A magnitude 4.7 (Md) earthquake struck the Campi Flegrei (Phlegraean Fields) volcanic caldera west of Naples, Italy, on the evening of July 31, 2026 (19:46 local time / 17:46 UTC).

It is the strongest instrumentally recorded event in the area during the current bradyseismic crisis (ongoing since around 2005) and the largest in about 40 years, exceeding prior peaks of Md 4.0 in the 1982–1984 crisis and recent Md 4.6 events in 2025.

Epicenter between Pozzuoli and Quarto (roughly 5 km from Pozzuoli and 9 km from Naples center), at a shallow depth of about 2.6–3 km. This shallow focus amplified shaking felt across the Phlegraean area, Naples, Ischia, Procida, and parts of the Sorrento Peninsula.

Part of ongoing volcanic-related (bradyseismic) unrest involving ground uplift (resumed after subsidence around 2005), fluid migration, and seismic swarms. It fits models of seismicity along a ring-like structure at 2–4 km depth channeling magmatic fluids; no immediate change in overall alert level (remains yellow) or key monitoring parameters (uplift rate, CO₂ flux, magma depth) was reported.

A seismic swarm followed, including a notable Md ~3.8 event and dozens of smaller ones (tens to over 100–170 in the following hours/day), with activity generally decreasing in intensity.

Reports indicated power outages (including from a high-voltage pylon issue affecting tens of thousands of customers, later restored), suspension of local rail/metro services (Cumana, Circumflegrea, and some high-speed checks), and temporary port/ferry disruptions at Pozzuoli.

Minor-to-moderate damage included cracks, fallen plaster/cornices, rockfalls/landslides from tuff cliffs (some crushing vehicles), and collapses of a few unoccupied or abandoned buildings/facades.

Injuries were mostly minor (falls, panic-related, cuts from glass/debris), with totals reported in the range of ~8–26 (some accounts note 21, including a couple more serious cases); limited evacuations of families occurred (dozens to a couple hundred people temporarily displaced or housed in waiting areas).

No widespread major structural failures or large-scale rescues were immediately needed.

Campi Flegrei is a large, densely populated caldera (last major eruption 1538) known for historical bradyseism (slow ground rise/fall).

Authorities (INGV Osservatorio Vesuviano, Civil Protection) continue intensive monitoring; residents are advised to follow official updates.

The event underscores the ongoing unrest but remains consistent with observed patterns rather than signaling an imminent major shift.

The bradyseism mechanisms

Bradyseism is the gradual, slow vertical movement of the Earth’s surface—either uplift (positive bradyseism) or subsidence (negative bradyseism)—typically occurring in volcanic calderas. The term comes from the Greek bradys (“slow”) and seismos (“movement”). It is especially well documented at Campi Flegrei (Phlegraean Fields) near Naples, Italy, where historical evidence (such as marine borings on Roman columns at the Macellum of Pozzuoli) shows multi-meter rises and falls over centuries.

Unlike rapid tectonic earthquakes or volcanic eruptions, bradyseism proceeds over years to millennia and is usually accompanied by swarms of small-to-moderate earthquakes, increased gas emissions (especially CO₂), and changes in hydrothermal activity.

Bradyseism results from pressure and volume changes in the subsurface, driven by the interaction of magmatic and hydrothermal processes. Two main families of models exist, and modern understanding emphasizes their interplay rather than pure magmatic intrusion or pure hydrothermal activity alone.

Magmatic processes

• Injection, recharge, or crystallization of magma in a chamber or mush zone increases or decreases pressure.
• As water-rich magma cools and crystallizes (“second boiling”), it exsolves aqueous fluids (H₂O, CO₂, and other volatiles). This generates mechanical energy (PΔV) that can inflate the overlying rocks.
• Fresh magma recharge is not always required; crystallization of already-emplaced magma can supply enough fluid and energy for observed uplifts.

Hydrothermal (fluid) processes — often the dominant short-term driver

• Hot, pressurized magmatic fluids (or mixtures of magmatic and meteoric water) migrate upward into a hydrothermal system.
• Increased pore pressure and temperature expand the rock matrix (thermo-poro-elastic effects), causing ground uplift.
• Fluids expand as they decompress (roughly isenthalpic / Joule–Thomson expansion), further contributing to volume change.
• When fluid pressure exceeds the strength of the rock, hydrofracturing occurs, producing earthquake swarms and allowing fluid ascent. Mineral precipitation (especially silica) can later seal fractures, reducing permeability and ending an uplift phase.

Two-Timescale Conceptual Model (particularly relevant to Campi Flegrei)

A widely cited framework distinguishes processes operating on very different timescales:

• Long timescale (10³–10⁴ years): Slow cooling and crystallization of a deeper magma body (or mush zone, often > ~5–8 km). This steadily generates magmatic fluids that accumulate in a low-permeability, lithostatically pressured reservoir (~3–5+ km depth), sealed by an impermeable carapace or “A-layer.”
• Short timescale (1–100 years): Episodic connection between the deep lithostatic reservoir and a shallower hydrostatic hydrothermal system (above ~2.5–3 km). An impermeable cap-rock layer (“B-layer,” often an anticlinal structure peaking near Pozzuoli) acts as a throttling valve.
  • When fractures open (triggered by fluid overpressure), fluids rush upward, expand, and cause rapid uplift + seismicity.
  • Precipitation of minerals reseals the pathways, isolating the reservoirs again and allowing subsidence as fluids cool, condense, or drain.

Uplift occurs while the deep and shallow systems are disconnected (pressure builds below the seal) or during active fluid transfer and expansion. Subsidence follows when connectivity allows pressure release or when fluids leave the system.

Additional Contributing Factors

Hybrid sources: Many researchers favor a combination—deep magmatic fluid input feeding a shallow pressurized hydrothermal reservoir—rather than pure magma emplacement at very shallow levels.

Structural controls: Caldera geometry, pre-existing faults, and layered permeability (weak tuff layers or past intrusion-damaged zones at ~2.5–3 km can trap fluids and concentrate deformation).

External modulation: Rainfall (pore-pressure changes in shallow aquifers), atmospheric pressure, and Earth tides can influence seismicity rates, especially when the system is already pressurized.

Uplift phases are typically accompanied by increased seismicity (as rocks fracture under rising stress/pore pressure), higher fumarolic gas output, and measurable ground deformation (often centered near Pozzuoli at Campi Flegrei). Subsidence phases are quieter. The process does not automatically lead to eruption; many bradyseismic cycles have occurred without one. Continuous monitoring of deformation, seismicity, gas chemistry, and temperature helps distinguish hydrothermal-dominated unrest from potential magmatic ascent.

In summary, bradyseism is fundamentally a pressure-driven deformation process rooted in the slow release and episodic migration of magmatic fluids through a complex hydrothermal system, modulated by the permeability structure of the crust.