The Karonga earthquake sequence in the southern East African Rift.
Juliet Biggs, Edwin Nissen, Tim Craig & James Jackson
In December 2009, a shallow earthquake sequence hit the Karonga region of northern Lake Malawi. Between 6-19th December, four earthquakes occurred with magnitudes over 5.5, accompanied by a five further events with magnitudes in the range 5.0-5.2. Over 1000 houses collapsed, a further 2900 were damaged, 300 people were wounded, and 4 were killed. Although these earthquakes were of moderate size only, the southern East African Rift has an unusually large thickness (35-40~km) of crust within which earthquakes can occur. This has resulted in wide tilted basins and extremely long faults with the potential for magnitude 7-8 normal-faulting earthquakes.
In conjunction with ESA’s Changing Earth Science Network, we used the satellites Envisat and ALOS to acquire radar images of the ground deformation associated with these earthquakes, which are among the first to be studied with satellite interferometry in the East African Rift. We use seismology and satellite interferometry to obtain earthquake source parameters and combined this with information on rift structure from geomorphology and seismic profiles providing an excellent opportunity to study active faulting within a rift setting.
The geomorphology of the area is dominated by the 100-km long Livingstone Fault; the down-thrown block to the west of the fault has created a topographic low, occupied by Lake Malawi and its sediments. The Karonga earthquakes lie 50 km west of the rift-bounding Livingstone Fault, just on the shore of Lake Malawi. This demonstrates that the down-thrown block of the Livingstone Fault is not intact. It is actively breaking up, reflecting either the temporal and spatial migration of activity or the release of stresses within it.
The deformation patterns seen in the satellite images are consistent with the rupture of a shallow, west-dipping fault. Although magmatism and dike intrusion are important components of continental rifting even in immature sections of the East African Rift System, and the earthquakes did not follow a simple mainshock-aftershock pattern, we see no evidence for the involvement of magmatic fluids.
Figure 1: Location of the 4 largest earthquakes in the December 2009 Karonga Malawi earthquake sequence from teleseismic records. The earthquakes were located on the western shore of the Lake, 50 km from the major rift-bounding Livingstone Fault. The white box is the location of the interferograms on the right which show deformation patterns from the satellites ALOS and Envisat. They are displayed such that each fringe (red to blue) represents 2.8 cm of motion away from the satellite in the satellite line of sight. The top panel contains deformation from the complete earthquake sequences whereas the lower panel only contains deformation from the last earthquake of the sequence which was the largest. The patterns are consistent with the rupture of a shallow, west-dipping fault represented by the black line.
Biggs, J., Nissen, E., Craig, T., Jackson, J. and Robinson, D.P. in press. Breaking up the hanging wall of a rift-border fault: the 2009 Karonga Earthquakes, Malawi. Geophysical Research Letters.