In the past two decades, the central portion of Campi Flegrei caldera has experienced ground uplift of up to 15 mm/month, and a consequent increase in the rate, magnitudes and extent of seismicity, especially in the past two years. We use a new method for multi-scale precise earthquake location to relocate the 2014-2023 seismicity and map in detail currently activated fault zones. We relate the geometry, extent, and depth of these zones with available structural reconstructions of the caldera. The current seismicity is mainly driven by the time-varying, ground-uplift induced stress concentration on pre-existing, weaker fault zones, not only related to the inner caldera, dome resurgence but also to ancient volcano-tectonic collapses and magma emplacement processes. The extent of imaged fault segments suggests they can accommodate ruptures up to magnitude 5.0, significantly increasing estimates of seismic hazard in the area.

Earthquake monitoring and many seismological studies depend on absolute earthquake locations from phase arrival-times. We present an absolute earthquake location procedure (NLL-SSST-coherence) which approaches the precision of waveform-based, relative location and is applicable with few seismic stations. NLL-SSST-coherence is based on the probabilistic, global-search NonLinLoc (NLL) location algorithm which defines a probability density function (PDF) in 3D space for absolute hypocenter location and is highly robust to outlier data. NLL-SSST-coherence location first reduces velocity model error through iteratively generated, smooth, source-specific, station travel-time corrections (SSST). Next, arrival-time error is reduced by consolidating location information across events based on inter-event waveform coherency. If the waveforms at a station for multiple events are very similar (have high coherency) up to a given frequency, then the distance separating these “multiplet” events is small relative to the seismic wavelength at that frequency. NLL-coherence relocation for a target event is a stack over 3D space of the NLL-SSST location PDF for the event and the PDF’s for other multiplet events, each weighted by its waveform coherency with the target. NLL-coherence relocation requires waveforms from only one or a few seismic stations, enabling precise, absolute relocation with sparse networks, for foreshocks and early aftershocks of significant events before installation of temporary stations, and for older data sets with few waveform data. We show the behavior and performance of NLL-SSST-coherence with synthetic and ground-truth tests, and through application and comparison to relative locations for California earthquake sequences with dense and sparse station coverage.