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ABSTRACT We present first results from a temporary seismic network located within the western bra... more ABSTRACT We present first results from a temporary seismic network located within the western branch of the East-African rift system. The project aims to constrain the development and uplift of the Rwenzori mountain range and its relation to the formation of the rift zone. Local and teleseismic earthquake recordings are used to image structures of the crust and upper mantle within the region. A temporary network consisting of 23 mobile broadband and short-period seismic sensors was operating during a period of about 16 months until October 2007. The stations were located mainly along two profiles in the westernmost part of Uganda near the Congo border. One profile was situated within the rift along the eastern flank of the Rwenzori Mountains, approximately between 0.2°S and 0.9°N. The second profile was located nearly perpendicular to the rift, extending from the eastern rift shoulder into the rift valley and further crossing the northern part of the Rwenzori Mountains. Additional seismic stations were placed on the rift shoulder in the South-East and the North-West to improve the localization of events. A number of seismological methods are used to study crustal and upper-mantle structure. The localization of sources and fault-plane solutions provide information on active fault zones and on current tectonic movements. Local and teleseismic tomography are applied to determine the 3D velocity structure in the area under investigation and to detect velocity anomalies in the crust and the upper mantle down to approximately 300 km depth. Moho depths are derived from converted phases using receiver functions. Furthermore, shear wave splitting is analyzed to detect regions of anisotropy and their relation to deformation processes and mantle flow. The first analysis of the data shows high seismic activity of the Rwenzori region. Approximately 500 events per month have been located. Focal depths are concentrated between 10 and 20 km. Local P and S-wave tomography using data of the first few months shows significant low velocity anomalies in the northern part of the network, correlated with active volcanic fields and hot springs. From receiver functions, crustal thicknesses of about 23 km have been found beneath the eastern flank of the rift. The interpretation of our results will provide constraints for the modeling of geodynamic processes responsible for the formation of the Rwenzori Mountains.

ABSTRACT Numerous seismological studies in East Africa have focused on the northern and eastern b... more ABSTRACT Numerous seismological studies in East Africa have focused on the northern and eastern branches of the East African Rift System (EARS). However, the seismic activity along the western branch is much more pronounced. Here, the Rwenzori Mountains are located within the Albertine rift valley, at the border between Uganda and D.R. Congo. During a seismic monitoring campaign between February 2006 and September 2007 we have recorded more than 800 earthquakes per month in the Rwenzori area. 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, were the mountains are in contact with the rift shoulders. The hypocentral depth distribution peaks at 16 km depth and extends down to the Moho which was found at 20 - 32 km depths by teleseismic receiver functions. Local magnitudes range from -0.5 to 5.1 with a b-value of 1.1. Fault plane solutions of 304 events were derived from P-polarities and SV/P amplitude ratios. More than 70% of the source mechanisms exhibit 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. The area of highest seismic activity NE of the Rwenzoris is characterized by the occurrence of several earthquake clusters in 5 -20 km depth. They have stable positions throughout time and form elongated pipes with 1-2 km diameter and vertical extensions of 3-5 km. From petrological considerations we presume that these earthquake swarms are triggered by fluids and gasses which originate from a magmatic source below the crust. The existence of a magmatic source within the lithosphere is supported by the detection of a shear-wave velocity reduction in 55-80 km depth from receiver-function analysis and the location of mantle earthquakes at about 60 km. 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.

ABSTRACT The 5000m high Rwenzori Mountains are situated within the western branch of the East Afr... more ABSTRACT The 5000m high Rwenzori Mountains are situated within the western branch of the East African Rift System (EARS), close to the equator at the border between Uganda and the Democratic Republic of Congo. They represent a basement block within the rift whose origin and relation to the development of the EARS are focus of the RiftLink project (www.riftlink.org). To investigate crustal and upper mantle structure in conjunction with seismic activity on a regional and local scale, a temporary seismic network was deployed over an area of roughly 80 x 140 km and operated from May 2006 to the end of September 2007. The analysis of the registered data revealed high microseismic activity in the region. On average more than 800 events per month were located during the registration period with local magnitudes ranging from -0.5 up to 5.1. Few earthquakes are located within the Rwenzori massive itself. Most of the events occur east and west of the mountains with a pronounced concentration of activity at a depth of about 15 km. Vertical sections across the northern parts of the Rwenzories show, that west of the mountains (towards the rift valley) the focal depths range from 10 to 20 km, whereas the hypocenters go as deep as 30 km on the eastern side. This is in good agreement with Moho depths that were derived from receiver functions and are close to 22 km west and 30 km east of the Rwenzories. There is one exception, however. Approximately 30 km east of the northern mountain ridge, we located a cluster of 7 events exhibiting an anomalous depth of about 60 km that occurred within 20 days in September 2006. These events are unique, up to now we located no other earthquakes at similar depths. P-wave polarities were used to determine fault plane solutions of events that were recorded by an adequate number of stations. Nearly all source mechanisms reveal normal faulting with strike directions more or less parallel to the rift axis and extension forces perpendicular to it. However, there is a group of events whose strike directions seem to be systematically tilted counter-clockwise supporting a numerical model of Koehn et al. (2007) who explain the Rwenzori block as a micro-plate that was captured during the approach of two rift segments and is rotating clockwise. Koehn, D., Aanyu, K., Haines, S., Sachau, T., 2007, Rift nucleation, rift propagation and the creation of basement micro-plates within active rifts, Tectonophysics, in press, doi:10.1016/j.tecto.2007.10.003

Journal of African Earth Sciences, 2014

International Journal of Earth Sciences, 2010

Bulletin of the Seismological Society of America, 2011

ABSTRACT We investigate the upper crustal velocity structure beneath the Rwenzori Mountains in we... more ABSTRACT We investigate the upper crustal velocity structure beneath the Rwenzori Mountains in western Uganda. This mountain range of nonvolcanic origin is situated within the western branch of the East African rift and reaches altitudes of more than 5000 m. The cause for the extreme uplift within a rifting environment is currently being debated. The local tomographic inversion described here is based on 2053 earthquakes recorded by a network of up to 35 stations covering an area of 140 x 90 km(2). The deployment was limited by the international border between Uganda and the Democratic Republic of the Congo, such that a number of recorded events lie outside the station perimeter. We perform synthetic tests to assess the effect of location uncertainty on the results. The tests show that the resolution is good between 3 and 15 km depth within a restricted area covered by the array. However, smearing can be significant in some parts. The inversion for P-and S-wave velocity anomalies is performed independently and agrees well. The interpretation of the results is based on a synthetic model that reproduces the same pattern of anomalies as that obtained after inversion of the real data. Our models exhibit a significant negative velocity anomaly (up to -8%) beneath the central Rwenzori Mountains. This could be an indication for active magmatic intrusions beneath the mountains in relation to the rifting. The presence of low velocities in the northwest of the range, within the rift, may be related to magmatic processes beneath the Buranga hot springs. Higher velocities are found elsewhere beneath the eastern rift shoulder and are thought to be related to old cratonic crust.

Solid Earth Discussions, 2012

The Rwenzori Mountains are located within the Albertine rift in Western Uganda with altitudes exc... more The Rwenzori Mountains are located within the Albertine rift in Western Uganda with altitudes exceeding 5000 m. We have carried out a passive-source seismological experiment in order to study the crust and mantle beneath the mountains within the framework of the multidisciplinary RiftLink research group. Local and teleseismic events have been recorded from May 2006 to September 2007. The project

The 5000m high Rwenzori Mountains are situated within the western branch of the East African Rift... more The 5000m high Rwenzori Mountains are situated within the western branch of the East African Rift System (EARS), at the border between Uganda and the Democratic Republic of Congo. They represent a basement block located within the rift valley whose origin and relation to the evolution of the EARS are highly puzzling. During a recent seismological campaign we located more than 800 earthquakes per month with magnitudes ranging from 0.5 to 5.1. Vertical sections across the northern parts of the Rwenzoris show, that west of the mountains (towards the rift valley) the focal depths range from 10 to 20 km, whereas the hypocentres go as deep as 30 km on the eastern side. This is in good agreement with Moho-depths derived from receiver functions and implies that all of these events are located within the crust. However, about 30 km east of the northern mountain ridge we located a cluster of 7 events that exhibit an anomalous depth of about 60 km. We can confidently locate these earthquakes within the mantle lithosphere beneath the rift. The existence of earthquakes at this depth is enigmatic, especially within a rifting regime were one expects hot and weak material relatively close to the surface. We think that these events are possibly related to the evolution of the Rwenzori Mountains. A recent hypothesis to explain the extreme uplift of the Rwenzori Mountains is rift induced delamination (RID) of mantle lithosphere. Here we show that the RID-process is indeed capable of explaining the seismic energy release in the mantle. However, in view of the specific hypocentral location of the event cluster, magmatic impregnation processes associated with dyke propagation into the mantle lithosphere may be a more realistic cause for seismic radiation at the observed depth. Crustal earthquakes northeast of the Rwenzori area are relocated with a double-difference algorithm to improve the spatial resolution of seismicity pattern. Several event clusters in the vicinity of the Fort Portal volcanic field form pipe-like structures with vertical extensions of 3 to 6 km and diameters of 1 to 2 km. In this region the rifting process is probably still in an early stage. The structures possibly indicate magmatic feeding channels through the crust that originate from the heated and impregnated lithospheric mantle.

International Journal of Earth Sciences, 2015

Geological Society, London, Special Publications, 2015

ABSTRACT Inferences of seismic anisotropy of the lithosphere and the deeper mantle are usually ba... more ABSTRACT Inferences of seismic anisotropy of the lithosphere and the deeper mantle are usually based on observations of shear-wave splitting from teleseismic phases. Therefore, depth variations of anisotropy are difficult to resolve. This is related to the fact that the effects of shear-wave splitting can be described by only two parameters, even in complex 3-dimensional media, provided that the dominant period of the incoming wave is sufficiently large. The Rwenzori region of the East African rift is characterized by strong seismic activity. Recently, we observed a cluster of unusually deep earthquakes with focal depths between 50 and 60 kilometers. Receiver-functions indicate a crustal thickness of about 32 km in this region, which is characterized by the extreme uplift (> 5000 m) of the Rwenzori Mountains. Numerical models show that lithospheric delamination induced by rifting may be the cause of the deep faulting. Measurements of shear-wave splitting from these earthquakes in combination with measurements from crustal and teleseismic events allow the discrimination between anisotropic contributions from the crust, mantle lithosphere, and asthenosphere. The results show that crustal anisotropy in the Rwenzori region is highly variable and relatively insignificant in comparison to anisotropy at greater depths that affects teleseismic phases. Delay-time measurements from the earthquakes in the upper mantle beneath the rift indicate that the most significant contribution to the observed shear-wave splitting for teleseismic phases is related to seismic anisotropy at depths greater than 60 kilometers.

Tectonophysics, 2012

ABSTRACT The Rwenzori Mountains are located within the Albertine Rift Valley in western Uganda. T... more ABSTRACT 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 20 months. The analysis of the recordings revealed several earthquake clusters within a restricted area NE of the mountain block. The clusters form elongated pipes with 1–2 km diameter and vertical extensions of 3–5 km. Most of them are located in 5–16 km depths; however one cluster reaches down to 22 km. 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 0.3 and 1 km/day. Local magnitudes range from ML = 0.5 to ML = 4.0 with b-values between 0.96 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 gases 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 55–80 km depth from receiver-function analysis and the location of mantle earthquakes at about 60 km. 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.

Geotectonic Research, 2008

Geophysical Journal International, 2011

Geochemistry, Geophysics, Geosystems, 2013

ABSTRACT [1] We have performed a tomographic study using a joint data set that includes local and... more ABSTRACT [1] We have performed a tomographic study using a joint data set that includes local and teleseismic events, recorded by a temporary network in the western branch of the East African rift system. From the travel time residuals, we derive a three-dimensional model of seismic P-wave velocity anomalies for the crust and upper mantle down to a depth of 80 km. Particular attention is paid to the verification of the inversion results by various resolution tests. The results show that the eastern rift shoulder is characterized by relatively high seismic velocities. Lower velocities are obtained beneath the entire length of the rift valley and the Rwenzori Mountains. A prominent feature is observed north-east of the mountain range: here we detected a vertically oriented, cylindrical low-velocity anomaly with maximum amplitudes in the middle crust and the upper mantle lithosphere. We suggest that this anomaly indicates reservoirs of molten material related to the ongoing rifting process within this segment of the rift. Just above this anomaly, at depths between 5 and 16 km, earthquake swarms exist. The observed reduction in P-wave velocity is used to provide constraints on the possible melt content and temperature anomaly in the uppermost mantle. The observed 3–5% P-velocity decrease can be explained by melt fraction up to 2%–3.3% or alternatively by a temperature increase of at least 248 to 376 K and even higher-temperature anomalies are possible if lower ambient temperatures in the reference mantle are assumed. Probably, the two effects act in combination.

Proceedings of Thirty- …, 2007