Seismology Unit

Pretoria, South Africa

Seismology Unit

Pretoria, South Africa
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Brandt M.B.C.,Seismology Unit | Brandt M.B.C.,University of Witwatersrand
Pure and Applied Geophysics | Year: 2013

I present the results of statistical hypothesis testing of Grand's (2002) global tomography model of three-dimensional shear velocity variations for the middle mantle underneath eastern and southern Africa. I apply an F test to evaluate the validity of a model where a tilted, slow-velocity anomaly in the deepest mantle under southern Africa, known as the African superplume, is continuous with a slow-velocity anomaly in the upper mantle under eastern Africa. This null hypothesis is tested against alternative hypotheses, in which various "obstruction volumes" in the middle mantle are constrained to zero perturbation from the one-dimensional reference velocity during the tomographic inversion. I find that there is an equal probability of accepting an alternative hypothesis with a thin "obstruction volume" at 850-1,000 km depth, whereas volumes at other depths are rejected. But the alternative hypothesis, where a connection is forced at 850-1,000 km depth, is rejected. I conclude that the African superplume rises to at least 1,150 km depth, and that the upper mantle slow-velocity anomaly continues from the surface to below the mantle transition zone. I interpret the "obstruction volume" as a weakening of the superplume in the middle mantle. © 2012 Springer Basel AG.

Arango M.C.,Imperial College London | Strasser F.O.,Seismology Unit | Bommer J.J.,Imperial College London | Boroschek R.,University of Chile | And 2 more authors.
Journal of Seismology | Year: 2010

Earthquake hazard along the Peru-Chile subduction zone is amongst the highest in the world. The development of a database of subduction-zone strong-motion recordings is, therefore, of great importance for ground-motion prediction in this region. Accelerograms recorded by the different networks operators in Peru and Chile have been compiled and processed in a uniform manner, and information on the source parameters of the causative earthquakes, fault-plane geometries and local site conditions at the recording stations has been collected and reviewed to obtain high-quality metadata. The compiled database consists of 98 triaxial ground-motion recordings from 15 subduction-type events with moment magnitudes ranging from 6.3 to 8.4, recorded at 59 different sites in Peru and Chile, between 1966 and 2007. While the database presented in this study is not sufficient for the derivation of a new predictive equation for ground motions from subduction events in the Peru-Chile region, it significantly expands the global database of strong-motion data and associated metadata that can be used in the derivation of predictive equations for subduction environments. Additionally, the compiled database will allow the assessment of existing predictive models for subduction-type events in terms of their suitability for the Peru-Chile region, which directly influences seismic hazard assessment in this region. © 2010 Springer Science+Business Media B.V.

Arango M.C.,Imperial College London | Strasser F.O.,Seismology Unit | Strasser F.O.,Arup | Bommer J.J.,Imperial College London | And 4 more authors.
Bulletin of the Seismological Society of America | Year: 2012

The applicability of existing ground-motion prediction equations (GMPEs) for subduction-zone earthquakes is an important issue to address in the assessment of the seismic hazard affecting the Peru-Chile and Central American regions. Few predictive equations exist that are derived from local data, and these do not generally meet the quality criteria required for use in modern seismic hazard analyses. This paper investigates the applicability of a set of global and regional subduction ground-motion models to the Peru-Chile and Central American subduction zones, distinguishing between interface and intraslab events, in light of recently compiled ground-motion data from these regions. Strong-motion recordings and associated metadata compiled by Arango, Strasser, Bommer, Boroschek, et al. (2011) and Arango, Strasser, Bommer, Hernandez, et al. (2011) have been used to assess the performance of the candidate equations following the maximum-likelihood approach of Scherbaum et al. (2004) and its extension to normalized intraevent and interevent residual distributions developed by Stafford et al. (2008). The results of this study are discussed in terms of the transportability of GMPEs for subduction-zone environments from one region to another, with a view to providing guidance for developing groundmotion logic trees for seismic hazard analysis in these regions.

Strasser F.O.,Seismology Unit | Albini P.,Italian National Institute of Geophysics and Volcanology | Flint N.S.,Seismology Unit | Beauval C.,French National Center for Scientific Research
Journal of Seismology | Year: 2015

The preparation of earthquake catalogues for seismic hazard analysis requires the use of uniform parameters, in particular for magnitudes, although the original data include a variety of formats, such as macroseismic intensities and various instrumental magnitude scales. In regions of low seismicity, such as South Africa, data are generally sparse and not always sufficient to develop locally calibrated conversion relations. They can nevertheless be used to test the applicability of imported conversion relations, as well as their consistency. The Koffiefontein region of South Africa provides a good test case in view of its somewhat higher level of seismicity, and central geographic location within the country. This paper reviews determinations of location and magnitude parameters for a suite of moderate-to-large earthquakes that have occurred in this region between 1903 and 1985. © 2015, Springer Science+Business Media Dordrecht.

Rietbrock A.,University of Liverpool | Strasser F.,Seismology Unit | Edwards B.,ETH Zurich
Bulletin of the Seismological Society of America | Year: 2013

Low-seismicity regions such as the United Kingdom (UK) pose a chal-lenge for seismic hazard analysis in view of the limited amount of locally recorded data available. In particular, ground-motion prediction is faced with the problem that most of the instrumental observations available have been recorded at large distances from small earthquakes. Direct extrapolation of the results of regression on these data to the range of magnitudes and distances relevant for the seismic hazard analysis of engineered structures generally leads to unsatisfactory predictions. The present study presents a new ground-motion prediction equation (GMPE) for the UK in terms of peak ground acceleration (PGA), peak ground velocity (PGV), and 5% damped pseudospectral acceleration (PSA), based on the results of numerical simulations using a stochastic point-source model calibrated with parameters derived from local weak-motion data. The predictions from this model are compared with those of previous GMPEs based on UK data, other GMPEs derived for stable continental regions (SCRs), as well as recent GMPEs developed for the wider European area.

Arango M.C.,Imperial College London | Strasser F.O.,Seismology Unit | Bommer J.J.,Imperial College London | Hernandez D.A.,SNET | Cepeda J.M.,International Center for Geohazards
Journal of Seismology | Year: 2011

Subduction earthquakes along the Pacific Coast of Central America generate considerable seismic risk in the region. The quantification of the hazard due to these events requires the development of appropriate ground-motion prediction equations, for which purpose a database of recordings from subduction events in the region is indispensable. This paper describes the compilation of a comprehensive database of strong ground-motion recordings obtained during subduction-zone events in Central America, focusing on the region from 8 to 14° N and 83 to 92° W, including Guatemala, El Salvador, Nicaragua and Costa Rica. More than 400 accelerograms recorded by the networks operating across Central America during the last decades have been added to data collected by NORSAR in two regional projects for the reduction of natural disasters. The final database consists of 554 triaxial ground-motion recordings from events of moment magnitudes between 5.0 and 7.7, including 22 interface and 58 intraslab-type events for the time period 1976-2006. Although the database presented in this study is not sufficiently complete in terms of magnitude-distance distribution to serve as a basis for the derivation of predictive equations for interface and intraslab events in Central America, it considerably expands the Central American subduction data compiled in previous studies and used in early ground-motion modelling studies for subduction events in this region. Additionally, the compiled database will allow the assessment of the existing predictive models for subduction-type events in terms of their applicability for the Central American region, which is essential for an adequate estimation of the hazard due to subduction earthquakes in this region. © 2010 Springer Science+Business Media B.V.

Midzi V.,Seismology Unit | Bommer J.J.,Imperial College London | Strasser F.O.,Seismology Unit | Albini P.,Italian National Institute of Geophysics and Volcanology | And 3 more authors.
Journal of Seismology | Year: 2013

A database of intensity observations from instrumentally recorded earthquakes in South Africa has been compiled as a contribution to the characterisation of seismic hazard. The database contains about 1,000 intensity data points (IDPs) that have been assigned from macroseismic observations retrieved from newspaper reports and questionnaires, and also digitised from previously published isoseismal maps. The database includes IDPs from 57 earthquakes with magnitudes in the range of M w 2.2 to 6.4, at epicentral distances up to 1,000 km. Sixteen events have 20 or more IDPs, with half of these events having more than 80 IDPs. The database is dominated by relatively low intensity values, mostly determined from human perception of shaking rather than structural damage. However, 19 IDPs correspond to intensity values greater than VI MMI-56. Using geological maps of South Africa, the sites of 60 % the IDPs were geologically classified as either 'rock' or 'soil', the uncertainty in locations precluding such a classification for the remaining data points. A few of the IDPs identified as being from soil sites appear to be strongly influenced by site effects, and these were removed from the trimmed database created for exploring ground-motion levels. The trimmed database includes 15 earthquakes which have a minimum of five useful IDPs, excluding those with intensity MMI = I and those based on a single observation. After removing such points, and those identified as clear 'outliers', a total of 436 useful IDPs were selected. © 2013 Springer Science+Business Media Dordrecht.

Roelofse F.,University of the Free State | Saunders I.,Seismology Unit
South African Journal of Science | Year: 2013

A bright meteor with an apparent magnitude of -18 was seen over large parts of southern Africa at ~23:00 South African Standard Time on 21 November 2009. Here we discuss the eye-witness accounts related to the meteor as well as the seismic signals generated by the meteor's passage through the atmosphere as detected by the Mussina seismograph station forming part of the South African National Seismograph Network. Two signals were identified on the seismogram; the first arrival is interpreted as a precursor coupled seismic wave and the second, which arrived ~138 s after the first, as a directly coupled airwave. The meteor is thought to have entered the atmosphere close to Mussina shortly before 22:55.06 local time, from where it proceeded in a westerly to northwesterly direction with an elevation angle not exceeding 43°. Our results presented here dispel the beliefs of many observers who thought that the meteor must have made landfall very close to their localities. In addition, this contribution documents the first instance of meteor-related seismic signals recorded by the South African National Seismograph Network. © 2013. The Authors.

Brandt M.B.C.,Seismology Unit
Journal of Seismology | Year: 2016

Quality factor Q, which describes the attenuation of seismic waves with distance, was determined for South Africa using data recorded by the South African National Seismograph Network. Because of an objective paucity of seismicity in South Africa and modernisation of the seismograph network only in 2007, I carried out a coda wave decay analysis on only 13 tectonic earthquakes and 7 mine-related events for the magnitude range 3.6 ≤ ML ≤ 4.4. Up to five seismograph stations were utilised to determine Qc for frequencies at 2, 4, 8 and 16 Hz resulting in 84 individual measurements. The constants Q0 and α were determined for the attenuation relation Qc(f) = Q0fα. The result was Q0 = 396 ± 29 and α = 0.72 ± 0.04 for a lapse time of 1.9*(ts − t0) (time from origin time t0 to the start of coda analysis window is 1.9 times the S-travel time, ts) and a coda window length of 80 s. This lapse time and coda window length were found to fit the most individual frequencies for a signal-to-noise ratio of at least 3 and a minimum absolute correlation coefficient for the envelope of 0.5. For a positive correlation coefficient, the envelope amplitude increases with time and Qc was not calculated. The derived Qc was verified using the spectral ratio method on a smaller data set consisting of nine earthquakes and one mine-related event recorded by up to four seismograph stations. Since the spectral ratio method requires absolute amplitudes in its calculations, site response tests were performed to select four appropriate stations without soil amplification and/or signal distortion. The result obtained for QS was Q0 = 391 ± 130 and α = 0.60 ± 0.16, which agrees well with the coda Qc result. © 2015, Springer Science+Business Media Dordrecht.

Brandt M.B.C.,Seismology Unit
Journal of the Southern African Institute of Mining and Metallurgy | Year: 2014

Focal depths of 15 tectonic earthquakes and 9 mine-related events were determined for South Africa using data recorded by the South African National Seismograph Network. These earthquakes and events were relocated by means of the Hypocenter program using direct P-waves (Pg) , critically refracted P-waves (Pn) , and first-arrival S-waves for the magnitude range 3.6 ≤ ML ≤ 4.4. Focal depths were first determined by means of the minimum root mean square (RMS) of the differences between the measured travel times and those predicted using the velocity model. The depths for tectonic earthquakes had a 2 km ≤ D ≤ 10 km range and an average depth and standard deviation of 6.9 ± 2.3 km. Depths for minerelated events ranged over 0 km ≤ D ≤ 7 km, averaging 3 ± 2.3 km. Next, arrival times for the additional regional depth phases sPn , PmP, sPmP, and SmP were measured. Focal depths were re-determined for the relocated epicentres, with the minimum variance (i.e. spread) of the differences between the measured travel times and travel times predicted by means of the Wentzel, Kramer, Brillouin, and Jeffreys (WKBJ) method for synthetic waveform modelling. Depth ranges were 4 km ≤ D ≤ 7 km (average 5.9 ± 1.2 km) and 1 km ≤ D ≤ 4 km (average 2.4 ± 1.2 km) for tectonic and mine-related events, respectively. The derived depths were verified for one tectonic earthquake with synthetic-to-recorded-waveform fits using the WKBJ synthetic seismogram software for the abovementioned regional phases. The focal mechanism parameters for this earthquake source were obtained from the National Earthquake Information Centre. Focal depths were estimated for nine stations by visually comparing synthetic waveform phases with recorded waveforms, ranging from 5 km to 8 km. © The Southern African Institute of Mining and Metallurgy, 2014.

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