I'm taking a geophysics class and the math makes sense but the context is lost on me. My understanding is that the primary use of seismic ray-tracing is to locate disturbances that cause waves to propagate radially. I also understand that 35km is the depth at which the earth's spherical shape starts to matter, and is also the depth of the Moho. What I don't understand is what kind of events would create observable waves that travel below the Moho. I feel like any human-related blasts would originate well inside the crust (what's the typical range of depth for say mining related blasts? I'm curious). Could you see a reflection off the CMB (or is there another major boundary between the Moho and CMB? I'm bad at this) from a disturbance in the crust? Are there disturbances that originate below the crust? Sorry, I have no sense of what 'normally happens' for this stuff whatsoever.