ALBERT

Description: We use traveltime data of local earthquakes and controlled sources observed by a large, temporary, amphibious seismic network to reveal the anatomy of the southcentral Chilean subduction zone (37–39°S) between the trench and the magmatic arc. At this location the giant 1960 earthquake (M = 9.5) nucleated and ruptured almost 1000 km of the subduction megathrust. For the three-dimensional tomographic inversion we used 17,148 P wave and 10,049 S wave arrival time readings from 439 local earthquakes and 94 shots. The resolution of the tomographic images was explored by analyzing the model resolution matrix and conducting extensive numerical tests. The downgoing lithosphere is delineated by high seismic P wave velocities. High v p/v s ratio in the subducting slab reflects hydrated oceanic crust and serpentinized uppermost oceanic mantle. The subducting oceanic crust can be traced down to a depth of 80 km, as indicated by a low velocity channel. The continental crust extends to approximately a 50-km depth near the intersection with the subducting plate. This suggests a wide contact zone between continental and oceanic crust of about 150 km, potentially supporting the development of large asperities. Eastward the crustal thickness decreases again to a minimum of about a 30-km depth. Relatively low v p/v s at the base of the forearc does not support a large-scale serpentinization of the mantle wedge. Offshore, low v p and high v p/v s reflect young, fluid-saturated sediments of forearc basins and the accretionary prism.

Description: Trees, as long living plants, are governed by environmental and/or climate changes within their habitat. Their growth rings record to a large extent the temporal dynamics of these changes either directly or through tree physiological reactions. They render the highest time resolution thus far possible for environmental or climate reconstructions of the past 10,000 years (exactly dated, annually resolved, see: [828], [994]). Trees are a substantial part of the human environment with a high socio-economic value. Their large geographical extension over various regions of the world, including those with greatest population densities but also marginal areas allows to gain unique informations about local and regional consequences of global climate change.

Description: Providing quantitative microzonation results that can be taken into account in urban land-use plans is a challenging task that requires collaborative efforts between the seismological and engineering communities. In this study, starting from the results obtained by extensive geophysical and seismological investigations, we propose and apply an approach to the Gubbio basin (Italy) that can be easily implemented for cases of moderate-to-low ground motion and that takes into account not only simple 1D, but also more complicated 3D effects. With this method, the sites inside the basin are classified by their fundamental resonance frequencies, estimated from the horizontal-to-vertical spectral ratio applied to noise recordings (HVNSR). The correspondence between estimates of the fundamental frequency from this method and those derived from earthquake recordings was verified at several calibration sites. The amplification factors used to correct the response spectra are computed by the ratio between the response spectra at sites within the basin and the response spectra at a hard-rock site using data from two seismic transects. Empirical amplification functions are then assigned to the fundamental frequencies after applying an interpolation technique. The suitability of the estimated site-specific correction factors for response spectra was verified by computing synthetic response spectra for stations within the basin, starting from the synthetic recording at a nearby rock station, and comparing them with observed ones.

Description: A temporal seismic network recorded local seismicity along a 130 km long segment of the transpressional dextral strike-slip Liquiñe-Ofqui fault zone (LOFZ) in southern Chile. Seventy five shallow crustal events with magnitudes up to M(tief)w 3.8 and depths shallower than 25 km were observed in an 11-month period mainly occurring in different clusters. Those clusters are spatially related to the LOFZ, to the volcanoes Chaitén, Michinmahuida and Corcovado, and to active faulting on secondary faults. Further activity along the LOFZ is indicated by individual events located in direct vicinity of the surface expression of the LOFZ. Focal mechanisms were calculated using deviatoric moment tensor inversion of body wave amplitude spectra which mostly yield strike-slip mechanisms indicating a NE–SW direction of the P-axis for the LOFZ at this latitude. The seismic activity reveals the present-day activity of the fault zone. The recent M(tief)w 6.2 event near Puerto Aysén, Southern Chile at 45.4°S on April 21, 2007 shows that the LOFZ is also capable of producing large magnitude earthquakes and therefore imposing significant seismic hazard to this region.

Description: We thank Dahlen & Nolet for the comments (DN05) on our paper (HH05). There are many points of agreement, as we think is clear from HH05, but we respectfully continue to differ in opinion on some fundamental aspects of the finite frequency sensitivity kernels known as 'banana doughnut' kernels—hereinafter BDKs, as per the original nomenclature of Dahlen et al.—and their benefit to global tomography. In contrast to DN05's summary statement, HH05's main concern about BDKs is not the effect of uncertainty in the earthquake source signature or origin time. HH05 argue that (i) the evaluation of sensitivity kernels in simple media has limitations for the interpretation of broad-band signals by means of (linearized) finite frequency tomography; (ii) finite frequency kernels are (indeed) oscillatory, but in general heterogeneity their structure will be complex and different from BD features; (iii) the resolved length scales of model variations are induced by the spectral scales present in the data, which makes the notion of 'hole' irrelevant; and (iv) with the need for 'damping' (regularization) and without a basis that matches properly the multi-scale aspects of finite frequency sensitivity, ray theory or finite frequency theory inversions are likely to yield results that are practically the same.

Description: Late Miocene to Quaternary volcanic rocks from the frontal arc to the back-arc region of the Central Volcanic Zone in the Andes show a wide range of δ11B values (+4 to −7 ‰) and boron concentrations (6 to 60 ppm). Positive δ11B values of samples from the volcanic front indicate involvement of a 11B-enriched slab component, most likely derived from altered oceanic crust, despite the thick Andean continental lithosphere, and rule out a pure crust-mantle origin for these lavas. The δ11B values and boron concentrations in the lavas decrease with increasing depth of the Wadati-Benioff Zone. This across-arc variation in δ11B values and decreasing B/Nb ratios from the arc to the back-arc samples are attributed to the combined effects of boron-isotope fractionation during progressive dehydration in the slab and a steady decrease in slab-fluid flux toward the back arc, coupled with a relatively constant degree of crustal contamination as indicated by similar Sr, Nd and Pb isotope ratios in all samples. Three-component mixing calculations for slab-derived fluid, the mantle wedge and the continental crust based on B, Sr and Nd isotope data indicate that the slab-fluid component dominates the boron composition of the fertile mantle and that the primary arc magmas were contaminated by an average addition of 15 to 30% crustal material. Modeling of fluid-mineral boron-isotope fractionation as a function of temperature shows that dehydration reactions liberate continuously changing fluid compositions from the slab during progressive subduction. A combination of a boron-isotope fractionation model and a temperature model for the Central Andean subduction zone fits the across-arc variation in δ11B and we conclude that the boron-isotope composition of arc volcanic rocks, especially in island arcs, is dominated by changing δ11B-composition of boron-rich slab-fluids during progressive dehydration. Owing to the decrease in slab-derived fluid flux crustal contamination becomes more important toward the back-arc. Because of the boron-isotope fractionation effect, across-arc variations in δ11B need not necessarily reflect different mixing proportions between boron derived from the slab-fluid and the mantle wedge.