Section 37
Chapter 36,511

Fluid-triggered earthquake swarms in the Rwenzori region, East African Rift – Evidence for rift initiation

Lindenfeld, M.; Ruempker, G.; Link, K.; Koehn, D.; Batte, A.

Tectonophysics 566-567


ISSN/ISBN: 0040-1951
DOI: 10.1016/j.tecto.2012.07.010
Accession: 036510561

The Rwenzori Mountains are located within the Albertine rift valley in western Uganda. To monitor the microseismic activity in the area we have deployed a seismic network of up to 35 stations for a period of about 2months. The analysis of the recordings revealed several earthquake clusters within a restricted area NE of the mountain block. The clusters form elongated pipes with 1 2km diameter and vertical extensions of 3 5km. Most of them are located in 5 16km depths; however one cluster reaches down to 22km. Each cluster is composed of a series of single earthquake swarms with durations between a few days and more than a week, interrupted by intervals of inactivity of up to several months. Some of the swarm events exhibit vertical migration tendencies with estimated velocities between .3 and 1km/day. Local magnitudes range from ML =.5 to ML =4. with b-values between .96 and 1.2. The source mechanisms of the swarm earthquakes are dominated by normal faulting with tension-axes orientations perpendicular to the rift axis. There are only few strike-slip events and no reverse mechanisms. From petrological considerations we presume that the earthquake swarms are triggered by fluids and gasses which originate from a magmatic source below the crust. Melt and/or CO2 are guided along the intersection lines of two steep fault sets that were identified by shear-wave splitting analysis and fault mapping in the Rwenzori area. The existence of a magmatic source within the lithosphere is supported by the detection of a shear-wave velocity reduction in 55 8km depth from receiver-function analysis and the location of mantle earthquakes at about 6km. We interpret these observations as indication for an initial rifting process that may eventually lead to the complete detachment of the Rwenzori block from the surrounding rift flanks. We recorded earthquake swarms in the western branch of the East African Rift. Earthquake clusters are located in the crust at 2 5km depth. Swarms form vertical oriented pipe-like shapes with 3 5km length and 1 2km diameter. Earthquakes are triggered by fluids and CO2 degassing from a magmatic body in the mantle. Swarm activity may be indicative for initial rifting stage of a propagating rift segment.

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