In order to investigate crustal and upper mantle structure in conjunction with the seismic activity on a regional and local scale, a temporary network of up to 27 seismic stations was deployed over an area of roughly 80 x 140 km and operated from May 2006 to the end of September 2007 (Figure 1, white triangles). A station consists of a seismic sensor, power supply by battery and solar panel, and an EDL datalogger (Figure 2, from left to right). Typically , the equipment was installed in a room that was rented from local residents for the duration of the project. At several locations we had to build small huts due to the poor conditions in order to place our instruments in a more secure way (Figures 3a and 3b).
Figure 1: Distribution of seismic stations (white triangles) and local seismicity from May 2006 to September 2007 (colored dots). Different colors indicate different source depths, see legend.
Figure 2: Installation of a seismic station: sensor, batteries, and datalogger (from left to right).
Figures 3a and 3b: Installation of a seismic station: construction of a station hut made of firebricks.
The analysis of the registered data revealed unexpectedly high microseismic activity in the region (Figure 1). On average more than 800 events per month could be located during the registration period with local magnitudes ranging from -0.5 up to 5.1 in some cases. Surprisingly , only 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 Rwenzoris 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 teleseismic receiver functions. The values range from about 21 km in the west to over 30 km in the eastern parts of the Rwenzori area, indicating that the vast majority of the local earthquakes is located within the crust (see below, results of receiver function analysis).
P-wave polarities are used to determine fault plane solutions of events that were recorded by an adequate number of stations (Figure 4). Most of the derived source mechanisms (> 75 %) reveal normal faulting with strike directions more or less parallel to the rift axis and extension forces perpendicular to it. Only few events have clear reverse or strike-slip mechanisms.
Figure 4: Fault plane solutions of about 70 events. Most of them indicate normal faulting with strike directions more or less parallel to the rift axis.
In order to resolve the 3D velocity structure of the crust and upper mantle beneath the Rwenzori area down to roughly 100 km, a joint tomography method has been applied using P arrival times of local and teleseismic events. The results are based on 2053 local and 284 teleseismic events that were recorded by our seismic network.
In general, we observe high seismic velocities below the eastern rift shoulder and lower velocities below the mountain range. A strong negative anomaly is seen in the northwest, close to the Buranga Hot Springs. Towards the southern end of the Rwenzoris there is an extended region of reduced seismic velocities that could be related to the sedimentary cover and a low velocity zone within the crust. There is also evidence for a deeper negative anomaly in the central eastern part of the region which could be related to melts rising from the earth mantle.
Receiver function analysis
Teleseismic events have been investigated with the receiver function method. With this technique it is possible to identify discontinuities in the velocity structure within the earth. Figure 5 shows a map with derived Moho depths: Beneath the eastern rift shoulder the crust reveals a rather simple structure with Moho depths of about 30 km. The Rwenzori range, however, is characterized by a complex inner crustal structure. Different methods have been applied to determine the Moho depth providing evidence for crustal thinning instead of a crustal root beneath the Rwenzori mountains. In this area, we derive Moho depths ranging between 28 and 21 km. Our study therefore indicates the absence of a crustal root beneath the Rwenzori block. Beneath the Lake Edward and Lake George basins we detect the top of a layer of significantly reduced S-wave velocity at 15 km depth. This low velocity layer may be attributed to the presence of partial melt beneath a region of recent volcanic activity.
Figure 5: Crustal thicknesses derived from teleseismic receiver function analysis. Beneath the eastern rift shoulder Moho depths are larger than 30 km. Beneath the Rwenzori mountains they are only slightly larger than 20 km.
New Station Network
In order to improve the resolution of our investigations and to extend the observation range, especially in the southern and western parts of the Rwenzori region, a new temporal seismic network was installed in the area (Figure 6). It was deployed during October 2009 and consists of 32 seismic stations equipped with broadband sensors. 10 of them are located west of the Rwenzoris, in the Democratic Republic of Congo.
Figure 6: Actual seismic network within the Rwenzori area as installed in October 2009. Different colors denote different instrument types.