Ana M. G. Ferreira

On the robustness of global radially anisotropic surface wave tomography

A number of recent global tomographic studies have modeled three dimensional variations in the parameters of radial anisotropy. As yet there is limited agreement among such studies, suggesting significant uncertainties in the models, which could lead to divergent geodynamical interpretations. In this study we assess the robustness of lateral variations in radial anisotropy globally in the upper mantle and in the transition zone to determine the extent to which anisotropic parameters are constrained by a data set of over 10,000,000 fundamental and higher mode surface wave dispersion measurements. We carry out inversions for isotropic and radially anisotropic shear wave velocity, systematically changing regularization and using three different crustal models to remove the effects of the crust on the data. Using crustal corrections from different crustal models has an impact on the data fit comparable or larger than that obtained by including lateral variations of radial anisotropy in the modeling. Moreover, the use of crustal corrections from different a priori crustal models may lead to different images of radial anisotropy suggesting divergent geodynamical interpretations. This work suggests that the three‐dimensional determination of global radial anisotropy in the Earths mantle using surface wave dispersion data is still an ongoing experiment.

Global compilation of interferometric synthetic aperture radar earthquake source models: 1. Comparisons with seismic catalogs

[1] While many earthquakes have now been studied using interferometric synthetic aperture radar (InSAR) data, a full assessment of the quality and additional value of InSAR source parameters compared to seismological techniques is still lacking. We compile a catalog of source models obtained using InSAR and estimate the corresponding centroid moment tensor (CMT) parameters; we refer to this compilation as the ICMT archive. We compare source parameters from over 70 InSAR studies of 57 global earthquakes with those in the Global CMT (GCMT), International Seismological Centre (ISC) and Engdahl‐Hilst‐Buland (EHB) seismic catalogs. We find an overall good agreement between fault strike, dip and rake values in the GCMT and ICMT archives. Likewise, the differences in seismic moment between these two archives are relatively small, and we do not find support for previously suggested trends of InSAR leading to larger moments than seismic data. However, epicentral locations show substantial discrepancies, which are larger for the GCMT (median differences of ∼21 km) than for the EHB and ISC catalogs (median differences of ∼10 km). Since InSAR data have a high spatial resolution, and thus should map epicentral locations accurately, this allows us to obtain a first independent estimate of epicentral location errors in the seismic catalogs. Earthquake depths from InSAR are systematically shallower than those in the EHB catalog, with a median of differences of ∼5 km. While this trend may be partly due to unmodeled crustal complexity, it is also compatible with the observation that the rupture of crustal earthquakes tends to propagate upward in the seismogenic layer.

Joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations

We present a new global whole-mantle model of isotropic and radially anisotropic S velocity structure (SGLOBE-rani) based on ~43,000,000 surface wave and ~420,000 body wave travel time measurements, which is expanded in spherical harmonic basis functions up to degree 35. We incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure. This is possible since we utilize short-period group-velocity data with a period range down to 16 s, which are strongly sensitive to the crust. The isotropic S velocity model shares common features with previous global S velocity models and shows excellent consistency with several high-resolution upper mantle models. Our anisotropic model also agrees well with previous regional studies. Anomalous features in our anisotropic model are faster SV velocity anomalies along subduction zones at transition zone depths and faster SH velocity beneath slabs in the lower mantle. The derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts, which has been difficult to constrain due to poor access compared with continental crusts.

Multiparameter monitoring of Fogo Island, Cape Verde, for volcanic risk mitigation

Fogo Island in the Cape Verde Archipelago (North Atlantic) is a stratovolcano of nearly conical shape that rises 2829 m above sea level and V6000 m above the surrounding seafloor. With a population of 40 000, the island has known intense historical volcanic activity since AD 1500, with an average interval between eruptions of the order of 20 years. Twentieth-century rates were more subdued, with only two flank eruptions in 1951 and 1995. Following the 1995 eruption, increased awareness of the volcanic hazard affecting the population of the island led to the deployment of the permanent VIGIL Network. Seismographic stations (both broadband and short-period), tiltmeters and a CO2 sensor where installed in Fogo, together with a telemetry infrastructure to allow remote real-time monitoring. A broadband seismographic station was installed in neighbour Brava Island. The operation of the network was complemented by the introduction of routine geodetic and microgravity surveying and the operation of an automatic meteorological station. In this paper, we describe the methodology adopted to monitor the volcanic activity, combining real-time data analysis (volcanotectonic and volcanic earthquakes, volcanic tremor and tilt) with repeated surveying at intervals of several months (GPS, microgravity). Examples of data from the first years of operation are presented. In particular, the data pertaining to a period of anomalous activity in September^October 2000 are discussed, in the context of the risk mitigation strategy currently being developed.

Constraint on the S-wave velocity at the base of the mantle

Some recent seismic studies have suggested the presence of a thin ultra-low P-wave velocity layer (ULVZ) at the base of the mantle, which is interpreted to be due to presence of partial melting. Partial melting would lead to a strong decrease of the S-wave velocity for which there is no seismic evidence. Such a decrease in the S-wave velocity would produce a strong precursor to SKS phase from the conversion of S to P at the upper boundary of the layer. We analyze records of events from the subduction zones in the south-west Pacific region obtained at stations in North America. At the source side, the converted phases propagate in the region, where the ultra-low P-wave velocity has been found earlier. Our analysis demonstrates that either the S-wave velocity drop in this layer is much smaller than predicted by the hypothesis of melting, or the layer is so thin (less than about 10–15 km) that it can not be detected with our technique.

Comparison of fluid tiltmeter data with long-period seismograms: Surface waves and Earth's free oscillations

We compare observations of long-period seismic surface waves and free oscillations recorded by high-resolution long-base fluid tube tiltmeters and by nearby broadband seismometers after large earthquakes. The quality of the tiltmeter data is comparable to that of the best horizontal component seismic data, recording some of the gravest free oscillations of the Earth, as well as successive passages of seismic surface waves circling the globe. We compare the observations with theoretical seismograms and with theoretical tilt. The predicted and observed surface wave tilt waveforms are very similar provided that we take into account horizontal acceleration effects on the tiltmeter. Phase and amplitude anomalies between the waveforms are well explained by the theoretical transfer function of the instrument. Likewise, observed horizontal seismograms converted into tilt match the tiltmeter data very well. Long-base fluid tube tiltmeters could potentially contribute to obtain high-quality measurements of the long-period seismic spectrum.

Upper- and mid-mantle interaction between the Samoan plume and the Tonga–Kermadec slabs

Mantle plumes are thought to play a key role in transferring heat from the core–mantle boundary to the lithosphere, where it can significantly influence plate tectonics. On impinging on the lithosphere at spreading ridges or in intra-plate settings, mantle plumes may generate hotspots, large igneous provinces and hence considerable dynamic topography. However, the active role of mantle plumes on subducting slabs remains poorly understood. Here we show that the stagnation at 660 km and fastest trench retreat of the Tonga slab in Southwestern Pacific are consistent with an interaction with the Samoan plume and the Hikurangi plateau. Our findings are based on comparisons between 3D anisotropic tomography images and 3D petrological-thermo-mechanical models, which self-consistently explain several unique features of the Fiji–Tonga region. We identify four possible slip systems of bridgmanite in the lower mantle that reconcile the observed seismic anisotropy beneath the Tonga slab (VSH>VSV) with thermo-mechanical calculations.

JAK/STAT controls organ size and fate specification by regulating morphogen production and signalling

A stable pool of morphogen-producing cells is critical for the development of any organ or tissue. Here we present evidence that JAK/STAT signalling in the Drosophila wing promotes the cycling and survival of Hedgehog-producing cells, thereby allowing the stable localization of the nearby BMP/Dpp-organizing centre in the developing wing appendage. We identify the inhibitor of apoptosis dIAP1 and Cyclin A as two critical genes regulated by JAK/STAT and contributing to the growth of the Hedgehog-expressing cell population. We also unravel an early role of JAK/STAT in guaranteeing Wingless-mediated appendage specification, and a later one in restricting the Dpp-organizing activity to the appendage itself. These results unveil a fundamental role of the conserved JAK/STAT pathway in limb specification and growth by regulating morphogen production and signalling, and a function of pro-survival cues and mitogenic signals in the regulation of the pool of morphogen-producing cells in a developing organ.

Toroidal free oscillations of the Earth observed by a ring laser system: a comparative study

In this study, we explore the potential of measuring systematically the Earths free oscillations using ring laser gyro (RLG) vertical axis rotational records. The RLG that we use is the vertical axis G-ring laser system of the Geodetic Observatory Wettzell (Germany). In 2009, its signal-to-noise ratio was considerably improved over the broadband frequency range of seismic measurements. Since then, three large magnitude earthquakes have occurred (Samoa Islands 2009; Maule, Chile, 2010; and Tohoku, Japan, 2011), leading to the first direct observations of rotational ground motions induced by toroidal free oscillations of the Earth. We compare these G-ring laser observations with synthetic seismograms computed by summing normal modes. We also analyse amplitude spectra of real and synthetic data to aid in the interpretation of the observations. We show that several toroidal modes are detected by the G-ring laser for earthquakes with a moment magnitude MW ≥ 8.0 and that our observations are in reasonable agreement with the synthetic spectra. We also report evidence for mode coupling in both translation and rotation spectra.

Resolution of rupture directivity in weak events: 1‐D versus 2‐D source parameterizations for the 2011, Mw 4.6 and 5.2 Lorca earthquakes, Spain

Resolving robustly source parameters of small-moderate magnitude earthquakes is still a challenge in seismology. We infer directivity from apparent source time functions (ASTFs) at regional distance and quantify the associated uncertainties. ASTFs are used for: (i) modeling a propagating 1D line source from the duration data; and, (ii) inverting the 2D slip distribution from the full signals. Slip inversion is performed through a Popperian scheme, where random trial models are either falsified on account of large misfit, or else become members of the solution set of the inverse problem. We assess the resolution of rupture directivity representing centroid shifts from the solution set in a rose diagram. Using as example an event with well-studied rupture directivity, the 2011 Mw 5.2 Lorca (Spain) earthquake, 1D and 2D parameterizations yield similar estimates for direction (N213°E and N220°E, respectively) and asymmetry (67:33, 65:35) of rupture propagation, as well as rupture length (2.1 km, 2.7 km) and speed (3.5 km/s, 3.25 km/s). The high rupture velocity ≥ 90% vS may be held primarily responsible for the strong directivity effect of this earthquake. We show that inversion of apparent source durations is intrinsically unable to resolve highly asymmetric bilateral ruptures, while inversion of full ASTFs misses part of the signals complexity, suggesting the presence of deconvolution artifacts. We extend the analysis to the Mw 4.6 foreshock of the Lorca earthquake, inferring similar directivity parameters and slip pattern as for the mainshock. The rupture towards SW of both earthquakes suggests that this direction could be inherent to the fault segment.