Earthquakes Explained — Causes, Seismic Waves, Hazards, and Prediction | Chapter 10 from Earth: Portrait of a Planet

Why do earthquakes strike without warning, and how do scientists measure and predict these violent pulses of the Earth? In Chapter 10 of Earth: Portrait of a Planet by Stephen Marshak, we journey into the heart of seismic science—discovering how earthquakes occur, what hazards they cause, and how societies can prepare. For a full podcast summary, watch the YouTube chapter video or explore the detailed written breakdown below.

How and Why Do Earthquakes Occur?

Earthquakes are sudden releases of energy caused by the stick-slip motion along faults, when built-up stress finally overcomes friction. This process is known as the elastic rebound theory. The initial rupture point is called the hypocenter, while the location directly above it at the surface is the epicenter. Earthquakes can be preceded by foreshocks and followed by aftershocks.

Types of Faults and Seismic Waves

• Fault Types: Normal, reverse, thrust, and strike-slip faults accommodate different kinds of tectonic stress.
• Seismic Waves:
  • P-waves (Primary): Fastest, compressional, travel through solids and liquids.
  • S-waves (Secondary): Slower, shear, travel only through solids.
  • Surface Waves (L and R): Cause most ground shaking and damage.

Measuring Earthquakes: Magnitude and Intensity

Earthquakes are recorded by seismometers and displayed on seismograms. The Modified Mercalli Intensity (MMI) scale measures observed effects, while the Richter scale and moment magnitude scale quantify energy release. Modern seismology relies on the moment magnitude scale for global comparisons.

Where Do Earthquakes Happen?

Most earthquakes occur at tectonic plate boundaries—subduction zones, transform faults (like the San Andreas), rift valleys, and collision zones. Some also strike within plates (intraplate earthquakes), including induced events (like Oklahoma’s recent seismicity).

Earthquake Hazards and Damage

• Ground Shaking: Causes building collapse, landslides, and ground ruptures.
• Liquefaction: Saturated soils lose strength, causing buildings to sink or tilt.
• Landslides and Avalanches: Triggered by seismic shaking in unstable slopes.
• Tsunamis: Massive sea waves from submarine earthquakes (e.g., 2004 Sumatra, 2011 Tōhoku).
• Fires: Broken gas lines and damaged infrastructure often ignite post-quake fires.

Earthquake Prediction, Early Warning, and Risk Reduction

• Prediction: Long-term forecasts are based on seismic gaps and recurrence intervals, but short-term prediction remains elusive.
• Early Warning Systems: Networks can give a few seconds of advance notice for automatic shutdowns and alerts.
• Engineering and Zoning: Earthquake-resistant building codes, retrofitting, and hazard mapping are essential for minimizing loss of life and property.

Conclusion: Living with Earthquakes

Earthquakes are natural, unpredictable, and sometimes catastrophic. Understanding their causes, behaviors, and hazards is vital for building resilient communities and reducing risk worldwide.

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