The 2016 Model for Sub-Saharan Africa

The 2016 Sub-Saharan Africa Earthquake Model is described by Poggi et. al. (2017). Here we present a short description of the OpenQuake-engine implementation of the model.

An up-to-date earthquake catalogue for Sub-Saharan Africa with homogenous magnitude representation (MW) was obtained by augmenting available global catalogues (e.g. ISC-Reviewed, ISC-GEM, GCMT, GEM Historical Catalogue) with information from local agencies and regional projects, particularly from AfricaArray temporary deployments (The Tanzanian Broadband Seismic Experiment; The Ethiopian Plateau Catalogue; The AfricaArray Eastern Africa Seismic experiment).

The homogeneous SSA-GEM catalogue was then purged from fore- and aftershock sequences and seismic swarms, using the declustering algorithm introduced by Gardner and Knopoff (1974). The declustered SSA-GEM catalogue consists of 7,259 events out of the original 29,803 in the magnitude range 3 ≤ Mw ≤ 7.53.

The proposed seismic source model for Sub-Saharan Africa is based on distributed seismicity sources, consisting of areal zones representing uniform temporal and spatial earthquake occurrence. The choice of this source type was mostly driven by the limited availability of current data, including local earthquake catalogues, faults, and focal mechanisms.

For the development of the area source model we then followed a mixed approach, which accounts for both observed seismicity and the geological/tectonic characteristics of the study region. The current area source model consists of a total of 19 zones distributed over 6 main tectonic group, which we assume to have comparable rheological and mechanical behaviour with respect to the underlying crustal geology.

Seismicity in each area source is assumed to follow a double truncated Gutenberg-Richter magnitude occurrence relation (or magnitude-frequency distribution, MFD). Lower truncation is arbitrarily assigned to Mw 4.5

Gutenberg-Richter b-values have been calibrated for the whole catalogue and independently for each source group. Conversely, occurrence rates (a-values) have been calculated separately for each source zone by imposing the previously calibrated b-values.

A different maximum magnitude (Mw-Max) estimate is derived independently for each source group as the largest observed event plus an arbitrary - although quite conservative - increment of 0.5 magnitude units.

The ground motion model distinguishes between two main tectonic domain:

• Active Shallow Crust (ASC)
• Stable Continental Crust (SCC)

While we used ASC for ground motion prediction equations (GMPEs) in areas involving plate boundary segmentation, SCC were used to model ground motion in all intra-plate areas.

The current logic-tree model was restricted to the use of four GMPEs, respectively two for active shallow crust:

• Chiou and Youngs, 2014 (CY)
• Akkar et al., 2014 (AK)

and two for stable continental conditions:

• Atkinson and Boore, 2006 (AB)
• Pezeshk et al., 2011 (PZ)

Source zones sharing the same weighting scheme for GMPEs have then been clustered into four main groups (named A to D) to reduce the total number of end-branches into the logic tree implementation.

Free rock conditions are assumed, with a fixed 30-metre averaged shear-wave velocity (Vs30) reference of 600 m/s (corresponding to stiff-soil transition in Eurocode8 [CEN, 2004] and NEHRP [BSSC, 2003] classification).

The figures below represent hazard maps for peak ground acceleration, for 10% of exceedance in 50 years and different spectral periods, calculated using the OpenQuake-engine.

1. Acceleration @ PGA
2. Acceleration @ 0.2s
3. Acceleration @ 1s

• Poggi, V., Durrheim, R., Mavonga Tuluka, G., Weatherill, G., Gee, R., Pagani, M., Nyblade, A., Delvaux, D., 2017. Assessing Seismic Hazard of the East African Rift: a pilot study from GEM and AfricaArray. Bulletin of Earthquake Engineering. DOI: 10.1007/s10518-017-0152-4 Paper

GEM platform is presently under reorganisation. The OpenQuake-engine input files (NRML format) for the Sub-Saharan Africa model will be released publicly as soon as in our possibilities.

For more information, please contact hazard@globalquakemodel.org