The phenomenon

Description. "Bradyseism" comes from the Greek bradýs (βραδύς), meaning "slow," and seismós (σεισμός), meaning "movement." It is a ground deformation involving slow lowering (subsidence) phases alternating with more rapid uplift phases. Although with specific characteristics, the phenomenon occurs in several volcanic calderas worldwide (e.g., Long Valley, USA; Rabaul, Papua New Guinea), including the Italian Phlegraean Fields. Rapid uplift can be followed by several earthquakes (earthquake swarms) that usually do not reach high magnitudes but are very shallow, easily perceived, and could sometimes damage buildings and infrastructure. 

Causes. The ground deformations that typify bradyseism, with which seismic activity is associated, are linked to the volcanic dynamic of the caldera.
Calderas are volcanic structures that are morphologically lowered, unlike the typical cone-building volcano. Specific studies of caldera activity suggest that periods of "unrest" of prolonged duration are often non-eruptive, meaning that bradyseismic crises often do not culminate in an eruption. However, sometimes, "unrest" phases can precede an eruption.
Concerning the causes of the uplift, there is still debate within the scientific community. Some researchers ascribe it to increased temperatures and pressure of fluids in the hydrothermal system, reacting to degassing events also caused by magma ascent; others believe, however, that the phenomenon is directly related, in whole or in part, to magma inputs at shallow magma chamber levels.

Effects. Slow ground deformation in the rapid uplift phase, combined with seismic activity, could compromise the functionality of some infrastructures (such as water and gas networks or harbor docks) but also cause minor damage to nonstructural elements of buildings, with cracks forming or plaster falling off and doors and windows losing their orthogonality, enough to affect their smooth opening/closing. On the other hand, seismic sequences of significant magnitude (M=4-4.5) and with high frequencies (several events per day) are likely to cause potentially more substantial damage.