Powerful enough earthquakes generate gravity perturbations that propagate at $c$ and can be detected by sensitive enough seismographers. See, e.g., this news in Nature:
In the latest paper, Vallée and his colleagues report many more observations of gravity signals immediately after the Tohoku quake. The signal was most apparent at monitoring stations between about 1,000 and 2,000 kilometres from the quake’s epicentre. At that distance, the fast-as-light signals had enough time to arrive and be clearly recorded before the seismic waves swamped them.
There isn't any fundamental difference between signals/pulses and waves (which can be described as a sequence of pulses). So, if spacetime can be pictured as a sort of a medium and gravitational waves as perturbations that propagate through it, these seismographic measurements would appear to constitute observations of gravitational waves.
But almost no one is referring to them as such, so this understanding should be mistaken. Why? Is the description above taking the mechanical wave analogy too far? Is this gravitational pulse an example of a change in the "quasistatic gravitational field" (see, e.g., this question)? If so, how exactly?