Source models


The earthquake sources capable of producing significant ground-motion at the site are identified and characterised from available source types. Source characterisation includes definition of source geometry (using a zone or polygon) and the probability distribution of potential rupture locations within the source. The source model selected for the site has been defined from four type of sources:

Finite fault sources represent a single tectonic fault. The complete fault geometry is defined by the upper and lower depth of the fault, dip and rake angles and strike direction. A finite fault source can be defined by its trace projection over the surface as a polyline, or by a 3D grid, representing subduction planes with varying dip and upper and lower depths. The occurrence of earthquakes is assumed to be uniformly distributed within the source. The size of the rupture area for a given earthquake can be estimated from the magnitude,27 or can be specified as covering the entire fault surface. All finite fault sources assume that rupture surfaces have a uniform probability distribution over the entire fault surface. Finally, the entire fault surface is divided in a regular mesh and all possible faults planes of a given size (magnitude) are considered. The productivity associated with that magnitude is assumed to be equally distributed between the corresponding planes.28

Regional source models represent regions of homogenous seismicity and are often used for modelling earthquake patterns with or without tectonic evidence. These models span over a single tectonic region representing a geological unit or tectonic feature: active shallow crust, stable continental crust, subduction in-slab, subduction interface or deep seismicity. The boundaries of the regional sources are adjusted to follow the surface projection of identified faults, avoiding the interruption of a fault system unless major differences are observed. A single productivity is assigned to each regional source under the assumption that there is a uniform likelihood of earthquake occurrence within the source. Therefore, the sources must be consistent with the earthquake catalogue, limiting their extent to regions where the productivity is uniform.

Background seismicity sources are used to incorporate the in-buffer seismicity. The background seismicity incorporates the earthquake pattern observed in regions with no identified faults. These models assume that the seismicity is spatially uniform over wide regions, and hence largely ignores the patterns shown by historical seismicity. The historical seismicity is distributed uniformly over broad regions.

Smoothed seismicity sources also called grid sources, are used to incorporate the in-buffer seismicity. The in-buffer seismicity includes the small magnitude events assumed to occur in the vicinity of the modelled faults. These models also assume that the historical seismicity patterns indicate the likely occurrence of future earthquakes, but rather than distributing the historical uniformly over the source zones, it performs spatial smoothing of the historical seismicity and does not require the definition of source zones.29 Grid sources are a collection of point sources distributed over a region. In principle each point can have a distinctive set of seismic properties (rupture plane geometry, maximum magnitude, ground-motion models, etc.). Grid sources assume that future earthquakes will tend to happen where previous earthquakes have occurred. The area to be modelled is covered in a uniform grid of evenly spaced cells. The productivity of each cell is based on the number of events recorded in its corresponding area. The productivities are then smoothed with a smoothing kernel, such as a Gaussian function with a correlation distance of three cells.30

A uniform probability of events occurrence is assumed inside the boundaries of each source. This assumption generally results in conservative estimates of seismic hazard but reflects the uncertainty that exists in identifying the precise geological features and locations where future earthquakes can occur. For areal sources, a grid of uniformly separated and randomly located epicentres is defined by a meshing algorithm. At each point of the grid the fault rupture geometry is modelled assuming a fault plane,31 its dimensions being derived from a regression analysis.32 Grid sources follow the same procedure, but the epicentre mesh is fixed by the source model.

In the hazard model prepared for the site, the sources are represented by a specific rupture plane associated with a moment magnitude. The minimum possible hypocentral depth is selected, keeping the rupture plane within the upper and lower depth boundaries specified by the source model. If the height of the rupture plane exceeds the maximum possible height for the source, the aspect ratio of the rupture surface is modified to accommodate the required area within the prescribed depth limits. The strike direction of the rupture plane is determined from the fault trace specified in the source model. Once the rupture plane of a scenario has been established, finite fault distances can be computed following the procedure described in Kaklamanos et al..33

Regional sources model

The study considers the seismic model of Western Africa developed by Poggi34 for the Global Earthquake Model (GEM). GEM created a Mw-homogenised earthquake catalogue by assembling globally (ISC review bulletin, GCMT, ISG-GEM, GHEC) and locally available sources.35 The earthquake catalogue includes 114 events with 4 ≤ \(M_w\) ≤ 6.5, covering a period from 1636 to 2013 (Figure 4). The limit of the catalogue selection area is marked by the dashed line and six seismic zones were adopted according to the events distribution.

Seismic zones of the Western Africa model and earthquake catalogue [@Poggi2018]

Figure: 4: Seismic zones of the Western Africa model and earthquake catalogue36

The seismic zones are areal sources with shallow hypocenter depths and the seismicity follows a double truncated Gutenberg-Richter magnitude occurrence relation with smoothed seismicity occurrence rates. The lower truncation of the magnitude occurrence relation is set to \(M_w=4.5\). The maximum magnitude of each zone is the largest observed event plus an arbitrary increment of 0.5 magnitude units. The Simandou mine location is in the southeastern part of Area 6 (Figure 5). The largest event of this zone is the December 1983, \(M_w\) 6.3 earthquake of northwestern Guinea.

Poggi37 adopted a unique b-value from all events in the region and calculated the occurrence rates (a-values) separately for each source zone. The annual occurrence rates obtained for each source zones were redistributed within each polygon accounting for the irregular spatial pattern of the observed events.

The modelling parameters are summarized in Table 2. Logic tree weights are given in parenthesis. For the parameters \(M_w\,max\) and b three branches were considered with symmetrical relative values and weights.

Simandou mine location and seismic zones

Figure: 5: Simandou mine location and seismic zones

Active faults

The site of interest lies ~45 km away from one of the megashear features mapped by Kadiri and Kijko39 (Figure 6). Along the shear zone, five seismic events have been recorded, between depths of 0-33 km, and seismic magnitudes between 4-5 over the past century. The shear zone strikes southwest to northeast with a bearing of 64 degrees.

Megashear and seismic events map of the Western African region

Figure: 6: Megashear and seismic events map of the Western African region

Around the site, there are both sub-vertical and inclined faults (Figure 7). The closest two recorded seismic events are situated ~65 km northeast of the site, with seismic magnitudes 4.1 and 4.9 respectively. These two seismic events are possibly associated with a sub-vertical fault, labeled A, that is subparallel to the megashear. This fault has a length of ~ 70 km and a strike direction of 96 degrees. Both events are reported at a depth of ~ 10 km. A third seismic event is mapped ~ 102 km to the north of the site. This seismic event has a magnitude of 4.1 and is possibly associated with another sub-vertical fault, labeled B, that is subparallel to the megashear and has a strike of 50 degrees. The depth of this seismic event is reported as ~0 km. A fourth seismic event is mapped ~144 km southwest of the site with a seismic magnitude of 4.3 and a reported depth of 10 km. It is not clear from the map with which seismic event the fault may be associated.

Fault map around the site of interest

Figure: 7: Fault map around the site of interest

These faults were considered for the scenarios analysis but they were not included in the source model as simple faults as they do not have a magnitude occurrence relation available.

  1. Wells and Coppersmith.↩︎

  2. Marco Pagani et al., “OpenQuake Engine: An Open Hazard (and Risk) Software for the Global Earthquake Model,” Seismological Research Letters 85 (2014): 692–702; “The OpenQuake-Engine User Manual.” (Global Earthquake Model (GEM) OpenQuake Manual for Engine version 3.5.0, 2019).↩︎

  3. P. Somerville et al., “Source and Ground Motion Models for Australian Earthquakes” (Proc. 2009 Annual Conference of the Australian Earthquake Engineering Society, 2009), 11–13.↩︎

  4. Arthur Frankel, “Mapping Seismic Hazard in the Central and Eastern United States,” Seismological Research Letters 66 (1995): 8–21.↩︎

  5. Keiiti Aki and Paul G. Richards, Quantitative Seismology, 2002.↩︎

  6. Wells and Coppersmith, “New Empirical Relationships Among Magnitude, Rupture Length, Rupture Width, Rupture Area, and Surface Displacement.”↩︎

  7. “Estimating Unknown Input Parameters When Implementing the NGA Ground-Motion Prediction Equations in Engineering Practice,” Earthquake Spectra 27 (2011): 1219–35.↩︎

  8. “OpenQuake-Engine PSHA Input Model for the Western Africa Region.”↩︎

  9. Paulina Amponsah, Günter Leydecker, and Rolf Muff, “Earthquake Catalogue of Ghana for the Time Period 1615–2003 with Special Reference to the Tectono-Structural Evolution of South-East Ghana,” Journal of African Earth Sciences 75 (2012): 1–13.↩︎

  10. Poggi, “OpenQuake-Engine PSHA Input Model for the Western Africa Region.”↩︎

  11. Poggi.↩︎

  12. “Seismicity and Seismic Hazard Assessment in West Africa.”↩︎