Passive seismology in southern Italy: the SAPTEX array

Abstract In this paper we describe the Southern APennines Tomography EXperiment (SAPTEX) temporary array deployed in southern Italy from June 2001 to December 2003. Five to twelve three-components seismic stations, all equipped with RefTek 72A07 digitizers in continuous mode recording and Lennartz 3D/5s sensors, were operating in the region during the three-year project. Many local, regional and teleseismic events have been recorded at 26 different recording sites, providing an invaluable data set for high-resolution seismological studies. Moreover, by the second half of 2002, two stations were installed in the Aeolian Islands with the main objective to record and better constrain the spatial distribution of the deep seismicity of the southern Tyrrhenian subduction zone. The preliminary analysis of the waveforms collected in the first two years includes phase identification and body wave arrival time estimation, local earthquakes (re)location and focal mechanisms computation, P -wave traveltime residuals, and resolution of crustal and upper mantle structure derived by teleseismic ray sampling

RECENT SEISMICITY (2000-2007) IN THE TIBURTINI-PRENESTINI MOUNTAINS REGION (LATIUM, ITALY)

A detailed knowledge of the seismicity distribution enables us to gain a better understanding of the recent tectonic evolution and the present-day state of stress of the Tiburtini and Prenestini Mountains region, whose seismic importance is related with the nearness to the town of Rome. It has been carried out an analysis of the seismicity of the region using the data recorded by the permanent stations belonging to the Italian National Seismic Network (RSNC) from 2000, and subsequently integrated with data of local seismic network from 2003 to 2007. The increase of the number of stations in the last years allow us to obtain better quality earthquake localization as before done. We have produced a large database of re-picked events collecting arrival times of P and S waves of local earthquakes with magnitude larger than 2.0, belonging to the 2000-2007 period. The work is subdivided into three step. The first one consists in the determination of the VP/VS ratio using the modified Wadati method. In the second step we perform the analysis of the 1D velocity model that better approximates the structure of the crust in the studied area using the VELEST code (Kissling et al., 1995). Finally, we have the step with earthquake locations and focal mechanisms computation using the first motion polarities method. Fault plane solutions are selected following the two quality factors defined by the FPFIT code (Reasenberg and Oppenheimer, 1985). For the determination of the regional stress field it is used the focal mechanism inversion method elaborated by Gephart and Forsyth (1984). These results are compared with those available from the historical seismicity that struck the region under study

The 2013–2018 matese and beneventano seismic sequences (Central–Southern apennines). New constraints on the hypocentral depth determination

The Matese and Beneventano areas coincide with the transition from the central to the southern Apennines and are characterized by both SW-and NE-dipping normal faulting seismogenic structures, responsible for the large historical earthquakes. We studied the Matese and Beneventano seismicity by means of high-precision locations of earthquakes spanning from 29 December 2013 to 4 September 2018. Events were located by using all of the available data from temporary and permanent stations in the area and a 1D computed velocity model, inverting the dataset with the Velest code. For events M > 2.8 we used P-and S-waves arrival times of the strong motion stations located in the study area. A constant value of 1.83 for Vp/Vs was computed with a modified Wadati method. The dataset consists of 2378 earthquakes, 18,715 P-and 12,295 S-wave arrival times. We computed 55 new fault plane solutions. The mechanisms show predominantly normal fault movements, with T-axis trends oriented NE–SW. Only relatively small E–W trending clusters in the eastern peripheral zones of the Apenninic belt show right-lateral strike-slip kinematics similar to that observed in the Potenza (1990–1991) and Molise (2002 and 2018) sequences. These belong to transfer zones associated with differential slab retreat of the Adriatic plate subduction beneath the Apennines. The Matese sequence (December 2013–February 2014; main shock Mw 5.0) is the most relevant part of our dataset. Hypocentral depths along the axis of the Apenninic belt are in agreement with previous seismological studies that place most of the earthquakes in the brittle upper crust. We confirm a general deepening of seismicity moving from west to the east along the Apennines. Seismicity depth is controlled by heat-flow, which is lower in the eastern side, thus causing a deeper brittle–ductile transition

Seismicity and seismogenic structures in Central Italy: new insigths from the SLAM passive experiment

We investigate the background seismicity of Central Italy in the area including southern Latium, Abruzzi and Molise (SLAM project). Within this region, the central Apenninic chain has been historically affected by many strong earthquakes, some of them very destructive such as the 1349 event (Mw ˜6.7) located at the border between southern Latium and western Molise, the 1654 event (Mw ˜6.4) in the southern Latium-Abruzzi area, and the 1805 Boiano earthquake in the northern Matese range (Mw 6.7). The last important seismic sequence occurred in May 1984 in the Comino Valley, southestern Latium (Mw 5.8). The recent activity is characterized by diffuse low-magnitude seismicity, punctuated by localised small sequences during 2009-2012. Our study focuses on the analysis of seismicity recorded in the period 2009-2013. We present earthquake locations and focal mechanism solutions obtained by standard procedures and an optimized regional 1D velocity model based on the Velest algorithm. The waveform data set was collected from the digital recordings of the permanent stations of the Italian national seismic network, the Abruzzi and Molise regional seismic networks, and from a dense seismic survey carried out in the region between November 2011 and May 2013. The temporary network consisted of 17 three-component seismic stations all equipped with Reftek RT130 digitizers and Lennartz 3D/5s sensors. The deployment of this array improved significantly the detection and location of background seismicity. We relocated more than 4300 events with magnitude ML ranging from about 0.5 to 4.2. Earthquakes distribution shows hypocentral depths concentrated within the upper crust, between 2 and 20 km of depth, and is mostly clustered along the Apenninic chain axis. The computed fault-plane solutions generally display normal fault mechanisms, confirming the extensional NE-SW processes active since Pleistocene in the study region

Study of recent seismicity in the area of Southern Apennines

In questo lavoro è stata effettuata una dettagliata analisi della sismicità dell’Italia meridionale in particolare della zona comprendente l’Appennino lucano e l’avanfossa bradanica. Sono stati utilizzati i tempi d’arrivo delle fasi P e S di terremoti locali registrati dalla Rete Sismica Nazionale (RSNC), dalla rete temporanea SAPTEX (2001-2004) (Cimini et al., 2006), e dalla rete locale dell’Eni-Agip operante nella Val d’Agri, registrate nel periodo 2001-2006. In questo modo è stato creato un database costituito da 7570 fasi P e 4956 fasi S, associate a 514 eventi con magnitudo maggiore di 2.0. Lo studio realizzato consiste nel: 1) Calcolo del rapporto VP/VS utilizzando il metodo di Wadati modificato (Chatelain, 1978), ottenendo un valore di 1.83 (Fig.1) leggermente superiore a quello ottenuto da studi precedenti; 2) Analisi del profilo di velocità 1D che meglio approssima la struttura crostale dell’area studiata (Fig.2) utilizzando il codice VELEST (Kissling et al., 1995) e tre modelli iniziali ottenuti da studi precedenti (Chiarabba and Frepoli, 1997; Cassinis et al., 2003; Chiarabba at al., 2005; Frepoli et al., 2005). 3) Localizzazione ipocentrale, calcolo dei meccanismi focali e campo di stress. Le localizzazioni ipocentrali calcolate sono prevalentemente di qualità A(243) e B(59), così come definite dal programma HYPOELLIPSE (Lahr, 1989). Rispetto a quelle ottenute utilizzando i dati della sola rete RSNC, esse risultano avere profondità ipocentrali piú vincolate ed errori di localizzazione inferiori. Considerando la distribuzione della sismicità in Fig.3 si possono distinguere tre zone principali: una situata nella parte ovest della catena appenninica, caratterizzata soprattutto da eventi piú superficiali; una situata in corrispondenza dell’avanfossa bradanica caratterizzata da una sismicità sparsa e da eventi piú profondi; ed infine un gruppo di eventi sparsi localizzati nell’area della Sila, separati da quelli esistenti nella zona del monte Pollino da un evidente gap sismico. Seguendo la procedura di calcolo dei meccanismi focali, col metodo delle polarità dei primi arrivi, ne sono stati selezionati 69 in base ai due fattori di qualità definiti dal codice FPFIT (Reasenberg and Oppenheimer, 1985). Le soluzioni ottenute sono in gran parte normali e trascorrenti con gli assi T che descrivono una generalizzata estensione dell’Appennino lucano in direzione NE-SW. Per la determinazione del campo di stress regionale è stata utilizzata la tecnica di inversione elaborata da Gephart & Forsyth (1984). I risultati ottenuti sono coerenti con i precedenti studi nella stessa area

A morphotectonic approach to the study of earthquakes in Rome

Rome has the world's longest historical record of felt earthquakes, with more than 100 events during the last 2600 years. However, no destructive earthquake has been reported in the sources, and all of the greatest damage suffered in the past has been attributed to far-field events. While this fact suggests that a moderate seismotectonic regime characterizes the Roman area, no study has provided a comprehensive explanation for the lack of strong earthquakes in the region. Through the analysis of the focal mechanism and the morphostructural setting of the epicentral area of a "typical" moderate earthquake (M-1 = 3.3) that recently occurred in the northern urban area of Rome, we demonstrate that this event reactivated a buried segment of an ancient fault generated under both a different and a stronger tectonic regime than that which is presently active. We also show that the evident structural control over the drainage network in this area reflects an extreme degree of fragmentation of a set of buried faults generated under two competing stress fields throughout the Pleistocene. Small faults and a present-day weaker tectonic regime with respect to that acting during the Pleistocene might explain the lack of strong seismicity in the long historical record, suggesting that a large earthquake is not likely to occur

Pattern of seismicity in the Lucanian Apennines and foredeep (Southern Italy) from recording by SAPTEX temporary array

The deployment of a temporary seismic network in Southern Italy during 2001-2004 (the SAPTEX array, Southern APennine Tomography EXperiment) allowed us to relocate the hypocenters of Southern Apennines earthquakes with low uncertainty among the location parameters. The best array distribution of the SAPTEX network for the analysis of seismicity in the Lucanian Apennines and foredeep was reached in the first two years of recording. The SAPTEX data were merged with those of the Italian National Seismic Network (RSNC) operated by the Istituto Nazionale di Geofisica e Vulcanologia (INGV). For the hypocenters computation of events in the upper Agri Valley we also included P- and S- waves arrivals from the local Eni-Agip network. The seismicity for the Lucanian Apennines and foredeep in the analyzed period has magnitudes ranging from 2.0 to 4.1. A major finding is the identification of two different crustal domains: the westernmost characterizing the chain, mostly with shallow earthquakes (within about 20 km of depth), and the easternmost one belonging to the outer margin of the chain and to the foredeep, with deeper seismicity (mostly between 20-40 km of depth). Thirty fault-plane solutions were computed and used for stress inversion; most of them are related to earthquakes within the chain sector and indicate a generalized NE-SW extension. Moreover, the dense network allowed us to improve the location of events relative to two low magnitude sequences which occurred in the study period

Emergenza sismica nel Frusinate (Ottobre 2009 – Gennaio 2010): l’intervento della Rete Sismica Mobile stand-alone e l’analisi dati

Tra il 30 settembre ed il 22 ottobre del 2009 una piccola area della provincia di Frosinone, presso la località di Campoli Appennino, non lontana dalla città di Sora e dal confine con l’Abruzzo, è stata interessata da uno sciame sismico la cui fase più intensa è stata raggiunta nella notte tra il 7 e l’8 di ottobre con due scosse di magnitudo locale (ML) 3.6 e 3.4. Nei primi 23 giorni della sequenza (30 settembre – 22 ottobre) sono state registrate ben 1075 scosse, tutte con magnitudo non superiore a 3.6. In precedenza, nei mesi di maggio e giugno del 2009, si era attivata una piccola area posta ad una quindicina di chilometri a NW di Campoli Appennino, esattamente nella zona montuosa che separa la Val Roveto dalla Vallelonga in territorio abruzzese. Questo piccolo sciame è stato caratterizzato da 64 eventi con ML non superiore a 2.7. Diverse sono state le ragioni che hanno indotto il team scientifico alla guida della Rete Sismica Mobile della sede di Roma [Re.Mo., Moretti et al., 2010a] a disporre nei primi giorni del mese di ottobre un intervento di emergenza nell’area che include i comuni di Sora, Atina, San Donato in Val Comino e Pescasseroli tra le provincie di Frosinone e de L’Aquila: 1) la relativa vicinanza delle due zone epicentrali sopra descritte alla regione dell’Aquilano colpita solo pochi mesi prima dal forte evento sismico del 6 aprile 2009 (ML 5.8, MW 6.31) [Chiarabba et al., 2009; Margheriti et al., 2010], 2) l’emotività della popolazione originatasi a seguito del forte trauma vissuto e 3) non ultimo la psicosi collettiva notevolmente alimentata dai media locali e nazionali. In tutto, sono state installate 4 stazioni sismiche temporanee ad integrazione delle permanenti già presenti in area epicentrale al fine di migliorarne il monitoraggio. In questo lavoro viene presentato l’intervento della Re.Mo. riportando le motivazioni che lo hanno guidato e la tempistica delle operazioni svolte. Inoltre, verrà fornita una breve descrizione delle caratteristiche geologico-strutturali e sismotettoniche dell’area e saranno mostrate alcune analisi eseguite sui dati acquisiti in campagna

Seismicity and seismogenic structures of Central Apennines (Italy): constraints on the present-day stress field from focal mechanisms – The SLAM (Seismicity of Lazio-Abruzzo and Molise) project

The aim of the SLAM (Seismicity of Lazio, Abruzzo and Molise region) project is to provide new insight on the seismotectonic and seismogenesis of a wide portion of central Italy situated between areas affected by recent destructive events such as the 2009, Mw = 6.3, L’Aquila earthquake to the north and the 2002, Mw = 5.8, Molise earthquake to the east. We present new results for the microseismic activity in the Central Apennines, occurred in the period 2009 – 2013, by analyzing seismogram recordings from two temporary networks of up to 17 stations in combination with data from three networks of permanent stations