Nicola Agostinetti - Academia.edu (original) (raw)

Papers by Nicola Agostinetti

Geophysical Journal International, 2013

The accurate definition of 3-D crustal structures and, in primis, the Moho depth, are the most im... more The accurate definition of 3-D crustal structures and, in primis, the Moho depth, are the most important requirement for seismological, geophysical and geodynamic modelling in complex tectonic regions. In such areas, like the Mediterranean region, various active and passive seismic experiments are performed, locally reveal information on Moho depth, average and gradient crustal V p velocity and average V p /V s velocity ratios. Until now, the most reliable information on crustal structures stems from controlled-source seismology experiments. In most parts of the Alpine region, a relatively large number of controlled-source seismology information are available though the overall coverage in the central Mediterranean area is still sparse due to high costs of such experiments. Thus, results from other seismic methodologies, such as local earthquake tomography, receiver functions and ambient noise tomography can be used to complement the controlled-source seismology information to increase coverage and thus the quality of 3-D crustal models. In this paper, we introduce a methodology to directly combine controlled-source seismology and receiver functions information relying on the strengths of each method and in relation to quantitative uncertainty estimates for all data to derive a well resolved Moho map for Italy. To obtain a homogeneous elaboration of controlledsource seismology and receiver functions results, we introduce a new classification/weighting scheme based on uncertainty assessment for receiver functions data. In order to tune the receiver functions information quality, we compare local receiver functions Moho depths and uncertainties with a recently derived well-resolved local earthquake tomography-derived Moho map and with controlled-source seismology information. We find an excellent correlation in the Moho information obtained by these three methodologies in Italy. In the final step, we interpolate the controlled-source seismology and receiver functions information to derive the map of Moho topography in Italy and surrounding regions. Our results show high-frequency undulation in the Moho topography of three different Moho interfaces, the European, the Adriatic-Ionian, and the Liguria-Corsica-Sardinia-Tyrrhenia, reflecting the complexity of geodynamical evolution.

Unexpected features in teleseismic receiver function reveal the presence of a dipping interface w... more Unexpected features in teleseismic receiver function reveal the presence of a dipping interface with pronounced seismic velocity contrast in the Campania-Lucania region, Southern Apennines (Italy). Locus of this finding is the Melandro-Pergola basin, located between the maximum intensity areas of two of the most destructive earthquakes reported in the Italian seismic catalogue, the magnitude ˜ 7.0 Val d'Agri (1857) and Irpinia (1980) earthquakes. This area is not associated with known historical events and for this reason is currently object of investigation as potential seismic gap, where the probability of future ruptures is higher than in surrounding regions. Could the enlightened interface be the seismogenic source which fills the gap? Could the findings of this study offer a new tool to investigate and constraint the source parameters of historical earthquakes?

The paradox of how horizontal contraction and extension can occur simultaneously in convergent mo... more The paradox of how horizontal contraction and extension can occur simultaneously in convergent mountain belts remains a fundamental and largely unresolved problem in continental dynamics. The Apennines represent one of the most accessible "type locality" areas of syn-convergent extension. Rollback - which describes the tendency of a subducting plate to retreat from the orogenic front - is commonly invoked as

In the framework of the italian PNRA (Progetto Nazionale di Ricerche in Antartide), we have start... more In the framework of the italian PNRA (Progetto Nazionale di Ricerche in Antartide), we have started to re-analize teleseismic waveforms recorded, using three-components seismometers (equipped with 5 seconds sensors, Lennartz 3D-5s), during five summer campaings, from 1993 to 2000. Seismic stations were deployed around Terra Nova Bay (TNB) italian base, from the sea to reach the interior of the Transantartic Moun- tains (TAM), the most striking example of nocontractional mountain belt. During the last campaingn (1999-2000) seismic stations were deployed deep into Northern Vic- toria Land to reach Rennik and Lillie Glaciers Area and George V coast region, the northest part of TAM. Our main goals were: to compute, using frequency-domanin deconvolution method by Di Bona [1998], Receiver Functions covering all the area around TNB italian antartic base; to map of Moho-depth and intercrustal S-waves ve- locity discontinuity from 1-D velocity model computed using Sambridge's inversion s...

Results from recent geophysical and mantle-xenolith geochemistry studies of the Kaapvaal Craton a... more Results from recent geophysical and mantle-xenolith geochemistry studies of the Kaapvaal Craton appear, at times, to provide disparate views of the physical, chemical and thermal structure of the lithosphere. Models from our recent SAMTEX magnetotelluric (MT) surveys across the Kaapvaal Craton indicate a resistive, 220-240 km thick lithosphere for the central core of the craton. One published S-wave receiver function (SRF) study and other surface-wave studies suggest a thinner lithosphere characterised by a ~160 km thick high-velocity "lid" underlain by a low-velocity zone (LVZ) of between 65-150 km in thickness. Other seismic studies suggest that the (high-velocity) lithosphere is thicker, in excess of 220 km. Mantle xenolith pressure-temperature arrays from Mesozoic kimberlites require that the base of the "thermal" lithosphere (i.e., the depth above which a conductive geotherm is maintained - the tLAB) is at least 220 km deep, to account for mantle geotherms i...

Tectonophysics, 2015

This study investigates the geometry of the crust-mantle boundary along the Northern Apennines (I... more This study investigates the geometry of the crust-mantle boundary along the Northern Apennines (Italy) and its relationship to the tectonic evolution of a retreating subduction system. Using receiver function analysis from teleseismic records, the Moho depth was computed beneath 26 seismic stations located in key positions along the orogen. The results show a relatively shallow (~20 km) Moho in the Tyrrhenian side, as expected from an extended overriding plate, and a deeper Moho (~40 km) in the subducted Adriatic plate, SW-dipping 15-45°beneath the chain. Stations deployed on the Northeastern edge of the Plio-Quaternary extensional basins provide maximum Moho depth estimates, thus supporting the hypothesis that these basins define the zone where the two plates separate at depth. In addition, an abrupt change in the Moho depth is recognized along strike of the orogen, attributed to a 45 km-wide, trench-normal deformation zone that crosses the Northern Apennines in the proximity of the Livorno-Sillaro lineament. This zone is interpreted as an incipient lithospheric tear fault, which accommodates varying rates of trench retreat along the length of the subduction zone. Tectonophysics j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / t e c t o Please cite this article as: Piana Agostinetti, N., The structure of the Moho in the Northern Apennines: Evidence for an incipient slab tear fault?, Tectonophysics (2015), http://dx.

Geophysical Journal International

Trans-dimensional (trans-D) algorithms have been recently introduced to the earth sciences to sol... more Trans-dimensional (trans-D) algorithms have been recently introduced to the earth sciences to solve inverse problems without having to impose a fixed spatial structure to the model parametrization (e.g. a fixed number of layers in a 1D structure). Trans-dimensional al-gorithms can be easily used for joint inversion of different data-set using a "Hierarchical Bayes" approach. However, some difficulties arise if the two observables display very dif-ferent resolving power. In this case, the structure of the target solution might be twisted toward one observable, introducing non resolved (i.e. over-complex) structure for the other investigated parameters. In this study, we develop a trans-D algorithm for joint inversion of two different data-sets to reconstruct the 1D structure of two different physical parameters. The "parsimony" of the trans-D algorithm produces common discontinuities (a "coupled" structure) for portions of the 1D profile where the two ob...

Geophysical Journal International, 2015

Local earthquake tomography is a non-linear and non-unique inverse problem that uses event arriva... more Local earthquake tomography is a non-linear and non-unique inverse problem that uses event arrival times to solve for the spatial distribution of elastic properties. The typical approach is to apply iterative linearization and derive a preferred solution, but such solutions are biased by a number of subjective choices: the starting model that is iteratively adjusted, the degree of regularization used to obtain a smooth solution, and the assumed noise level in the arrival time data. These subjective choices also affect the estimation of the uncertainties in the inverted parameters. The method presented here is developed in a Bayesian framework where a priori information and measurements are combined to define a posterior probability density of the parameters of interest: elastic properties in a subsurface 3-D model, hypocentre coordinates and noise level in the data. We apply a trans-dimensional Markov chain Monte Carlo algorithm that asymptotically samples the posterior distribution of the investigated parameters. This approach allows us to overcome the issues raised above. First, starting a number of sampling chains from random samples of the prior probability distribution lessens the dependence of the solution from the starting point. Secondly, the number of elastic parameters in the 3-D subsurface model is one of the unknowns in the inversion, and the parsimony of Bayesian inference ensures that the degree of detail in the solution is controlled by the information in the data, given realistic assumptions for the error statistics. Finally, the noise level in the data, which controls the uncertainties of the solution, is also one of the inverted parameters, providing a first-order estimate of the data errors. We apply our method to both synthetic and field arrival time data. The synthetic data inversion successfully recovers velocity anomalies, hypocentre coordinates and the level of noise in the data. The Bayesian inversion of field measurements gives results comparable to those obtained independently by linearized inversion, reconstructing the geometry of the main seismic velocity anomalies. The quantification of the posterior uncertainties, a crucial output of Bayesian inversion, allows for visualizing regions where elastic properties are closely constrained by the data and is used here to directly compare our results to the ones obtained with the linearized inversion. In the case we examined the results of two inversion techniques are not significantly different.

Geophysical Journal International, 2014

This work presents a teleseismic P-wave receiver function study on 34 stations deployed across Ir... more This work presents a teleseismic P-wave receiver function study on 34 stations deployed across Ireland in order to determine the first-order crustal properties, thickness (H) and mean crustal V p /V s , over the entire island. We apply the H − V p /V s stacking method, which exploits the information contained in both the Ps and the multiple phases from the free-surface. In this way, we obtain the first Moho depth and V p /V s maps of Ireland based on a uniform distribution of measurements. The results are used to examine in detail the lateral variation of crustal thickness and V p /V s ratio across the major terrane boundaries in Ireland. Our results show a good agreement with the available previous estimates from onshore wide-angle/refraction experiments and add new information in poorly constrained areas such as Northern Ireland and the NW coast of Ireland. The mean V p /V s ratio is 1.73 ± 0.05 with a consistently low (≈1.70) value in the Leinster domain and in central Ireland. The mean crustal thickness is 30.9 ± 2.3 km. The southern portion of the island shows a nearly flat Moho at a depth of 32-33 km, while north of the Southern Uplands Fault, a relatively higher spatial frequency variation in Moho topography exists with values ranging from 28 to 32 km. This reflects the complex history of multiphase terranes accretion during the Caledonian orogeny, although locally, the superposition of more recent geological processes is not excluded. Crossing the Iapetus Suture Zone, our results support the presence of a 'transitional' Moho, that is, a 3-4 km smooth seismic transition between crust and mantle, while Moho depth remains constant. Anomalous values in Northern Ireland are interpreted as evidence of a 5-to 6-kmthick high S-wave velocity layer just above the Moho.

Earth and Planetary Science Letters, 2014

In this study, we use teleseismic receiver function analysis to image the seismic structure of th... more In this study, we use teleseismic receiver function analysis to image the seismic structure of the Juan de Fuca oceanic plate during its subduction beneath the North American plate. Seismic data have been recorded at 58 seismic stations deployed along the northern Cascadia subduction zone. Harmonic decomposition of the receiver function data-set along a trench-normal profile allows us to image both the isotropic and the anisotropic structure of the plate (slab). Our images highlight the presence of a highly anisotropic region at 40-70 km depths across the Cascadia subduction zone. The detected seismic anisotropy is interpreted to be related to both metamorphic facies (e.g. blueschists) and fluid released during the dehydration of the subducting mantle. The processes of dehydration and metamorphism produce the variations of the seismic properties within each lithologic unit that constitutes the subducted slab, i.e. basalts, gabbro layer and upper mantle, as the oceanic plate sinks in the upper mantle. Such variations make it almost impossible to recognize the "plate boundary" as a characteristic "velocity-jump" at depth (neither positive nor negative) along the Cascadia subduction zone. Based on the comparative interpretation of both the isotropic and the anisotropic structures retrieved, we propose a 4-stage model of the evolution of the Juan de Fuca oceanic plate during its subduction beneath the North American plate.

Energy Procedia, 2014

We report the preliminary results from a project (GAPSS-Geothermal Area Passive Seismic Sources),... more We report the preliminary results from a project (GAPSS-Geothermal Area Passive Seismic Sources), aimed at testing the resolving capabilities of passive exploration methods on a well-known geothermal area, namely the Larderello-Travale Geothermal Field (LTGF). Located in the western part of Tuscany (Italy), LTGF is the most ancient geothermal power field of the world. GAPSS consisted of up to 20 seismic stations deployed over an area of about 50 x 50 Km. During the first 12 months of measurements, we located more than 2000 earthquakes, with a peak rate of up to 40 shocks/day. Preliminary results from analysis of these signals include: (i) analysis of Shear-Wave-Splitting from local earthquake data, from which we determined the areal distribution of the most anisotropic regions; (ii) local-earthquake travel-time tomography for both P-and S-wave velocities; (iii) telesismic receiver function aimed at determining the high-resolution (<0.5km) S-velocity structure over the 0-20km depth range, and seismic anisotropy using the decomposition of the angular harmonics of the RF data-set; (iv) S-wave velocity profiling through inversion of the dispersive characteristics of Rayleigh waves from earthquakes recorded at regional distances. After presenting results from these different analyses, we eventually discuss their potential application to the characterisation and exploration of the investigated area.

Journal of Geodynamics, 2014

While the upper crustal structure of the Southern Apennines is known, lack of control on the deep... more While the upper crustal structure of the Southern Apennines is known, lack of control on the deep structure allows competing thin-skinned and thick-skinned models of the orogen. In thin-skinned models, the detachment decouples a stack of rootless nappes from the basement. In thick-skinned models, basement is involved in the most recent phase of thrusting. To examine crustal structure, we use teleseismic data from the Calabria-Apennine-Tyrrhenian/Subduction-Accretion-Collision Network (CAT/SCAN) array in southern Italy. We use receiver functions (RF) processed into a common conversion point stack to generate images of the crust. Interpretation and correlation to geological structure are done using inversions of individual station RFs. We focus on a shallow discontinuity where P-to-S conversions occur. In the foreland, it corresponds to velocity jumps between carbonate and clastic strata with basement. A similar interpretation for the Apennines provides the most parsimonious explanation and supports a thick-skinned interpretation. In a thick-skinned reconstruction, the amount of shortening is much smaller than for a thin-skinned model. This implies considerably less Pliocene-Pleistocene shortening across the Apennines and suggests an east-southeast motion of the Calabrian arc subparallel to the southern Apennines rather than a radial expansion of the arc.

We investigate crustal structure of the northern Apennines using broadband seismic data from a de... more We investigate crustal structure of the northern Apennines using broadband seismic data from a dense linear transect. Receiver functions reveal significant differences between the responses of the Tyrrhenian and Adriatic sides of the Apennines, and determine the spatial location of a change in this response, just west of the high crest of the mountains. We find a clear signature of

ABSTRACT The Iapetus Ocean closure during the Caledonian Orogeny in the Paleozoic led to a multi-... more ABSTRACT The Iapetus Ocean closure during the Caledonian Orogeny in the Paleozoic led to a multi-phase continental collision between different paleocontinents in the North Atlantic region. It is believed that evidence of these past geological events could be preserved in the fabric of the Ireland&#39;s crust and upper mantle, especially across the Iapetus Suture Zone which cuts through the island with a NE-SW trend. In the framework of the SIM-CRUST project, we studied this open matter through the use of classical teleseismic receiver function (RF) method, in order to map Ireland&#39;s Moho depth and highlight intra-crustal seismic discontinuities. Moreover, thanks to the recently developed RF harmonic decomposition technique, the presence of crustal anisotropy has been investigated analysing the changes in the amplitudes of the P-to-S converted phases as a function of backazimuth. In this study, we analysed teleseismic records from about 40 broadband seismic stations deployed across Ireland in the last decade. We used data from both permanent (Irish National Seismic Network , INSN) and temporary (Ireland Array, IA; Irish Seismic Lithospheric Experiment, ISLE; and Irish Seismological Upper Mantle Experiment, ISUME) stations, with a recording period of minimum two years. This represents a suitable data-set for RF studies and for the harmonic decomposition technique, providing a full range of backazimuth and a uniform coverage of Ireland. Preliminary results from RF analysis show strong consistency of signals from the different stations, with a clear signal of the P-to-S converted phase generated from an almost flat Moho, arriving around 4s after the direct P pulse. We also observed an intra-crustal positive pulse that matches what previously described by others authors. Finally, thanks to the harmonic decomposition analysis some anisotropic features have been found in the upper and lower crust. These results give a new insight into Ireland&#39;s lithospheric structure and evolution in the context of regional geodynamics.

The presence of subducted slabs in the Mediterranean has been well documented with seismic tomogr... more The presence of subducted slabs in the Mediterranean has been well documented with seismic tomography, however, these images, which are produced by smoothed, damped inversions, underestimate the sharpness of the structures. The position and extent of the slabs and the presence possible tears or gaps in the subducted lithosphere are still debated, yet the shape and location these structures are important for kinematic reconstructions and evolution of the entire subduction zone system. Extensive distribution of broadband seismic instrumentation in the Mediterranean (Italian National Seismic Network in Italy and the NSF--PICASSO project in Spain and Morocco) has allowed us to use alternative methodologies to detect the position of the slabs and slab tears beneath the Central and Western Mediterranean. Using S receiver functions we are able to identify S-to--p conversions from the bottom of the subducted slab and a lack of these signals where there are gaps or tears in the slab. We also analyze broadband waveforms for changes in P wave coda from deep (> 300 km depth) local earthquakes. The waveform records for stations in southern Italy and around the Betic--Rif show large amplitude, high frequency (f > 5 Hz) late arrivals with long coda after relatively low--frequency onset. High frequency arrivals are the strongest from events whose raypaths travel within the slab to the stations where they are recorded allowing for mapping of where the subducted material is located within the upper mantle. These two methods, along with inferring the slab position from fast P--wave velocity perturbations in tomography and intermediate depth seismicity, provide additional geophysical evidence to aid in interpretation of the complex, segmented slab structure beneath the Mediterranean.

Earth and Planetary Science Letters, 2014

Recordings of teleseismic earthquakes from a dense set of temporary and permanent broadband seism... more Recordings of teleseismic earthquakes from a dense set of temporary and permanent broadband seismic stations reveal the lithospheric structure of the northern Apennines and support the scenario of a retreating detachment within the mid-crust. Lithospheric delamination appears crucial to the formation and evolution of the Apennines orogen. Receiver-function (RF) stacks outline a continuous west-dipping Ps converted phase from a positive velocity jump that we interpret as the top of the lower crust and mantle of the Adria continental lithosphere, which is descending into the shallow mantle. The correlation of seismicity with two RF profiles across the northern Apennines suggests distinct stages of lithospheric delamination. Active penetration of the detachment into the Adria lithosphere seems evident in the south/east, with induced shallow-mantle flow facilitated by slab dehydration. Penetration of the detachment in the north/west seems to have arrested, and is possibly marked by crustal underplating. This layer atop the Apennines slab is visible only down to 80 km depth and suspends above an oppositely-dipping paired positive/negative Ps converted phase in stacked receiver functions. The break in the west-dipping Adria lithosphere conflicts with a westward-subduction scenario continuous from the Oligocene. Lateral changes of deep structure and seismicity along the northern Apennines suggest that underplating of crustal material and delamination-retreat are distinct mechanisms active today in the western and eastern sectors, respectively, of the northern Apennines. Negative Ps-pulses at 100-120 km depth help to define a seismic lithosphere-asthenosphere boundary (LAB), but cross-cut a volume of high-velocity mantle rock, as inferred from tomographic models. We hypothesize that this seismic LAB is a rheological discontinuity that affects the frequency band of seismic body waves, but not the long-term viscous response that governs the evolution and eventual detachment of the continental slab.

Geophysical Journal International, 2013

The accurate definition of 3-D crustal structures and, in primis, the Moho depth, are the most im... more The accurate definition of 3-D crustal structures and, in primis, the Moho depth, are the most important requirement for seismological, geophysical and geodynamic modelling in complex tectonic regions. In such areas, like the Mediterranean region, various active and passive seismic experiments are performed, locally reveal information on Moho depth, average and gradient crustal V p velocity and average V p /V s velocity ratios. Until now, the most reliable information on crustal structures stems from controlled-source seismology experiments. In most parts of the Alpine region, a relatively large number of controlled-source seismology information are available though the overall coverage in the central Mediterranean area is still sparse due to high costs of such experiments. Thus, results from other seismic methodologies, such as local earthquake tomography, receiver functions and ambient noise tomography can be used to complement the controlled-source seismology information to increase coverage and thus the quality of 3-D crustal models. In this paper, we introduce a methodology to directly combine controlled-source seismology and receiver functions information relying on the strengths of each method and in relation to quantitative uncertainty estimates for all data to derive a well resolved Moho map for Italy. To obtain a homogeneous elaboration of controlledsource seismology and receiver functions results, we introduce a new classification/weighting scheme based on uncertainty assessment for receiver functions data. In order to tune the receiver functions information quality, we compare local receiver functions Moho depths and uncertainties with a recently derived well-resolved local earthquake tomography-derived Moho map and with controlled-source seismology information. We find an excellent correlation in the Moho information obtained by these three methodologies in Italy. In the final step, we interpolate the controlled-source seismology and receiver functions information to derive the map of Moho topography in Italy and surrounding regions. Our results show high-frequency undulation in the Moho topography of three different Moho interfaces, the European, the Adriatic-Ionian, and the Liguria-Corsica-Sardinia-Tyrrhenia, reflecting the complexity of geodynamical evolution.

Unexpected features in teleseismic receiver function reveal the presence of a dipping interface w... more Unexpected features in teleseismic receiver function reveal the presence of a dipping interface with pronounced seismic velocity contrast in the Campania-Lucania region, Southern Apennines (Italy). Locus of this finding is the Melandro-Pergola basin, located between the maximum intensity areas of two of the most destructive earthquakes reported in the Italian seismic catalogue, the magnitude ˜ 7.0 Val d'Agri (1857) and Irpinia (1980) earthquakes. This area is not associated with known historical events and for this reason is currently object of investigation as potential seismic gap, where the probability of future ruptures is higher than in surrounding regions. Could the enlightened interface be the seismogenic source which fills the gap? Could the findings of this study offer a new tool to investigate and constraint the source parameters of historical earthquakes?

The paradox of how horizontal contraction and extension can occur simultaneously in convergent mo... more The paradox of how horizontal contraction and extension can occur simultaneously in convergent mountain belts remains a fundamental and largely unresolved problem in continental dynamics. The Apennines represent one of the most accessible &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot;type locality&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;quot; areas of syn-convergent extension. Rollback - which describes the tendency of a subducting plate to retreat from the orogenic front - is commonly invoked as

In the framework of the italian PNRA (Progetto Nazionale di Ricerche in Antartide), we have start... more In the framework of the italian PNRA (Progetto Nazionale di Ricerche in Antartide), we have started to re-analize teleseismic waveforms recorded, using three-components seismometers (equipped with 5 seconds sensors, Lennartz 3D-5s), during five summer campaings, from 1993 to 2000. Seismic stations were deployed around Terra Nova Bay (TNB) italian base, from the sea to reach the interior of the Transantartic Moun- tains (TAM), the most striking example of nocontractional mountain belt. During the last campaingn (1999-2000) seismic stations were deployed deep into Northern Vic- toria Land to reach Rennik and Lillie Glaciers Area and George V coast region, the northest part of TAM. Our main goals were: to compute, using frequency-domanin deconvolution method by Di Bona [1998], Receiver Functions covering all the area around TNB italian antartic base; to map of Moho-depth and intercrustal S-waves ve- locity discontinuity from 1-D velocity model computed using Sambridge's inversion s...

Results from recent geophysical and mantle-xenolith geochemistry studies of the Kaapvaal Craton a... more Results from recent geophysical and mantle-xenolith geochemistry studies of the Kaapvaal Craton appear, at times, to provide disparate views of the physical, chemical and thermal structure of the lithosphere. Models from our recent SAMTEX magnetotelluric (MT) surveys across the Kaapvaal Craton indicate a resistive, 220-240 km thick lithosphere for the central core of the craton. One published S-wave receiver function (SRF) study and other surface-wave studies suggest a thinner lithosphere characterised by a ~160 km thick high-velocity "lid" underlain by a low-velocity zone (LVZ) of between 65-150 km in thickness. Other seismic studies suggest that the (high-velocity) lithosphere is thicker, in excess of 220 km. Mantle xenolith pressure-temperature arrays from Mesozoic kimberlites require that the base of the "thermal" lithosphere (i.e., the depth above which a conductive geotherm is maintained - the tLAB) is at least 220 km deep, to account for mantle geotherms i...

Tectonophysics, 2015

This study investigates the geometry of the crust-mantle boundary along the Northern Apennines (I... more This study investigates the geometry of the crust-mantle boundary along the Northern Apennines (Italy) and its relationship to the tectonic evolution of a retreating subduction system. Using receiver function analysis from teleseismic records, the Moho depth was computed beneath 26 seismic stations located in key positions along the orogen. The results show a relatively shallow (~20 km) Moho in the Tyrrhenian side, as expected from an extended overriding plate, and a deeper Moho (~40 km) in the subducted Adriatic plate, SW-dipping 15-45°beneath the chain. Stations deployed on the Northeastern edge of the Plio-Quaternary extensional basins provide maximum Moho depth estimates, thus supporting the hypothesis that these basins define the zone where the two plates separate at depth. In addition, an abrupt change in the Moho depth is recognized along strike of the orogen, attributed to a 45 km-wide, trench-normal deformation zone that crosses the Northern Apennines in the proximity of the Livorno-Sillaro lineament. This zone is interpreted as an incipient lithospheric tear fault, which accommodates varying rates of trench retreat along the length of the subduction zone. Tectonophysics j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / t e c t o Please cite this article as: Piana Agostinetti, N., The structure of the Moho in the Northern Apennines: Evidence for an incipient slab tear fault?, Tectonophysics (2015), http://dx.

Geophysical Journal International

Trans-dimensional (trans-D) algorithms have been recently introduced to the earth sciences to sol... more Trans-dimensional (trans-D) algorithms have been recently introduced to the earth sciences to solve inverse problems without having to impose a fixed spatial structure to the model parametrization (e.g. a fixed number of layers in a 1D structure). Trans-dimensional al-gorithms can be easily used for joint inversion of different data-set using a "Hierarchical Bayes" approach. However, some difficulties arise if the two observables display very dif-ferent resolving power. In this case, the structure of the target solution might be twisted toward one observable, introducing non resolved (i.e. over-complex) structure for the other investigated parameters. In this study, we develop a trans-D algorithm for joint inversion of two different data-sets to reconstruct the 1D structure of two different physical parameters. The "parsimony" of the trans-D algorithm produces common discontinuities (a "coupled" structure) for portions of the 1D profile where the two ob...

Geophysical Journal International, 2015

Local earthquake tomography is a non-linear and non-unique inverse problem that uses event arriva... more Local earthquake tomography is a non-linear and non-unique inverse problem that uses event arrival times to solve for the spatial distribution of elastic properties. The typical approach is to apply iterative linearization and derive a preferred solution, but such solutions are biased by a number of subjective choices: the starting model that is iteratively adjusted, the degree of regularization used to obtain a smooth solution, and the assumed noise level in the arrival time data. These subjective choices also affect the estimation of the uncertainties in the inverted parameters. The method presented here is developed in a Bayesian framework where a priori information and measurements are combined to define a posterior probability density of the parameters of interest: elastic properties in a subsurface 3-D model, hypocentre coordinates and noise level in the data. We apply a trans-dimensional Markov chain Monte Carlo algorithm that asymptotically samples the posterior distribution of the investigated parameters. This approach allows us to overcome the issues raised above. First, starting a number of sampling chains from random samples of the prior probability distribution lessens the dependence of the solution from the starting point. Secondly, the number of elastic parameters in the 3-D subsurface model is one of the unknowns in the inversion, and the parsimony of Bayesian inference ensures that the degree of detail in the solution is controlled by the information in the data, given realistic assumptions for the error statistics. Finally, the noise level in the data, which controls the uncertainties of the solution, is also one of the inverted parameters, providing a first-order estimate of the data errors. We apply our method to both synthetic and field arrival time data. The synthetic data inversion successfully recovers velocity anomalies, hypocentre coordinates and the level of noise in the data. The Bayesian inversion of field measurements gives results comparable to those obtained independently by linearized inversion, reconstructing the geometry of the main seismic velocity anomalies. The quantification of the posterior uncertainties, a crucial output of Bayesian inversion, allows for visualizing regions where elastic properties are closely constrained by the data and is used here to directly compare our results to the ones obtained with the linearized inversion. In the case we examined the results of two inversion techniques are not significantly different.

Geophysical Journal International, 2014

This work presents a teleseismic P-wave receiver function study on 34 stations deployed across Ir... more This work presents a teleseismic P-wave receiver function study on 34 stations deployed across Ireland in order to determine the first-order crustal properties, thickness (H) and mean crustal V p /V s , over the entire island. We apply the H − V p /V s stacking method, which exploits the information contained in both the Ps and the multiple phases from the free-surface. In this way, we obtain the first Moho depth and V p /V s maps of Ireland based on a uniform distribution of measurements. The results are used to examine in detail the lateral variation of crustal thickness and V p /V s ratio across the major terrane boundaries in Ireland. Our results show a good agreement with the available previous estimates from onshore wide-angle/refraction experiments and add new information in poorly constrained areas such as Northern Ireland and the NW coast of Ireland. The mean V p /V s ratio is 1.73 ± 0.05 with a consistently low (≈1.70) value in the Leinster domain and in central Ireland. The mean crustal thickness is 30.9 ± 2.3 km. The southern portion of the island shows a nearly flat Moho at a depth of 32-33 km, while north of the Southern Uplands Fault, a relatively higher spatial frequency variation in Moho topography exists with values ranging from 28 to 32 km. This reflects the complex history of multiphase terranes accretion during the Caledonian orogeny, although locally, the superposition of more recent geological processes is not excluded. Crossing the Iapetus Suture Zone, our results support the presence of a 'transitional' Moho, that is, a 3-4 km smooth seismic transition between crust and mantle, while Moho depth remains constant. Anomalous values in Northern Ireland are interpreted as evidence of a 5-to 6-kmthick high S-wave velocity layer just above the Moho.

Earth and Planetary Science Letters, 2014

In this study, we use teleseismic receiver function analysis to image the seismic structure of th... more In this study, we use teleseismic receiver function analysis to image the seismic structure of the Juan de Fuca oceanic plate during its subduction beneath the North American plate. Seismic data have been recorded at 58 seismic stations deployed along the northern Cascadia subduction zone. Harmonic decomposition of the receiver function data-set along a trench-normal profile allows us to image both the isotropic and the anisotropic structure of the plate (slab). Our images highlight the presence of a highly anisotropic region at 40-70 km depths across the Cascadia subduction zone. The detected seismic anisotropy is interpreted to be related to both metamorphic facies (e.g. blueschists) and fluid released during the dehydration of the subducting mantle. The processes of dehydration and metamorphism produce the variations of the seismic properties within each lithologic unit that constitutes the subducted slab, i.e. basalts, gabbro layer and upper mantle, as the oceanic plate sinks in the upper mantle. Such variations make it almost impossible to recognize the "plate boundary" as a characteristic "velocity-jump" at depth (neither positive nor negative) along the Cascadia subduction zone. Based on the comparative interpretation of both the isotropic and the anisotropic structures retrieved, we propose a 4-stage model of the evolution of the Juan de Fuca oceanic plate during its subduction beneath the North American plate.

Energy Procedia, 2014

We report the preliminary results from a project (GAPSS-Geothermal Area Passive Seismic Sources),... more We report the preliminary results from a project (GAPSS-Geothermal Area Passive Seismic Sources), aimed at testing the resolving capabilities of passive exploration methods on a well-known geothermal area, namely the Larderello-Travale Geothermal Field (LTGF). Located in the western part of Tuscany (Italy), LTGF is the most ancient geothermal power field of the world. GAPSS consisted of up to 20 seismic stations deployed over an area of about 50 x 50 Km. During the first 12 months of measurements, we located more than 2000 earthquakes, with a peak rate of up to 40 shocks/day. Preliminary results from analysis of these signals include: (i) analysis of Shear-Wave-Splitting from local earthquake data, from which we determined the areal distribution of the most anisotropic regions; (ii) local-earthquake travel-time tomography for both P-and S-wave velocities; (iii) telesismic receiver function aimed at determining the high-resolution (<0.5km) S-velocity structure over the 0-20km depth range, and seismic anisotropy using the decomposition of the angular harmonics of the RF data-set; (iv) S-wave velocity profiling through inversion of the dispersive characteristics of Rayleigh waves from earthquakes recorded at regional distances. After presenting results from these different analyses, we eventually discuss their potential application to the characterisation and exploration of the investigated area.

Journal of Geodynamics, 2014

While the upper crustal structure of the Southern Apennines is known, lack of control on the deep... more While the upper crustal structure of the Southern Apennines is known, lack of control on the deep structure allows competing thin-skinned and thick-skinned models of the orogen. In thin-skinned models, the detachment decouples a stack of rootless nappes from the basement. In thick-skinned models, basement is involved in the most recent phase of thrusting. To examine crustal structure, we use teleseismic data from the Calabria-Apennine-Tyrrhenian/Subduction-Accretion-Collision Network (CAT/SCAN) array in southern Italy. We use receiver functions (RF) processed into a common conversion point stack to generate images of the crust. Interpretation and correlation to geological structure are done using inversions of individual station RFs. We focus on a shallow discontinuity where P-to-S conversions occur. In the foreland, it corresponds to velocity jumps between carbonate and clastic strata with basement. A similar interpretation for the Apennines provides the most parsimonious explanation and supports a thick-skinned interpretation. In a thick-skinned reconstruction, the amount of shortening is much smaller than for a thin-skinned model. This implies considerably less Pliocene-Pleistocene shortening across the Apennines and suggests an east-southeast motion of the Calabrian arc subparallel to the southern Apennines rather than a radial expansion of the arc.

We investigate crustal structure of the northern Apennines using broadband seismic data from a de... more We investigate crustal structure of the northern Apennines using broadband seismic data from a dense linear transect. Receiver functions reveal significant differences between the responses of the Tyrrhenian and Adriatic sides of the Apennines, and determine the spatial location of a change in this response, just west of the high crest of the mountains. We find a clear signature of

ABSTRACT The Iapetus Ocean closure during the Caledonian Orogeny in the Paleozoic led to a multi-... more ABSTRACT The Iapetus Ocean closure during the Caledonian Orogeny in the Paleozoic led to a multi-phase continental collision between different paleocontinents in the North Atlantic region. It is believed that evidence of these past geological events could be preserved in the fabric of the Ireland&#39;s crust and upper mantle, especially across the Iapetus Suture Zone which cuts through the island with a NE-SW trend. In the framework of the SIM-CRUST project, we studied this open matter through the use of classical teleseismic receiver function (RF) method, in order to map Ireland&#39;s Moho depth and highlight intra-crustal seismic discontinuities. Moreover, thanks to the recently developed RF harmonic decomposition technique, the presence of crustal anisotropy has been investigated analysing the changes in the amplitudes of the P-to-S converted phases as a function of backazimuth. In this study, we analysed teleseismic records from about 40 broadband seismic stations deployed across Ireland in the last decade. We used data from both permanent (Irish National Seismic Network , INSN) and temporary (Ireland Array, IA; Irish Seismic Lithospheric Experiment, ISLE; and Irish Seismological Upper Mantle Experiment, ISUME) stations, with a recording period of minimum two years. This represents a suitable data-set for RF studies and for the harmonic decomposition technique, providing a full range of backazimuth and a uniform coverage of Ireland. Preliminary results from RF analysis show strong consistency of signals from the different stations, with a clear signal of the P-to-S converted phase generated from an almost flat Moho, arriving around 4s after the direct P pulse. We also observed an intra-crustal positive pulse that matches what previously described by others authors. Finally, thanks to the harmonic decomposition analysis some anisotropic features have been found in the upper and lower crust. These results give a new insight into Ireland&#39;s lithospheric structure and evolution in the context of regional geodynamics.

The presence of subducted slabs in the Mediterranean has been well documented with seismic tomogr... more The presence of subducted slabs in the Mediterranean has been well documented with seismic tomography, however, these images, which are produced by smoothed, damped inversions, underestimate the sharpness of the structures. The position and extent of the slabs and the presence possible tears or gaps in the subducted lithosphere are still debated, yet the shape and location these structures are important for kinematic reconstructions and evolution of the entire subduction zone system. Extensive distribution of broadband seismic instrumentation in the Mediterranean (Italian National Seismic Network in Italy and the NSF--PICASSO project in Spain and Morocco) has allowed us to use alternative methodologies to detect the position of the slabs and slab tears beneath the Central and Western Mediterranean. Using S receiver functions we are able to identify S-to--p conversions from the bottom of the subducted slab and a lack of these signals where there are gaps or tears in the slab. We also analyze broadband waveforms for changes in P wave coda from deep (> 300 km depth) local earthquakes. The waveform records for stations in southern Italy and around the Betic--Rif show large amplitude, high frequency (f > 5 Hz) late arrivals with long coda after relatively low--frequency onset. High frequency arrivals are the strongest from events whose raypaths travel within the slab to the stations where they are recorded allowing for mapping of where the subducted material is located within the upper mantle. These two methods, along with inferring the slab position from fast P--wave velocity perturbations in tomography and intermediate depth seismicity, provide additional geophysical evidence to aid in interpretation of the complex, segmented slab structure beneath the Mediterranean.

Earth and Planetary Science Letters, 2014

Recordings of teleseismic earthquakes from a dense set of temporary and permanent broadband seism... more Recordings of teleseismic earthquakes from a dense set of temporary and permanent broadband seismic stations reveal the lithospheric structure of the northern Apennines and support the scenario of a retreating detachment within the mid-crust. Lithospheric delamination appears crucial to the formation and evolution of the Apennines orogen. Receiver-function (RF) stacks outline a continuous west-dipping Ps converted phase from a positive velocity jump that we interpret as the top of the lower crust and mantle of the Adria continental lithosphere, which is descending into the shallow mantle. The correlation of seismicity with two RF profiles across the northern Apennines suggests distinct stages of lithospheric delamination. Active penetration of the detachment into the Adria lithosphere seems evident in the south/east, with induced shallow-mantle flow facilitated by slab dehydration. Penetration of the detachment in the north/west seems to have arrested, and is possibly marked by crustal underplating. This layer atop the Apennines slab is visible only down to 80 km depth and suspends above an oppositely-dipping paired positive/negative Ps converted phase in stacked receiver functions. The break in the west-dipping Adria lithosphere conflicts with a westward-subduction scenario continuous from the Oligocene. Lateral changes of deep structure and seismicity along the northern Apennines suggest that underplating of crustal material and delamination-retreat are distinct mechanisms active today in the western and eastern sectors, respectively, of the northern Apennines. Negative Ps-pulses at 100-120 km depth help to define a seismic lithosphere-asthenosphere boundary (LAB), but cross-cut a volume of high-velocity mantle rock, as inferred from tomographic models. We hypothesize that this seismic LAB is a rheological discontinuity that affects the frequency band of seismic body waves, but not the long-term viscous response that governs the evolution and eventual detachment of the continental slab.