Rayleigh waves from correlation of seismic noise in Great Island of Tierra del Fuego,Argentina:Constraints on upper crustal structure

In this study, the ambient seismic noise cross-correlation technique is applied to estimate the upper structure of the crust beneath Great Island of Tierra del Fuego（TdF）, Argentina, by the analysis of shortperiod Rayleigh wave group velocities. The island, situated in the southernmost South America, is a key area of investigation among the interaction between the South American and Scotia plates and is considered as a very seismically active one. Through cross-correlating the vertical components of ambient seismic noise registered at four broadband stations in TdF, we were able to extract Rayleigh waves which were used to estimate group velocities in the period band of 2.5-16 s using a timefrequency analysis. Although ambient noise sources are distributed in homogeneously, robust empirical Green＇s functions could be recovered from the cross-correlation of 12 months of ambient noise, The observed group velocities were inverted considering a non-linear iterative damped least-squares inversion procedure and several 1-D shear wave velocity models of the upper crust were obtained.According to the inversion results, the S-wave velocity ranges between 1.75 and 3,7 km/s in the first10 km of crust, depending on the pair of stations considered. These results are in agreement to the major known surface and sub-surface geological and tectonic features known in the area. This study represents the first ambient seismic noise analysis in TdF in order to constraint the upper crust beneath this region.It can also be considered as a successful feasibility study for future analyses with a denser station deployment for a more detailed imaging of structure.

Different canopy openings affect underground traits in herbaceous plants of a southern forest in Patagonia

Aims Forest canopy openings modify the natural environment,producing changes in light quality and intensity,precipitation and temperature.In turn,these changes promote the acclimation of understory species.However,little work has been done on underground responses to those environmental changes.The objective of this work was to determine how Osmorhiza depauperata,Phleum alpinum and Poa pratensis change its root length density and root colonization by mycorrhiza as a function of light availability in a Nothofagus pumilio(i.e.lenga)forest harvested following the variable retention prescription.Methods We selected three microenvironments in an old growth forest harvested by the variable retention prescription:aggregated retention,dispersed retention with influence of aggregated retention and dispersed retention.A non-harvested primary forest(PF),similar to the harvested one,was used as a control.Every 2 months,from October 2008 to April 2009,we took soil cores from randomly selected plants.From these soil cores,root length density and colonization percentage(CP)by arbuscular mycorrhizae were estimated.Important Findings Light availability changed significantly among the microenvironments.In general,root length density was significantly greater in P.pratensis than in P.alpinum and both species greater than in O.depauperata.Light availability increased root length density in all species,although the magnitude of these increases difference among species.Root length density was 187%greater in P.pratensis,101%in P.alpinum and 94%in O.depauperata in the disperse retention system than in the PF.Mycorrhiza CP was higher in O.depauperata than in P.alpinum and P.pratensis.Also,it was lower in the PF than in the harvested microenvironments.CPs were very low.