Not a nuclear test because there's no initial transient.
This is more like a forced resonance - the geological equivalent of an organ pipe.
So I'd guess there's a pipe-like feature or a chamber in the area (an old magma tunnel? - it doesn't have to be empty, it just needs to have a constant density significantly different to its surroundings) and "noise" from moving magma made it to ring.
With a 17s period and 6km/s velocity, for lambda/4 resonance the pipe/chamber would be around 30km long - which looks not-completely-insanely-wrong, possibly.
This is more like a forced resonance - the geological equivalent of an organ pipe.
So I'd guess there's a pipe-like feature or a chamber in the area (an old magma tunnel? - it doesn't have to be empty, it just needs to have a constant density significantly different to its surroundings) and "noise" from moving magma made it to ring.
With a 17s period and 6km/s velocity, for lambda/4 resonance the pipe/chamber would be around 30km long - which looks not-completely-insanely-wrong, possibly.