Early Stage Geohazard Assessments in Tectonically Active Areas: Application to Potential Developments in South-East Asia

Abstract

A methodology is outlined for performing desktop studies in geohazard-prone areas to estimate feasibility for subsea cables and pipelines. Various global cases studies are referenced with relevance to development areas in South-East Asia. This involves a review of available geological, geophysical and geotechnical data, to provide guidance for routing, planning of surveys, and geohazard mitigation. Seismic hazard is not discussed in detail as it is comprehensively dealt with elsewhere. Enhanced subsea construction activity results from increased demand for energy supply and communication networks. The need for transportation of these products often necessitates the crossing of tectonically active areas that may be prone to geohazards which may result in pipeline rupture or cable breakage. Implications of tectonic activity may be direct (e.g. earthquakes), as well as shaping the terrain through imposition of structural controls, and providing potential trigger mechanisms for slope instability and density flows. Seafloor morphology is characterized on a regional-scale to understand structural controls, and local-scale to identify characters indicative of seafloor rupture, past slope instability, or future flow conduits. Interpretation of geophysical data provides information to feed an evolutionary geological model. This rationalizes periods of tectonic activity with geohazard scenarios, e.g. regional-scale catastrophic slope failures during intense rifting, compared with smaller, discrete landslides during periods of relative quiescence. Integration with available geotechnical data provides information to understand sedimentary processes and inform forward-looking analyses, such as debris-flow modeling. Thus, an estimation can made of magnitude and duration of forces impacting on structures. Examples are provided of how slope failures and sediment density flows may vary on slopes that have been modified by different tectonic processes. Conduits laterally offset by faults, or uplifted by toe-thrusts may limit the downslope run-out of flows; however, linear, fault controlled canyons may permit longer travel distances. A phased and integrated framework is outlined for early stage geohazard assessment in tectonically active areas to inform feasibility and concept routing studies. The intention of this approach is to ensure early stage decisions can be made with respect to route selection and to maximize the efficiency and effectiveness of future data acquisition.