Seismic Waves

What is Seismic Waves?

Every earthquake sends three distinct types of waves radiating outward through Earth. Primary (P) waves are compressional: rock alternately squeezes and expands in the direction of travel, much like sound moving through air — they travel at roughly 6–8 km/s through the crust and can pass through both solid rock and liquid. Secondary (S) waves shear rock perpendicular to their path; they are slower (~3.5–4.5 km/s) and cannot travel through liquid at all. Surface waves (Love and Rayleigh) travel along Earth's outer shell; they are slowest but carry the most destructive energy — the rolling and sideways ground motion that topples buildings. The fact that S-waves disappear on the far side of Earth was the first evidence that the outer core is liquid. The simulation shows all three wave types leaving an epicenter, arriving at a seismograph, and how earthquake depth and magnitude affect propagation.

Parameters explained

Earthquake Depth(km)

Earthquake Depth sets how far below Earth's surface the rupture begins, from 5 km to 700 km. Shallow earthquakes place the source close to people and buildings, so P-waves, S-waves, and surface waves can arrive with stronger local shaking. Deeper earthquakes are usually linked to subduction zones, where cold slabs sink into the mantle. Their P-waves and S-waves still spread through Earth, but the extra distance to the surface often reduces damage near the epicenter. Keep in mind that depth works together with magnitude: the Magnitude (×10) slider uses the HTML control's direct 10× scale, so 72 means M7.2, not magnitude 72.

Magnitude (×10)

Magnitude (×10) controls earthquake size using the same scale stored in the HTML slider: 40 displays as M4.0, 72 displays as M7.2, and 95 displays as M9.5. This 10× scaling avoids decimal slider values while still representing familiar earthquake magnitudes. Larger magnitudes release much more energy, producing stronger P-waves and S-waves and often more damaging surface waves. In real seismology, magnitude is logarithmic, so each whole-number increase represents far greater wave amplitude and energy release. Compare the same magnitude at different depths to separate source size from source location, then watch how the seismograph records the P-wave and S-wave arrivals.

Common misconceptions

• MisconceptionAll seismic waves are the same — they just travel at different speeds.

  CorrectP-waves, S-waves, and surface waves differ fundamentally in how they deform rock. P-waves compress and expand rock in the direction of travel; S-waves shear rock side-to-side perpendicular to travel; surface waves combine motions in complex patterns confined to the near-surface layer. These different deformation modes explain why only P-waves travel through liquids and why surface waves are most destructive despite being slowest.

• MisconceptionThe most dangerous seismic waves are the P-waves because they arrive first.

  CorrectArriving first does not mean most destructive. P-waves cause relatively small ground motion. Surface waves arrive last but have the largest amplitudes and longest duration at the surface where structures stand. Most building damage in major earthquakes is caused by surface waves, particularly Love waves (horizontal shearing) and Rayleigh waves (elliptical rolling motion).

• MisconceptionSeismic waves slow down as they travel farther from the earthquake because they lose energy.

  CorrectWave speed depends on the elastic properties and density of the rock the wave is passing through, not on distance traveled. Waves do lose amplitude with distance (geometric spreading and absorption), but their propagation speed is a property of the medium. In fact, seismic waves often speed up as they travel deeper where rock is denser and more rigid.

• MisconceptionEarthquakes happen only at plate boundaries.

  CorrectMost large earthquakes occur at plate boundaries, but intraplate earthquakes happen in the interior of plates along ancient fault zones. The 1811–1812 New Madrid earthquakes in the central United States were magnitude ~7.5 events far from any active boundary. The simulation's epicenter represents any location where rock fractures suddenly under stress, not exclusively a plate boundary.

How teachers use this lab

1. Natural-hazards comparison: have students run the Shallow Crustal Quake, Deep-Focus Quake, and Megathrust Earthquake presets, then record the Earthquake Depth and Magnitude (×10) values for each. Students explain which scenario would likely create the highest local hazard and cite P-wave, S-wave, and surface-wave evidence (HS-ESS2-1).
2. Depth-only investigation: keep Magnitude (×10) fixed at 72, then move Earthquake Depth from 10 km to 700 km. Students compare arrival patterns and shaking at shallow versus deep sources, connecting earthquake distribution to plate interactions and subduction zones (HS-ESS2-3).
3. Magnitude scaling mini-lesson: keep Earthquake Depth fixed, sweep Magnitude (×10) from 40 to 95, and require students to translate slider values into displayed magnitudes such as 40 = M4.0 and 95 = M9.5. This supports hazard reasoning because students must distinguish the slider's 10× data format from real earthquake magnitude labels.
4. Preset-based claim-evidence-reasoning: assign groups one of the three presets and ask them to defend whether it best represents a crustal fault, a deep subduction-zone event, or a plate-interface megathrust. Students must cite both sliders and the P-wave/S-wave sequence as evidence.
5. Risk-mitigation discussion: after comparing all three presets, ask students what monitoring, building design, or warning-system choices would matter most for each hazard. The activity links model observations from the Earthquake Depth and Magnitude (×10) sliders to reducing impacts from natural hazards.