Ellitticità delle onde di Rayleigh e struttura crostale superficiale.

The axes ratio of elliptically-polarised Rayleigh-wave particle motion may provide information about shallow crustal structure beneath a seismographic station. This seldom-used parameter, often called ellipticity, or H/V, is different from the popular horizontal to vertical amplitude ratio of ambient seismic noise used for microzonation and similar studies, as we refer to deterministic measurements on Rayleigh-wave, fundamental-mode, wave groups originated by distant earthquakes. This measure is not influenced either by the source, or the propagation, and may yield robust information on crustal structure at shallower depth than group or phase velocity at the same frequency. Ellipticity is a potentially excellent tool to retrieve seismic parameters in sedimentary basins. We intend to examine feasibility and robustness of ellipticity measurements, and their application to retrieval of shallow crustal structure in northern Italy. The most striking structural element is the Po Plain sedimentary basin, but also sedimentary and crystalline rocks in the Northern Apennines and Alps. The Po Plain hides a complex system of active thrusts and folds that caused the 2012 seismic sequence. Our aim is to set a reliable system to measure and use H/V ratio to study the shallow crustal structure of this region. We implement and test automatic measurements, for 95 seismic stations in northern Italy. Comparison between observations and predictions from a reference crustal model show substantial fit, particularly for T ≈ 38s data. Discrepancy for shorter periods suggests that slight modifications of the model are needed. Analysis of synthetic and real data indicates the possibility of prograde (inverse) particle motion, but the influence of ambient noise at the predicted, short, transition periods, makes such observations less reliable. We further invert ellipticity curves to retrieve Vs profiles beneath each station, and finally build a high-resolution crustal model of the Po Plain

Extraction and applications of Rayleigh wave ellipticity in polar regions

Seismic Rayleigh wave ellipticity measurements are the horizontal-to-vertical ratio of the Rayleigh wave particle motion, and are sensitive to the subsurface structure beneath a seismic station. H/V ratios measured from the ambient vibrations of the Earth are being increasingly used in glaciological applications to determine glacier and ice sheet thickness, seismic velocities and firn properties. Using the newly developed degree-of-polarisation (DOP-E) method which exploits the polarisation properties of seismic noise, we identify and extract Rayleigh waves from seismic stations in Greenland, and relate them to sea ice processes and the geology of the upper crust. Finally, we provide some suggestions for future applications of DOP-E method to gain greater insight into seasonal and long-term variability of sea ice formation and breakup as well as the monitoring of ice sheet thickness, subglacial environment and firn layers in the poles

Constraining subglacial geology using ambient noise Rayleigh wave ellipticity

EGU General Assembly 2020 Online, 4-8 May 2020B asal slip is an important mechanism by which glaciers and ice-sheets flow, and is a major source of uncertainty in simulations of ice-mass loss and sea level rise from the Greenland Ice Sheet (GrIS). Sub-ice geology is a dominant control on ice flow velocity with fast flow often coinciding with the presence of deformable subglacial till eroded from underlying sedimentary rocks. The subglacial geology of Greenland has received relatively little attention thus far and its control on ice flow is poorly understood. Seismic studies of the crust beneath the GrIS have been limited due to a lack of seismic stations and the reliance on earthquake event data. However, in the past decade, there has been a rapid increase in the number of both permanent and temporary seismic stations deployed in Greenland as well developments in ambient noise methods, allowing for improved spatial resolution of crustal geology. Ellipticity measurements, the ratio of the horizontal to vertical component of a Rayleigh wave, have been shown to be particularly sensitive to the geological structure directly beneath the station. Ambient noise H/V measurements have been used for decades in geotechnical and civil engineering for site characterisation, making them a well-suited technique to determine the subglacial geology of the GrIS. Using all available broadband stations deployed on Greenland from 2012 to 2018 we extract Rayleigh wave ellipticity measurement from ambient noise data using the degree-of-polarization (DOP) method where meaningful signals are defined as a waveform with an arbitrary polarization which remains stable for a given time window. We invert these ellipticity measurements in the period range of 4 ¿ 9 s to generate Vs profiles of the first 5 km beneath each station. Our inversions indicate that: (1) off-ice stations along the margins of the GrIS produce a good agreement with the litho1.0 model to within error and (2) an additional subglacial layer 1.0 - 2.0km thick with a Vs < 3.0km is necessary to match the data recorded at several of the on-ice stations. We attribute these observations to the widespread presence of sedimentary rocks beneath the GrIS, potentially capable of sustaining extensive subglacial till layers that can support enhanced basal slip

Retrieving sea ice drag coefficients and turning angles from in situ and satellite observations using an inverse modeling framework

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124(8), (2019): 6388-6413, doi: 10.1029/2018JC014881.For ice concentrations less than 85%, internal ice stresses in the sea ice pack are small and sea ice is said to be in free drift. The sea ice drift is then the result of a balance between Coriolis acceleration and stresses from the ocean and atmosphere. We investigate sea ice drift using data from individual drifting buoys as well as Arctic‐wide gridded fields of wind, sea ice, and ocean velocity. We perform probabilistic inverse modeling of the momentum balance of free‐drifting sea ice, implemented to retrieve the Nansen number, scaled Rossby number, and stress turning angles. Since this problem involves a nonlinear, underconstrained system, we used a Monte Carlo guided search scheme—the Neighborhood Algorithm—to seek optimal parameter values for multiple observation points. We retrieve optimal drag coefficients of CA=1.2×10−3 and CO=2.4×10−3 from 10‐day averaged Arctic‐wide data from July 2014 that agree with the AIDJEX standard, with clear temporal and spatial variations. Inverting daily averaged buoy data give parameters that, while more accurately resolved, suggest that the forward model oversimplifies the physical system at these spatial and temporal scales. Our results show the importance of the correct representation of geostrophic currents. Both atmospheric and oceanic drag coefficients are found to decrease with shorter temporal averaging period, informing the selection of drag coefficient for short timescale climate models.The scripts developed for this publication are available at the GitHub (https://github.com/hheorton/Freedrift_inverse_submit). The Neighborhood Algorithm was developed and kindly supplied by M. Sambridge (http://www.iearth.org.au/codes/NA/). Ice‐Tethered Profiler data are available via the Ice‐Tethered Profiler program website (http://whoi.edu/itp). Buoy data were collected as part of the Marginal Ice Zone program (www.apl.washington.edu/miz) funded by the U.S. Office of Naval Research. The ice drift data were kindly supplied by N. Kimura. H. H. was funded by the Natural Environment Research Council (Grants NE/I029439/1 and NE/R000263/1). M. T. was partially funded by the SKIM Mission Science Study (SKIM‐SciSoc) Project ESA RFP 3‐15456/18/NL/CT/gp. T. A. was supported at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. M. T. and H. H. thank Dr. Nicolas Brantut for early discussions on the implementation of inverse modeling techniques.2020-02-1

Crustal structure of northern Italy from the ellipticity of Rayleigh waves

Northern Italy is a diverse geological region, including the wide and thick Po Plain sedimentary basin, which is bounded by the Alps and the Apennines. The seismically slow shallow structure of the Po Plain is difficult to retrieve with classical seismic measurements such as surface wave dispersion, yet the detailed structure of the region greatly affects seismic wave propagation and hence seismic ground shaking. Here we invert Rayleigh wave ellipticity measurements in the period range 10–60 s for 95 stations in northern Italy using a fully non linear approach to constrain vertical and density profiles of the crust beneath each station. The ellipticity of Rayleigh wave ground motion is primarily sensitive to shear-wave velocity beneath the recording station, which reduces along-path contamination effects. We use the 3D layering structure in MAMBo, a previous model based on a compilation of geological and geophysical information for the Po Plain and surrounding regions of northern Italy, and employ ellipticity data to constrain and density within its layers. We show that ellipticity data from ballistic teleseismic wave trains alone constrain the crustal structure well. This leads to MAMBo-E, an updated seismic model of the region’s crust that inherits information available from previous seismic prospection and geological studies, while fitting new seismic data well. MAMBo-E brings new insights into lateral heterogeneity in the region’s subsurface. Compared to MAMBo, it shows overall faster seismic anomalies in the region’s Quaternary, Pliocene and Oligo-Miocene layers and better delineates the seismic structures of the Po Plain at depth. Two low velocity regions are mapped in the Mesozoic layer in the western and eastern parts of the Plain, which seem to correspond to the Monferrato sedimentary basin and to the Ferrara-Romagna thrust system, respectively.Published1-147T. Struttura della Terra e geodinamicaJCR Journa

Ellipticity of Rayleigh waves in basin and hard-rock sites in Northern Italy

We measure ellipticity of teleseismic Rayleigh waves at 95 seismic stations in Northern Italy, for wave period between 10 and 110 s, using an automatic technique and a large volume of high-quality seismic recordings from over 500 global earthquakes that occurred in 2008– 2014. Northern Italy includes a wide range of crustal structures, from the wide and deep Po Plain sedimentary basin to outcropping sedimentary and crystalline rocks in the Northern Apennines and Alps. It thus provides an excellent case for studying the influence of shallow earth structure on polarization of surface waves. The ellipticity measurements show excellent spatial correlation with geological features in the region, such as high ellipticity associated with regions of low seismic velocity in the Po Plain and low ellipticity values in faster, hard rock regions in the Alps and Apennine mountains. Moreover, the observed ellipticity values also relate to the thickness of the basement, as highlighted by observed differences beneath the Alps and the Apennines. Comparison between observations and predicted ellipticity from a reference crustal model of the region show substantial fit, particularly for T ∼ 38 s data. Discrepancy for shorter wave period suggests that slight modifications of the model are needed, and that the ellipticity measurements could help to better constrain the shallow crustal structure of the region. Predictions for the Po Plain are larger than the observations by a factor of four or more and transition from retrograde to prograde Rayleigh wave motion at the surface for periods of T ∼ 10–13 s is predicted for seismic stations in the plain. Analysis of corresponding real data indicates a possible detection of teleseismic prograde particle motion, but the weak teleseismic earthquake signals are mixed with ambient noise signals at the predicted, short, transition periods. Detection of the period of polarity inversion from the joint analysis of earthquake and ambient noise ellipticity measurements may provide further, stringent, constraints on the structure of sedimentary basins.Published395–4074T. Sismologia, geofisica e geologia per l'ingegneria sismicaJCR Journa

Crustal structure beneath Portugal from teleseismic Rayleigh Wave Ellipticity

Up until now, Portugal lacked a countrywide shear velocity model sampling short length-scale crustal structure, which limits interpretations of seismicity and tectonics, and predictions of strong ground motion. In turn, such interpretations and predictions are important to help mitigate risk of destruction from future large on- and offshore earthquakes similar to those that Portugal has experienced in the past (e.g. the Mw 8.5–8.7 tsunamigenic event in 1755). In this study, we measured teleseismic Rayleigh Wave Ellipticity (RWE) from 33 permanent and temporary seismic stations in Portugal with wave periods between 15 s and 60 s, and inverted it for 1-D models of shear wave velocity (Vs) structure beneath each station using a fully non-linear Monte Carlo method. Because RWE is strongly sensitive to the uppermost few kilometres of the crust, both RWE measurements and Vs models are spatially correlated with surface geology in Portugal. For instance, we find that sedimentary basins produced by rifting that had begun in the Mesozoic such as the Lusitanian Basin (LB) and the Lower Tagus-Sado Basin (LTSB) are characterised by higher RWE (lower Vs). Interestingly, we observe similar RWE (and Vs) values in the interior of the Central Iberian Zone (CIZ), which is a metamorphic belt of Paleozoic age. Together with reduced crustal thickness previously estimated for the same parts of the CIZ, this suggests that the CIZ might have experienced an episode of extension possibly simultaneous to Mesozoic rifting. The Galicia-Tras-os-Montes-Zone (GTMZ) that has undergone polyphased deformation since the Paleozoic is characterised by the lowest RWE (highest Vs) in Portugal. Ossa Morena Zone and the South Portuguese Zone exhibit intermediate Vs values when compared to that of basins and the GTMZ. Our crustal Vs model can be used to provide new insights into the tectonics, seismicity and strong ground motion in Portugal.Published344-3617T. Struttura della Terra e geodinamicaJCR Journa

Investigating the Antarctic subglacial liquid water layer using the ellipticity of Rayleigh waves from polarization analysis of seismic noise

EGU General Assembly in Viena, Austria,7–12 April 2019We investigate the seismic structure of the uppermost ice and crustal layers beneath the Concordia station in Antarctica using a new method based on the inversion of ellipticity of Rayleigh waves from ambient noise by degree-of-polarization analysis (DOP-E). The new technique, validated by various synthetic tests, shows a good capability of separating ambient noise containing polarized Rayleigh waves from noise containing Love waves and uncorrelated noise. It also gives information on the azimuthal direction of the sources, giving the possibility to better characterise the ambient noise sources. We apply this technique to 1 month of continuous noise record in the period band 2 – 10 s, and complement such analysis with measurement of Rayleigh-wave ellipticity on earthquake data (in the period band 10 – 60 s). Results show no evidence of a liquid water layer beneath the ice directly beneath the station confirming the results from previous studies. To further validate this result we perform a synthetic test demonstrating that this technique is able to resolve a thin (>100m) liquid water layer at the base of the ice (3.5km). The DOP-E technique could be a new tool to better illuminate the uppermost crustal layers. Since DOP-E is a completely single-station technique, it can be used when a dense seismic array is not available. It can also be used to monitor possible transients in the shear-waves velocity in a wide range of geological settings such as volcanoes, fault zones and glaciers

Constraining S -wave velocity using Rayleigh wave ellipticity from polarization analysis of seismic noise

We develop a new method for measuring ellipticity of Rayleigh waves from ambient noise records by degree-of-polarization (DOP) analysis. The new method, named DOP-E, shows a good capability to retrieve accurate ellipticity curves separated from incoherent noise. In order to validate the method we perform synthetic tests simulating noise in a 1-D earth model. We also perform measurements on real data from Antarctica and Northern Italy. Observed curves show a good fit with measurements from earthquake records and with theoretical ellipticity curves. The inversion of real data measurements for v S structure shows a good agreement with previous models. In particular, the shear-wave structure beneath Concordia station shows no evidence of a significant layer of liquid water at the base of the ice. The new method can be used to measure ellipticity at high frequency and therefore it will allow the imaging of near-surface structure, and possibly of temporal changes in subsurface properties. It promises to be useful to study near-surface processes in a wide range of geological settings, such as volcanoes, fault zones and glaciers. © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society.This work received a fundamental contribution by TIDES COST Action ES1401 (in particular by Short Term Scientific Mission program). A.M.G. Ferreira and A. Berbellini are also grateful for funding from NERC project NE/N011791/1. M. Schimmel acknowledges funding by the Spanish MISTERIOS project (CGL2013-48601-C2-1-R).Peer reviewe

The anatomy of uppermost mantle shear-wave speed anomalies in the western U.S. from surface-wave amplification

We build SWUS-amp, a three-dimensional shear-wave speed model of the uppermost mantle of the western U.S. using Rayleigh wave amplification measurements in the period range of 35–125 s from teleseismic earthquakes. This represents the first-ever attempt to invert for velocity structures using Rayleigh wave amplification data alone. We use over 350,000 Rayleigh wave amplitude measurements, which are inverted using a Monte Carlo technique including uncertainty quantification. Being a local seismic observable, Rayleigh wave amplification is little affected by path-averaged effects and in principle has stronger depth resolution than classical seismic observables, such as surface wave dispersion data. SWUS-amp confirms shallow mantle heterogeneities found in previous models. In the top 100 km of the mantle, we observe low-velocity anomalies associated with Yellowstone and the Basin & Range province, as well as a fast-velocity anomaly underneath the Colorado Plateau, where a strong velocity gradient at its edges shows a drastic contrast with its surroundings. SWUS-amp also gives additional insights into the current state of the uppermost mantle in the region. We image a high-velocity anomaly beneath the high-topography Wyoming province with a maximum depth extent of about 150–170 km, which is shallower than in previous tomographic models, and resolves previous inconsistencies with geological information. Beneath the Snake River Plain, a finger-like low-velocity anomaly dips to the west, suggesting lateral flow in the region. Below about 150 km depth, SWUS-amp shows a north-south dichotomy in shear-wave speed structure, with the northern region showing mostly high-velocity anomalies, whereas the southern region shows low-velocity anomalies. This is consistent with the continuous subduction history of the western U.S. and with the recent extension and uplift of the southern region.Published1158221T. Struttura della TerraJCR Journa