ALBERT

Description: Fault zones are major sources of hazard for many populated regions around the world. Earthquakes still occur unanticipated, and research has started to observe fault properties with increasing spatial and temporal resolution, having the goal of detecting signs of stress accumulation and strength weakening that may anticipate the rupture. The common practice is monitoring source parameters retrieved from measurements; however, model dependence and strong uncertainty propagation hamper their usage for small and microearthquakes. Here, we decipher the ground motion (i.e., ground shaking) variability associated with microseismicity detected by dense seismic networks at a near-fault observatory in Irpinia, Southern Italy, and obtain an unprecedentedly sharp picture of the fault properties evolution both in time and space. We discuss the link between the ground-motion intensity and the source parameters of the considered microseismicity, showing a coherent spatial distribution of the ground-motion intensity with that of corner frequency, stress drop, and radiation efficiency. Our analysis reveals that the ground-motion intensity presents an annual cycle in agreement with independent geodetic displacement observations from two Global Navigation Satellite System stations in the area. The temporal and spatial analyses also reveal a heterogeneous behavior of adjacent fault segments in a high seismic risk Italian area. Concerning the temporal evolution of fault properties, we highlight that the fault segment where the 1980 Ms 6.9 Irpinia earthquake nucleated shows changes in the event-specific signature of ground-motion signals since 2013, suggesting changes in their frictional properties. This evidence, combined with complementary information on the earthquake frequency–magnitude distribution, reveals differences in fault segment response to tectonic loading, suggesting rupture scenarios of future moderate and large earthquakes for seismic hazard assessment.

Description: This paper reports the first results obtained thanks to the installation of a dedicated one-km fiber-optic cable, integrated within the Irpinia Near Fault Observatory regional network in Southern Italy. The cable was installed in a dry lake, located near the active faults responsible for the M6.9 earthquake that occurred in 1980. A distributed acoustic sensing (DAS) interrogator was deployed over a period of 4 months and a half and allowed to record tens of local events. To model the seismic phases observed in the recordings, simple seismic refraction experiments were conducted along with analyses of earthquakes and numerical simulations. Results show that in this peculiar geological context, DAS is mainly sensitive to waves guided horizontally by the subsurface low-velocity layered structure of the site. This leads to considerable site amplification but also wavefront deformation and allows to detect local microearthquakes without any stacking or other array processing techniques. Magnitude estimation was performed using a dedicated amplitude correction process, along with detection threshold estimation. The benefit of using stacking of DAS channels for improving signal-to-noise ratio was also estimated.

Description: Nowadays, Tsunami Early Warning Systems (TEWS) issue the first alert between three and five minutes after the occurrence of a potentially tsunamigenic earthquake. For offshore earthquakes, for which a significant network azimuthal gap limits the accuracy of standard techniques for source location, this time is spent to obtain stable estimations of the event magnitude and depth. This large warning time affects the efficiency of TEWS for near-coastal large earthquakes. Recent developments in Earthquake Early Warning Systems (EEWS) mitigates this issue. Here, we consider a P-wave, shaking-forecast based EEW method (Zollo et al., 2023) to provide fast and accurate estimations of event location and magnitude along with the Potential Damage Zone (PDZ). As a first test of a combined E- and T- EWS, we applied the method by playing-back the simulated records of two events of Mw 6 and 7 in the Messina strait, with source characteristics that mimic the 1908 Messina earthquake. The events are simulated at the INGV and RAN seismic stations along the Sicily and Calabria coasts. We estimated stable and accurate hypocenter locations and magnitude determinations in 20-25 seconds for both events. The shape of the PDZ obtained after 30 s from the earthquake origin well reproduces the geometry of the rupture surface. These first results show that combining EEWS and TEWS can speed up the tsunami forecasting, thus increasing the lead-time available for actions to protect the exposed population.

Description: In the context of Earthquake Early Warning (EEW) applications, the high-speed rail networks are of particular interest due to their strategic role for private/public transportation. In Italy, in the frame of an industrial partnership, the University of Naples Federico II was recently commissioned by Rete Ferroviaria Italiana (RFI) S.p.A. to design, develop, and implement a prototype EEW system on the High-Speed Railway line between Rome and Naples. In case of a relevant earthquake, the system has the main goal of issuing an automatic alert that makes it possible to slow down high-speed trains, by resorting to a specific interface with existing signalling systems. The EEW methodology implements an evolutionary, P-wave based approach combining the single station and the network-based approaches. A probabilistic decision scheme for the alert declaration is implemented, accounting for the exceedance probability of a given PGA threshold value and for the uncertainty associated to the empirical scaling relationships. A final decision module evaluates whether and where the predicted PGA exceeds the established threshold and declares the alert at the nodes of the line where the most relevant shaking is expected. Here we will overview the basic principles and methodologies of the EEW platform and introduce a quantitative performance assessment, based on a retrospective off-line analysis of the system outputs. The performance analysis allows evaluating both the system rapidity in providing alerts and the quality/reliability of these predictions and represents the key element for stakeholders and end-users, to properly configure and setup the operational system.

Description: Recent estimates of the rupture scenarios of moderate-to-large earthquakes highlights the need of incorporating segmented fault surfaces into seismic hazard models rather than individual, planar ones. The 2014 Northern Nagano earthquake sequence provides an opportunity to rethink the fault system geometrical complexities of the Itoigawa-Shizuoka tectonic line, one of the most inland active faults in Japan, and their impact on the fault rupture nucleation and development. For this study we used continuous records of 37 seismic stations to detect and locate 2500 events that occurred from 1st October 2014 to 31 December 2014. By refining the automatic arrival time picks, based on the cross-correlation and the hierarchical clustering, we relocated the earthquake hypocenters with the double-difference location and a three-dimensional (3D) velocity model. We obtained a high-quality earthquake location catalog and the composite focal mechanisms which served for constraining the 3D geometry of the segmented fault system. We found that the early aftershocks are distributed along two sub-parallel, NNE-SSW striking, en-echelon, left-stepping fault segments and analyses on the spatio-temporal evolution of fault activation suggest that the rupture propagates in a clockwise direction, involving first the shallow northern portion of the segmented fault system, then the deeper northern portion of it, and finally the deeper and the shallower parts of the southern portion. Results suggest that a relationship exists between the rupture mechanism and the 3D segmented fault geometry. This relationship between initial structure of the segmented fault controls the earthquake rupture becomes crucial to assess reliable scenarios in seismic hazard.