Seismicity from February 2006 to September 2007 at the Rwenzori Mountains, East African Rift: earthquake distribution, magnitudes and source mechanisms

Abstract

Abstract.We have analysed the microseismic activity within the Rwenzori Mountains area in the western branch of the East African Rift. Seismogram recordings from a temporary array of up to 27 stations reveal approximately 800 events per month with local magnitudes ranging from -0.5 to 5.1. The earthquake distribution is highly heterogeneous. The majority of located events lie within faults zones to the east and west of the Rwenzoris with the highest seismic activity observed in the northeastern area, where the mountains are in contact with the rift shoulders. The hypocentral depth distribution exhibits a pronounced peak of seismic energy release at 15 km depth. The maximum extent of seismicity ranges from 20 to 32 km and correlates well with Moho depths that were derived from teleseismic receiver functions. We observe two general features: (i) beneath the rift shoulders, seismicity extends from the surface down to ca. 30 km depth; (ii) beneath the rift valley, seismicity is confined to depths greater than 10 km. From the observations there is no indication for a crustal root beneath the Rwenzori Mountains. The magnitude frequency distribution reveals a b-value of 1.1, which is consistent with the hypothesis that part of the seismicity is caused by magmatic processes within the crust. Fault plane solutions of 304 events were derived from P-polarities and SV/P amplitude ratios. More than 70 % of the source mechanisms exhibit pure or predominantly normal faulting. T-axis trends are highly uniform and oriented WNW-ESE, which is perpendicular to the rift axis and in good agreement with kinematic rift models. At the northernmost part of the region we observe a rotation of the T-axis trends to NEN-SWS, which may be indicative of a local perturbation of the regional stress field.

The Rwenzori Mountains, a landslide-prone region?

“…The last major earthquake (M=4.5, USGS 2014b) on the horst mountain occurred on 31 October 2014. Concentrations of earthquakes in this inventory coincide with the observations of Lindenfeld et al (2012b) and cluster near the major faults bordering the horst mountain.…”

“…Around the center of the eastern flank, a more complex faulting structure is apparent (Koehn et al 2010). The majority of the tectonic activity takes place in the faults' zones bordering the eastern and western flanks, and the most active area is located in the NE of the Rwenzori Mountains (Lindenfeld et al 2012b). This tectonic activity is related to active rifting processes, under an overall WNW-ESE extension (Delvaux and Barth 2010).…”

Variations in the Seismogenic Thickness of East Africa

“…As stated in our introduction, in this study we have excluded the consideration of seismicity clearly representing the brittle failure of high-temperature material driven by the high-strain-rate deformation associated with the migration of magmatic fluids (Illsey-Kemp et al, 2018;Keir et al, 2009;Lindenfeld et al, 2012a;Oliva et al, 2019). However, we should not neglect that, despite the link between high-temperature, lower crustal seismicity and an anhydrous, dry bulk crustal rheology, crustal fluids do play a role in crustal CRAIG AND JACKSON 10.1029/2020JB020754 9 of 15 Priestley et al (2018), which has a horizontal resolution of ∼200 km.…”

“…Temporary local networks of seismometers, allowing the detection and location of smaller‐magnitude earthquakes, have been operating across East Africa since the early 1990s, although, due to the logistical requirements, they remain sparse. We compile the results from 13 high‐quality seismic surveys, covering regions including Afar and the Main Ethiopian Rift (Illsey‐Kemp et al., 2018; Keir et al., 2006), the Eastern Branch through Kenya and Tanzania (Ibs‐von Seht et al., 2001; Langston et al., 1998; Mulibo & Nyblade, 2016; Weinstein et al., 2017; Young et al., 1991), and the Western Branch from Lake Albert (Lindenfeld et al., 2012a; F. A. Tugume & Nyblade, 2009), through Tanganyika (Lavayssière et al., 2019) and Rukwa (Camelbeeck & Iranga, 1996), to northern Malawi (Ebinger et al., 2019; Gaherty et al., 2019): all shown on Figure 3.…”

The Impact of Complex Volcanic Plumbing on the Nature of Seismicity in the Developing Magmatic Natron Rift, Tanzania

“…Tectonic thinning is commonly accompanied by decompression melting and the rise of volatiles and magma through the lithosphere (White and McKenzie, 1989;Weinlich et al, 1999;Rooney, 2010;Lee et al, 2016). Therefore, intrusion of magma and volatile release are thought to play a pivotal, yet interacting, role in the continental rifting process (Lindenfeld et al, 2012a;Wright et al, 2012;Muirhead et al, 2016;Weinstein et al, 2017).…”