Breaking up the hanging wall of a rift‐border fault: The 2009 Karonga earthquakes, Malawi

Abstract

[1] The southern East African Rift has an unusually large seismogenic thickness (35-40 km), which is responsible for wide tilted basins and extremely long faults with the potential for M7-8 normal-faulting earthquakes. From 6-19 December 2009, a shallow earthquake sequence (four of M w > 5.5) hit the Karonga region of northern Lake Malawi. The location is 50 km west of the rift-bounding Livingstone Fault, within the hanging-wall. We used seismology and InSAR to obtain source parameters and combined this with information on rift structure from geomorphology and seismic profiles. The deformation is consistent with rupture of a shallow, west-dipping fault, with no evidence for the involvement of magmatic fluids. Although the Livingstone Fault dominates local geomorphology, the Karonga earthquakes demonstrate that the hanging-wall block is actively breaking up, reflecting temporal and spatial migration of activity or the release of stresses within it.

Acquisition of a Unique Onshore/Offshore Geophysical and Geochemical Dataset in the Northern Malawi (Nyasa) Rift

“…The long border faults within thick strong crust may be capable of producing M w ∼ 8 earthquakes (Jackson and Blenkinsop, 1997;Hodge et al, 2015). As demonstrated by the Karonga earthquake sequence (Biggs et al, 2010;Fagereng, (Farr et al, 2007) and lake depth from Lyons et al (2011). Key defines symbols.…”

“…InSAR data are collected over the region infrequently by most satellites, and some satellites have made no useful observations. But sufficient data from several satellites exist to observe ground deformation-principally from the European Space Agency's Envisat and Sentinal-1 satellites and the Japanese Space Agency's ALOS-1 satellite (Biggs et al, 2010;Hamiel et al, 2012). The GPS network includes 12 continuous GPS stations deployed in Malawi, Tanzania, and Zambia (Fig.…”

“…However, a number of intrabasin faults are present as well. The occurrence of a sequence of earthquakes in 2009 with magnitudes up to M w 6 on one or more west-dipping intrabasin faults near Karonga in northern Malawi clearly indicates that some of these faults remain active (Biggs et al, 2010;Fagereng, 2013;Macheyeki et al, 2015; see Fig. 1a for locations).…”

Assessing infrequent large earthquakes using geomorphology and geodesy: the Malawi Rift

“…For the assumption of independent segmented rupture, only the Bilila-Mtakataka and Livingstone faults would be capable of producing M W 7.8 earthquakes, but M W 7.0 or larger earthquakes would be expected to occur every 50 years with our model compared to 170 years from the GR parameters. However, it is more likely that the stress transfer from the rupture of a single segment would bring neighbouring segments closer to failure, thus initiating a sequence of events that cluster in time (Stein 1999), especially considering the 2009 Karonga sequence (Biggs et al 2010;Fagereng 2013). A probabilistic approach, such as PSHA, gives a more appropriate estimate than a simple repeat interval calculation, but further investigations on temporal clustering of segmented faults are needed before this can be formally included in the calculation.…”

“…The Karonga sequence occurred between 6 and 9 December 2009, when a succession of earthquakes hit the Karonga region of Malawi. Four of these earthquakes had M W [ 5.5 and five more events with a body wave magnitude (m b ) between 5.0 and 5.2 occurred (Biggs et al 2010;Fagereng 2013). Four deaths were attributed directly to the earthquakes with 300 people injured and 1,000 houses collapsed (United Nations Office of the Resident Coordinator 2009).…”