Presets
Playback
Earth model and source
Speed, site and view
Camera
Layers
Quality
Model check
The science
What is simulated
The page integrates the scalar wave equation ∂2u/∂t2 = β(x,z)2 ∇2u on a grid with a second-order leapfrog scheme. In a medium of constant density this is exactly the equation of SH waves (shear waves polarised horizontally), with β the shear-wave speed. The earth model is a crust (β1) over a faster mantle (β2) separated by the Mohorovicic discontinuity at depth H, plus an optional soft sediment basin. The source is a point force with a Ricker wavelet of dominant frequency f0. The top surface is free (waves reflect with the same sign, doubling the surface amplitude), the other faces absorb with a Cerjan sponge.
Ray theory the graphs compare against
Direct wave t = √(X2 + zs2) / β1. Moho reflection from the image source t = √(X2 + (2H - zs)2) / β1. Head wave (refracted along the top of the mantle) t = X/β2 + (2H - zs) cosθc / β1 with the critical angle sinθc = β1/β2 (Snell's law). Beyond the crossover distance the head wave arrives first, which is how Mohorovicic discovered the boundary in 1909.
Earthquake size
The magnitude slider sets the amplitude and the readouts: radiated seismic energy log10 E = 1.5 Mw + 4.8 (Gutenberg-Richter, in joules), seismic moment log10 M0 = 1.5 Mw + 9.1 (N m) and the typical rupture length log10 L = -2.44 + 0.59 Mw km (Wells and Coppersmith 1994). One magnitude unit is a factor 31.6 in energy. These numbers are empirical averages, and the figures here are about the physics of wave propagation, nothing about damage to people.
Honest simplifications
Real earthquakes radiate P and S waves that convert into each other at interfaces and into surface waves; only SH is modelled here, which still shows travel times, reflection, refraction, head waves, free-surface doubling and basin amplification. The source is a point, not a rupturing fault. The earth is flat, layers are homogeneous, there is no attenuation except in the sponge, and the grid (0.25 km in 2D) disperses the shortest wavelengths slightly. Basin amplification shows as slower, larger, longer-lasting shaking at the soft-soil station, qualitatively like real site response.