Abstract The mantle beneath the Central Mediterranean is thermally and compositionally heterogeneous, as indicated by fragmented and locally stagnant slabs in tomographic images and HIMU‐like anorogenic magmatism. Mantle discontinuities are sensitive to both temperature and composition, and thus provide a way to quantify these heterogeneities. We investigate mantle discontinuities using 1798 high‐quality teleseismic P receiver functions. P‐to‐S converted phases are identified at individual stations through back‐azimuth stacking across multiple frequency bands and converted from time to depth using locally constrained velocity models. Discontinuity depth, sharpness, and impedance contrasts are used to infer mantle temperature and composition. Robust converted phases are detected at three major discontinuities: the X, 410, and 660. The X discontinuity is at an average depth of 272 8 km. The 410 averages 399 4 km, making it shallower than the global reference depth, while the 660 is deeper at 669 4 km. All three discontinuities are relatively sharp (thickness 15 5 km). The X reflects silica‐rich phase transitions linked to HIMU‐like anorogenic volcanism. Synthetic tests show that comparable PXs and P410s amplitudes imply high basalt fractions (∼70%–80%), well above pyrolitic values (∼18%). Variations in 410 and 660 depths produce significant mantle transition zone thickening. We infer thermal anomalies from the 410 topography. Yet, temperature alone cannot explain large 660 depressions of ∼30 10 km. The 660 depth, sharpness, and P410s/P660s amplitude ratios indicate a strong compositional control, with post‐garnet transition in basalt‐enriched material likely contributing to the observed deepening and aiding slab stagnation.

Abstract High-precision seismic phase arrivals are a prerequisite for building reliable velocity models with travel-time tomography. There has recently been a growing use of seismic phase arrival data obtained through deep learning techniques in travel-time tomography research. Nevertheless, a significant challenge that has emerged pertains to the assessment of the quality of these automatic arrivals. In this article, we used PhaseNet, a deep learning method, to automatically detect the arrival times of the P wave and S wave of 3086 seismic events recorded by dense seismic arrays, obtaining 87,553 high-quality arrivals. To evaluate the quality of the arrival times subsequently used for travel-time tomography inversion, we applied a weighting scheme that includes both detection probability value and signal-to-noise ratio. This new weighting scheme can effectively reduce the overall travel-time residual by 7%. The weighted data were then used in the double-difference tomography method to invert for the crustal velocity structure of the Anninghe–Xiaojiang fault zone. The resulting new model exhibits a lateral resolution of up to 0.25° and reveals velocity anomalies that exhibit a strong correlation with major geological features and block boundaries. Notably, the presence of low-VP and low-VS in the middle crust of the Ludian–Qiaojia seismic zone suggests the existence of hot and weak felsic rocks, as well as possible fluid presence beneath the seismogenic layer of this area. This study not only validates the practicality of using deep learning-based phase picking arrivals in travel-time tomography but also proposes a new weighting scheme to refine the tomographic velocity models.

Double-Difference Tomography witha Deep Learning-Based Phase ArrivalWeighting Scheme and Its Applicationto the Anninghe-Xiaojiang Fault ZoneTing Yang1,2, Lihua Fang*1,2, Jianping Wu1,2, Stephen Monna3, and Weimin Xu1,2AbstractCite this article asYang, T., L. Fang,J. Wu, S. Monna, and W. Xu (2024).Double-Difference Tomography with aDeep Learning-Based Phase ArrivalWeighting Scheme and Its Application tothe Anninghe-Xiaojiang Fault Zone,Seismol. Res. Lett.95, 3681-3695,doi:10.1785/0220230362.Supplemental MaterialHigh-precision seismic phase arrivals are a prerequisite for building reliable velocitymodels with travel-time tomography. There has recently been a growing use of seismicphase arrival data obtained through deep learning techniques in travel-time tomogra-phy research. Nevertheless, a significant challenge that has emerged pertains to theassessment of the quality of these automatic arrivals. In this article, we usedPhaseNet, a deep learning method, to automatically detect the arrival times of thePwave andSwave of 3086 seismic events recorded by dense seismic arrays, obtaining87,553 high-quality arrivals. To evaluate the quality of the arrival times subsequentlyused for travel-time tomography inversion, we applied a weighting scheme thatincludes both detection probability value and signal-to-noise ratio. This new weightingscheme can effectively reduce the overall travel-time residual by 7%. The weighted datawere then used in the double-difference tomography method to invert for the crustalvelocity structure of the Anninghe-Xiaojiang fault zone. The resulting new modelexhibits a lateral resolution of up to 0.25 degrees and reveals velocity anomalies that exhibita strong correlation with major geological features and block boundaries. Notably, thepresence of low-VPand low-VSin the middle crust of the Ludian-Qiaojia seismic zonesuggests the existence of hot and weak felsic rocks, as well as possible fluid presencebeneath the seismogenic layer of this area. This study not only validates the practicalityof using deep learning-based phase picking arrivals in travel-time tomography but alsoproposes a new weighting scheme to refine the tomographic velocity models.

The Algerian offshore earthquake of 18 March 2021, Mw 6.0, was felt by people in various Italian regions, also at large epicentral distance. This unusual human perception far from the source prompted us to analyze the waveforms recorded by land seismic stations installed along the Iberian, French, and Italian coasts. On some seismograms of the selected network, prominent T phases are detected. T waves can travel in the SOund Fixing And Ranging (SOFAR) channel over great distances (thousands of kilometers) with little loss in signal strength and be recorded by near-coastal seismometers after the P (primary) and S (secondary) phases (hence Tor tertiary phases). To explain the subjective perception of ground shaking with quantities that are measured on the seismogram, we estimated the empirical macroseismic intensities for both body and T phases and we calculated the body-wave seismic attenuation. The P-wave anelastic attenuation analysis shows two main wave propagation patterns that reflect lithosphere heterogeneity of the Algerian, Liguro-Proven & ccedil;al, and Tyrrhenian basins. We find that in some cases, in particular along the Italian and French coasts, the largest ground shaking is caused by the T phase. Our observations confirm that the central-western Mediterranean Sea is a favorable site for T-wave propagation and suggest that the T phases should be taken into account in ground-shaking hazard assessment for the central-western Mediterranean.

We use seismic waveform data from the AlpArray Seismic Network and three other temporary seismic networks, to perform receiver function (RF) calculations and time-to-depth migration to update the knowledge of the Moho discontinuity beneath the broader European Alps. In particular, we set up a homogeneous processing scheme to compute RFs using the time-domain iterative deconvolution method and apply consistent quality control to yield 112 205 high-quality RFs. We then perform time-to-depth migration in a newly implemented 3D spherical coordinate system using a European-scale reference P and S wave velocity model. This approach, together with the dense data coverage, provide us with a 3D migrated volume, from which we present migrated profiles that reflect the first-order crustal thickness structure. We create a detailed Moho map by manually picking the discontinuity in a set of orthogonal profiles covering the entire area. We make the RF dataset, the software for the entire processing workflow, as well as the Moho map, openly available; these open-access datasets and results will allow other researchers to build on the current study.

The Qinling orogenic belt and surrounding areas are a junction zone in Central China that connects the North China craton, Yangtze craton and northeastern Tibetan plateau. This area is in a key position within the more general framework where there is eastward extrusion of the Tibetan plateau and westward subduction of the Pacific plate. There is an ongoing scientific debate on how these two main processes are driving the geodynamic evolution of the Qinling orogenic belt and surrounding areas. Specific mechanisms have been proposed, such as lower crustal flow, crustal shortening and big mantle wedge tectonics. Knowledge of the seismic velocity structure helps us understand how these two driving forces affect the tectonic blocks of the study area, as well as test how well the proposed mechanisms to fit the regional velocity structure. For this purpose, a high-resolution P-wave velocity model of the crust and uppermost mantle was obtained by inverting arrival times from local earthquakes that occurred in the Qinling and surrounding region. Our three-dimensional (3D) model suggests that crustal channel flow cannot exist in the whole Qinling orogenic belt, and the localized low-velocity zone generated by the horizontal compressive stress caused by the collision of the Indian and Eurasian plates, is a possible explanation for the growth of the northeastern Tibetan plateau. The contrast in seismic velocities found under the east-Qinling and the west-Qinling orogenic belts implies a different response of these two terranes to the eastward extrusion of the Tibetan plateau. Our velocity model points to relatively stable structural features in the east-Qinling and the Weihe graben, while a low-velocity anomaly in the uppermost mantle of the southernmost Ordos plateau suggests that the lithosphere of the southern Ordos plateau is undergoing destruction.

In spite of numerous active and passive seismological investigations, the existence of continuous or interrupted continental subduction below the Western Alps is still open to debate. Many of the observations focus on the Moho or the deeper part of the mantle, while reliable information on the Lithosphere‐Asthenosphere Boundary (LAB) below the Alpine region is scarce. Exploiting the data from the dense, broadband AlpArray Seismic Network we present a set of Receiver Function (RF) measurements on the Moho and LAB of a region encompassing the Western Alps, which includes the Ivrea Geophysical Body (IGB), a fragment of mantle placed at a few kilometers depth at the collision margin between Eurasia and Adria plates. We derive seismic velocity profiles of the crust‐uppermost mantle below each station down to about 250 km, through the joint inversion of P and S RF. We constrain the lateral variations of the Moho and LAB topographies across the colliding plates, and quantify the errors related to our measurements. Our results allow us to considerably expand the published data of the Moho depth and to add a unique set of new measurements of the LAB. Our observations show that Eurasia and Adria lithospheres have a comparable thickness (on average 90–100 km), and are colliding below the IGB, and that Eurasia is not presently continuously subducting below Adria. These observations suggest that there is a gap between the superficial (continental) European lithosphere and the deep (oceanic) lithosphere, confirming the discontinuous structure imaged by some seismic tomography models.

T-waves are acoustic waves generated by earthquakes at the land-water interface. They can propagate efficiently for thousands of kilometers within the ocean's low-velocity waveguide-the SOund Fixing And Ranging (SOFAR) channel. In the present work, we studied T-waves that propagate in the Ionian basin and are generated by regional earthquakes (epicentral distance <1000 km) located in the Hellenic Arc (Greece). The Ionian Sea is a small basin that has strong bathymetric variations and is limited at its western edge by a steep continental slope-the Malta escarpment. T-waves from Greece were recorded by a broadband seismometer onboard one of seafloor-observing units of the Western Ionian Regional Facility of EMSO-European Multidisciplinary Seafloor and water column Observatory Research Infrastructure (see Data and Resources) deployed in the western Ionian Sea (Italy) at about 2100 m water depth. By studying the particle motion and T-phase energy flux (TPEF) of the T-waves recorded at the observatory, we find that the western Ionian Sea bathymetry is an efficient reflector for T-waves within the SOFAR channel. To investigate whether factors other than T-wave path effects drive TPEF levels, we also study the source part of the T-wave generation process.

Seismological data recorded in the Ionian Sea by a network of seven Ocean Bottom Seismometers (OBSs) during the 2017–2018 SEISMOFAULTS experiment provides a close-up view of seismogenic structures that are potential sources of medium-high magnitude earthquakes. The high-quality signal-to-noise ratio waveforms are observed for earthquakes at different scales: teleseismic, regional, and local earthquakes as well as single station earthquakes and small crack events. In this work, we focus on two different types of recording: 1) local earthquakes and 2) Short Duration Events (SDE) associated to micro-fracturing processes. During the SEISMOFAULTS experiment, 133 local earthquakes were recorded by both OBSs and land stations (local magnitude ranging between 0.9 and 3.8), while a group of local earthquakes (76), due to their low magnitude, were recorded only by the OBS network. We relocated 133 earthquakes by integrating onshore and offshore travel times and obtaining a significant improvement in accuracy, particularly for the offshore events. Moreover, the higher signal-to-noise ratio of the OBS network revealed a significant seismicity not detected onshore, which shed new light on the location and kinematics of seismogenic structures in the Calabrian Arc accretionary prism and associated to the subduction of the Ionian lithosphere beneath the Apennines. Other signals recorded only by the OBS network include a high number of Short Duration Events (SDE). The different waveforms of SDEs at two groups of OBSs and the close correlation between the occurrence of events recorded at single stations and SDEs suggest an endogenous fluid venting from mud volcanoes and active fault traces. Results from the analysis of seismological data collected during the SEISMOFAULTS experiment confirm the necessity and potential of marine studies with OBSs, particularly in those geologically active areas of the Mediterranean Sea prone to high seismic risk.

Reply| January 07, 2020 Reply to “Comment on ‘An Alternative View of the Microseismicity along the Western Main Marmara Fault’ by E. Batsi et al.” by Y. Yamamoto et al. Evangelia Batsi; Evangelia Batsi 1Marine Geosciences Research Unit, Institut français de recherche pour l’exploitation de la mer (Ifremer), Plouzané, France Search for other works by this author on: GSW Google Scholar Anthony Lomax; Anthony Lomax 2ALomax Scientific, Mouans Sartoux, France Search for other works by this author on: GSW Google Scholar Jean‐Baptiste Tary; Jean‐Baptiste Tary 3Departamento de Geociencias, Universidad de los Andes, Bogota DC, Columbia Search for other works by this author on: GSW Google Scholar Frauke Klingelhoefer; Frauke Klingelhoefer 1Marine Geosciences Research Unit, Institut français de recherche pour l’exploitation de la mer (Ifremer), Plouzané, France Search for other works by this author on: GSW Google Scholar Vincent Riboulot; Vincent Riboulot 1Marine Geosciences Research Unit, Institut français de recherche pour l’exploitation de la mer (Ifremer), Plouzané, France Search for other works by this author on: GSW Google Scholar Shane Murphy; Shane Murphy 1Marine Geosciences Research Unit, Institut français de recherche pour l’exploitation de la mer (Ifremer), Plouzané, France Search for other works by this author on: GSW Google Scholar Stephen Monna; Stephen Monna 4Istituto Nazionale di Geofisica e Vulcanologia (INGV), Rome, Italy Search for other works by this author on: GSW Google Scholar Nurcan Meral Özel; Nurcan Meral Özel 5Kandili Observatory and Earthquake Research Institute (KOERI), Cengelkoy, Istanbul, Turkey Search for other works by this author on: GSW Google Scholar Hakan Saritas; Hakan Saritas 6Dokuz Eylül University (DEU), Alsancak, İzmir, Turkey Search for other works by this author on: GSW Google Scholar Günay Cifçi; Günay Cifçi 6Dokuz Eylül University (DEU), Alsancak, İzmir, Turkey Search for other works by this author on: GSW Google Scholar Namik Çagatay; Namik Çagatay 7Faculty of Mines, Department of Geology, Istanbul Technical University (ITU), Maslak, Istanbul, Turkey Search for other works by this author on: GSW Google Scholar Luca Gasperini; Luca Gasperini 8CNR, Institute of Marine Sciences (ISMAR), Bologna, Italy Search for other works by this author on: GSW Google Scholar Louis Géli Louis Géli * 1Marine Geosciences Research Unit, Institut français de recherche pour l’exploitation de la mer (Ifremer), Plouzané, France *Corresponding author: Louis.Geli@ifremer.fr Search for other works by this author on: GSW Google Scholar Bulletin of the Seismological Society of America (2020) 110 (1): 383–386. https://doi.org/10.1785/0120190052 Article history first online: 07 Jan 2020 Connected Content Companion: An Alternative View of the Microseismicity along the Western Main Marmara Fault Companion: Comment on “An Alternative View of the Microseismicity along the Western Main Marmara Fault,” by E. Batsi et al. Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Evangelia Batsi, Anthony Lomax, Jean‐Baptiste Tary, Frauke Klingelhoefer, Vincent Riboulot, Shane Murphy, Stephen Monna, Nurcan Meral Özel, Hakan Saritas, Günay Cifçi, Namik Çagatay, Luca Gasperini, Louis Géli; Reply to “Comment on ‘An Alternative View of the Microseismicity along the Western Main Marmara Fault’ by E. Batsi et al.” by Y. Yamamoto et al.. Bulletin of the Seismological Society of America 2020;; 110 (1): 383–386. doi: https://doi.org/10.1785/0120190052 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyBulletin of the Seismological Society of America Search Advanced Search In their comment, Yamamoto et al. (2019) are primarily concerned with the existence and effect of large values of minimum and maximum phase residuals in our analysis and locations using the 2014 observations, as listed in tables S7 and S8 in the supplementary material of our paper (Batsi et al., 2018). We retain these large residuals in the tables and analysis because they have vanishingly small effect on the NonLinLoc locations, as the used, equal differential time location algorithm (Lomax, 2008; Lomax et al., 2009) is highly robust to outlier readings. In the... You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

We investigate the upper mantle discontinuities in the central Mediterranean region by applying the P and S receiver function techniques on waveforms recorded at broadband stations located around the Tyrrhenian basin. P and S wave velocity profiles (down to 300-km depth) are calculated with joint inversion of P and S receiver functions. We could identify the Moho, lithosphere-asthenosphere boundary, and an underlying low-velocity layer between similar to 60- and similar to 200-km depth. The low-velocity layer is interpreted as asthenospheric material, and its lower boundary is identified below the western Ionian and Tyrrhenian basins as a sharp Lehmann discontinuity. Although the stations are located on different lithospheric domains we find a strong correlation between Moho and the lithosphere-asthenosphere boundary depths, which suggests ubiquitous coupling of the crust and lithospheric mantle, consistently with the southward opening of the Tyrrhenian basin. The Tyrrhenian and western Ionian basins present thinning of the transition zone of similar to 14 km, as inferred from a reduced P660s-P410s differential time. Below the southern Apennines we observe a standard differential time that implies an average mantle transition zone thickness. We explain these mantle transition zone thickness variations as due to temperature heterogeneity linked to the area's subduction history. Finally, under central Europe (the location of the deep S-to-P conversion points) two strong signals from nonstandard discontinuities within the mantle transition zone are observed. These signals can be explained as being generated at the boundaries of high seismic velocity layers that are spatially correlated with stagnant slabs in the transition zone detected by seismic tomography.

A detailed study, based on ocean-bottom seismometers (OBSs) recordings from two recording periods (3.5 months in 2011 and 2 months in 2014) and on a high-resolution, 3D velocity model, is presented here, which provides an alternative view of the microseismicity along the submerged section of the North Anatolian fault (NAF) within the western Sea of Marmara (SoM). The nonlinear probabilistic software packages of NonLinLoc and NLDiffLoc were used for locating earthquakes. Only earthquakes that comply with the following location criteria (e.g., representing 20% of the total amount of events) were considered for analysis: (1) number of stations >= 5; (2) number of phases >= 6, including both P and S; (3) root mean square (rms) location error <= 0.5 s; and (4) azimuthal gap <= 180 degrees. P and S travel times suggest that there are strong velocity anomalies along the Western High, with low V-P, low V-S, and ultra-high V-P/V-S in areas where mud volcanoes and gas-prone sediment layers are known to be present. The location results indicate that not all earthquakes occurred as strike-slip events at crustal depths (> 8 km) along the axis of the Main Marmara fault (MMF). In contrast, the following features were observed: (1) a significant number of earthquakes occurred off-axis (e.g., 24%), with predominantly normal focal mechanisms, at depths between 2 and 6 km, along tectonically active, structural trends oriented east-west or southwest-northeast, and (2) a great number of earthquakes was also found to occur within the upper sediment layers (at depths < 2 km), particularly in the areas where free gas is suspected to exist, based on high-resolution 3D seismics (e.g., 28%). Part of this ultra-shallow seismicity appears to occur in response to deep earthquakes of intermediate (M-L similar to 4-5) magnitude. Resolving the depth of the shallow seismicity requires adequate experimental design ensuring source-receiver distances of the same order as hypocentral depths. To reach this objective, deep-seafloor observatories with a sufficient number of geophone sensors near the fault trace are needed.

Understanding micro-seismicity is a critical question for earthquake hazard assessment. Since the devastating earthquakes of Izmit and Duzce in 1999, the seismicity along the submerged section of North Anatolian Fault within the Sea of Marmara (comprising the “Istanbul seismic gap”) has been extensively studied in order to infer its mechanical behaviour (creeping vs locked). So far, the seismicity has been interpreted only in terms of being tectonic-driven, although the Main Marmara Fault (MMF) is known to strike across multiple hydrocarbon gas sources. Here, we show that a large number of the aftershocks that followed the M 5.1 earthquake of July, 25 th 2011 in the western Sea of Marmara, occurred within a zone of gas overpressuring in the 1.5–5 km depth range, from where pressurized gas is expected to migrate along the MMF, up to the surface sediment layers. Hence, gas-related processes should also be considered for a complete interpretation of the micro-seismicity (~M < 3) within the Istanbul offshore domain.