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.

We look for repeating earthquakes at Shishaldin volcano (Alaska) by a multi-stage clustering of the waveforms of a large catalog of Long-Period (LP) earthquakes (about 330,000 events) occurring between October 2003 and July 2004, that includes a minor eruption. We find about 5,550 LP earthquakes with at least on repeater in the catalog; they mostly occur in the interval December 2003 - March 2004. Repeaters are composed of two ∼5-s wave packets. The first packet is a direct source effect and its polarization dip angle points towards the nucleation depth of the pressurization phenomenon triggering the conduit resonance. The second packet is in the coda of the first one and is dominated by scattered waves. Repeaters are LP events with common pressurization position and triggering mechanism. The nucleation depths of the overall LP seismicity and of the repeaters (inferred by the polarization dip angle) show a sharp increase in January 2004 that temporary matches the onset of the eruptive crisis, followed by a slow nucleation depth increase in January-May 2004, that we interpret as a fast pressure drop followed by a slow pressure build up in the feeding system. During the latter stage, coda wave interferometry applied to pairs of repeaters indicates a medium velocity increase. By simple macroscopic modelling of the feeding system as a homogeneous porous medium and a velocity increase as due to squeezing of fluid-filled cracks, we estimate a pressure increase of 40 MPa in Dec 2003 - Apr 2004.

The Irpinia region is one of the most seismically active areas of Italy due to ongoing, late-orogenic extension in the axial zone of the Apennines mountain belt. However, the 3D architecture and the nature of the faults that drive this extension are still uncertain, posing challenges to seismic hazard assessment. Here, we address these uncertainties by integrating a new catalog of high-resolution micro-seismicity (ML<3.5) complemented by earthquake focal mechanisms, with existing 3D seismic velocity models and geological data. We found that micro-seismicity is primarily taking place along a segmented, approximately 60 km long, deep-seated, Mesozoic normal fault that was inverted during the shortening stages of the Apennine orogeny and then extensionally reactivated during the Quaternary. These findings suggest that multiple events of reactivation of long-lived faults can weaken their strength, making them prone to co-seismic remobilization under newly-imposed strain fields in active mountain belts.

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.

The reliable determination of earthquake source parameters is a relevant task of seismological investigations which ground nowadays on high quality seismic waveforms collected by near-source dense arrays of ground motion sensors. Here we propose a parametric modelling technique which analyzes the time-domain P-wave signal recorded in the near-source range of small-to-large size earthquakes. Assuming a triangular moment-rate function and a uniform speed, circular rupture model, we develop the equations to estimate the seismic moment, rupture radius and stress-drop from the corner-time and plateau level of the average logarithm of the P-wave displacement vs time curves (LPDT). The constant-Q, anelastic attenuation effect is accounted by a post-processing procedure that evaluates the Q-unperturbed moment-rate triangular shape. The methodology has been validated through the application to the acceleration records of the 2016-2017 Central Italy and 2007-2019 Japan earthquake sequences covering a wide moment magnitude range (Mw 2.5 - 6.5) and recording distance < 100 km. After correcting for the anelastic attenuation function, the estimated average stress-drop and the confidence interval (〈∆σ〉=0.60 (0.42-0.87) MPa and 〈∆σ〉=1.53 (1.01-2.31) for crustal and subcrustal events of Japan and 〈∆σ〉=0.36(0.30-0.44) MPa for Central Italy) show, for both regions, a self-similar, constant stress-drop scaling of the rupture duration/radius with seismic moment. The smaller sensitivity of the spatially averaged, time-varying peak displacement amplitude to the radiation from localized high slip patch on the fracture surface, could explain the retrieved smaller average stress-drops for sub-crustal earthquakes in Japan and M>5.5 events in Central Italy relative to previous estimates using spectral methods.

Abstract. Here we propose a methodology for Earthquake Early Warning able issuing the alert based on the real-time estimation of the epicentral area where a ground Intensity measure is expected to exceed a user-set ground shaking level. The method provides in output a P-wave-based, time-evolutive “early” shake map. The P-wave displacement, velocity and acceleration amplitudes are jointly measured on a progressively expanded time window while the earthquake location and magnitude are evaluated using data at near source stations. A retrospective analysis of the 2016, Mw 6.5 Central Italy earthquake records shows that the method naturally accounts for effects related to the earthquake rupture directivity and spatial variability of strong ground motion related to source and path and site effects. Five seconds after the origin time the simulated performance of the system in predicting the event impact is very high: in the 40 km-radius area that suffered an Intensity MCS VIII-IX, 41 over 42 strong-motion instrumented sites would have been successfully alerted, with only one false alert. Even considering the 15-km-radius blind-zone, a 15-55 km wide annular area would have received the alert 2-14.5 sec before the occurrence of the strong ground shaking.The proposed EEW method evolves with time in a way that it minimizes the missed alarms while increasing successful alarms and to a lesser extent false alarms, so it is necessary for the end-user to accept these eventualities and account for them in a probabilistic decision scheme depending on the specific safety actuation measure to be undertaken in real-time

The moderate earthquake occurred at the volcanic island of Ischia, south-west of Naples (Italy) caused several buildings collapse, two victims, and several tens of injured people. This event generated a large amplitude ground shaking and long-lasting S-wave signal, longer than those expected for an earthquake. To investigate the event rupture complexity and its radiated wave field, we used finite-fault modelling to invert the near-source (< 1 km epicentral distance), three-component velocity records of the accelerometric station (IOCA), and searched for the best-fit kinematic rupture parameters. This analysis showed that the rupture nucleated at about 600 m west of IOCA and 1.1 km depth, along a 1 km, NW-SE striking fault (thrust-strike slip with right-lateral component), with a rupture velocity 0.8 km/s. The retrieved rupture model coupled with multi-path reverberations effects related to a thin, low-velocity near-surface volcanic sedimentary layer, allowed us to explain the observed long ground motion duration and the large amplitudes recorded all over the island. The actual fault location, mechanism, and the spatial correlation between the simulated peak ground motion zone and the area where the maximum vertical displacement has been determined by DInsar images suggest that the latter is associated with strong-shaking locally generated by land-slide phenomena caused by co-seismic slip. Our source model is consistent with the earthquake located near the border of the caldera resurgent block, which is likely still active, where mass rock creeps evolved into widespread collapses at NW of Monte Epomeo.

A primary task of a network-based, earthquake early warning system is the prompt event detection and location, needed to assess the magnitude of the event and its potential damage through the predicted peak ground shaking amplitude using empirical attenuation relationships. Most of real-time, automatic earthquake location methods ground on the progressive measurement of the first P-wave arrival time at stations located at increasing distances from the source but recent approaches showed the feasibility to improve the accuracy and rapidity of the earthquake location by using the additional information carried by the P-wave polarization or amplitude, especially unfavorable seismic network lay-outs. Here we propose an evolutionary, Bayesian method for the real-time earthquake location which combines the information derived from the differential P-wave arrival times, amplitude ratios and back-azimuths measured at a minimum of two stations. As more distant stations record the P-wave the posterior pdf is updated and new earthquake location parameters are determined along with their uncertainty. To validate the location method we performed a retrospective analysis of mainshocks (M>4.5) occurred during the 2016-2017 Central Italy earthquake sequence by simulating the typical acquisition layouts of in-land, coastal and linear array of stations. Results show that with the combined use of the three parameters, 2-4 sec after the first P-wave detection, the method converges to stable and accurate determinations of epicentral coordinates and depth even with a non-optimal coverage of stations. The proposed methodology can be generalized and adapted to the off-line analysis of seismic records collected by standard local networks.

In this work we propose and apply a straightforward methodology for the automatic characterization of the extended earthquake source, based on the progressive measurement of the P-wave displacement amplitude at the available stations deployed around the source. Specifically, we averaged the P-wave peak displacement measurements among all the available stations and corrected the observed amplitude for distance attenuation effect to build the logarithm of amplitude vs. time function, named LPDT curve. The curves have an exponential growth shape, with 31 an initial increase and a final plateau level. By analyzing and modelling the LPDT curves, the information about earthquake rupture process and earthquake magnitude can be obtained. We applied this method to the Chinese strong motion data from 2007-2015 with MS ranging between 4 and 8. We used a refined model to reproduce the shape of the curves and different source models based on magnitude to infer the source-related parameters for the study dataset. Our study shows that the plateau level of LPDT curves has a clear scaling with magnitude, with no saturation effect for large events. By assuming a rupture velocity of 0.9Vs, we found a consistent self-similar, constant stress drop scaling law for earthquakes in China with stress drop mainly distributed between a lower level (0.23Mpa) and a higher level (3.74Mpa). The derived relation between the magnitude and rupture length can be used for probabilistic hazard analyses and real-time applications of Earthquake Early Warning systems.