Deep structures of the southern Tyrrhenian basin and P-wave residuals at Messina

1975 ◽  
Vol 65 (4) ◽  
pp. 1013-1021
Author(s):  
Antonio Bottari
Keyword(s):  
Upper Mantle ◽  
P Wave ◽  
Tyrrhenian Sea ◽  
Travel Times ◽  
Deep Earthquakes ◽  
First Motion ◽  
Southern Tyrrhenian Sea ◽  
Epicentral Location ◽  
Foregoing Result ◽  
Tyrrhenian Basin

Abstract In this article, P travel-time residuals for the Messina station are analyzed in order to investigate the Tyrrhenian upper mantle, which is considered to be crossed by a lithospheric slab. A first set of 24 residuals derived from deep earthquakes of the southern Tyrrhenian Sea show early arrivals of, on the average, −1.3 sec at Messina. In addition, these negative residuals are associated with initial motion of the dilatation type. On the contrary, the few deep earthquakes which produce, as first motion, a compression at the Messina station, are associated with late arrivals of about 1 sec. These results are considered and discussed in order to analyze the hypocentral mechanism and P-wave transmission through the lithospheric slab. A second and wider analysis is then extended to 206 earthquakes which have, with respect to Messina, an epicentral location in the distance range 16° to 103° and azimuthal orientation Z in the interval 180° to 380°. The first conclusion from this analysis is that the P travel times observed at Messina for epicentral distances in the range 20° to 103° and 245° ≦ Z ≦ 380° are generally 0.5 to 3 sec less than those given in the Jeffreys-Bullen tables. Finally, a further improvement on the foregoing result has been obtained. This gives further confirmation of the consistency of regional variations of the P travel times with a slab model for the Tyrrhenian deep structures. As a matter of fact, the comparison between the travel times of Messina and a standard provided by observations in the stations of Rome and Trieste confirms early arrivals of about 1 sec on the seismic paths which cross the upper mantle in the southern Tyrrhenian region.

scholarly journals Upper mantle velocity structure beneath Italy from direct and secondary P-wave teleseismic tomography

1997 ◽  
Vol 40 (1) ◽  
Author(s):  
G. B. Cimini ◽  
P. De Gori
Keyword(s):  
Upper Mantle ◽  
Velocity Structure ◽  
Velocity Model ◽  
P Wave ◽  
Depth Range ◽  
Tyrrhenian Sea ◽  
Tomographic Images ◽  
3D Velocity Model ◽  
Southern Tyrrhenian Sea ◽  
Velocity Anomalies

High-quality teleseismic data digitally recorded by the National Seismic Network during 1988-1995 have been analysed to tomographically reconstruct the aspherical velocity structure of the upper mantle beneath the Italian region. To improve the quality and the reliability of the tomographic images, both direct (P, PKPdf) and secondary (pP,sP,PcP,PP,PKPbc,PKPab) travel-time data were used in the inversion. Over 7000 relative residuals were computed with respect to the IASP91 Earth velocity model and inverted using a modified version of the ACH technique. Incorporation of data of secondary phases resulted in a significant improvement of the sampling of the target volume and of the spatial resolution of the heterogeneous zones. The tomographic images show that most of the lateral variations in the velocity field are confined in the first ~250 km of depth. Strong low velocity anomalies are found beneath the Po plain, Tuscany and Eastern Sicily in the depth range between 35 and 85 km. High velocity anomalies dominate the upper mantle beneath the Central-Western Alps, Northern-Central Apennines and Southern Tyrrhenian sea at lithospheric depths between 85 and 150 km. At greater depth, positive anomalies are still observed below the northernmost part of the Apenninic chain and Southern Tyrrhenian sea. Deeper anomalies present in the 3D velocity model computed by inverting only the first arrivals dataset, generally appear less pronounced in the new tomographic reconstructions. We interpret this as the result of the ray sampling improvement on the reduction of the vertical smearing effects.

P-wave travel times from shallow earthquakes recorded in India and inferred upper mantle structure

1968 ◽  
Vol 58 (6) ◽  
pp. 1879-1897
Author(s):  
K. L. Kaila ◽  
P. R. Reddy ◽  
Hari Narain
Keyword(s):  
Upper Mantle ◽  
P Wave ◽  
Mantle Structure ◽  
Travel Times ◽  
The North ◽  
Shallow Earthquakes ◽  
Upper Mantle Structure ◽  
Wave Travel ◽  
Excess Value ◽  
Velocity Changes

ABSTRACT P-wave travel times of 39 shallow earthquakes and three nuclear explosions with epicenters in the North in Himalayas, Tibet, China and USSR as recorded in Indian observatories have been analyzed statistically by the method of weighting observations. The travel times from Δ = 2° to 50° can be represented by four straight line segments indicating abrupt velocity changes around 19°, 22° and 33° respectively. The P-wave velocity at the top of the mantle has been found to be 8.31 ± 0.02 km/sec. Inferred upper mantle structure reveals three velocity discontinuities in the upper mantle at depths (below the crust) of 380 ± 20, 580 ± 50 and 1000 ± 120 km with velocities below the discontinuities as 9.47 ± 0.06, 10.15 ± 0.07 and 11.40 ± 0.08 km/sec respectively. The J-B residuals up to Δ = 19° are mostly negative varying from 1 to 10 seconds with a dependence on Δ values indicating a different upper mantle velocity in the Himalayan region as compared to that used by Jeffreys-Bullen in their tables (1940). Between 19° to 33° there is a reasonably good agreement between the J-B curve and the observation points. From Δ = 33° to 50° the J-B residuals are mostly positive with an average excess value of about 4 sec.

Teleseismic P-wave travel time tomography of the Alpine upper mantle using AlpArray seismic network data

2020 ◽  
Author(s):  
Marcel Paffrath ◽  
Wolfgang Friederich ◽  
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
High Velocity ◽  
Eastern Alps ◽  
Alpine Region ◽  
Seismic Network ◽  
P Wave ◽  
Travel Times ◽  
Travel Time Tomography ◽  
Wave Travel

<p>We perform a teleseismic P-wave travel time tomography to examine geometry and slab structure of the upper mantle beneath the Alpine orogen. Vertical component data of the extraordinary dense seismic network AlpArray are used which were recorded at over 600 temporary and permanent broadband stations deployed by 24 different European institutions in the greater Alpine region, reaching from the Massif Central to the Pannonian Basin and from the Po plain to the river Main. Mantle phases of 347 teleseismic events between 2015 and 2019 of magnitude 5.5 and higher are evaluated automatically for direct and core diffracted P arrivals using a combination of higher-order statistics picking algorithms and signal cross correlation. The resulting database contains over 170.000 highly accurate absolute P picks that were manually revised for each event. The travel time residuals exhibit very consistent and reproducible spatial patterns, already pointing at high velocity slabs in the mantle.</p><p>For predicting P-wave travel times, we consider a large computational box encompassing the Alpine region up to a depth of 600 km within which we allow 3D-variations of P-wave velocity. Outside this box we assume a spherically symmetric earth and apply the Tau-P method to calculate travel times and ray paths. These are injected at the boundaries of the regional box and continued using the fast marching method. We invert differences between observed and predicted travel times for P-wave velocities inside the box. Velocity is discretized on a regular grid with an average spacing of about 25 km. The misfit reduction reaches values of up to 75% depending on damping and smoothing parameters.</p><p>The resulting model shows several steeply dipping high velocity anomalies following the Alpine arc. The most prominent structure stretches from the western Alps into the Apennines mountain range reaching depths of over 500 km. Two further anomalies extending down to a depth of 300 km are located below the central and eastern Alps, separated by a clear gap below the western part of the Tauern window. Further to the east the model indicates a possible high-velocity connection between the eastern Alps and the Dinarides. Regarding the lateral position of the central and eastern Alpine slabs, our results confirm previous studies. However, there are differences regarding depth extent, dip angles and dip directions. Both structures dip very steeply with a tendency towards northward dipping. We perform various general, as well as purpose-built resolution tests, to verify the capabilities of our setup to resolve slab gaps as well as different possible slab dipping directions.</p>

Can Teleseismic Travel-Times Constrain 3D Anisotropic Structure in Subduction Zones? Insights from Realistic Synthetic Experiments

2020 ◽  
Author(s):  
Brandon VanderBeek ◽  
Manuele Faccenda
Keyword(s):  
Upper Mantle ◽  
Subduction Zones ◽  
Seismic Anisotropy ◽  
Joint Inversion ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Elastic Model ◽  
Anisotropic Structure ◽  
Travel Times ◽  
Delay Times

<p>Despite the well-established anisotropic nature of Earth’s upper mantle, the influence of elastic anisotropy on teleseismic tomographic images remains largely ignored. In subduction zones, unmodeled anisotropic heterogeneity can lead to substantial isotropic velocity artefacts that may be misinterpreted as compositional heterogeneities (e.g. Bezada et al., 2016). Recent studies have demonstrated the possibility of inverting P-wave delay times for the strength and orientation of seismic anisotropy assuming a hexagonal symmetry system (e.g. Huang et al., 2015; Munzarová et al., 2018). However, the ability of P-wave delay times to constrain complex anisotropic patterns, such as those expected in subduction settings, remains unclear as the aforementioned methods are tested using ideal self-consistent data (i.e. data produced using the assumptions built into the tomography algorithm) generated from simplified synthetic models. Here, we test anisotropic P-wave imaging methods on data generated from geodynamic simulations of subduction. Micromechanical models of polymineralic aggregates advected through the simulated flow field are used to create an elastic model with up to 21 independent coefficients. We then model the teleseismic wavefield through this fully anisotropic model using SPECFEM3D coupled with AxiSEM. P-wave delay times across a synthetic seismic array are measured using conventional cross-correlation techniques and inverted for isotropic velocity and the strength and orientation of anisotropy using travel-time tomography methods. We propose and validate approximate analytic finite-frequency sensitivity kernels for the simplified anisotropic parameters. Our results demonstrate that P-wave delays can reliably recover horizontal and vertical changes in the azimuth of anisotropy. However, substantial isotropic artefacts remain in the solution when only inverting for azimuthal anisotropy parameters. These isotropic artefacts are largely removed when inverting for the dip as well as the azimuth of the anisotropic symmetry axis. Due to the relative nature of P-wave delay times, these data generally fail to reconstruct anisotropic structure that is spatially uniform over large scales. To overcome this limitation, we propose a joint inversion of SKS splitting intensity with P-wave delay times. Preliminary results demonstrate that this approach improves the recovery of the magnitude and azimuth of anisotropy. We conclude that teleseismic P-wave travel-times are a useful observable for probing the 3D distribution of upper mantle anisotropy and that anisotropic inversions should be explored to better understand the nature of isotropic velocity anomalies in subduction settings.</p>

Teleseismic P-wave travel time tomography of the Alpine upper mantle using AlpArray seismic network data

2021 ◽  
Author(s):  
Marcel Paffrath ◽  
Wolfgang Friederich ◽  
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
High Velocity ◽  
Alpine Region ◽  
Seismic Network ◽  
P Wave ◽  
Travel Times ◽  
Massif Central ◽  
Travel Time Tomography ◽  
Wave Travel

<p>We perform a teleseismic P-wave travel time tomography to examine geometry and slab structure of the upper mantle beneath the Alpine orogen. Vertical component data of the extraordinary dense seismic network AlpArray are used which were recorded at over 600 temporary and permanent broadband stations deployed by 24 different European institutions in the greater Alpine region, reaching from the Massif Central to the Pannonian Basin and from the Po plain to the river Main. Mantle phases of 370 teleseismic events between 2015 and 2019 of magnitude 5.5 and higher are evaluated automatically for direct and core diffracted P arrivals using a combination of higher-order statistics picking algorithms and signal cross correlation. The resulting database contains over 170.000 highly accurate absolute P picks that were manually revised for each event. The travel time residuals exhibit very consistent and reproducible spatial patterns, already pointing at high velocity slabs in the mantle.</p><p>For predicting P-wave travel times we consider a large computational box encompassing the Alpine region up to a depth of 600 km within which we allow 3D-variations of P-wave velocity. To account for influences of the strongly heterogeneous crust that cannot be resolved with teleseismic data, we integrate a complex three-dimensional crustal model directly into our model. Outside the box we assume a spherically symmetric earth and apply the Tau-P method to calculate travel times and ray paths. These are injected at the boundaries of the regional box and continued using the fast marching method (Rawlinson et al. 2005). We invert differences between observed and predicted traveltimes for P-wave velocities inside the box. Velocity is discretized on a regular grid with a spacing of about 25x25x15 km. The misfit reduction reaches values of over 80% depending on damping and smoothing parameters.</p><p>The resulting model shows several steeply dipping high velocity anomalies following the Alpine arc. The most prominent structure stretches from the western Alps into the Apennines mountain range reaching depths of over 500 km. Two further anomalies of high complexity extending down to a depth of 300 km are located below the central and eastern Alps, both being detached from the lithosphere and separated by a clear gap below the western part of the Tauern window. The central anomaly shows mainly southwards dipping, whereas the eastern anomaly is mainly dipping to the northeast. We compare our results to former studies, confirming lateral positions of the anomalies. However, the new results can benefit from the superior resolution capabilities of the dense AlpArray seismic network, providing more accurate insights into depth extent, dip angle and directions. We perform various general, as well as purpose-built resolution tests, to verify the capabilities of our setup to resolve slab gaps as well as different possible slab dipping directions.</p>

scholarly journals A study of the Northwestern Pacific upper mantle

1965 ◽  
Vol 55 (5) ◽  
pp. 925-939
Author(s):  
Daniel A. Walker
Keyword(s):  
Upper Mantle ◽  
Fundamental Problem ◽  
Body Wave ◽  
Lower Boundary ◽  
P Wave ◽  
Northwestern Pacific ◽  
Travel Times ◽  
Low Velocity ◽  
S Waves ◽  
Wave Channel

abstract A fundamental problem of earthquake seismology is the occurrence of the upper mantle low-velocity channel. This study is intended to examine its existence in the upper mantle below the Northwestern Pacific on the basis of body-wave arrivals at a bottom-mounted hydrophone near Wake Island. A comparison of the observed travel times and the Jeffreys-Bullen travel times shows an extreme anomaly in the 21- to 33-degree range for both P and S waves. Assumed linear paths suggest a P-wave-channel upper boundary between 165 km and 185 km, and a lower boundary between 290 km and 542 km. Travel times for P and S waves indicate that the velocities in the channel remain constant at 8.1 km/sec and 4.65 km/sec respectively.

scholarly journals Extensional tectonics during the Tyrrhenian back-arc basin formation synthetized in a new morpho-tectonic map

2020 ◽  
Author(s):  
Maria Filomena Loreto ◽  
Nevio Zitellini ◽  
César Rodriguez Ranero ◽  
Camilla Palmiotto ◽  
Manel Prada
Keyword(s):  
Seismic Velocity ◽  
Single Channel ◽  
Regional Scale ◽  
P Wave ◽  
Tyrrhenian Sea ◽  
Seismic Structure ◽  
Velocity Models ◽  
Mantle Exhumation ◽  
Back Arc ◽  
Tyrrhenian Basin

<p>A new tectonic map is presented focused upon the extensional style accompanying the formation of the Tyrrhenian back-arc basin. Our basin-wide analysis synthetizes the interpretation of vintage multichannel and single channel seismic profiles integrated with modern seismic images and P-wave velocity models, and with a new morpho-tectonic map of the Tyrrhenian (Palmiotto & Loreto, 2019). Four distinct evolutionary opening stages have been constrained: 1) the initial Langhian(?)/Serravallian opening phase actives offshore central/southern Sardinia and offshore western Calabria; 2) the Tortonian/Messinian phase dominated by extension offshore North Sardinia-Corsica, and by oceanic accretion in the Cornaglia and Campania Terraces; 3) the Pliocene phase, dominated by mantle exhumation which was active mainly in the central Tyrrhenian and led to the full opening of Vavilov Basin; and 4) the Quaternary phase characterized by the opening of the Marsili back-arc basin. Listric and planar normal faults and their conjugates bound a series of horst and graben, half-graben and triangular basins. Distribution of extensional faults, active since Middle Miocene, throughout the basin allowed us to define a faults arrangement in the northern / central Tyrrhenian mainly related to in a pure shear which evolved a simple shear opening of continental margins. At depth, faults accommodate over a Ductile-Brittle Transitional zone cut by a low-angle detachment fault possibly responsible for mantle exhumation in the Vavilov and Magnaghi abyssal plains. In the southern Tyrrhenian, normal, inverse and transcurrent faults appear to be related to a large shear zone located along the continental margin of the northern Sicily. Extensional style variationthroughout the back-arc basin combined with wide-angle seismic velocity models, from Prada et al. (2014; 2015), allow to explore the relationship between shallow deformation, represented by faults distribution throughout the basin, and crustal-scale processes, subduction of Ionian slab and exhumation.</p><p> </p><p><strong>REFERENCES</strong></p><p>Palmiotto, C., & Loreto, M. F., (2019). Regional scale morphological pattern of the Tyrrhenian Sea: New insights from EMODnet bathymetry. Geomorphology, 332, 88-99.</p><p>Prada, M., Sallarès, V., Ranero, C.R., Vendrell, M.G., Grevemeyer, I., Zitellini, N. & De Franco, R., 2014. Seismic structure of the Central Tyrrhenian basin: Geophysical constraints on the nature of the main crustal domains. J. Geophys. Res.: Solid Earth, 119(1), 52-70.</p><p>Prada, M., Sallarès, V., Ranero, C.R., Vendrell, M.G., Grevemeyer, I., Zitellini, N. & De Franco, R., 2015. The complex 3-D transition from continental crust to backarc magmatism and exhumed mantle in the Central Tyrrhenian basin. Geophys. J. Int., 203(1), 63-78.</p>

scholarly journals Imaging the Mediterranean upper mantle by p- wave travel time tomography

1997 ◽  
Vol 40 (4) ◽  
Author(s):  
C. Piromallo ◽  
A. Morelli
Keyword(s):  
Travel Time ◽  
Upper Mantle ◽  
Three Dimensional ◽  
Velocity Model ◽  
Structural Heterogeneity ◽  
Linear Interpolation ◽  
P Wave ◽  
Travel Times ◽  
P Waves ◽  
Tectonic Lineament

Travel times of P-waves in the Euro-Mediterranean region show strong and consistent lateral variations, which can be associated to structural heterogeneity in the underlying crust and mantle. We analyze regional and tele- seismic data from the International Seismological Centre data base to construct a three-dimensional velocity model of the upper mantle. We parameterize the model by a 3D grid of nodes -with approximately 50 km spacing -with a linear interpolation law, which constitutes a three-dimensional continuous representation of P-wave velocity. We construct summary travel time residuals between pairs of cells of the Earth's surface, both inside our study area and -with a broader spacing -on the whole globe. We account for lower mantle heterogeneity outside the modeled region by using empirical corrections to teleseismic travel times. The tomo- graphic images show generai agreement with other seismological studies of this area, with apparently higher detail attained in some locations. The signature of past and present lithospheric subduction, connected to Euro- African convergence, is a prominent feature. Active subduction under the Tyrrhenian and Hellenic arcs is clearly imaged as high-velocity bodies spanning the whole upper mantle. A clear variation of the lithospheric structure beneath the Northem and Southern Apennines is observed, with the boundary running in correspon- dence of the Ortona-Roccamonfina tectonic lineament. The western section of the Alps appears to have better developed roots than the eastern, possibly reflecting à difference in past subduction of the Tethyan lithosphere and subsequent continental collision.

Upper Mantle velocity structure in the Hindukush region from travel time studies of deep earthquakes using a new analytical method

1969 ◽  
Vol 59 (5) ◽  
pp. 1949-1967
Author(s):  
K. L. Kaila ◽  
V. G. Krishna ◽  
Hari Narain
Keyword(s):  
Analytical Method ◽  
Upper Mantle ◽  
Velocity Structure ◽  
Focal Depth ◽  
P Wave ◽  
S Wave ◽  
Deep Earthquakes ◽  
Calibration Curves ◽  
Depth Determination ◽  
Mantle Velocity

abstract Upper Mantle velocity structure in the Hindukush region has been determined from the P- and S-wave travel times of 28 deep earthquakes making use of a new analytical method given by Kaila (1969). From a depth of 45 to 230 kms, the present analysis reveals a continuous linear increase of P-wave velocity from 8.21 to 8.52 km/sec. For S waves, however, the velocity increases linearly from 4.58 km/sec at a depth of 85 kms to 4.77 km/sec at 230 kms depth. Upper mantle velocities in the Hindukush region are found to be considerably higher in comparison to those for other regions of the Earth. Within the accuracy of the velocity determination from the present method, no inferences can be drawn regarding the existence or otherwise of the low-velocity channel in this region. Calibration curves for focal depth determination in the Hindukush region are also drawn. The accuracy of focal depth determination from these calibration curves is of the same order as that obtained in the focal depths determined by making use of pP, sS and other reflected phases.

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