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.

Neglecting fault segmentation in hazard assessments leads to underestimated potential hazard. Moreover, integrating the temporal evolution of fault segments activations in hazard assessment improve scenario’s reliability. In this view, enhanced seismic catalogs have potential in revealing previously neglected fault complexities. Past efforts were restricted to the 2D view analysis without involving the segment temporal activation. Our work provides a comprehensive approach, reconstructing 3D fault fine-scale geometry and segments activation evolution. We analyzed the 2014 Northern Nagano (Japan) (Mw 6.2) earthquake sequence using high-resolution seismic catalogs. We automatically detected and located about 2500 events between October and December 2014. We refined the automatic picks, based on cross-correlation and hierarchical clustering, and we relocated the hypocenters with the double-difference in 3D velocity models optimized for the area. Moreover, we calculated the composite focal mechanisms of the main clusters, crucial to constrain the 3D geometry of the fault segments, and rupture directivity that we interpreted jointly with the seismicity and the fault slip. We found that the multi-segmented fault system, is comprised of, at least, 9 distinct segments, that ruptured during 3 successive phases. Different segments exhibit a different rupture mechanism based on their spatial and temporal occurrence, influencing seismicity evolution and rupture length. The presented analysis can be used to improve the reliability of probabilistic hazard assessment in the high seismic potential area of the Itoigawa-Shizuoka fault system. The possibility of fault segment interaction and mutual triggering processes should be considered when drawing reliable seismic hazard scenarios.

We use seismic ambient noise correlation and coda wave interferometry to estimate velocity variations at high temporal resolution, during the pre-eruptive period and the onset of the 2018 eruption of Kilauea volcano. A progressive velocity increase is observed from March to the end of April. It is followed by rapid decrease starting a few days before the onset of the East Rift Zone (ERZ) eruption and then by sharp velocity drop when the eruption started. The change of trend from velocity increase to decrease is progressively delayed by a few days from the summit caldera toward the ERZ. The location of the velocity perturbations shows a migration of the sources of velocity changes from the summit caldera toward the ERZ before the eruption. Using a model of pressure source, we show that the simultaneous caldera inflation and velocity increase probably result from an anisotropic distribution of fault and crack orientations. The velocity decrease could be due to damaging processes above the shallow reservoir and to plastic deformations around the caldera. We introduce a forward model of rock damage associated with the volcano-tectonic seismicity to calculate the velocity decrease. The good agreement between the calculated and the observed velocity variations shows that a large part of the velocity decrease results from damage of the medium. The delayed onsets of velocity decrease and the source migration of velocity perturbations are probably related to progressive fault openings in the Southern and Eastern parts of the caldera and to magma transfer toward the ERZ.