Guust Nolet

Low S velocities under the Tornquist-Teisseyre zone: Evidence for water injection into the transition zone by subduction

A strong low S velocity anomaly at 300–500 km depth coincides with the western boundary of the Russian Platform. The anomaly is too large to be explained by a simple temperature anomaly or by compositional variations, nor is it an artifact induced by seismic anisotropy. We present a model that explains the anomaly through the injection of water at the time of closure of the Tomquist Ocean that separated the continents of Avalonia and Baltica in the early Paleozoic. Dense hydrous magnesium phases are the most likely agents for transporting water to the transition zone, but an important role may also be played by nominally anhydrous clinopyroxene. When these minerals are brought out of their stability field, water is released. It may accumulate in β-spinel or K-amphibole and may be released when the mantle warms up after subduction halts. The effect of this water is to induce weakening of the shear modulus or even creation of a heavy melt. By a conservative estimate, a subduction episode lasting 85 m.y. would inject enough water to saturate a large volume of mantle rock with 0.3% H2O. Low velocities or low Q anomalies have also been observed in the transition zones near currently active slabs.

Finite frequency whole mantle P wave tomography: Improvement of subducted slab images

Received 23 July 2013; revised 14 October 2013; accepted 17 October 2013. [1] We present a new whole mantle P wave tomographic model GAP_P4. We used two data groups; short-period data of more than 10 million picked-up onset times and long-period data of more than 20 thousand differential travel times measured by waveform cross correlation. Finite frequency kernels were calculated at the corresponding frequency bands for both long- and short-period data. With respect to an earlier model GAP_P2, we find important improvements especially in the transition zone and uppermost lower mantle beneath the South China Sea and the southern Philippine Sea owing to broadband ocean bottom seismometers (BBOBSs) deployed in the western Pacific Ocean where station coverage is poor. This new model is different from a model in which the full data set is interpreted with classical ray theory. BBOBS observations should be more useful to sharpen images of subducted slabs than expected from simple raypath coverage arguments. Citation: Obayashi, M., J. Yoshimitsu, G. Nolet, Y. Fukao, H. Shiobara, H. Sugioka, H. Miyamachi, and Y. Gao (2013), Finite frequency whole mantle P wave tomography: Improvement of subducted slab images, Geophys. Res. Lett., 40, doi:10.1002/ 2013GL057401.

Structure of North American mantle constrained by simultaneous inversion of multiple‐frequency SH, SS, and Love waves

[1] We simultaneously invert for the velocity and attenuation structure of the North American mantle from a mixed data set: SH wave traveltime and amplitude anomalies, SS wave differential traveltime anomalies, and Love wave fundamental mode phase delays. All data are measured for multiple frequency bands, and finite frequency sensitivity kernels are used to explain the observations. In the resulting SH velocity model, a lower mantle plume is observed to originate at about 1500 km depth beneath the Yellowstone area, tilting about 40° from vertical. The plume rises up through a gap in the subducting Farallon slab. The SH velocity model confirms high‐level segmentation of the Farallon slab, which was observed in the recent P velocity model. Attenuation structure is resolvable in the upper mantle and transition zone; in estimating it, we correct for focusing. High‐correlation coefficients between dlnVS and dlnQS under the central and eastern United States suggest one main physical source, most likely temperature. The smaller correlation coefficients and larger slopes of the dlnQS − dlnVS relationship under the western United States suggest an influence of nonthermal factors such as the existence of water and partial melt. Finally, we analyze the influence of the different components of our data set. The addition of Love wave phase delays helps to improve the resolution of both velocity and attenuation, and the effect is noticeable even in the lower mantle.

Slab temperature and thickness from seismic tomography: 1. Method and application to Tonga

Delay times of compressional body wave phases from both teleseismic and local events are used to invert for a high-resolution P wave velocity model in the Tonga subduction zone. The images obtained show a high-velocity subducting slab with velocity deviations of the order of 3–4%. Assuming to first order that the positive velocity anomalies within the slab are caused by a temperature effect, a theoretical slab temperature model based on the diffusion equation is used to explain velocity anomalies within the tomographic slab. Temperature differences between the interior of the slab and the ambient mantle are converted to velocity perturbations using the scaling parameter dVp/dT ≈ 4.8 × 10−4 km s−1 °C−1 for lithosphere material. The optimal values for the parameters in the temperature model are found using a nonlinear optimization that compares the integrated velocity anomalies in the tomographic slab region to integral of high velocities in a synthetic slab derived from a temperature model. The parameters for slab thickness and mantle potential temperature are not uniquely determined; therefore a fixed value for the mantle potential temperature based on laboratory values for the temperature of the spinel-to-perovskite transition at 660 km is used. Using 1180°C as the potential temperature, the theoretical temperature model gives an optimal slab thickness of 82 km for a region near 29°S in Tonga. The uncertainty in the thickness is dominated by the uncertainty in the mantle temperature and would be 8 km for an uncertainty of 100° in mantle temperature, but nonsystematic errors are less. In order to enhance the tomographic result the velocity model is biased towards the theoretical slab model. However, a posteriori changes made to the tomogram will most likely violate the fit to the delay time data. To prevent this, the difference between the tomogram and the predicted slab model is projected onto the null-space of the inversion to remove components which do not satisfy the seismic data. Using only null-space components to modify the minimum-norm solution, an enhanced model is obtained which has been biased toward the theoretical solution but has the same data misfit as the minimum-norm solution. The final image shows a very narrow and continuous slab with maximum velocity anomalies of the order of 6–7%; many of the gaps within the slab, as well as artifacts around the slab which were present in the minimum-norm solution, are absent in the biased image.

Slab temperature and thickness from seismic tomography: 2. Izu-Bonin, Japan, and Kuril subduction zones

Delay times from teleseismic and local P wave arrivals are used to invert for a high-resolution three-dimensional velocity model beneath the northwest Pacific. The model shows high-velocity slabs with average velocity anomalies of the order of 3–4%. Assuming the positive velocity deviations in the subducting lithosphere are to first order due to a temperature anomaly, the results of a theoretical slab temperature profile based on the diffusion equation are converted to a synthetic slab velocity model. Temperature variations between the ambient mantle and the interior of the slab are converted to P wave velocity perturbations using dVp/dT ≈ 4.8 × 10−4 km s−1 °C−1. A nonlinear optimization scheme compares the tomograms obtained via tomography to the theoretically predicted models in order to determine the optimal values for slab thickness and mantle potential temperature. Using 1180±100°C as the potential temperature, thickness estimates of 88±8 km, 85±8 km, and 84±8 km are obtained for the Izu-Bonin, Japan, and Kuril slabs, respectively. A correlation exists between slab thickness and age, which is strong if mantle temperature variations along the slab strike can be ruled out. In the process of estimating slab thickness the predicted slab velocity model is used as a filter to enhance the initial minimum-norm tomographic result. The initial tomogram is modified to closely resemble the synthetic slab tomogram by using only null-space components. The use of the null-space components guarantees that the enhanced solution will satisfy the original seismic delay times. The enhanced slab images show very continuous and narrow slabs compared to the initial tomographic results.

Nonlinear regularization techniques for seismic tomography

The effects of several nonlinear regularization techniques are discussed in the framework of 3D seismic tomography. Traditional, linear, @?2 penalties are compared to so-called sparsity promoting @?1 and @?0 penalties, and a total variation penalty. Which of these algorithms is judged optimal depends on the specific requirements of the scientific experiment. If the correct reproduction of model amplitudes is important, classical damping towards a smooth model using an @?2 norm works almost as well as minimizing the total variation but is much more efficient. If gradients (edges of anomalies) should be resolved with a minimum of distortion, we prefer @?1 damping of Daubechies-4 wavelet coefficients. It has the additional advantage of yielding a noiseless reconstruction, contrary to simple @?2 minimization (Tikhonov regularization) which should be avoided. In some of our examples, the @?0 method produced notable artifacts. In addition we show how nonlinear @?1 methods for finding sparse models can be competitive in speed with the widely used @?2 methods, certainly under noisy conditions, so that there is no need to shun @?1 penalizations.

Crustal thickness map of the western United States by partitioned waveform inversion

The partitioned waveform inversion method [Nolet, 1990] was used to obtain a new crustal thickness map of the western United States. We fitted 200 seismograms containing higher- and fundamental mode Rayleigh waves for events and stations in the western United States, which provided average crustal thickness constraints between each source-receiver pair. All these constraints were combined in a linear inversion to obtain a crustal thickness map. Local crustal thickness estimates from published seismic reflection/refraction work were used as additional constraints to overcome deficiencies of path coverage in some areas. The resulting crustal thickness map shows thicker (∼45 km) crust beneath the Cascades, thick (∼40 km) crust beneath the Sierra Nevada, thin (∼30 km) crust beneath the Basin and Range, and thick (greater than 45 km) crust beneath the Colorado Plateau. The boundary between the Colorado Plateau and the southern leg of the Basin and Range is a prominent feature of the model. Thick crust is seen at the bend of the San Andreas fault near Los Angeles. Our model is broadly consistent with the crustal thickness map of Mooney and Weaver [1989]. A quantitative comparison with the crustal thickness model 3SMAC [Nataf and Ricard, 1996] was performed by inverting for a model that deviates least from it. Results show that we are able to retain most of the features of that model while maintaining a good fit to the surface wave and point constraint data.

Comparison of ray- and adjoint-based sensitivity kernels for body-wave seismic tomography

[1]xa0We compare finite-frequency sensitivity kernels computed from ray-theoretical wavefields (“banana-doughnut” kernels) and from full waveform computations (often called “adjoint” kernels) in order to evaluate resolution, accuracy and computational cost. We focus here on body-wave seismic tomography at regional and local scales. Our results show that: (1) for homogeneous reference media, ray-based and adjoint kernels agree except for the expected differences in the regions close to the source and the receiver, where near-field contributions are neglected in the ray-based kernels; (2) for a smooth 3D background velocity model, the differences in predicted delay times for the two methods are generally well below 10 % of the delay for P waves, though as much as 20 % for S-waves, suggesting that extra care should be taken when performing S-wave tomography with ray-based “banana doughnut” kernels.

Automatic discrimination of underwater acoustic signals generated by teleseismic P‐waves: A probabilistic approach

[1]xa0We propose a new probabilistic scheme for the automatic recognition of underwater acoustic signals generated by teleseismic P-waves recorded by hydrophones in the ocean. The recognition of a given signal is based on the relative distribution of its power among different frequency bands. The signals power distribution is compared with a statistical model developed by analyzing relative power distributions of many signals of the same origin and a numerical criterion is calculated, which can serve as a measure of the probability for the signal to belong to the statistical model. Our recognition scheme was applied to 6-month-long continuous records of seven ocean bottom hydrophones (OBH) deployed in the Ligurian Sea. A maximum of 94% of all detectable teleseismic P-waves recorded during the deployment of the OBHs were recognized correctly with no false recognitions. The proposed recognition method will be implemented in autonomous underwater robots dedicated to detect and transmit acoustic signals generated by teleseismic P-waves.

Seismic monitoring in the oceans by autonomous floats.

Our understanding of the internal dynamics of the Earth is largely based on images of seismic velocity variations in the mantle obtained with global tomography. However, our ability to image the mantle is severely hampered by a lack of seismic data collected in marine areas. Here we report observations made under different noise conditions (in the Mediterranean Sea, the Indian and Pacific Oceans) by a submarine floating seismograph, and show that such floats are able to fill the oceanic data gap. Depending on the ambient noise level, the floats can record between 35 and 63% of distant earthquakes with a moment magnitude M≥6.5. Even magnitudes <6.0 can be successfully observed under favourable noise conditions. The serendipitous recording of an earthquake swarm near the Indian Ocean triple junction enabled us to establish a threshold magnitude between 2.7 and 3.4 for local earthquakes in the noisiest of the three environments.