Browsing by Author "Andreoli, Marco"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Open Access
    Orientation of the maximum horizontal stress (SHmax) in the Orange basin and Outeniqua sub-basins of South Africa : neotectonic implications
    (2015) Logue, Andrew Selkirk; Le Roex, Anton; Ben-Avraham, Ben; Heidbach, Oliver; Andreoli, Marco
    South Africa lies within an intraplate setting, characterised by sparse, scattered, low-magnitude seismicity not easily correlated with known neotectonic features. Recent seismicity has repeatedly been shown to result from the reactivation of pre-existing geological structures, typically large faults or shear zones, which are preferentially-aligned to the contemporary stress field. Expanding the catalogue of stress data for the region is therefore vital to better understand the regional stress field pattern and aid in identifying potentially seismogenic structures. In these intraplate regions, reliable high-quality indicators of maximum horizontal compressive stress (SHmax) are difficult to obtain in the absence of earthquake-derived focal mechanism solutions (so called "beach balls"). In South Africa however, extensive hydrocarbon exploration drilling in the Mesozoic offshore provide an opportunity to utilise the borehole breakout technique to derive quality SHmax orientation data. In collaboration with the World Stress Map Project (WSM), and utilising borehole logs provided by the Petroleum Agency South Africa (PASA), training and software was provided to review, analyse and visualise borehole breakouts observed in 4-arm calliper logs from exploration areas in the Outeniqua and Orange basins. Digital music software can limit the forms of music we create by using interfaces that directly copy those of the analogue instruments that came before. In this study we report on a new multi-touch interface that affords a completely new form of drum sequencing. Based on ideas from Avant-guard music and embodied interaction, a technology probe was created and then evaluated by a wide range of users. We found that for users with no musical training, and for users with a large amount of musical training, the software did allow them to be more creative. However, users with limited training on existing sequencing software found the new interface challenging.
  • Thumbnail Image
    Item
    Open Access
    Patterns of stress and strain rate in southern Africa
    (2006) Bird, Peter; Ben-Avraham, Zvi; Schubert, Gerald; Andreoli, Marco; Viola, Giulio
    The southward propagation of the East Africa rift presents an opportunity to study plate boundary formation. We tabulate orientation data which confirm the province of NW-SE directed most compressive horizontal principal stress (Wegener stress anomaly) earlier tentatively attributed to ridge push. We also collect information on stress regime, described by the associated Andersonian fault type(s). We use thin shell finite element models with realistic rheology to test three causes of stress: (1) lateral variations in density moment, (2) resistance of unbroken lithosphere to relative plate rotation, and (3) stress concentration ahead of a crack tip. Models with stress due primarily to variations in density moment are unsuccessful in their predictions (59-73% incorrect regimes; 32-40° azimuth errors). Models in which Africa-Somalia spreading is regulated at realistic rates by remote boundary conditions are more accurate (18-41% incorrect regimes; 25-35° azimuth errors). Treating the East Africa rift as a frictionless crack degrades the fit in either case. Apparently, the Wegener stress anomaly is caused primarily by resistance to the relative rotation between the Somalia and Africa plates. The East Africa rift north of 21°S may be weakened by strain but has residual friction ≥0.1. Greater strength of oceanic lithosphere is likely to cause stress increases, reorientations, and regime changes offshore. The predicted strain rate map has high rates along the rift, curving at 12°S into a western arc through Angola-Namibia-South Africa. Seismic hazard in Namibia may be greater than the instrumental catalog suggests. However, a number of unfit data indicate that these models represent only a first step.