Description: Providing a quantitative estimate of earthquake location quality is not a simple task. Traditional methods, used in literature, are not exhaustive and depend on a subjective point of view, because they consist of empirical choice of quality thresholds of different estimators of location uncertainty. However, we notice that these estimators are correlated with one another, implying the need to combine them to obtain a numerical and impartial estimate of the quality of a seismic location. Therefore, we provide a formula that associates a quality factor (⁠qf⁠) value with a seismic location, which is based on the combination of a set of uncertainty estimators, suitably normalized. We apply the criterion to two different‐type and different‐scale earthquake catalogs, located by two different methods, obtaining encouraging results. The qf parameter definition is a fast, simple, and objective instrument to provide a user‐friendly classification of location quality. Thus, the qf could represent a powerful tool for routine monitoring location computation.

Description: This work focuses on the observation of data recorded by the seismic temporary network installed in the Amatrice area, under the umbrella of the Italian Center for Seismic Microzonation (http://www.centromicrozonazionesismica.it), following the M 6.0 earthquake of August 24, 2016 in Central Italy. The aim was studying the presence of an evident late low frequency wave packet observed in some of the recorded aftershocks. In order to interpret this phenomenon, we combined a beam-forming analysis performed on these data with the statistics on residuals of localizations related to the same events, recorded by the Italian Seismic Network (RSN). The total number of analyzed events, characterized by M≥=3 and epicentral distances between 30 and 55 Km, is 356. By observing the seismic traces of these events there was an evidence, in some of them, of a low frequency packet appearing 10 seconds after the first arrival. The evidence of this packet was correlated with epicentral distance and focal depth. For a subset of stations, considered as an array, a beam-forming analysis was performed by using the ObsPy toolbox (M. Beyreuther et al., 2010). Results of this analysis gave information in terms of slowness and azimuth to distinguish the main seismic phases of the considered events. In addition, by using locations of the RSN records (Chiaraluce et al., 2017), we performed a parallel analysis within the subset of events with clear evidence of the low frequency packet. We relocated these events by using the NonLinLoc code, with a fixed 1D P-wave velocity model, and varying the Vp/ Vs ratio in the range 1.6–2.0. We found that the P phases residuals are not influenced by the Vp/Vs ratio changes whereas the higher the Vp/Vs the lower are the S phases residuals. Higher values of Vp/Vs ratio, fixing Vp values, could mean a decrease of Vs connected to particular effects during the seismic waves path, that are probably due to geological heterogeneities at local or larger scale

Description: Published

Description: Miami, Florida

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: The Alto Tiberina normal fault (ATF) in central Italy is a 50-km-long crustal structure that dips at a low angle (15–20◦). Events on the fault plane are about 10 times less frequent than those located in its shallower syn- and antithetic hanging-wall splays. To enhance ATF catalog and achieve a better understanding of the degree of coupling in the fault system, we apply a template matching technique in the 2010–2014 time window.We augment by a factor 5 the detections and decrease the completeness magnitude to negative values. Contrary to what previously observed on ATF, we highlight intermittent seismic activity and long-lasting clusters interacting with sequences on the shallower splays. One of these episodes of prolonged seismic activity, detected at the end of 2013 on a 30-km-long ATF segment, suggest the ATF active role during an aseismic transient unraveled by geodetic data.

Description: Published

Description: e2020GL089039

Description: 2T. Deformazione crostale attiva

Description: JCR Journal

Description: The 2016–17 central Italy earthquake sequence began with the first mainshock near the town of Amatrice on August 24 (MW 6.0), and was followed by two subsequent large events near Visso on October 26 (MW 5.9) and Norcia on October 30 (MW 6.5), plus a cluster of 4 events with MW 〉 5.0 within few hours on January 18, 2017. The affected area had been monitored before the sequence started by the permanent Italian National Seismic Network (RSNC), and was enhanced during the sequence by temporary stations deployed by the National Institute of Geophysics and Volcanology and the British Geological Survey. By the middle of September, there was a dense network of 155 stations, with a mean separation in the epicentral area of 6–10 km, comparable to the most likely earthquake depth range in the region. This network configuration was kept stable for an entire year, producing 2.5 TB of continuous waveform recordings. Here we describe how this data was used to develop a large and comprehensive earthquake catalogue using the Complete Automatic Seismic Processor (CASP) procedure. This procedure detected more than 450,000 events in the year following the first mainshock, and determined their phase arrival times through an advanced picker engine (RSNI-Picker2), producing a set of about 7 million P- and 10 million S-wave arrival times. These were then used to locate the events using a non-linear location (NLL) algorithm, a 1D velocity model calibrated for the area, and station corrections and then to compute their local magnitudes (ML). The procedure was validated by comparison of the derived data for phase picks and earthquake parameters with a handpicked reference catalogue (hereinafter referred to as ‘RefCat’). The automated procedure takes less than 12 hours on an Intel Core-i7 workstation to analyse the primary waveform data and to detect and locate 3000 events on the most seismically active day of the sequence. This proves the concept that the CASP algorithm can provide effectively real-time data for input into daily operational earthquake forecasts, The results show that there have been significant improvements compared to RefCat obtained in the same period using manual phase picks. The number of detected and located events is higher (from 84,401 to 450,000), the magnitude of completeness is lower (from ML 1.4 to 0.6), and also the number of phase picks is greater with an average number of 72 picked arrival for a ML = 1.4 compared with 30 phases for RefCat using manual phase picking. These propagate into formal uncertainties of ± 0.9km in epicentral location and ± 1.5km in depth for the enhanced catalogue for the vast majority of the events. Together, these provide a significant improvement in the resolution of fine structures such as local planar structures and clusters, in particular the identification of shallow events occurring in parts of the crust previously thought to be inactive. The lower completeness magnitude provides a rich data set for development and testing of analysis techniques of seismic sequences evolution, including real-time, operational monitoring of b-value, time-dependent hazard evaluation and aftershock forecasting.

Description: Published

Description: 555–571

Description: 3T. Fisica dei terremoti e Sorgente Sismica

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

Description: The 2016 central Italy seismic sequence consists so far of a series of moderate-to-large earthquakes activating within a fewmonths along a 60-km-long and Apenninic-trending normal-fault system. Regrettably, the high vulnerability of the local infrastructure and the shallowness of the largest events (depth around 8 km) resulted in 299 casualties and more than 20,000 homeless, with great difficulties in the disaster management. The sequence evolved around its largest event (Mw 6.5, 30 October) that occurred right in the middle of a fault system already activated two months before with a first Mw 6.0 mainshock (on 24 August) located to the south near the town of Amatrice. Then, another Mw 5.9 mainshock occurred just four days before the largest mainshock (26 October) at the northernmost extent of the sequence, near the town of Visso.We analyze the space–time evolution of the first four months of seismic activity through the relocation of ∼26; 000 earthquakes and the kinematic source models of the three mainshocks. All the main events nucleated at the base of a southwest-dipping normal-fault system segmented by the presence of crosscutting compressional structures. The presence of these inherited faults, separating diverse geological domains, appears to modulate evolution of the sequence interfering with coseismic slip distribution and fault segments interaction. Several secondary antithetic and synthetic faults are located at a shallow depth (〈4 km), both in the hanging wall and footwall. The whole normal fault system, confined within the first 8 km of the upper crust, is bounded below by a shallow east-dipping and 2–3-km-thick layer in which small events plus a series of large extensional aftershocks (≈Mw 4) occur, possibly decoupling the upper and lower crusts.

Description: Published

Description: 757-771

Description: 2T. Sorgente Sismica

Description: JCR Journal

Description: Three devastating earthquakes of MW ≥ 5.9 activated a complex system of high-angle normal, antithetic, and sub-horizontal detachment faults during the 2016–2017 central Italy seismic sequence. Waveform cross-correlation based double-difference location of nearly 400,000 aftershocks illuminate complex, fine-scale structures of interacting fault zones. The Mt. Vettore–Mt. Bove (VB) normal fault exhibits wide and complex damage zones, including a system of bookshelf faults that intersects the detachment zone. In the Laga domain, a comparatively narrow, shallow dipping segment of the deep Mt. Gorzano fault progressively ruptures through the detachment zone in four subsequent MW ∼ 5.4 events. Reconstructed fault planes show that the detachment zone is fragmented in four sub-horizontal, partly overlaying shear planes that correlated with the extent of the mainshock ruptures. We find a new, deep reaching seismic barrier that coincides with a bend in the VB fault and may play a role in controlling rupture evolution.

Description: Published

Description: e2021GL092918

Description: 4T. Sismicità dell'Italia

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal

Description: We present an analysis of the reliability of focal mechanisms obtained through moment tensor inversion at the Irpinia Seismic Network, southern Italy. Our analysis is based on the methods proposed by Zahradník and Custódio (2012) and Sokos and Zahradník (2013). We present two different studies: (1) we compute maps of theoretical focal mechanism resolution for the Irpinia region and (2) we study the reliability of the solutions obtained from waveform inversion of five regional earthquakes. Theoretically, we find that when data error is the dominant source of error, focal mechanism resolution is better close to the spots of higher station density rather than at the center of the network. Using real data, we were able to successfully study four of the five regional events, in spite of the large source–station distances (up to ∼280  km) and significant azimuthal gaps (〉319°). We used variance reduction, double‐couple percentage, signal‐to‐noise ratio, condition number, and focal mechanism variability index to assess the quality of the solutions. Our quality assessment is validated by comparison with independent focal mechanism solutions.

Description: Published

Description: 1348–1357

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal

Description: The University of Bari (Italy), in cooperation with the National Institute of Geophysics and Volcanology (INGV) (Italy), has installed the OTRIONS micro-earthquake network to better understand the active tectonics of the Gargano promontory (Southern Italy). The OTRIONS network operates since 2013 and consists of 12 short period, 3 components, seismic stations located in the Apulian territory (Southern Italy). This data article releases the waveform database collected from 2013 to 2018 and describes the characteristics of the local network in the current configuration. At the end of 2018, we implemented a cloud infrastructure to make more robust the acquisition and storage system of the network through a collaboration with the RECAS-Bari computing centre of the University of Bari (Italy) and of the National Institute of Nuclear Physics (Italy). Thanks to this implementation, waveforms recorded after the beginning of 2019 and the station metadata are accessible through the European Integrated Data Archive (EIDA, https://www.orfeus-eu.org/data/eida/nodes/INGV/).

Description: Published

Description: 106783

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: JCR Journal

Description: The 2016–2017 central Italy seismic sequence occurred on an 80 km long normal‐fault system. The sequence initiated with the Mw 6.0 Amatrice event on 24 August 2016, followed by the Mw 5.9 Visso event on 26 October and the Mw 6.5 Norcia event on 30 October. We analyze continuous data from a dense network of 139 seismic stations to build a high‐precision catalog of ∼900,000 earthquakes spanning a 1 yr period, based on arrival times derived using a deep‐neural‐network‐based picker. Our catalog contains an order of magnitude more events than the catalog routinely produced by the local earthquake monitoring agency. Aftershock activity reveals the geometry of complex fault structures activated during the earthquake sequence and provides additional insights into the potential factors controlling the development of the largest events. Activated fault structures in the northern and southern regions appear complementary to faults activated during the 1997 Colfiorito and 2009 L’Aquila sequences, suggesting that earthquake triggering primarily occurs on critically stressed faults. Delineated major fault zones are relatively thick compared to estimated earthquake location uncertainties, and a large number of kilometer‐long faults and diffuse seismicity were activated during the sequence. These properties might be related to fault age, roughness, and the complexity of inherited structures. The rich details resolvable in this catalog will facilitate continued investigation of this energetic and well‐recorded earthquake sequence.

Description: Published

Description: 11–19

Description: 5T. Sismologia, geofisica e geologia per l'ingegneria sismica

Description: N/A or not JCR