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Stankiewicz J.,German Research Center for Geosciences | Stankiewicz J.,European Center for Geodynamics and Seismology | De Wit M.,Nelson Mandela Metropolitan University
Tectonophysics | Year: 2013

According to some previous studies, Archean continental crust is, on global average, apparently thinner than Proterozoic crust. Subsequently, the validity of this statement has been questioned. To provide an additional perspective on this issue, we present analyses of Moho signatures derived from recent seismic data along swaths 2000. km in length across southern Africa and its flanking ocean. The imaged crust has a near continuous age range between ca. 0.1 and 3.7. billion years, and the seismic data allow direct comparison of Moho depths between adjacent Archean, Proterozoic and Phanerozoic crust. We find no simple secular change in depth to Moho over this time period. In contrast, there is significant variation in depth to Moho beneath both Archean and Proterozoic crust; Archean crust of southern Africa displays as much crustal diversity in thickness as the adjacent Proterozoic crust. The Moho beneath all crustal provinces that we have analysed has been severely altered by tectono-metamorphic and igneous processes, in many cases more than once, and cannot provide unequivocal data for geodynamic models dealing with secular changes in continental crust formation. These results and conclusions are similar to those documented along ca. 2000. km swaths across the Canadian Shield recorded by Lithoprobe. Tying the age and character of the Precambrian crust of southern Africa to their depth diversities is clearly related to manifold processes of tectono-thermal 'surgery' subsequent to their origin, the details of which are still to be resolved, as they are in most Precambrian terranes. Reconstructing pristine Moho of the early Earth therefore remains a formidable challenge. In South Africa, better knowledge of 'fossilised' Archean crustal sections 'turned-on-edge', such as at the Vredefort impact crater (for the continental crust), and from the Barberton greenstone belt (for oceanic crust) is needed to characterize potential pristine Archean Moho transitions. © 2013 Elsevier B.V.

Wauthier C.,Royal Museum for Central Africa | Wauthier C.,University of Liège | Cayol V.,CNRS Magmas and Volcanoes Laboratory | Kervyn F.,Royal Museum for Central Africa | And 2 more authors.
Journal of Geophysical Research: Solid Earth | Year: 2012

On 17 January 2002, Nyiragongo volcano erupted along a 20 km-long fracture network extending from the volcano to the city of Goma. The event was captured by InSAR data from the ERS-2 and RADARSAT-1 satellites. A combination of 3D numerical modeling and inversions is used to analyze these displacements. Using Akaike Information Criteria, we determine that a model with two subvertical dikes is the most likely explanation for the 2002 InSAR deformation signal. A first, shallow dike, 2 km high, is associated with the eruptive fissure, and a second, deeper dike, 6 km high and 40 km long, lies about 3 km below the city of Goma. As the deep dike extends laterally for 20 km beneath the gas-rich Lake Kivu, the interaction of magma and dissolved gas should be considered as a significant hazard for future eruptions. A likely scenario for the eruption is that the magma supply to a deep reservoir started ten months before the eruption, as indicated by LP events and tremor. Stress analysis indicates that the deep dike could have triggered the injection of magma from the lake and shallow reservoir into the eruptive dike. The deep dike induced the opening of the southern part of this shallow dike, to which it transmitted magma though a narrow dike. This model is consistent with the geochemical analysis, the lava rheology and the pre-and post-eruptive seismicity. We infer low overpressures (1-10 MPa) for the dikes. These values are consistent with lithostatic crustal stresses close to the dikes and low magma pressure. As a consequence, the dike direction is probably not controlled by stresses but rather by a reduced tensile strength, inherited from previous rift intrusions. The lithostatic stresses indicate that magmatic activity is intense enough to relax tensional stresses associated with the rift extension. Copyright 2012 by the American Geophysical Union.

Samsonov S.,Natural Resources Canada | Samsonov S.,European Center for Geodynamics and Seismology | d'Oreye N.,European Center for Geodynamics and Seismology | Smets B.,European Center for Geodynamics and Seismology
International Journal of Applied Earth Observation and Geoinformation | Year: 2013

We present a novel methodology for integration of multiple InSAR data sets for computation of two dimensional time series of ground deformation. The proposed approach allowscombination of SAR data acquired with different acquisition parameters, temporal and spatial sampling and resolution, wavelength and polarization. Produced time series have combined coverage, improved temporal resolution and lower noise level. We apply this methodology for mapping coal mining related ground subsidence and uplift in the Greater Region of Luxembourg along the French-German border. For this we processed 167 Synthetic Aperture Radar ERS-1/2 and ENVISAT images acquired between 1995 and 2009 from one ascending (track 29) and one descending (track 337) tracks and created over five hundred interferograms that were used for time series analysis. Derived vertical and east-west linear deformation rates show with remarkable precision a region of localized ground deformation located above and caused by mining and post-mining activities. Time series of ground deformation display temporal variability: reversal from subsidence to uplift and acceleration of subsidence in the vertical component, and horizontal motion toward the center of the subsidence on the east-west component. InSAR results are validated by leveling measurements collected by the French Geological Survey (BRGM) during 2006-2008. We determined that deformationrate changes are mainly caused by water level variations in the mines. Due to higher temporal and spatial resolution the proposed space-borne method detected a larger number of subsidence and uplift areas in comparison to leveling measurements restricted to annual monitoring of benchmark points along roads. We also identified one deformation region that is not precisely located above the mining sites. Comparison of InSAR measurements with the water levels measured in the mining pits suggest that part of the water that filled thegalleries after termination of the dewatering systems may come from this region. Providing that enough SAR data is available, this method opens new opportunities for detecting and locating man-made and natural ground deformation signals with high temporal resolution and precision. © 2012 Published by Elsevier B.V.

Samsonov S.,Natural Resources Canada | Samsonov S.,European Center for Geodynamics and Seismology | d'Oreye N.,European Center for Geodynamics and Seismology
Geophysical Journal International | Year: 2012

A novel, multidimensional small baseline subset (MSBAS) methodology is presented for integration of multiple interferometric synthetic aperture radar (InSAR) data sets for computation of 2- or 3-D time-series of deformation. The proposed approach allows the combination of all possible air-borne and space-borne SAR data acquired with different acquisition parameters, temporal and spatial sampling and resolution, wave-band and polarization. The produced time-series have improved temporal resolution and can be enhanced by applying either regularization or temporal filtering to remove high-frequency noise. We apply this methodology to map 2003-2010 ground deformation of the Virunga Volcanic Province (VVP), North Kivu, Democratic Republic of Congo. The horizontal and vertical time-series of ground displacement clearly identify lava compaction areas, long-term deformation of Mt Nyamuragira and 2004, 2006 and 2010 pre- and coeruptive deformation. Providing that enough SAR data is available, the method opens new opportunities for detecting ground motion in the VVP and elsewhere. © Her Majesty the Queen in right of Canada 2012.

Samsonov Sergey S.,Institute of Geological & Nuclear Sciences | Samsonov Sergey S.,European Center for Geodynamics and Seismology | van der Kooij M.,MDA Corporation | Tiampo K.,University of Western Ontario
Computers and Geosciences | Year: 2011

We demonstrate here a computer code for calculation of time series and also mean and linear deformation rates from a set of coregistered unwrapped differential interferograms using a linear least-squares inversion technique based on the small baseline subset (SBAS) algorithm. The computer code is written in C and uses a singular value decomposition (SVD) routine from the LAPACK library and the fast Fourier transform for spatial filtering from the FFTW library. Various offset estimation and topographic correction algorithms are implemented, including simultaneous inversion for deformation rates and residual topographic error. This approach is particularly useful when applied to ALOS PALSAR interferograms that are coherent even at large perpendicular baselines and acquired with orbital parameters correlated with the time of acquisition. This methodology is applied to produce time series of ground deformation at Tauhara and Wairakei geothermal fields (Taupo Volcanic Zone, North Island, New Zealand) from 12 ALOS PALSAR images acquired between July 2007 and December 2009. We also present here a high-resolution deformation map of the ground subsidence caused by the extraction of geothermal groundwater for power generation, with maximum rates of subsidence of about 7. cm/y. © 2011 Elsevier Ltd.

Oth A.,European Center for Geodynamics and Seismology | Bindi D.,Helmholtz Center Potsdam | Parolai S.,Helmholtz Center Potsdam | di Giacomo D.,Helmholtz Center Potsdam | di Giacomo D.,International Seismological Center
Bulletin of the Seismological Society of America | Year: 2011

In this study we apply a nonparametric spectral inversion scheme to a data set of accelerograms recorded by the K-NET and KiK-net networks in Japan in order to derive attenuation characteristics, source spectra, and site response. For this purpose, we use a total of more than 67,000 S-wave records from 2178 earthquakes (MJMA 2.7-8) obtained at 1555 stations at the Earth's surface and more than 29,000 records from 1826 events recorded at 637 borehole stations at depths of 100 to 3000 m. Attenuation characteristics are investigated in five separate regions, showing that crustal Q depicts lower values in central compared to southern Japan, and a significant frequency dependence is observed in every region. The source spectra follow the ω2 model with higher stress drops for subcrustal earthquakes as compared with crustal ones. While strong amplification effects dominate the site contributions for the surface sensors, those for the borehole sensors are characterized by smaller variability. Nevertheless, consistent with observations from deconvolution of borehole/surface recording pairs, downgoing wave effects are visible in the site contributions for many borehole stations. Finally, the site amplification functions obtained at the surface are compared with surface-to-borehole (S/B) and horizontal-to-vertical (H/V) spectral ratios, showing that the S/B ratios generally provide better estimates of the horizontal amplification than the H/V ratios due to amplification of the vertical component of ground motion.

Oth A.,European Center for Geodynamics and Seismology | Bindi D.,Helmholtz Center Potsdam | Parolai S.,Helmholtz Center Potsdam | Di Giacomo D.,Helmholtz Center Potsdam | Di Giacomo D.,International Seismological Center
Geophysical Research Letters | Year: 2010

We investigate earthquake source characteristics and scaling properties using the results of a spectral inversion of more than 29,000 accelerometric borehole recordings from 1,826 earthquakes (MJMA 2.7-8) throughout Japan. We find that the calculated source spectra can be well characterized by the omega-square model and show on average self-similar scaling over the entire magnitude range, with median stress drops of 1.1 and 9.2 MPa for crustal and subcrustal events, respectively. The seismic energy-to-moment ratio, as theoretically expected if the omega-square model is valid, shows a strong dependency on stress drop only, which, in conjunction with data selection practice in some studies to cope with limited recording bandwidth, can explain the often observed apparent scale-dependence. Our observations suggest that there is no significant deviation from similarity of the energy radiation in the investigated magnitude range and that the observed scatter is mainly related to the scatter in stress drop. © 2010 by the American Geophysical Union.

Oth A.,European Center for Geodynamics and Seismology | Parolai S.,Helmholtz Center Potsdam | Bindi D.,Helmholtz Center Potsdam
Bulletin of the Seismological Society of America | Year: 2011

The wealth of accelerometric recordings collected by the K-NET and KiK-net networks in Japan since 1996 provides a unique opportunity to improve our understanding of many important seismological research questions. Subsets of these data have been used for many case studies, most of them, however, not focusing specifically on the best practices for data selection and giving relatively little attention to the properties and peculiarities directly observable from the data. Yet for many applications, these steps are an important prerequisite for successful and reliable analysis. For this reason, we devote this article to the extraction of a large data set of surface and borehole recordings from the K-NET and KiK-net databases with strong emphasis on data quality and reliability. The final data set available for subsequent work consists of 78,840 records from 2201 earthquakes covering the Japan Meteorological Agency (JMA) magnitude range 2.7-8, observed at 1681 sites throughout Japan. We explain how this data set has been compiled, including automatic phase picking and relocation of events. We also present an overview of the general features of the data set, providing important information for subsequent analysis. Strong amplification effects at high frequencies are immediately visible on the surface recordings. Furthermore, there is a clear presence of downgoing waves in the borehole records, as deconvolution of borehole/surface recording pairs indicates.

Oth A.,European Center for Geodynamics and Seismology | Kaiser A.E.,Institute of Geological & Nuclear Sciences
Pure and Applied Geophysics | Year: 2013

The Canterbury earthquake sequence beginning with the 2010 MW 7.2 Darfield earthquake is one of the most notable and well-recorded crustal earthquake sequences in a low-strain-rate region worldwide and as such provides a unique opportunity to better understand earthquake source physics and ground motion generation in such a tectonic setting. Ground motions during this sequence ranged up to extreme values of 2.2 g, recorded during the February 2011 MW 6.2 event beneath the city of Christchurch. A better understanding of the seismic source signature of this sequence, in particular the stress release and its scaling with earthquake size, is crucial for future ground motion prediction and hazard assessment in Canterbury, but also of high interest for other low-to-moderate seismicity regions where high-quality records of large earthquakes are lacking. Here we present a source parameter study of more than 200 events of the Canterbury sequence, covering the magnitude range MW 3–7.2. Source spectra were derived using a generalized spectral inversion technique and found to be well characterized by the ω−2 source model. We find that stress drops range between 1 and 20 MPa with a median value of 5 MPa, which is a factor of 5 larger than the median stress drop previously estimated with the same method for crustal earthquakes in much more seismically active Japan. Stress drop scaling with earthquake size is nearly self-similar, and we identify lateral variations throughout Canterbury, in particular high stress drops at the fault edges of the two major events, the MW 7.2 Darfield and MW 6.2 Christchurch earthquakes. © 2013, Springer Basel.

Oth A.,European Center for Geodynamics and Seismology
Earth and Planetary Science Letters | Year: 2013

The amount of stress released during earthquakes is a fundamental characteristic of the earthquake rupture process. As such, it represents a key parameter for improving our understanding of earthquake source physics and for reliable ground motion prediction. Large earthquake populations usually show variations in stress release as large as three orders of magnitude, but the underlying mechanisms have remained largely elusive, and particularly the dependence of stress release on earthquake size is still a matter of debate. Here I use a unique dataset from Japan encompassing earthquakes of a wide magnitude range and spatial coverage to show that stress release variations of crustal earthquakes are strongly correlated with heat flow variations, indicating that they are thermally controlled. In contrast, subcrustal events depict highest stress release in regions of strong subduction-interface coupling and overall less pronounced variability as compared with crustal earthquakes. Stress release is overall only weakly dependent on earthquake size, but at local scales (i.e., within individual earthquake sequences) the dependence can be very strong and apparently varies with stress regime. Accounting for these systematic variations reduces the stress release variability on local scale by a factor of two to three as compared with the full earthquake population, a finding that is of key significance in the endeavor to reduce the uncertainties in future ground motion predictions. © 2013 Elsevier B.V.

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