ALBERT

Description: The stress release (or stress drop) during an earthquake is an important element of seismic hazard forecasting; high stress drop earthquakes radiate more high frequency energy, causing stronger ground shaking. The stress drop also provides information about the energy budget, and the size of fault ruptured, and consequently, earthquake triggering and rupture dynamics. Reliable estimates of stress release are difficult to make, largely because of the ambiguity in removing the distorting propagation effects experienced by waves traveling from earthquake to seismometer from recorded seismograms. Most measurements are made using frequency amplitude spectra. We use two methods to estimate earthquake stress drop for 30 of the larger earthquakes in central Italy (2016–2017) and compare them with the results of previous studies. We find that the variation between absolute values estimated in different studies is much larger than the reported formal inversion errors. The relative values are more reliable, with different studies consistently finding a particular earthquake has relatively high or low stress drop. Direct comparison of the similar-sized, damaging Amatrice and Visso earthquakes reveals that the relative spectral stress drop estimates reflect the relative strength of high-frequency ground motion, but may indicate more complex rupture rather than higher average stress release.

Description: Estimates of spectral stress drop are fundamental to understanding the factors controlling earthquake rupture and high frequency ground motion, but are known to include large, poorly-constrained uncertainties. We use earthquakes from the 2016–2017 sequence in the Italian Appenines (largest event at Norcia, Mw 6.3) to investigate these uncertainties and their causes. The similarly-sized events near Amatrice (Mw 6.0) and Visso (Mw 5.9) enable better constrained relative analysis. We calculate S wave source spectra, corner frequencies, and spectral stress drop for 30 of the larger events. We compare both empirical and modeling approaches to isolate the source spectra and calculate source parameters; we also compare our results with those from published studies. Both random and systematic inter-study variations are larger than the standard errors reported by any individual study. The reported magnitude dependence of stress drop varies between studies, being largest for generalized inversions and smallest for more individual event based approaches. The relative spectral estimates of inter-event stress drop are more consistent; all approaches estimated higher stress drop in the Amatrice earthquake than the similar-sized Visso earthquake. In contrast, finite fault inversions of these two earthquakes found that the Visso earthquake had the larger region of concentrated, higher slip, whereas the Amatrice earthquake had multiple, lower slip, subevents. The Amatrice spectra contain more high frequency energy than those of the Visso earthquake. This comparison suggests that consistent measurement of a higher spectral stress drop indicates greater high-frequency ground motion but may correspond to greater rupture complexity rather than higher stress drop.

Description: Published

Description: e2022JB025022

Description: OST2 Deformazione e Hazard sismico e da maremoto

Description: JCR Journal

Description: The incompleteness of earthquake catalogs is a well-known issue caused by our technical limitation in detecting the small-to very small-magnitude seismicity falling near or below the background seismic noise. The detection of small-magnitude events is fundamental for improving our knowledge of geometry and kinematics of seismogenic sources and the spatio-temporal characteristics of seismicity, thus leading to better models for seismic hazard. Template-matching (TM) is a powerful technique that, based on similarity measure (cross-correlation) of seismic waveforms, allows to detect hidden earthquakes that are similar to known events (called templates). The high computational effort often limits such technique to small areas and for short time frames (less than 1 year). In this work, we present the first application of template-matching at regional scale for the Italian Peninsula, focusing on the Southern Apennines. We use about 3,600 high-quality events as templates, scanning 6-year long continuous recordings (2009–2014), at more than 180 stations of the INGV network. About 20,000 new events are found, showing a comparable quality to the template catalog in terms of hypocentral solution, reaching a decrease of the magnitude of completeness of about one unit. To highlight the improved quality of the TM catalog, we report two main examples regarding the Sannio-Matese area, where TM allowed us to unravel relevant details on the spatio-temporal distribution of the local seismicity.

Description: - PRIN-2017 project MUSE 4D (2017KT2MKE) Overtime tectonic, dynamic and rheologic control on destructive multiple seismic events—Special Italian Faults and Earthquakes: From real 4-D cases to models. - FURTHER project “The role of FlUids in the pReparaTory pHase of EaRthquakes in Southern Apennines” funded by the Strategic Earthquake Department of Istituto Nazionale di Geofisica e Vulcanologia (Italy).

Description: Published

Description: e2023GC011160

Description: OST5 Verso un nuovo Monitoraggio

Description: JCR Journal

Description: Abstract Tomographic images of the lithosphere are the first step to constrain the evolution of mountain belts and their interaction. By inverting new high-quality P- and S-wave arrivals that sample the entire lithosphere, we determined Vp and Vp/Vs models with reliable resolution in the critical depth range (40–80 km) where plates of the central Mediterranean area interact. This data set yields homogeneous representation of the 3D structure over a critical area at a regional scale. Here, we show that the Alps derive from a laterally continuous underthrusting of the European plate and that the Adria lithosphere was delaminated after the collision. Tomograms resolve the lateral changes of the continental versus oceanic subduction along the Alpine belt and identify original evidence of fluids beneath the orogens able to facilitate the current deformation. Plain Language Summary A high resolution imaging of the lithosphere/asthenosphere system is crucial to understand tectonic processes of orogens and subductions. The Alpine chain is an exemplary case of complexity, with its lateral heterogeneity and changes. The largest seismic array ever developed in the Alpine chain (Alparray Seismic Network) has enabled the creation of a high-quality seismic data set contributing to new images of the entire central Mediterranean area. The novelty of this work lies in the enhanced resolution of velocity anomalies in a critical depth range (35–80 km) and with optimal homogeneity at the regional scale. The new 3D Vp and Vp/Vs models allow us to get insights into many open questions about the structure and evolution of the circum-Mediterranean orogens.

Description: Published

Description: e2023JB026411

Description: OST1 Alla ricerca dei Motori Geodinamici

Description: JCR Journal

Description: Abstract We present the first rupture models of the two mainshocks of the 2012 northern Italy sequence, determined by jointly inverting seismic and geodetic data. We aim at providing new insights into the mainshocks for which contrasting seismotectonic interpretations are proposed in literature. Sources' geometric parameters were constrained by seismic reflection profiles, 3-D relocations and focal mechanisms of mainshocks/aftershocks. Site-specific velocity profiles were used to model accelerograms affected by strong propagation effects related to the Po basin. Our source models differ significantly from previous ones relying on either seismic or geodetic data. Their comparison against geological sections and aftershock distribution provides new insights about the ruptured thrust faults. The May 20th Mw6.1 mainshock activated the Middle Ferrara thrust-ramp dipping ∼45° SSW-wards, breaking a main eastern slip patch 4–15 km deep in Mesozoic carbonates (maximum slip 0.7–0.8 m) and Paleozoic-Triassic basement rocks, and a small western patch in the basement. The May 29th Mw6.0 mainshock featured two separated asperities along the Mirandola thrustramp dipping ∼42° S-wards: an eastern asperity 4–15 km deep in Mesozoic carbonates and basement rocks (maximum slip 0.7 m) and a deeper western one (7–16 km depth) mainly in the basement (slip peak 0.8 m). On-fault aftershocks were concentrated within the basement and Mesozoic carbonates, devoiding highslip zones. Slip and aftershock distribution was controlled by the rheological transition between Mesozoic carbonates and Cenozoic sediments. Unlike previous thin-skinned tectonic interpretations, our results point to a complex rupture process along moderately dipping (40°–45°) thrust-ramps deeply rooted into the Paleozoic crystalline basement. Plain Language Summary The two M6 mainshocks of the 2012 Italy sequence are the strongest earthquakes ever observed in the Po Plain, a strategic region for the Italian economy. The mainshocks ruptured blind thrust-faults, however their source models and seismotectonic interpretation are still debated because the thrust-system architecture is controversial. Contrasting thick-skinned and thin-skinned tectonic models are proposed. In thick-skinned interpretations, shortening is accommodated by thrust-ramps rooted into the crystalline basement that represent main seismogenic structures, whereas in thin-skinned interpretations, shortening and seismicity are controlled by listric faults splaying out from dècollement levels in the sedimentary crust. A comprehensive analysis of the mainshocks' source represents an opportunity to provide new insights into the seismogenesis in northern Italy and on a broader scale into seismotectonics of thrust-and-fold belts. We get a complete picture of the mainshocks kinematics by jointly inverting, for the first time, seismic and geodetic data, and unravel rupture heterogeneities not resolved by previous studies. By integrating source models with aftershock locations and geological models, we propose a comprehensive seismotectonic interpretation of the sequence. We conclusively identify the ruptured faults that correspond to thrust-ramps rooted into the crystalline basement and evidence the key role played by lithological changes in the rupture process.

Description: Published

Description: e2022JB026278

Description: OST3 Vicino alla faglia

Description: JCR Journal

Description: Foreshocks are spatially clustered seismic events preceding large earthquakes. Since the dawn of seismology, their occurrence has been identified as a possible mechanism leading to further crustal destabilization, hence, to major failures. However, several cases occurred without any previous anomalous seismic activity, so that the hypothesis of foreshocks as reliable seismic precursors fails to pass statistical tests. Here, we perform an all‐round statistical comparative analysis of seismicity in Southern California to assess whether any differences can be identified between swarms and foreshocks. Our results suggest that extremely variable seismic patterns can forerun mainshocks, even though they tend to be preceded by clusters with more numerous events spread over larger areas than swarms and with a wider range of magnitudes. We provide a physical explanation of such dissimilarity and conclude, despite it, that foreshocks can hardly be reliable short‐ term precursors of large earthquakes in California.

Description: Published

Description: e2023JB027337

Description: OST4 Descrizione in tempo reale del terremoto, del maremoto, loro predicibilità e impatto

Description: JCR Journal

Description: A catalogue of precisely located micro-seismicity is fundamental for investigating seismicity and rock physical properties in active tectonic and volcanic regions and for the definition of a ‘baseline’ seismicity, required for a safe future exploitation of georesource areas. In this study, we produce the first manually revised catalogue of micro-seismicity for Co. Donegal region (Ireland), an area of about 50K M2 of on-going deformation, aimed at localizing natural micro-seismic events occurred between 2012 and 2015. We develop a stochastic method based on a Markov chain Monte Carlo (McMC) sampling approach to compute earthquake hypocentral location parameters. Our results indicates that micro-seismicity is present with magnitudes lower than 2 (the highest magnitude is 2.8).The recorded seismicity is almost clustered along previously mapped NE-SW trending, steeply dipping faults and confined within the upper crust (focal depth less than 10 km). We also recorded anthropogenic seismicity mostly related to quarries' activity in the study area.

Description: Published

Description: 62-76

Description: OST1 Alla ricerca dei Motori Geodinamici

Description: JCR Journal

Description: We evaluate, by numerical tests, whether surface-wave attenuation can be determined from ambient-noise data. We generate synthetic recordings of numerically simulated ambient seismic noise in several experimental setups, characterized by different source distributions and different values of attenuation coefficient. We use them to verify that the source spectrum can be reconstructed from ambient recordings (provided that the density of sources and the attenuation coefficient are known) and that true attenuation can be retrieved from normalized cross correlations of synthetic signals. We then apply the so validated method to real continuous recordings from 33 broadband receivers distributed within the Colorado Plateau and Great Basin. A preliminary analysis of the signal-to-noise ratio as a function of azimuth reveals a SW-NE preferential directionality of the noise sources within the secondary microseism band (6–8s), consistent with previous studies. By nonlinear inversion of noise data we find the attenuation coefficient in the area of interest to range from ~1×10^-5 m^-1 at 0.3Hz to 4.5×10^-7 m^-1 at 0.065Hz, and confirm the statistical robustness of this estimate by means of a bootstrap analysis. The result is compatible with previous observations based on both earthquake-generated and ambient Rayleigh waves. In this regard, the method proves to be promising in accurately quantifying surface-wave attenuation at relatively high frequencies.

Description: Published

Description: e2020JB019865

Description: 1T. Struttura della Terra

Description: JCR Journal

Description: We explore the three‐dimensional structure of the 2016–2017 Central Italy sequence using ~34,000 ML ≥ 1.5 earthquakes that occurred between August 2016 and January 2018. We applied cross‐correlation and double‐difference location methods to waveform and parametric data routinely produced at the Italian National Institute of Geophysics and Volcanology. The sequence activated an 80 km long system of normal faults and near‐horizontal detachment faults through the MW 6.0 Amatrice, the MW 5.9 Visso, and the MW 6.5 Norcia mainshocks and aftershocks. The system has an average strike of N155°E and dips 38°–55° southwestward and is segmented into 15–30 km long faults individually activated by the cascade of MW ≥ 5.0 shocks. The two main normal fault segments, Mt. Vettore‐Mt. Bove to the North and Mt. della Laga to the South, are separated by an NNE‐SSW‐trending lateral ramp of the Sibillini thrust, a regional structure inherited from the previous compressional tectonic phase putting into contact diverse lithologies with different seismicity patterns. Space‐time reconstruction of the fault system supports a composite rupture scenario previously proposed for the MW 6.5 Norcia earthquake, where the rupture possibly propagated also along an oblique portion of the Sibillini thrust. This dissected set of normal fault segments is bounded at 8–10 km depth by a continuous 2 km thick seismicity layer of extensional nature slightly dipping eastward and interpreted as a shear zone. All three mainshocks in the sequence nucleated along the high‐angle planes at significant distance from the shear zone, thus complicating the interpretation of the mechanisms driving strain partitioning between these structures.

Description: Published

Description: e2019JB018440

Description: 3T. Sorgente sismica

Description: JCR Journal