Well, "quakes" and "plate quakes" are the exact same thing.
ALL earthquakes are caused by tectonic plate motion. Basically it involves the edges of the plates grinding together, eventually they will slip and that is when the quake occurs.
If you want an example of how this works, push your palms together tightly and then attempt to slide one away from the other. Eventually with enough force your hand will suddenly slip. This is what happens with the edges of the plates during an earthquake.
And yes, the tsunami in 2004 altered the rotation of the earth slightly. A tsunami is caused when the ocean floor is displaced (which is usually caused by an earthquake).
By the way, do we know anything about the geology of Cythera anyway? Is active plate motion happening? (There isn't a lot of evidence for it recently on Venus). If there is no plate drift then earthquakes are not going to happen. (On New Terra, there originally wasn't but the terraforming experiment caused the plates to become geologically active again).