Earthquake location in tectonic structures of the Alpine Chain: the case of the Constance Lake (Central Europe) seismic sequence

AbstractA set of four magnitude Ml ≥ 3.0 earthquakes including the magnitude Ml = 3.7 mainshock of the seismic sequence hitting the Lake Constance, Southern Germany, area in July–August 2019 was studied by means of bulletin and waveform data collected from 86 seismic stations of the Central Europe-Alpine region. The first single-event locations obtained using a uniform 1-D velocity model, and both fixed and free depths, showed residuals of the order of up ± 2.0 s, systematically affecting stations located in different areas of the study region. Namely, German stations to the northeast of the epicenters and French stations to the west exhibit negative residuals, while Italian stations located to the southeast are characterized by similarly large positive residuals. As a consequence, the epicentral coordinates were affected by a significant bias of the order of 4–5 km to the NNE. The locations were repeated applying a method that uses different velocity models for three groups of stations situated in different geological environments, obtaining more accurate locations. Moreover, the application of two methods of relative locations and joint hypocentral determination, without improving the absolute location of the master event, has shown that the sources of the four considered events are separated by distances of the order of one km both in horizontal coordinates and in depths. A particular attention has been paid to the geographical positions of the seismic stations used in the locations and their relationship with the known crustal features, such as the Moho depth and velocity anomalies in the studied region. Significant correlations between the observed travel time residuals and the crustal structure were obtained

Automatic inspection and analysis of digital waveform images by means of convolutional neural networks

AbstractAnalyzing seismic data to get information about earthquakes has always been a major task for seismologists and, more in general, for geophysicists. Recently, thanks to the technological development of observation systems, more and more data are available to perform such tasks. However, this data "grow up" makes "human possibility" of data processing more complex in terms of required efforts and time demanding. That is why new technological approaches such as artificial intelligence are becoming very popular and more and more exploited. In this paper, we explore the possibility of interpreting seismic waveform segments by means of pre-trained deep learning. More specifically, we apply convolutional networks to seismological waveforms recorded at local or regional distances without any pre-elaboration or filtering. We show that such an approach can be very successful in determining if an earthquake is "included" in the seismic wave image and in estimating the distance between the earthquake epicenter and the recording station

High-resolution aeromagnetic survey of Calabria (Southern Italy)

We present a 1:350,000 high-resolution magnetic anomaly map of Calabria (Southern Italy), obtained by merging the results from two low-altitude aeromagnetic surveys performed in southern and northern Calabria. Magnetic anomalies of Calabria are of low intensity, and mostly range from 11 to –9 nT. Northern Calabria is characterized by positive anomalies in the Tyrrhenian margin (Coastal Chain) that turn into negative values moving eastward in the Sila Massif. Southern Calabria is characterized by slightly positive anomaly values, interrupted by a null magnetic anomaly corridor roughly corresponding to the eastern margin of the Gioia Tauro basin. Finally, anomaly values turn systematically negative in the Messina Straits. Due to the unprecedented resolution (low flying height, spatial sampling along the flight line of ∼5 m and 1–2 km flight line spacing), the new map highlights, in detail, the geometry and setting of the upper crustal features. As Calabria is one of the most seismically active regions in Italy, hit by several high-magnitude earthquakes in recent centuries, the interpretation of this new map will hopefully contribute to new insights into the crustal geological setting, location and dimension of the main seismogenic sources.Published116-1231A. Geomagnetismo e Paleomagnetismo3SR. AMBIENTE - Servizi e ricerca per la SocietàJCR Journa

Active and capable fault? The case study of Prata D'Ansidonia (L'Aquila, Central Apennine)

The study deals with the morphogenetic meaning of several linear scarps that carved the paleo-landsurface of Valle Daria, an extended geomorphological feature located between Barisciano (AQ) and Prata D'Ansidonia (AQ). These villages are situated in the southern termination of the L'Aquila intermontane basin (one of the largest basin of the central Apennines), nearby the epicentral area of the 6th April 2009 earthquake (Mw 6.1). These scarps, up to 3 meters high and up to 1.5 km long, define narrow/elongated flat-bottom depressions, filled by colluvial deposits. These depressions are carved into fluvial-deltaical conglomerates, dated back to the lower Pleistocene. Even if different authors have interpreted these shapes as a paleodrainage or secondary faults, a morphometrical study of the Valle Daria paleo-landsurface provided several information which cast doubt on these two interpretations. In order to better understand the nature and the state of activity of these lineaments, geological, geomorphological and geophysical surveys were carried out. A paleoseismological trench pointed out two events of deformation. The curvilinear shape of the shear plane seems to be related to a slow deformation, attributable to collapse-phenomena. Three GPR profiles, two ERT profiles and two microgravimetrical profiles seem to corroborate this interpretation. Therefore, this study permits to attribute the genesis of these scarps to tectono-karstic phenomena, excluding the presence of an active and capable fault.Published346-3494T. Sismologia, geofisica e geologia per l'ingegneria sismicaN/A or not JC

Imaging the Bracciano caldera system by aeromagnetic data inversion (Sabatini Volcanic District, Central Italy)

Bracciano Lake, located in central Italy, 30 km northwest of Rome, is a volcano-tectonic depression developed in Pleistocene time representing the Sabatini Volcanic District (SVD) western eruptive center. Little is known about Bracciano caldera deep structure and potential field methods are appropriate to probe through a volcanic pile. In this paper, we present a 3D magnetic inversion model of the Bracciano caldera system based upon new aeromagnetic data and paleomagnetic measurements carried out by Istituto Nazionale di Geofisica e Vulcanologia (INGV). Magnetic modelling provides a reconstruction of Bracciano caldera morphology whose extension is about 20 km north-south and 15 km east-west; the maximum collapse depth is up to 1 km and the depocentre area appears localised to the north of the present Bracciano Lake shoreline. Considering the geological records of the main Bracciano caldera forming events, we have interpreted the resulting magnetic model as an overlap of two collapse structures generating a nested system whose location was strictly influenced by tectonic processes. Our 3D magnetic model confirms and consolidates the validity of the magnetic method to highlight deep structures in volcanic environments.Published1066802TR. Ricostruzione e modellazione della struttura crostaleJCR Journa

Constraining age and volume of lava flow invasions of the Alcantara valley, Etna volcano (Italy). New insights from paleomagnetic dating and 3D magnetic modeling

The Alcantara Rivermarks the northern border of Etna volcano, and along its NE sector, it forms a narrowNW-SE elongated valley that is carved in the Mts. Peloritani sedimentary sequences. The valley has been invaded by several lava flows during the growth of the main bulk of the stratovolcano edifice in the past 60 ky (Ellittico and Mongibello volcanoes). In this paper, we have constrained the age of the main lava flows forming the valley floor and reconstructed the length and volume of the main lava flow that impacted this area through a multidisciplinary approach integrating stratigraphic and aeromagnetic data analysis together with new paleomagnetic and 14C dating. The new age determinations showed that the evolution of the present-day geological and hydrographic setting of the valleywasmainly conditioned by a fewflank eruptions that occurred in the lowerNflank of Etna during the activity of theMongibello volcano. In particular, between 13.9 and 9.7 ka BP the valley floor was filled by the so-called Alcantara lava flow for its entire length up to the river mouth, reaching amaximumlength of about 24 km. Later, two other flank eruptions – producing the so-calledMt. Dolce and Solicchiata lava flows – occurred at 9.1–7.2 and 7.3–7.2 ka BP (respectively), covering the eruptive fissure and the proximal portion of the Alcantara lava field and generated themost recent northward shift of the riverbed. Finally, 3Dmagneticmodeling allowed us to calculate the total on- and off-shore volume of the Alcantara lava flow as 1 km3, a value comparable to those of the long-lasting historical eruptions of Etna. Conversely, the 24 kmlength reached by this lava flow is anomalously high when compared to lava flows of the past 15 ky, likely because its emplacement occurred within the valley floor, preventing any possibility of lateral expansion.Published13-251V. Storia eruttivaJCR Journa

Unravelling Mount Etna’s early eruptive history by three-dimensional magnetic modeling

Evidence of Mount Etna’s early volcanism is hidden by the products of its continuously intense volcanic activity. Etna’s volcanism has buried most of the onshore geological record, and the poorly known offshore extent of these volcanic deposits further limits understanding the volcano’s history. We obtained and analyzed new high-resolution offshore aeromagnetic data along the Ionian continental margin of Mount Etna and combined these with recent paleomagnetic data and available geological information. Specifically, we calculated a three-dimensional (3-D) magnetic inversion model of the thickness variations of the offshore Etnean volcanic system. The inversion model highlights E-W and N-S tabular lava flows associated with the Basal Tholeiitic (ca. 500 ka) and Timpe (ca. 220–110 ka) volcanic phases. These early Etna products, considering the onshore and offshore sectors, achieved total volumes of ~26 km3 and 77 km3, respectively. Moreover, the magnetic anomalies reveal several gravitational collapses that may be indicative of the submarine instabilities of Etna’s eastern flank. The findings shed new light on the history of Mount Etna and its early offshore phases. The observed geometry of these early products links the magma distribution to a recently discovered system of faults that can be associated with the presence of a lateral slab-tearing mechanism during the Ionian subduction.Published1664-16742TR. Ricostruzione e modellazione della struttura crostaleJCR Journa

High-resolution aeromagnetic investigation for volcanic and seismo-tectonic characterization of the upper Crust: Examples from Mt. Etna and Calabria (Italy)

High-resolution aeromagnetic surveys of Mt. Etna Volcano and southern Calabria were carried out during the last three years by the Airborne Geophysics Science Team of Istituto Nazionale di Geofisica e Vulcanologia (INGV). Different datasets of the total intensity of the Earth’s Magnetic Field were collected at different altitudes aimed at taking into account the huge topographic variations. Flight line spacing was kept constant at 1 km, or less. In the Etna volcano area we interpreted the magnetic anomaly pattern as due to the effects of thickness variation of magnetized volcanic products due to the complex pre-volcanic basement morphology of Etna. The survey of southern Calabria and the Messina Straits was aimed at locating the strongly debated seismogenic sources of the 1908 and 1783 M~7 earthquakes, likely the most destructive events of Italy in the last centuries. Low-intensity magnetic anomalies of Calabria clearly show a set on en-echelon lineaments that correspond to some of the faults putative for the 1908 and 1783 earthquakes. Modeling shows that lineaments are likely due to NW-dipping normal faults disrupting an upper crustal crystalline layer with moderate magnetic susceptibility in the order of 1x10-3.PublishedChenghu, China2TR. Ricostruzione e modellazione della struttura crostal

Aeromagnetic Investigation of the Central Apennine Seismogenic Zone (Italy): From Basins to Faults

We report on a high-resolution, low-altitude aeromagnetic investigation of the central Apennine extensional seismogenic zone, hit by destructive historical earthquakes including the 2009 L’Aquila seismic sequence. Central Apennines are predominantly made by thick (>4 and possibly up to 12 km) packages of shelf and deep marine limestones and dolomites of Mesozoic age, unconformably covered by upper Pliocene-Holocene continental sediments lying on (often active) normal fault hanging walls. Seismogenic faults cut the carbonates down to 10- to 12-km depth, where the brittle-ductile transition occurs. Aeromagnetic data were collected during June 2014 with a cesium magnetometer, along 200-m-spaced flight lines. Apart from a regional 80-nT anomaly that we modeled at 30- to 40-km depths in the lower crust of the Adria plate, weak magnetic residuals are observed. As expected, normal faults cutting the diamagnetic carbonates lack any magnetic fingerprint. However, shallow continental basins yield clear anomalies of 2- to 8-nT intensity, as they contain both residual soils and tephra erupted after 0.7 Ma by volcanoes from the Tyrrhenian margin of Italy. Basin margins imaged by aeromagnetism mirror the geometry of their causative normal faults. Thus, aeromagnetic residuals document many of the central Apennine normal faults that were active during the last ~3 Ma. Most prominent anomalies reflect basins formed after 0.7 Ma, as their magnetization is significantly higher than that of older continental basins. We conclude that rectilinear boundaries of most prominent anomalies reflect faults formed after 0.7 Ma, thus probably seismogenic.Published1435-14531A. Geomagnetismo e PaleomagnetismoJCR Journa