Earth's magnetic field.
