ALBERT

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: Magnetotelluric (MT) data from 66 sites along a 45-km-long profile across the San Andreas Fault (SAF) were inverted to obtain the 2-D electrical resistivity structure of the crust near the San Andreas Fault Observatory at Depth (SAFOD). The most intriguing feature of the resistivity model is a steeply dipping upper crustal high-conductivity zone flanking the seismically defined SAF to the NE, that widens into the lower crust and appears to be connected to a broad conductivity anomaly in the upper mantle. Hypothesis tests of the inversion model suggest that upper and lower crustal and upper-mantle anomalies may be interconnected.We speculate that the high conductivities are caused by fluids and may represent a deep-rooted channel for crustal and/or mantle fluid ascent. Based on the chemical analysis of well waters, it was previously suggested that fluids can enter the brittle regime of the SAF system from the lower crust and mantle. At high pressures, these fluids can contribute to fault-weakening at seismogenic depths. These geochemical studies predicted the existence of a deep fluid source and a permeable pathway through the crust. Our resistivity model images a conductive pathway, which penetrates the entire crust, in agreement with the geochemical interpretation. However, the resistivity model also shows that the upper crustal branch of the high-conductivity zone is locatedNEof the seismically defined SAF, suggesting that the SAF does not itself act as a major fluid pathway. This interpretation is supported by both, the location of the upper crustal highconductivity zone and recent studies within the SAFOD main hole, which indicate that pore pressures within the core of the SAF zone are not anomalously high, that mantle-derived fluids are minor constituents to the fault-zone fluid composition and that both the volume of mantle fluids and the fluid pressure increase to the NE of the SAF.We further infer from the MT model that the resistive Salinian block basement to the SW of the SAFOD represents an isolated body, being 5–8 km wide and reaching to depths 〉7 km, in agreement with aeromagnetic data. This body is separated from a massive block of Salinian crust farther to the SW. The NE terminus of resistive Salinian crust has a spatial relationship with a near-vertical zone of increased seismic reflectivity ∼15 km SW of the SAF and likely represents a deep-reaching fault zone.

Description: We propose a new rapid procedure for determining the energy magnitude Me for shallow events from broadband teleseismic P-wave signals within the distance range 20°–98°. To accomplish this task, we compute spectral amplitude decay functions for different periods using numerical simulations based on the reference Earth model AK135Q. By means of these functions, we correct the spectra of the teleseismic recordings for the propagation path effects, and calculate the radiated seismic energy ES, and hence Me. We use cumulative P-wave windows for simulating a real- or near real-time procedure and test it for 61 shallow earthquakes. The results show that our approach is able to provide a rapid and reliable Me determination within 7–15 minutes after the earthquake origin time, and is therefore suitable for implementation in rapid response systems.

Description: Detailed images of the lithosphere beneath the western Bohemian Massif were obtained by analysis of more than 8500 P receiver functions. At the intersection of Regensburg-Leipzig-Rostock zone and Eger Rift, crustal thickness decreases to 26 km from approx. 31 km in the surroundings. The receiver functions display a positive phase at about 6 s delay time and a strong negative phase at 7 to 8 s, which coincides with an area of Moho updoming, CO2 mantle-derived degassing and earthquake swarm activity. These phases can be modeled by a velocity increase at 50 km and a velocity decrease at 65 km depth. The velocity decrease, observed over an area of 5300 km2, gives evidence for local asthenospheric updoming and/or a confined body of partial melt, which might be the cause for high CO2 mantle fluid flow and earthquake swarm activity in this recently nonvolcanic, intracontinental rift area.

Description: Compound-specific isotope analysis has become an important tool in environmental studies and is an especially powerful way to evaluate biodegradation of hydrocarbons. Here, carbon isotope ratios of light hydrocarbons were used to characterise in-reservoir biodegradation in the Gullfaks oil field, offshore Norway. Increasing biodegradation, as characterised, for example, by increasing concentration ratios of Pr/n-C17 and Ph/n-C18, and decreasing concentrations of individual light hydrocarbons were correlated to 13C-enrichment of the light hydrocarbons. The δ13C values of C4 to C9n-alkanes increase by 7–3‰ within the six oil samples from the Brent Group of the Gullfaks oil field, slight changes (1–3‰) being observed for several branched alkanes and benzene, whereas no change (〈1‰) in δ13C occurs for cyclohexane, methylcyclohexane, and toluene. Application of the Rayleigh equation demonstrated high to fair correlation of concentration and isotope data of i- and n-pentane, n-hexane, and n-heptane, documenting that biodegradation in reservoirs can be described by the Rayleigh model. Using the appropriate isotope fractionation factor of n-hexane, derived from laboratory experiments, quantification of the loss of this petroleum constituent due to biodegradation is possible. Toluene, which is known to be highly susceptible to biodegradation, is not degraded within the Gullfaks oil field, implying that the local microbial community exhibits rather pronounced substrate specificities. The evaluation of combined molecular and isotopic data expands our understanding of the anaerobic degradation processes within this oil field and provides insight into the degradative capabilities of the microorganisms. Additionally, isotope analysis of unbiodegraded to slightly biodegraded crude oils from several oil fields surrounding Gullfaks illustrates the heterogeneity in isotopic composition of the light hydrocarbons due to source effects. This indicates that both source and also maturity effects have to be well constrained when using compound-specific isotope analysis for the assessment of biodegradation.

Description: As part of the South American Geodynamic Activities project we observed the present day deformation field in the territories of Chile and Argentina using the Global Positioning System. The results clearly show that the earthquake cycle dominates the contemporary surface deformation of the central and southern Andes. Compared to geological timescales, the transient elastic deformation related to subduction earthquakes presents a short-term signal which can be explained by interseismic, coseismic, and postseismic phases of interplate thrust earthquakes. We constructed the Andean Elastic Dislocation Model (AEDM) in order to subtract the interseismic loading from the observed velocities. The estimated parameters of the AEDM, and the amount and depth of coupling between the subducting Nazca and overriding South American Plates, represent long-term features and show that the seismogenic interface between both plates is fully locked and that the depth of coupling increases from north to south. The prominent signals in the residual velocity field (i.e. observed velocities minus AEDM) are obviously due to postseismic relaxation processes; they are visible in the area of the 1995 Mw 8.0 Antofagasta earthquake and in the area of the 1960 Mw 9.5 Valdivia earthquake. Although postseismic deformations, compared to geologic timescales, are short-term signals, those signals are valuable constraints on important long-term features of Andean evolution, i.e., the viscosity of the upper mantle and lower crust. The observed surface data are best fitted with a three-dimensional finite element model in which we incorporate a mantle viscosity of 4 × 1019 Pa s. The most obvious long-term deformation signal is manifested in the back-arc of the subduction zone where the Brazilian Shield thrusts beneath the Subandean zone. The style and amount of backarc shortening changes along strike of the orogen, increasing from zero in the south (latitude 〈 −38° S) to values in the order of 10 mm yr−1 close to the Bolivian Orocline. In the fore-arc, whilst we see indications for long-term E-W extension, we did not find any apparent slip partitioning. In addition to this long-term signal, we suggest that the asymmetry of interseismic and coseismic deformation may lead to tectonic structures in the fore-arc. If the coseismic deformation does not release all of the accumulated deformation, then, over many earthquake cycles, part of the interseismic deformation may be transformed into permanent long-term plastic deformation.

Description: The lithospheric structure of the Aegean region is investigated by analysis of Rayleigh-wave fundamental mode dispersion measurements. Isotropic 1-D models for almost 100 two-station ray paths across the region display distinct variations in the Moho depth and crustal S-wave velocities. The descending slab of the subducting African plate can be resolved down to 120 km depth beneath the volcanic arc. Three different regions are distinguished in terms of Moho depth: (1) The forearc, with large crustal thicknesses between 38 and 48 km and an average of 43 km, (2) the northern Aegean, with an average Moho depth of 28 km and (3) the southern Aegean (central volcanic arc, i.e. Cyclades, and Sea of Crete) with an even thinner crust of around 25 km. Lateral variations in structure between 25 and 55 km depth indicate a marked difference between the western and eastern forearc, collocated with pronounced changes in trench and slab geometry as well as published deformation rates. S velocities between 25 and 55 km depth are low everywhere beneath the forearc but increase from the Peleponnesus to Crete. An abrupt change occurs between western and central Crete in terms of the visibility of the Aegean Moho and the seismic structure of the lithospheric mantle wedge: An Aegean mantle wedge with S velocities above 4.4 km s−1 is only observed to the east of central Crete, whereas to the west velocities of less than 4.0 km s−1 occur down to the plate contact. These low velocities above the slab may indicate the presence of a melange of metamorphic rocks at the depths. A low-velocity asthenospheric layer is observed beneath the Sea of Crete and the Cyclades below 40 km depth, between the thinned lithosphere above and the slab below. The observed radial anisotropy in the northern part of the Aegean is likely to be due to preferred orientation of anisotropic minerals within the lower crust, possibly caused by lateral ductile flow associated with recent lithospheric extension.