Quakes are caused either by the movement of parts of a planet's crust, or by pressure from molten rock and gases from its core. Earth's crust, for example, consists of several tectonic plates in motion relative to each other. When one plate shifts or is pressed beneath another in a process called subduction, quakes occur. Volcanic activity, on Earth or on other worlds, also causes quakes, as hot liquid from the core, propelled by gases, pushes upward through fissures on the crust.
Although the Earth is the most seismically active planet in the Solar System, observation and data from satellites and deep-space probes reveal evidence of seismic activity on other planets too. The "terrestrial" planets Mercury, Venus and Mars --- those having Earth-like compositions with hard exterior crusts --- show evidence of massive volcanic activity. Among the outer planets, larger moons such as Jupiter's Io, Ganymede and Euridyce, Saturn's Enceladus and Neptune's Titan show evidence of seismic activity in the form of cryovolcanoes, which spew icy matter rather than hot lava.
The third-largest planet in the Solar System, Uranus consists of a dense inner core surrounded by an extended liquid mantle of water, ammonia and methane, which accounts for 80 percent of the planet's mass. Above this "ammonia ocean," Uranus' outer layer is made up of icy hydrogen and helium gases. Uranus' solid core appears to be relatively small and dense, although its composition is unknown, and the planet has no solid surface crust.
Because Uranus lacks a crust layer subject to pressure and movement from shifting plates or upwellings of hot liquid from deep within, this gas giant does not experience quakes. Like its neighbors Jupiter and Saturn, this gas giant possesses no known structural features which create conditions for seismic activity.