E. Beucler

High-resolution imaging of the Pyrenees and Massif Central from the data of the PYROPE and IBERARRAY portable array deployments

The lithospheric structures beneath the Pyrenees, which holds the key to settle long-standing controversies regarding the opening of the Bay of Biscay and the formation of the Pyrenees, are still poorly known. The temporary PYROPE and IBERARRAY experiments have recently filled a strong deficit of seismological stations in this part of western Europe, offering a new and unique opportunity to image crustal and mantle structures with unprecedented resolution. Here we report the results of the first tomographic study of the Pyrenees relying on this rich data set. The important aspects of our tomographic study are the precision of both absolute and relative traveltime measurements obtained by a nonlinear simulated annealing waveform fit and the detailed crustal model that has been constructed to compute accurate crustal corrections. Beneath the Massif Central, the most prominent feature is a widespread slow anomaly that reflects a strong thermal anomaly resulting from the thinning of the lithosphere and upwelling of the asthenosphere. Our tomographic images clearly exclude scenarios involving subduction of oceanic lithosphere beneath the Pyrenees. In contrast, they reveal the segmentation of lithospheric structures, mainly by two major lithospheric faults, the Toulouse fault in the central Pyrenees and the Pamplona fault in the western Pyrenees. These inherited Hercynian faults were reactivated during the Cretaceous rifting of the Aquitaine and Iberian margins and during the Cenozoic Alpine convergence. Therefore, the Pyrenees can be seen as resulting from the tectonic inversion of a segmented continental rift that was buried by subduction beneath the European plate.

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.

The Snake River Plain experiment revisited. Relationships between a Farallon plate fragment and the transition zone

We reconsider the analysis of the 93 PASSCAL-OREGON SRP experiment data. The techniques applied to the processing of Pds waves, which are P to S conversions at seismic discontinuities at depth d in the receiver region, differ from those employed by Dueker & Sheehan [1997]. We perform global move-out corrections and migration, taking advantage of the alignment of seismicity along the great circle described by the array (South America, and the Kuril and Aleutians islands). We do not detect a deep signature of a mantle plume. The 410 km discontinuity exhibits a fragmented aspect, which prevents us from reliably estimating the transition zone thickness. The 660 km discontinuity is clearly more visible, and largely deflected in a large part of the profile. We associate this anomalous behaviour and the noisy aspect of the discontinuities with the presence of a Farallon plate fragment.