2. SEISMICITY AND TECTONICS

2.1 Tectonics

In Malawi, tectonic features fall into two age groups. The first group constitutes Pre Cenozoic age structures. These are faults, shear zones and dyke swarms that are generally of Mesozoic age. These structures developed during the Karoo rifting of Permian to Triassic (〜280-195Ma) and Post-Karoo rifting of Jurassic-Cretaceous period (〜195-65Ma)[10]. Chimaliro fault and a NE-SW trending shear zone which is reflected by faults in Ruhuhu trough, in southern Tanzania are examples of Mesozoic structures (Fig.2. 1.1). NE-SW trending dyke swarms of Jurassic age (〜195-140Ma) are common in southern Malawi. The dykes are not shown in Figure 2.1.1. The second group of structures is of Cenozoic age. These are faults related to the EARS and are collectively referred to as the Malawi rift. In Malawi, rifting was initiated in Late Tertiary (6-5Ma) [11]. Tectonic activity is related to Cenozoic rift structures except where Pre-Cenozoic structures are reactivated.

The Malawi rift extends for about 800km from Rungwe in southern Tanzania to the middle of Shire valley in southern Malawi (Fig. 3.1.1). The rift structures extend further south by the Shire trough in Malawi and Urema graben in Mozambique. The Malawi rift is largely occupied by the lake which is 40-90km wide. It is composed of curvilinear N-S trending boundary faults which define half grabens and horsts, step faults with riftward tilted blocks and monoclinal structures [ 12,13]. Further south in the Shire trough, the rift trend changes from N-S to NW-SE at about latitude 16°S and continues with this trend into Urema graben in Mozambique.

2.2 Seismicity

An epicentral map of Malawi and the surrounding areas for earthquakes of M ≥ 3.0 is shown in Figure 2.2.1. Rukwa - Malawi - Urema is the most seismically active belt. The area east of Rukwa is characterised by isolated big earthquakes (M ≥ 6.0) although the activity in this area is low. In general, magnitudes ≥5.5 earthquakes are common north of latitude 14.50°S while the area south of this latitude is dominated by low magnitude earthquakes(M<5.0).

Within Malawi seismic activity is concentrated within or close to Lake Malawi particularly in the extreme north and centre of the lake. Shire trough shows low seismic activity which increases tremendously at the southern tip of the country and continues at that rate into Urema graben.

Earthquakes of this area generally occur at shallow depths of ≤ 15km. However, depths  ≤ l5km have been reported in the region. Microseismic surveys done in the nearby regions in central and southern Tanzania [15,16] and north-eastern Tanzania [17] confirm the 0-40km depth ranges. Body wave modelling of the 10th march 1989 Salima earthquake gave a focal depth of 30km [18] further confirming depths of ≥ 15km in the study area.

Data source (Country report)

1.      Name: Lostina Serrah CHAPOLA

2.      Organization: Exploration Geologist,Geological Survey Department, Ministry of Forestry and Natural Resources

3.      Course: 2000 S

4.      Title: COUNTRY REPORT IN MALAWI