This is a real 2D finite-difference time-domain (FDTD) solver for the damped scalar wave equation on a cross-section of regolith:
∂²u/∂t² + γ·∂u/∂t = c(x,y)²·∇²u
γ = 2π·f / Q (intrinsic attenuation rate)
Two independent, physically distinct mechanisms combine to build the coda (the long "ringing" tail after the direct wave), exactly as in real seismic coda-wave theory (Aki & Chouet):
- Scattering — the wave speed field c(x,y) is not uniform: it carries randomly placed fractures and impedance patches. Every one of them partially reflects and mode-converts the wavefront, so energy keeps bouncing around the slab long after the source pulse has passed. Raise the fracture / scattering strength slider and the medium gets rougher.
- Intrinsic attenuation (Q) — even scattered energy eventually turns into heat, at a rate set by the quality factor Q. Wet, jointed Earth-like rock has Q ≈ 50–300 and drains scattered energy fast; the Moon's bone-dry, impact-fractured megaregolith reaches Q ≈ 3000–5000 and barely loses any, so the same scattered energy keeps ringing for tens of simulated minutes.
The absorbing sponge layer around the domain edge removes energy that reaches the boundary, so what you see ringing in the seismogram is genuinely the interior scattering + Q at work, not an artifact of the simulation box.
- Shallow moonquake — tectonic slip near the surface; sharp, higher-frequency source.
- Deep moonquake — tidal flexing deep in the slab; weaker, lower-frequency source.
- Meteorite impact — largest initial energy, shortest pulse, richest in high frequencies.
- Seismometer marker — a fixed receiver whose live trace is drawn on the strip chart below, exactly like an Apollo lunar seismometer station.
Real-world relevance: this scattering + low-attenuation combination is precisely why Apollo seismograms "rang like a bell" for hours after an event, why lunar-base engineers plan for long-duration low-amplitude shaking rather than one sharp jolt, and why deep-listening missions like the proposed Farside Seismic Suite need months, not minutes, to map the interior.