Seismotectonic and seismic hazard studies for Madagascar
Date
2022
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
Madagascar is an island that was once situated in the centre of the Gondwana supercontinent but is now located 1000 km from the East African Rift and about 2000 km from the mid-ocean ridge of the Indian Ocean. Its separation from the African continent (ca. 165-130 Ma) and the Indian sub-continent (ca. 90 Ma) and mantle upwelling that gave rise to the recent volcanism (ca. 28-0.5 Ma) has had significant impact on the geological structure and neotectonics of the island. This thesis focuses on the seismotectonic evolution of Madagascar and assesses the seismic hazard. Data for this project was derived from the 28-station MAdagascar – COmoros-MOzambique (MACOMO) array, as well as stations from the concurrent SEismological signatures in the Lithosphere/Asthenosphere system of SOuthern MAdagascar (SELASOMA) and Réunion Hotspot and Upper Mantle - Réunions Unterer Mantel (RHUM-RUM) projects, and permanent stations of the national seismological network. The combined 56 stations provided a better distribution and density than any previous study.
The thesis is presented in three main parts. The first part focuses on seismicity and focal mechanisms. The 695 events recorded and located during the 23-month period of MACOMO observations in 2011-2013 have magnitudes that vary from M1 to M5.3. Twenty-three focal mechanism solutions were determined using Pwave first motion polarities and some clear S-wave polarities. In agreement with previous studies, it was found that central Madagascar is the most seismicallyactive region. Most earthquakes are located near known faults. Normal faulting dominates the central area, while strike-slip and reverse fault mechanisms are predominantly observed near the Antsaba and Sandrakota shear zones in the north, and the Bekily and Ampanihy shear zones in the south. The focal mechanism studies revealed extensional faulting in central Madagascar, which can be explained by the reactivation of older faults by a thermal bulge underlying the region. The faulting style was not well defined in the northern and southern parts as few solutions were determined due to the lower density of stations.
In the second part of this thesis, 182 focal mechanism solutions were used to study the characteristics of the stress field. Calculations were performed using a linear inversion technique based on the Michael (1984) method and using a recently calculated velocity model of Madagascar. Results give shape ratios (i.e., relative magnitude of stress axes) that vary from 0.7 to 0.9; a maximum principal stress axis (σ1) that is near-vertical and a minimum principal stress axis (σ3) that is nearhorizontal with a NW-SE orientation. This corresponds to a normal faulting regime. The regional variation of stress and the characteristics of the shape ratio and friction values were determined. The average direction of the stress field is approximately NW-SE in central Madagascar, and trends W-E in the southern and northern parts of the island. In central Madagascar the shape ratio and friction vary from 0.83 to 0.97 and 0.45 to 0.65, respectively; while in the northern and southern parts of Madagascar they vary from 0.3 to 0.5 and 0.5 to 0.7, respectively.
The third part of the thesis reports on the assessment of the seismic hazard in Madagascar using data from a catalogue that combines data from the NDC (National Data Center), bulletins produced by the current study for the period between 1975 and 2016, as well as historical earthquake data. The analysis reveals that Madagascar has moderate seismic activity with local magnitude ML ≤ 6. The relationship between the moment magnitude MW and local magnitude ML values was estimated in order to unify magnitude characteristics and create the homogeneous catalogue that is required for seismic hazard studies. A Probabilistic Seismic Hazard Assessment approach (PSHA) was performed using the Kijko and Sellevoll (KS) and Cornell-McGuire methods with two different ground motion models. In order to implement the alternative parameter values in seismic hazard calculation and to attribute uncertainties associated to the input parameter, a logic tree formalism was applied. The Peak Ground Acceleration (PGA) values near major towns in the central region for 10% and 2% probability of occurrence in 50 years are 0.04 g and 0.11 g, respectively. The spectral acceleration for 1.0 s and 3.0 s vary from 0.001 g to 0.01 g and 0.001 g to 0.005 g for 10% probability of vii occurrence in 50 years, respectively; and from 0.02 g to 0.10 g and 0.002 g to 0.01 g for 2% probability of occurrence in 50 years.
This work demonstrates that the deployment of additional temporary seismic stations significantly improved our understanding of the seismotectonics of Madagascar, particularly in the northern and southern regions. In summary, we obtained a new magnitude relation, more accurate epicenter locations, better focal mechanism descriptions, and improved the assessment of seismic hazard.
Description
A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Philosophy to the Faculty of Science, University of the Witwatersrand, 2022
Keywords
Seismotectonic hazard, Seismic hazard, Madagascar