Earthquakes and Earth’s Interior

Definition: An earthquake is the sudden release of energy in the lithosphere experiencing deformation stress due to the motion of Earth's tectonic plates.
Focus: The point underground where an earthquake starts and where energy is released.
Epicenter: The point on Earth's surface directly above the focus, where ground shaking is felt first.
Elastic Rebound Hypothesis: Crust deforms under stress until it breaks, snapping back toward its original shape (but not its original position) and releasing energy as seismic waves.
Foreshocks and Aftershocks:
- Foreshocks: Smaller earthquakes that occur before the main earthquake.
- Aftershocks: Smaller earthquakes that occur after the main earthquake as the crust adjusts.
Faults and Stress Types:
- Fault: A break or fracture in Earth's crust along which movement takes place (e.g., the San Andreas Fault).
- Normal Faults: Caused by tension stress (pulling apart) at divergent boundaries.
- Reverse/Thrust Faults: Caused by compression stress (pushing together) at convergent boundaries.
- Strike-Slip Faults: Caused by shear stress (sliding past each other) at transform boundaries.

Seismic Waves: Waves of energy that travel through Earth.
Body Waves: Travel through Earth's interior.
- Primary Waves (P-waves): Push-and-pull waves where vibration is in the same direction as wave travel. They move fastest and travel through both solids and liquids.
- Secondary Waves (S-waves): Waves with vibrational movement perpendicular to the direction of wave travel. They move slower than P-waves and can travel only through solids.
Surface Waves: Move along Earth's surface and cause the most damage to structures.

Seismograph: An instrument used to measure the frequency and magnitude of seismic waves.
Seismogram: The visual record of ground motion produced by a seismograph.
Measurement Scales:
- Richter Scale: Measures ground motion height on a seismogram using a logarithmic scale, where each whole number step represents a $10\times$ increase in shaking.
- Moment Magnitude Scale: Measures the total energy released by an earthquake; it is the scale preferred by scientists.
- Modified Mercalli Intensity Scale: Rates earthquake damage to human structures; used by emergency responders and officials.

Travel-Time Graph: A graph used to determine the distance from a seismic station to the epicenter based on the arrival times of P-waves and S-waves.
Triangulation: Finding the exact location of an epicenter using distance data from at least three different seismic stations.

Primary Hazards:
- Seismic Shaking: Ground shaking that causes buildings to collapse, power lines to break, water and gas pipes to burst, and roads to buckle.
- Liquefaction: Occurs when earthquake vibrations cause water in saturated soil to move upward, turning solid ground into a liquid-like slurry.
- Landslides and Mudflows: Seismic shaking destabilizes steep slopes.
- Tsunamis: Large, fast-moving ocean waves created by seafloor displacement from underwater earthquakes, volcanic eruptions, or landslides.
Earthquake Safety and Engineering:
- Building Design: Constructing flexible structures using steel frames, cross-bracing, and base isolators to absorb seismic shock.
- Safety Measures: Practicing "Drop, Cover, and Hold On" during shaking.

Direct sampling is limited because humans cannot drill through the crust.
Most of our understanding of Earth's interior comes from analyzing seismic waves.
Refraction and Speed Changes: Seismic waves change speed and direction when passing through rock layers of different densities. A geologist named Mohorovičić first discovered this boundary between the crust and mantle.
Shadow Zones:
- P-wave Shadow Zone: Caused by P-waves bending (refracting) as they pass through the liquid outer core.
- S-wave Shadow Zone: Caused by S-waves being blocked completely because they cannot travel through liquids, proving the outer core is liquid.

Crust: The thin, rocky outer layer composed of silicate minerals.
- Oceanic Crust: Composed mostly of basalt with a density of $3.0\,g\,cm^{-3}$ .
- Continental Crust: Composed mostly of granite with a density of $2.7\,g\,cm^{-3}$ ; thicker and older than oceanic crust.
Mantle: The middle layer composed of peridotite with a density of $3.4\,g\,cm^{-3}$ .
Core: The innermost layer composed mostly of iron and nickel with a density of $13\,g\,cm^{-3}$ .

Lithosphere: The cool, rigid, thin outer layer that includes the crust and uppermost mantle.
Asthenosphere: A soft, weak layer in the upper mantle where rising temperatures partially melt rock, allowing it to deform and flow slowly under pressure.
Lower Mantle: A rigid rock layer where high pressure prevents melting despite extreme temperatures.
Outer Core: A liquid layer of molten iron and nickel; movement here creates convection currents that generate Earth's magnetic field.
Inner Core: A solid sphere of iron and nickel compressed into a solid phase by extreme pressure.