ALBERT

Beschreibung: We describe and discuss some activities of our working group to disseminate scientific issues concerning seismology to the students of the Region Liguria.

Beschreibung: Published

Beschreibung: Rimini, Italy

Beschreibung: 5.9. Formazione e informazione

Beschreibung: open

Beschreibung: This report describes a preliminary analysis of borehole seismic data to determine VLF/Sub-bottom Seismic Noise in the Atlantic and the preliminary results of finite difference modelling for a Cape Fear environment. Noise levels were not a simple function of depth and there are indications that noise levels may depend on local geology about a given receiver position and/or on clamping. Coherency of the noise was generally poor (〈0.8) and was independent of depth. There is no evidence for distinct polarizations or directionality of the noise. The lowest determined value for ambient noise power on the vertical component was 10-4 nm2/Hz in the frequency range from 5-50 Hz. The better clamped horizontal component had comparable power values. In conclusion, although the drill ship was on the site and drill pipe was in the hole, analysis of the data for a large number of windows can provide meaningful upper bounds on the ambient noise levels in the upper crust.

Beschreibung: Prepared for the Naval Ocean Research and Development Activity as the final report for Contract Purchase Order No. N62306-86-l4-7589

Beschreibung: Reexploring convection and its various transitions to chaotic behavior were the central themes of GFD 1981. Our principal lecturer, Dr. Edward A. Spiegel, provided both a rich historical picture and stimulating hours at the current frontiers of this topic. Before the summer was out his research lecture on "A Tale of Two Methods" elegantly merged Pierre Coullet's canonical formalism for studying dynamical systems in a central manifold and the more traditional two-timing amplitude expansions near critical points. Other lecture sequences on convection and its relation to simpler dynamical systems ranged from the fine presentations of John Guckenheimer on bifurcation theory to Fritz Busse's survey of his immense contributions to our understanding of nonlinear convection. The list of other lectures found on the following pages attests to our summer-long exposure to convection in the ocean, the atmosphere, the earth's core and mantle, and in the sun. August brought lectures on new observations of convection in the laboratories of physicists. Albert Libchaber's precise experiments on the many routes convection can take to turbulence, with parallel laboratory and numerical experiments described by J. Gollub and E. Siggia, added much to our language of inquiry.

Beschreibung: Office of Naval Research under Contract N00014-81-G-0089.

Beschreibung: The strong lithospheric dichotomy between eastern and western Antarctica origins from the West Antarctic Rift. The rheological implications are therefore a reduction of elastic-lithosphere thickness by more than a factor of 2 from East to West Antarctica, and strongly reduced mantle viscosities below West Antarctica and the Antarctic Peninsula. We apply a spectral finite element model which enables the consideration of lateral viscosity variations in the upper mantle. Variations in seismic velocity are transformed to viscosity variations applying scaling laws, and the glaciation model IJ05 (Ivins & James, 2005, Ant. Sci.) is used for parameterizing the deglaciation of Antarctica. Considering different parameterizations of lithosphere structure we study the implications of lateral variability on the glacial-isostatic adjustment of Antarctica.

Beschreibung: Lateral heterogeneities in the Earth’s crust and mantle are demanded from seismic tomographic models, surface data and constraints derived from mantle dynamics. Nevertheless, such structural features are often neglected in GIA and only a 1D structure is assumed for the prediction of the earth’s response to glacial loading as for the inversion of mantle viscosity. 1D model assumption is valid when focussing on vertical motions which are less sensitive to lateral variations in mantle structure but it is questionable for the prediction of horizontal motions. In this presentation, we discuss the consequences which arise for the deformational behaviour of the Earth’s interior if we consider lateral viscosity variations. In particular, our study focusses on viscosity variations in the upper mantle including the mantle lithosphere and implications for plate motions as for the global gravity field are discussed.

Beschreibung: We compute P and S receiver functions to investigate the crustal and lithospheric thickness as well as the Vp/Vs ratio beneath the Northwest of Iran and map out the lateral variations of these discontinuities under this region.We selected data from teleseismic events (Mb ≥ 5.5, epicentral distance between 30°-95° for P receiver functions and Mb 〉 5.7, epicentral distance between 60°-85° for S receiver functions) recorded since 1995 to present at 8 three component short period stations from Tabriz Telemetry Network. First of all, we calculated PRFs for each station and then the Moho depth will be estimated only from the delay times of the Moho p-to-s conversion phases. Then, we used an H-Vp/Vs stacking algorithm of Zhu & Kanamori to estimate crustal thickness and Vp/Vs ratio under each station. The average Moho depth is estimated to be about ˜48 Km and varies from 38.5 to 53 km. Deeper and shallower Moho are found under the western and eastern stations, beneath SHB and SRB, respectively. The obtained average Vp/Vs ratio is 1.76, with higher ratio of 1.82 beneath the TBZ station and lower ratio of 1.73 beneath the AZR station. The crustal structure beneath these stations is also determined by the modeling of P receiver functions.We obtained a 3 layered model for the crust beneath this region for all stations. The thickness of layers are estimated to be 7-11, 18-35 and 38-53 km, respectively. The average of the crustal shear wave velocity is 3.39 km/s and it reaches 4.32 km/s below the Moho discontinuity. The crustal thicknesses derived from these data are in good agreement with the results obtained from our S receiver functions. In addition, Clear negative signals from the Lithosphere-Asthenosphere boundary (LAB) are also observed (˜8.7 s) in our S receiver functions. They map a clear LAB showing a thin continental lithospheric thickness of 84 Km and varies from 77 to 103 km. Deeper and shallower lithosphere–astenosphere boundary are found under the western and eastern parts of the study area. The P to S converted phases from 410 and 660 km upper mantle discontinuities are delayed more than 2 and 1 s with respect to the IASP91 global reference model, indicating an slower upper mantle and show higher temperature than the standard earth model. Because the 410 and 660 km discontinuities do not show the same delay, the transition zone is also considered to be thinner than that predicted by the IASP91 reference model. This could mean that the upper mantle in the region is still influenced by several geodynamical processes involving rifting, uplifting and magmatism.

Beschreibung: 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.

Beschreibung: New Zealand was the last major landmass in the world to be colonised by people. Despite the short time-depth of human presence in the country there is ongoing debate about the date of earliest arrival of people that resulted in the emergence of two contrasting colonisation hypotheses: the Early hypothesis and the Short prehistory. To decide between both hypotheses we employed a range of multi-proxy investigations (geochemistry, stable isotopes and mineral magnetism) on a lacustrine sequence from Lake Pupuke, a maar lake in the city of Auckland and an area potentially among the first places in New Zealand to have been colonised by early people. The environmental history reconstructed from the multi-proxy evidence identified a clear lack of catchment disturbance from c. 2000 B.P. until several decades before the eruption of the Rangitoto volcano. The nature and abruptness of disturbance unambiguously point to anthropogenic forcings and are likely to mark the onset of prehistoric human colonisation at the site. Linear interpolation between independently-dated tephrochronostratigraphic marker beds present in the sediments allowed to date this event to c. 610 cal yr B.P. This date is in compliance with the Short prehistory for this region.

Beschreibung: Global continental hydrological models use atmospheric weather data like precipitation and evaporation to force the simulation of continental water storage variations and river discharge. The predominant dependency of the modelled hydrological results from the incoming recipitation-evaporation budget is especially obvious when calculating global geodetic parameters such as Earth rotation excitation and gravity field changes. Many geodetically oriented hydrological studies are based on forcing data from the ERA-40 re-analysis of the European Centre for Medium-Range Weather Forecasts (ECMWF). In 2006 ECMWF started to develop a new interim re-analysis system derived from the latest version of their operational system. The ERA-Interim re-analysis starts 1989 and is now available until 2005. Using an improved assimilation background model and additional observation data several of the problems experienced in ERA-40 have been eliminated or significantly reduced in ERA-Interim, most notably the too-strong tropical oceanic precipitation from the early 1990s onwards. Nevertheless precipitation over tropical continental regions like in Africa is still higher than the estimates from the Global Precipitation Climate Centre (GPCC). The differences between ERA-40 and ERA-Interim forcing data significantly change hydrological Earth rotation excitation. The hydrological land surface discharge model LSDM was used to determine these differences in polar motion, length of day and low degree gravity coefficients. The detected biases indicate that the overall continental water storage is reduced, and part of the water masses are shifted between continents and seasons. The trends in excitation time series due to unbalanced precipitation-evaporation budgets vanish, whereas the seasonal timing of regional water storage events remains almost unaffected. Additional results from regional studies like in the Nile basin help to analyse the quality of the new ERA-Interim data and to classify their benefits for geodetic Earth system models.

Beschreibung: A novel numerical model based on solid deformation is presented in this paper. This thermo-mechanical model can simulate the tectonic evolution of crust and (lithospheric and asthenospheric) mantle under different conditions. Our implementation uses the finite element method (FEM) in order to solve the equations. As a Lagrangian approach is employed, remeshing techniques are implemented to avoid distortion problems when a certain deformation threshold is reached. The translation of the state between the old and new mesh is achieved by means of the information stored on Lagrangian particles, which minimizes the diffusion. The model is able to represent elastic, viscous and plastic behaviour inside the studied domain. Three types of creep mechanism (diffusion, dislocation and Peierls) are included. Two different quadrilateral isoparametric elements were implemented and can be employed to perform the calculations. The first one is an element with 4 nodes, selective reduced integration and a stabilization operator to diminish hourglass modes, which reduces the computational time needed. The second one has 8 nodes located in standard positions, uses full integration scheme and has no hourglass modes as it satisfies the Inf-Sup condition. Several test cases with known solutions were run to validate the different aspects of the implementation.

Beschreibung: The Aral Sea, located in a semi-arid environment, undergoes substantial annual (1200 mm/yr) and decadal lake-level fluctuations due to extreme seasonality in evaporation and precipitation along with steadily reduced river discharge. To trace the source of the lake water and understand the internal dynamics of the lake, we used oxygen and deuterium isotope composition of the lake water collected at different depths during spring and autumn from 2004 to 2006. We collected data from both the western (W) and eastern (E) basins of the Large Aral Sea as well as the channel connecting the two basins. The oxygen and hydrogen isotope ratios of the lake water vary widely (+ 4.6 to − 5.3‰ and − 48 to + 11‰, respectively). We further measured isotopic ratios of groundwater leakage near the shoreline of the W basin of the Large Aral Sea and released in an artesian well on the Kulandy Peninsula. These ratios range from − 16 to + 3.4‰ (δ18O) and − 120 to + 2.2‰ (δD). The river water displays ratios of − 12‰ (δ18O) and − 81.3‰ (δD). Precipitation from winter and early spring 2006 exhibit δ18O values of − 14‰ (snow) and + 0.2‰ (rain) and δD values of − 97‰ (snow) and + 3.6‰ (rain). The oxygen and hydrogen isotope snapshots show that in addition to evaporation, groundwater effluent flows at different depths are major contributors to the lake in spring and autumn. The d-excess, ranging between − 25‰ (lake water) and + 10‰ (groundwater), further demonstrates the impact of both effluent groundwater and evaporation on the isotopic composition of the lake water. Thus, stable isotope ratios can provide a first insight into seasonally triggered hydrologic interactions in the western part of the endorheic Aral Sea region. Remote sensing studies prove that major groundwater leakage occurs along the entire shoreline, except for the western shore where spatial resolution was too low.

Beschreibung: Cluster/EDI electron drift observations above the Northern and Southern polar cap areas for more than seven and a half years (2001-2008) have been used to derive a statistical model of the high-latitude electric potential distribution for summer condi5 tions. Based on potential pattern for different orientations of the interplanetary magnetic field (IMF) in the GSM y-z-plane, basic convection pattern (BCP) were derived, that represent the main characteristics of the electric potential distribution in dependence on the IMF. The BCPs comprise the IMF-independent potential distribution as well as patterns, which describe the dependence on positive and negative IMF Bz and IMF By variations. 10 The full set of BCPs allows to describe the spatial and temporal variation of the high-latitude electric potential (ionospheric convection) for any solar wind IMF condition near the Earth's magnetopause within reasonable ranges. The comparison of the Cluster/EDI model with the IZMEM ionospheric convection model, which was derived from ground based magnetometer observations, shows a good agreement of the basic patterns and its 15 variation with the IMF. According to the statistical models, there is a two-cell anti sunward convection within the polar cap for northward IMF Bz≤2 nT, while for increasing northward IMF Bz+ there appears a region of sunward convection within the high-latitude daytime sector, which assumes the form of two additional cells with sunward convection between them for IMF Bz+ ≈ 4-5 nT. This results in a four-cell convection pattern of the 20 high-latitude convection. In dependence of the +_ IMF By contribution during sufficiently strong northward IMF Bz conditions, a transformation to three-cell convection patterns takes place.

Beschreibung: Possible contributions of the non-advective processes in generating the geomagnetic secular variation (SV) are sought by isolating its parts that are inconsistent with the frozen-flux condition. This condition derives from the diffusionless radial induction equation and defined explicitly in the spatial domain: radial flux changes within a closed null-flux curve at the core surface are not allowed at any instant. We here study this condition in the spectral domain, relying on the spherical harmonic expansion of the diffusionless equation, i.e. the observation equation often used in the core surface flow inversion. SV models at a certain epoch are separated into the advective and non-advective parts, each satisfying and not satisfying the frozen-flux condition in the spectral domain. The non-uniqueness of the separation is avoided by assuming the orthogonality of the two parts in terms of the radial SV energy at the core-mantle boundary (CMB). From the recent geomagnetic models, GRIMM and CM4, we find that the non-advective part preferentially appears within smaller reverse patches of the radial field at the CMB, though it also exhibits more than such a simple configuration as a single signed flux density change within a patch. As long as no restrictions are imposed on the core flow configuration, time variations of the non-advective part are not correlated with those of the original SV models, characteristic of the geomagnetic jerks. However, jerks have to be partly non-advective, if the flow is restricted to be tangentially geostrophic. In this limited flow configuration, it is also found that a large part of the secular decrease of the axial dipole does not originate in the advection.

Beschreibung: One of the mysteries regarding Earth’s climate system response to variations in solar output is how the relatively small fluctuations of the 11-year solar cycle can produce the magnitude of the observed climate signals in the tropical Pacific associated with such solar variability. Two mechanisms, the top-down stratospheric response of ozone to fluctuations of shortwave solar forcing and the bottom-up coupled ocean-atmosphere surface response, are included in versions of three global climate models, with either mechanism acting alone or both acting together. We show that the two mechanisms act together to enhance the climatological off-equatorial tropical precipitation maxima in the Pacific, lower the eastern equatorial Pacific sea surface temperatures during peaks in the 11-year solar cycle, and reduce low-latitude clouds to amplify the solar forcing at the surface.

Beschreibung: Beginning in September 2006, a temporary network of 30 broadband and 45 short-period seismic stations has been set up on both sides of the Dead Sea Basin (DSB). During one and a half year of successful operation, data were continuously recorded in the field at 100 Hz and 200 Hz sample frequency for the broadband and short-period seismic stations, respectively. The raw data were converted to miniseed format and archived as full seed volume in the GEOFON data center of the GFZ. In the present work, the Receiver Function Method has been applied to the three component passive source data to investigate seismic discontinuities from the crust down to the upper mantle. Unusual negative phases at about 1s delay time have been observed at several stations in the Dead Sea region on the top of the assumed salt diapir. First preliminary receiver function analysis reveals a crustal thickness of about 30 -35 km in the investigated area and possibly low-velocity layer beneath the Moho. It also shows a basin which is possibly filled with salt about 10 km thick beneath the Lisan peninsula (Dead Sea).

Beschreibung: We present direct observations of the structure of the tectonic plates beneath the Tibetan plateau using seismic waves converted at discontinuities beneath Tibet. By assembling sections of P-S and S-P receiver function sections we map the lithosphere-asthenosphere boundary (LAB), the Moho and the 410 and 660 discontinuities along a number of profiles that traverse the plateau. We have identified a Tibetan plate that is sandwiched between the Asian and Indian plates. The seismic velocities in the Tibetan plate are slower than in the Indian and Asian plates, suggesting a significant temperature difference and hence different rheological behaviour of the Tibetan plate. In the westernmost part of Tibet, the Asian plate is underthrusting beneath the Indian plate, while in central and eastern Tibet the Indian plate is underthrusting beneath the Tibetan plate. There are also indications of Asian plate subduction in northern Tibet that are connected with a thickening of the Tibetan plate. There is no indication of subduction at the boundary between eastern Tibet and the Sichuan Basin.

Beschreibung: The Earth's magnetic field is generated mainly by a self-sustaining dynamo in the fluid outer core. Known as the core or main field, the dynamo's magnetic field is not constant but changes with time, a phenomenon denoted as secular Variation. Unfortunately, no common agreement exists about the definition of secular Variation: While some use this term for the temporal changes of the core field in general, others use the term only for its linear part (first time derivative). Two more terms are linked to core field temporal variations: geomagnetic jerks and archeomagnetic jerks. They are used to describe specific magnetic field signatures in the observations, implying a phenomenological classification. We suggest that a characterization of magnetic field changes based on the physics of the underlying core process may be more useful. Such a classification is proposed here to help avoid further misunderstanding through terminology.

Beschreibung: 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.

Beschreibung: In the last years the application of high-quality terrestrial gravity observations to studies of mass transport has become more and more a focus of interest. This goes hand in hand with efforts to enhance the signal-to-noise ratios in the time series by more comprehensive and sophisticated reductions for geodynamic studies. In addition a precondition for a consistent combination of terrestrial and satellite-derived gravity field variations comprehends not only to apply reductions to the data sets as good as possible, but also to ensure that reductions for the same influences either are applied or evidence is provided that the respective effect is negligible. One of the effects which thus moved into focus encompasses non-tidal mass shifts in the oceans. A major benefit of including this effect in the reductions is the appropriate response of the oceans with respect to changes in atmospheric loading, when sea surface pressure anomalies are applied as additional forcing in the ocean simulations, redundantising assumptions with regard to inverted or non-inverted barometer response. For example from a study by Fratepietro et al. (2006) a significant influence of storm surge-related effects in the North Sea on gravity even for inland sites emerged. Based on the OMCT and the ECCO model the gravity effect of non-tidal oceanic mass shifts is computed for various sites equipped with a superconducting gravimeter (SG) esp. with a view on seasonal variations. A five year-long period covering the years 2002 through 2006 is considered. The results so far are ambiguous: The systematic seasonal change of about 10 nm/s2 peak-to-peak in gravity found for mid-European stations is presently not found in the observed gravity variations. Generally, the order of magnitude of the total effect of 22 to 27 nm/s2 peak-to-peak is quite large for stations at a distance of some 100 km from the coast. In some data sections an agreement between observed and modelled gravity variation can be found which then results in the removal of larger residuals. For the South-African station Sutherland a different result is obtained. Here the seasonal variation caused by the non-tidal oceanic mass shifts and gravity residuals correlate. In this instance the application of the additional reduction leads to an overall substantial improvement of the signal-to-noise ratio in the gravity observations. One explanation for the different results might be found in the principle accuracy of the global continental hydrological models. Such a model is needed in order to remove the effect of large-scale variations in continental water storage in the gravity observations in order to obtain residuals which contain mainly variations related to the non-tidal oceanic mass shifts. This reduction plays a greater role for European stations than for the South African site. A possible critical impact of the land-sea-mask of the oceanic models resp. the negligence of the shelf areas could not be confirmed. The OMCT and the ECCO model have with 1.875° and 1° a different spatial resolution. The incorporation of a regional high resolution model (5’ x 3’, resp. 50” x 30”) for the North and Baltic Sea from the BSH (Bundesamt für Seeschifffahrt und Hydrographie) does not change the principle order of magnitude of the seasonal effect. Thus, there are presently strong indications that the order of magnitude of the long-period contribution is real. This means the influence of non-tidal mass shifts in the oceans should not only be considered for studies in the short periods, but also in the long-period spectral range.