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Potsdam, Germany

Earthquake early warning systems (EEWS) are considered an effective and viable tool for seismic risk reduction in cities. Innovative directions for the development of novel EEWS include designing new methods for structure specific EEWS, where regional and on-site networks of seismic stations and seismological and earthquake-engineering knowledge and experience are combined. Furthermore, the design of novel EEWS can nowadays take advantage of recent technological advances involving the development of low-cost networks of wireless accelerometric stations. In this work we present a first attempt at designing a structure specific EEW procedure, which is based on the analysis of early P-wave signals recorded by wireless accelerometers placed at different levels of a structure. The procedure exploits the interferometric analysis of the recorded P-wave signals to obtain the response of the structure with respect to an impulsive input signal. This latter piece of information can be combined with an estimation of the incoming event size in order to obtain a real-time early assessment of the structural response at the different structure's levels. The procedure, named Tailor-made Earthquake Early Warning (TEEW), is composed of four steps: (1) The early event characterization through the inversion of the accelerometer spectra recorded at the base of the structure, (2) the early building response estimation through the deconvolution of P-wave signals recorded at selected levels of the structure, (3) the estimation of the expected shaking at the different levels through the convolution of the results from steps (1) and (2), and (4) a simple decisional rule based on the exceedance of a threshold value of the early estimation of the inter-story drift (i.e. the relative horizontal displacement of two adjacent floors in a building, expressed as a percentage of the story height separating the two adjacent floors) to determine whether or not to issue an alarm.A first test of the TEEW procedure is presented using recordings of three aftershocks of the L'Aquila sequence in 2009 that have been collected at the city hall of Navelli, Italy. © 2012 Elsevier Ltd. Source


Korte M.,Helmholtz Center Potsdam | Constable C.,University of California at San Diego
Physics of the Earth and Planetary Interiors | Year: 2011

Global geomagnetic field reconstructions on millennial time scales can be based on comprehensive paleomagnetic data compilations but, especially for older data, these still suffer from limitations in data quality and age controls as well as poor temporal and spatial coverage. Here we present updated global models for the time interval 0-3. ka where additions to the data basis mainly impact the South-East Asian, Alaskan, and Siberian regions. We summarize recent progress in millennial scale modelling, documenting the cumulative results from incremental modifications to the standard algorithms used to produce regularized time-varying spherical harmonic models spanning 1000. BC to 1990. AD: from 1590 to 1990. AD gauss coefficients from the historical gufm1 model supplement the paleomagnetic information; in addition to absolute paleointensities, calibrated relative paleointensity data from sediments are now routinely included; iterative data rejection and recalibration of relative intensity records from sediments ensure stable results; bootstrap experiments to generate uncertainty estimates for the model take account of uncertainties in both age and magnetic elements and additionally assess the impact of sampling in both time and space. Based on averaged results from bootstrap experiments, taking account of data and age uncertainties, we distinguish more conservative model estimates CALS3k.nb representing robust field structure at the core-mantle boundary from relatively high resolution models CALS3k.n for model versions n= 3 and 4. We assess the impact of newly available data and modifications to the modelling method by comparing the previous CALS3k.3, the new CALS3k.4, and the conservative new model, CALS3k.4b. We conclude that with presently available data it is not feasible to produce a model that is equally suitable for relatively high-resolution field predictions at Earth's surface and robust reconstruction of field evolution, avoiding spurious structure, at the core-mantle boundary (CMB). We presently consider CALS3k.4 the best high resolution model and recommend the more conservative lower resolution version for studies of field evolution at the CMB. © 2011 Elsevier B.V. Source


The collapse of Mount St. Helens (United States) on 18 May 1980 is one of the only incidents of its kind that was visually witnessed and instrumentally recorded. Previous analyses determined that the northern fl ank of the mountain failed, which resulted in a rockslide, and disintegrated during its mobilization to form a blocky facies and hummocky terrain in the downslope region. Gary Rosenquist's iconic photographs of the initial 18 May collapse were here analyzed by using a modern photogrammetric method to track portions of the moving fl ank across the photographs in the sequence. Thirty years after the 1980 Mount St. Helens rockslide, the digital image correlation technique enabled a precise investigation of the associated displacement vectors and strain. A listric basal detachment and several associated faults at the lateral edge of the rockslide were identified. In addition, the heterogeneous movements and a number of shear zones were detected within the sliding block, which had previously been assumed to be cohesive. The results of my study demonstrate the value of using optical images for strain analyses of active volcanoes-even decades after recording. © 2011 Geological Society of America. Source


Walter T.R.,Helmholtz Center Potsdam
Geophysical Journal International | Year: 2011

This paper describes an innovative method of volcano deformation measurements, applied to camera images taken from the 2004-2008 eruption period at Mount St. Helens. Dome growth was thought to be characterized by sustained, near-linear rates of a solid dacite plug. Through spatial digital image correlation (DIC) analysis of the camera images, new evidences arise that the deformation and strain rate of the spine was more complex. DIC yielded cumulative and incremental displacements, strain and shear planes at decimetre resolution. It was found that dome extrusion rates are highly non-linear, decelerating prior to partial collapse, followed by a pronounced dome extrusion increase and direction change. Associated processes have been identified through DIC, such as shallow landslides and reworking of talus apron material. The work highlights the strengths of camera strain monitoring, and illustrates that dome growth and collapse is a very dynamic process complexly interplaying with the surrounding. © 2011 The Author Geophysical Journal International © 2011 RAS. Source


The African continent inherits a long history of continental accretion and breakup. The stage of "making" a continent goes back to the Archean, when the first continental masses formed cratons which mostly remained stable ever since. Subsequent collision of weaker continental masses was followed by several extension and compression episodes that resulted in the formation of super-continents. After the assemblage of Gondwana, a period of predominantly "breaking", i.e., the breakup of super-continents, took over. The modern-day African continent exhibits different types of margins; continental rifting occurs side by side with recent collision. Since the late 1960s, magnetotelluric (MT) experiments have played an important role in studies of the electrical conductivity structure of Africa. The early results significantly shaped the MT community's understanding of continental-scale conductivity belts and basic characteristics of cratons and mobile belts on both crustal and lithospheric mantle scales for some decades. Modern MT studies in Africa have generally supported earlier results with high resistivities observed on cratons and low resistivities observed across mobile belts. Advances in instrumentation, data processing and interpretation resulted in higher-resolution images of the lithosphere, which in consequence induce an improved understanding of tectonic processes and geological prerequisites for the occurrence of natural resources. The high electrical conductivity of mobile belts and their relation to reactivated fault and detachment zones were often interpreted to characterize mobile belts as tectonic weak zones, which can accommodate stress and constitute zones along which continents can break. Recent breaking of the African continent can be studied on land across the East African rift; however, the lack of amphibian MT experiments across today's margins does not allow for good resolution of remnants of continental breakup processes. Naturally, the regions and the focus of the MT studies in Africa are diverse, but they all contribute to the story of making and breaking a continent. © 2011 Springer Science+Business Media B.V. Source

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