Seismic Wave Propagation Simulator

The fundamental behavior governing how seismic waves change direction at a boundary between two layers with different wave speeds is described by Snell's Law.

Here, $i$ is the angle of incidence, $r$ is the angle of refraction, and $v_1$ and $v_2$ are the seismic wave velocities in the first and second layer, respectively. If $v_2 \gt v_1$, the wave bends away from the normal line.

The existence of the shadow zone is a direct consequence of two physical principles combined with Earth's layered structure.

S-waves require shear strength to propagate, which a fluid lacks, so they are absorbed at the core-mantle boundary. The P-wave is refracted so strongly at the core boundary that it emerges on the other side at a specific minimum angle, leaving a gap (the shadow zone) between 103° and 143° from the epicenter where no direct P-waves are detected.

Earthquake Early Warning Systems: The predictable difference in speed between P and S-waves is exploited for early warnings. Sensitive detectors can feel the fast, less-damaging P-wave and trigger alarms seconds before the destructive S-wave arrives, allowing trains to brake and gas valves to shut.

Oil & Gas Exploration (Seismic Reflection Surveying): The same principles are used on a smaller scale. Trucks send artificial seismic waves into the ground, and the reflected signals are analyzed to map subsurface rock layers and identify potential hydrocarbon reservoirs, essentially creating an ultrasound of the Earth's crust.

Nuclear Test Ban Treaty Verification: A global network of seismic stations monitors for the unique seismic signatures of underground nuclear explosions, which differ from natural earthquakes. Understanding wave propagation is crucial to pinpoint the location and nature of the event.

Planetary Science: By placing seismometers on other celestial bodies (like the InSight lander on Mars), scientists can use the recorded seismic waves to infer the internal structure, composition, and layer thicknesses of these planets, just as we did with Earth.

There are a few key points you should be aware of when starting to use this simulator. First, understand the premise that wave velocity is fixed per layer. In the actual Earth, pressure, temperature, and density change with depth even within the same mantle, causing velocity to increase continuously. The simulator simplifies this into "layers," which makes the abrupt refraction at boundaries appear more pronounced. Real data often shows smoother ray paths.

Next, don't confuse "epicentral distance" with "distance to the observation point". The simulator's "epicentral distance" is the angular distance from the epicenter to a point on the Earth's surface (e.g., 103 degrees). However, with a deep earthquake source, the path the wave takes before reaching the surface changes. For example, for an earthquake at 600 km depth, the shadow zone's range differs from the simulator's (which assumes a surface source). Always keep the source depth in mind.

Finally, avoid overinterpreting amplitude (magnitude) changes. While you can change the amplitude with the slider, this is a simplified representation. The amplitude of real seismic waves varies greatly not only with travel distance but also due to site amplification effects, scattering, and attenuation. Even if you increase the S-wave amplitude in the simulator, the fundamental principle that it cannot propagate through the liquid outer core remains unchanged. The key to learning is to focus on this qualitative difference of "whether it propagates or not."