Click image to open full size
Earthquake and Seismic Waves
Earthquake and Seismic Waves
Related Tools
Related Worksheets
Related Cheat Sheets
Earthquakes happen when stress builds up in rocks and is suddenly released along a fault. That release sends energy through Earth as seismic waves and can shake the ground strongly enough to damage buildings, roads, and other structures. Studying earthquakes helps scientists map plate boundaries, understand Earth's interior, and improve hazard planning. It also helps communities prepare for aftershocks, landslides, and tsunamis.
Most earthquakes begin at a point underground called the focus, or hypocenter, where rock rupture starts. The point directly above it on the surface is the epicenter, which is often used to describe the quake's location. Seismic waves travel outward from the focus in different ways: P waves compress material, S waves shear material, and surface waves move along Earth's exterior and often cause the most damage. Seismographs record these waves, and the time difference between their arrivals helps scientists locate the earthquake and estimate its size.
Key Facts
- Focus or hypocenter = underground point where rupture begins; epicenter = point on the surface directly above the focus.
- P waves are compressional waves and travel through solids, liquids, and gases.
- S waves are shear waves and travel only through solids.
- Surface waves travel along Earth's surface and usually produce the strongest ground motion near the epicenter.
- Wave speed relation: distance = speed x time
- Magnitude measures energy released, while intensity describes observed shaking and damage at a location.
Vocabulary
- Fault
- A fault is a fracture in Earth's crust where blocks of rock move relative to each other.
- Focus
- The focus is the point inside Earth where an earthquake starts.
- Epicenter
- The epicenter is the point on Earth's surface directly above the focus.
- Seismic wave
- A seismic wave is a vibration that carries energy away from an earthquake through Earth or along its surface.
- Seismograph
- A seismograph is an instrument that detects and records ground motion during an earthquake.
Common Mistakes to Avoid
- Confusing the focus with the epicenter, which is wrong because the focus is underground and the epicenter is the surface point above it.
- Thinking the largest shaking always happens exactly at the epicenter, which is wrong because local geology, building conditions, and wave type can make other areas shake more strongly.
- Assuming S waves can travel through liquids, which is wrong because shear waves require a solid material to pass through.
- Using magnitude and intensity as if they mean the same thing, which is wrong because magnitude describes total energy released while intensity describes effects at a specific place.
Practice Questions
- 1 A P wave travels at 6 km/s and reaches a station in 20 s. How far is the station from the earthquake focus if you assume a straight path?
- 2 At one seismic station, the P wave arrives 12 s before the S wave. If P waves travel at 6 km/s and S waves travel at 3.5 km/s, how far away is the earthquake source?
- 3 Explain why the absence of S waves in Earth's outer core is evidence that the outer core is liquid.