Matthieu Sylvander

Mapping S-velocity heterogeneities in the D″ region, from SmKS differential travel times

Abstract Seismic velocity heterogeneities in the D″ region are studied using differential travel time residuals between SmKS waves. Previous studies indicate that regional variations in the propagation times of these waves are not due to the propagation through the liquid core. Furthermore, they are only weakly influenced by the topography of the core-mantle boundary (CMB). Thus, the differential travel time anomalies are interpreted as the result of lower-mantle heterogeneity. We compiled all the available published data, and added new observations of travel times from GEOSCOPE stations. We thus obtained 807 S2KS-SKS and 209 S3KS-S2KS differential times. Differential travel time residuals with respect to the mean Earth model PREM (Preliminary Reference Earth Model) are corrected for ellipticity and for mantle heterogeneity down to 2590 km depth (300 km above the CMB) using the 3-dimensional S-wave velocity model SH12-WM13. This model gives a 10% variance reduction of the residuals. The remaining variance of residuals is higher for S2KS-SKS than for S3KS-S2KS, which is consistent with a D″ origin for the heterogeneities. The data are inverted for regional variations using a least-squares analysis, assuming a single-layer D″ structure 300 km thick. The Earth is divided in 150 equal-area sectors, about 115 of which are sampled by four rays or more. We performed inversions using 18 different geographic grids and averaged the results, so as to minimize the artefacts induced by imposing artificial block boundaries. The velocity heterogeneity map, though affected by the poor sampling of the oceans, reveals significant long-wavelength structure, with velocity perturbations exceeding ±3% with respect to a reference velocity, which is slightly lower than the PREM one. This heterogeneity map is consistent with the results from most of previous studies, global as well as regional. The location of some of the high-velocity anomalies coincides with places where subducted slab material may have accumulated at the CMB. These high-velocity heterogeneities are also generally correlated with places where a discontinuous velocity increase of several per cent has been detected at the top of D″.

Seismic velocities at the core-mantle boundary inferred from P waves diffracted around the core

The very base of the mantle is investigated with core-diffracted P-wave (Pdiff) travel times published by the International Seismological Centre (ISC) for the period 1964–1987. Apparent slownesses are computed for two-station profiles using a difference method. As the short-period Pdiff mostly sample a very thin layer above the core-mantle boundary (CMB), a good approximation of the true velocity structure at the CMB can be derived from the apparent slownesses. More than 27000 profiles are built, and this provides an unprecedented Pdiff sampling of the CMB. The overall slowness distribution has an average value of 4.62 s/deg, which corresponds to a velocity more than 4% lower than that of most mean radial models. An analysis of the residuals of absolute ISC P and Pdiff travel times is independently carried out and confirms this result. It also shows that the degree of heterogeneities is significantly higher at the CMB than in the lower mantle. A search for lateral velocity variations is then undertaken; a first large-scale investigation reveals the presence of coherent slowness anomalies of very large dimensions of the order of 3000 km at the CMB. A tomographic inversion is then performed, which confirms the existence of pronounced (±8–10%) lateral velocity variations and provides a reliable map of the heterogeneities in the northern hemisphere. The influence of heterogeneity in the overlying mantle, of noise in the data and of CMB topography is evaluated; it seemingly proves minor compared with the contribution of heterogeneities at the CMB. Our results support the rising idea of a thin, low-velocity laterally varying boundary layer at the base of the D″ layer. The two principal candidate interpretations are the occurrence of partial melting, or the presence of a chemically distinct layer, featuring infiltrated core material.

P-velocity structure of the core-mantle boundary region inferred from PKP(AB)-PKP(BC) differential travel times

PKP(AB)-PKP(BC) differential travel time residuals from the bulletins of the International Seismological Centre are inverted to retrieve the P-velocity anomalies in the lowermost 300 km of the mantle. We show that a realistic core-mantle boundary (CMB) topography may have only a marginal contribution to the residuals. The block parametrization used allows us to discard poorly sampled areas prior to a least-squares inversion. The main features are negative anomalies beneath Eurasia and North America and a positive anomaly beneath the Southwest Pacific. The inferred map is not consistent with other maps resulting from previous P- and S-velocity studies, which are correlated at long wavelengths with each other. The preferred explanation for this challenging inconsistency is the presence of a thin heterogeneous layer just above the CMB.

Observation of deep water microseisms in the North Atlantic Ocean using tide modulations

Ocean activity produces continuous and ubiquitous seismic energy mostly in the 2–20 s period band, known as microseismic noise. Between 2 and 10 s period, secondary microseisms (SM) are generated by swell reflections close to the shores and/or by opposing swells in the deep ocean. However, unique conditions are required in order for surface waves generated by deep-ocean microseisms to be observed on land. By comparing short-duration power spectral densities at both Atlantic shoreline and inland seismic stations, we show that ocean tides strongly modulate the seismic energy in a wide period band except between 2.5 and 5 s. This tidal proxy reveals the existence of an ex situ short-period contribution of the SM peak. Comparison with swell spectra at surrounding buoys suggests that the largest part of this extra energy comes from deep ocean-generated microseisms. The energy modulation might be also used in numerical models of microseismic generation to constrain coastal reflection coefficients.

Identifying an asperity through 3‐D Mapping of the frequency‐magnitude distribution

Following the idea introduced by Wiemer and Wyss [1997], here is an attempt to map the seismic hazard, in terms of the b value in the frequency-magnitude distribution, in a region with moderate seismic activity (Bearn region, French Pyrenees). In this area, the complexity of the geology makes it necessary to perform a 3-D mapping, b values are computed within 5 km radius spherical volumes, that contain an average of 140 events. Strong lateral variations, between 0.45 and 1.8, are observed at short scales in the whole investigated volume. The minimum of the b value is clearly correlated with a historically active zone, which last ruptured in 1980 (Mb=5.1). Interpretating low b values in terms of high stresses may lead us to consider this minimum-b patch as a 5 km radius asperity. This study allows to define its location and dimensions.