If rock is stressed (extension, compression), then do seismic waves propagate there-through differently? Can seismic data indicate where rock is being compressed, vs. stretched? Inexpertly, differences in density (compression increases, tension decreases) might be observable

Elastic moduli increase much faster than density, so the effect is bigger than expected from density changes and in the opposite direction.

 

Though, I doubt the effect can be used, because:

- Compression or traction at a fault is much smaller tan the hydrostatic pressure

- Dislocations at a fault have more influence on sound propagation than density there

- You wont' make the difference between a compressed rock and a different chemical composition.

Rock in the elastic regime has a quasi-constant-valued "spring constant". If so, then extension or compression would not affect the elastic properties of the rock. Ergo, no change in pressure-wave propagation (except for density, more/less mass to move in compressed/stretched rock). However, pressure waves could possibly push rock beyond the elastic regime, into plastic / failure regimes. I.e. earthquakes could induce secondary, sympathetic, "copy-cat quakes". (Cp. earth's moon may be able to tidally induce quakes.) So, secondary sympathetic quakes would indicate regions where the rock had been under extreme loads. However, are those spring-constants the same, for both compression / extension? Or, is one spring constant different than the other? If so, the seismic data could possibly discriminate between compressed vs. stretched rock, based upon inferred elastic spring constant? (Perhaps Californians could artificially trigger quakes along the San Andreas, so as to "make small ones", rather than "waiting for the big one", i.e. for pressures to predictably inexorably build, as they are of course, this very moment.)