Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Solid Earth Discussions 1 , and A. Schumann 566 565 2 , A. Batte 1 ¨ umpker , G. R 1 This discussion paper is/has been under review for the journal Solid Earth (SE). Please refer to the corresponding final paper in SE if available. Institute of Geosciences, Goethe-University, Frankfurt amDepartment Main, of Germany Geology, Makerere University, Kampala, Uganda and represents one2006; of Chorowicz, the 2005; Ebinger, world’s 1989). largest Starting at continental the rift Afar triple structures junction in (Bendick the et north, al., nantly normal faulting. T-axis trends areis highly perpendicular uniform to and the rift orientednorthernmost axis WNW-ESE, and which part in of good the agreement region with wewhich observe kinematic may a rift be rotation models. indicative of At of the the T-axis a trends to local NEN-SWS, perturbation of the regional stress1 field. Introduction The East African Rift System (EARS) separates the Nubian and the Somalian plates observations there is no indicationThe for magnitude a frequency crustal distribution root revealsthe a beneath hypothesis b-value the that of Rwenzori 1.1, part Mountains. which ofcrust. is the Fault consistent seismicity with plane is causedamplitude solutions by ratios. of magmatic More 304 processes than within events 70 % the were of derived the source from mechanisms P-polarities exhibit and pure or SV/P predomi- jority of located eventswith lie the within highest faults seismic zonestains activity to are observed the in in East contact thea and with northeastern pronounced the West area, of rift peak where shoulders. the of theof The Rwenzoris seismic seismicity moun- hypocentral ranges energy depth from release distribution 20derived at exhibits to from 15 32 km teleseismic km depth. receiver andneath functions. correlates The the well We maximum rift with observe extent shoulders Moho two(ii) seismicity depths general beneath extends that the features: from were rift (i) the valley be- surface seismicity is down confined to to ca. depths 30 greater km than depth; 10 km. From the We have analysed the microseismicwestern activity branch within of the the Rwenzori East Mountains Africanof Rift. area up Seismogram in recordings to the from 27 a temporary stationsranging array from reveal –0.5 approximately to 800 5.1. events The per earthquake month distribution is with highly local heterogeneous. magnitudes The ma- Abstract 1 2 Received: 14 March 2012 – Accepted: 12Correspondence April to: 2012 M. – Lindenfeld Published: ([email protected]) 8 MayPublished 2012 by Copernicus Publications on behalf of the European Geosciences Union. Seismicity at the Rwenzori Mountains, East African Rift: earthquake distribution, magnitudes and source mechanisms M. Lindenfeld Solid Earth Discuss., 4, 565–598,www.solid-earth-discuss.net/4/565/2012/ 2012 doi:10.5194/sed-4-565-2012 © Author(s) 2012. CC Attribution 3.0 License. 5 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | × sharply ff ¨ umpker (2011) have 568 567 ). Despite of the high activity, there is only little knowl- erent sites during June to September 1973 moving the ff www.riftlink.org in the Rwenzori Mountains region. The instruments were installed in the 2 In the present paper we provide a comprehensive characterization of the seismic The 5000 m high Rwenzori Mountains are situated within the Albertine rift, in an area Globally recorded earthquakes (Fig. 1) as well as compilations on regional and local stations were equipped withwith broadband short-period instruments (Guralp seismometers CMG-3T), (Mark the L-4C3D). remainder Occasionally some instruments were 80 km framework of theBGR/GSMD RiftLink GEOTHERM-Subprogram Project, (German however,cal Geological some Survey Survey, data BGR, and werework Geologi- Mines of also Department, nine provided 3-component GSMD,of by short-period Uganda). the instruments the Until area was Aprilgered running equipped mode. 2006 in with Starting the a in Reftek northern MayEDL small data 2006 part data the loggers net- net loggers sampling was recording expanded the continuously by at 20 data stations a at equipped sample with 100 rate Hz of in 100 Hz. trig- Seven of these 2 Seismic network and data analysis The data weber present 2007 in by this a study temporary were network recorded of 29 from seismic February stations 2006 covering to an Septem- area of 140 up. activity within the whole Rwenzoriperiod region, covering than a previous substantially investigations. largertions area For and and the time reliable first source timedetailed mechanisms we discussion of of present a the magnitudesential complex large distribu- for hypocentral set a depth of better distribution. earthquakes, understandingwill These of improve together data our the with are general rifting es- a insight processes into in the this mechanisms part of of active the rifts. EARS and distribution exhibits a maximumat number 22 of km earthquakes extending at downof 16 to km seismicity 31 and is km. falls restricted In o reported to all the the of detection crust. the of However, above Lindenfeldtains, mantle investigations and earthquakes which the R to are depth the likely extent northeast related of to the magmatic Rwenzori intrusions Moun- in relation to lithospheric break depth extent and its implication on the heat flow in the area. The observed focal depth and June based on a network of eight stations and they investigate the hypocentral in the Rwenzori areaThey were (Maasha, operated 1975) at wasinstruments 28 based after di on a fourevents period portable per of day seismic 4 was stations. 4 to recorded and 20 in focal days the depth tohibit reaching area large down new with uncertainties to magnitude locations. because 25 estimates Ancomposite of to fault between average the 40 plane km. –2 limited of solutions However, number and indicated these 5recent of normal depth to study faulting stations. estimates by in 20 A ex- Tugume the small and RwenzoriRwenzori number Nyblade area. Mountains. (2009) of A was The more restricted authors to present the northern the end seismic of activity the between 2006 March edge on the local seismicity in the Rwenzori region. An early microearthquake survey with the highest seismic activity ofbasement the block whose whole rift origin system. and TheyRiftLink relation represent to project a the ( non-volcanic evolution of the EARS are focus of the earthquake studies have showncantly that higher the seismic western activity branch than1997). the of Numerous eastern the publications branch EARS focus (Midzi on exhibits etout the al., signifi- to 1999; hypocentral depth Twesigomwe, extend distribution, down whichnental to turns regions the where crustal earthquakes base are2009; Camelbeek – restricted and in to Iranga, 1996; the contrast Foster upper1995; to and crust Shudofsky Jackson, other 1998; et (e.g. seismically Nyblade al., Albaric and active 1987; Langston, et conti- Young al., et al., 1991). connecting rifts segments (Fig.up 1). into South two of the branches,They Main the enclose Ethiopian Albertine the Rift Tanzania Rift(Nyblade the Craton and in system which Brazier, splits the is 2002). westand Further surrounded terminates and at south by the the the Proterozoic coast Kenya system mobile of Rift southern continues belts in Mozambique. as the the east. Malawi Rift it extends approximately in N-S direction through East Africa consisting of a series of 5 5 25 15 20 10 25 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent ff 0.05 s. ± 49.2 km). ∆ = 0.9). The plotted 2.8 km) and KISA = 0.1 s. This is due to L ± 5 km which is 76 % of 3.0 was chosen to trig- M ∆ = ≤ = available for the Rwenzori is the hypocentral distance ¨ dist oller, personal communication, phases travelling predominantly 12.9 km) and KISA ( , n 3 km, see below). Furthermore, 3D d ects, P waves on the vertical compo- (a, b, c, d) < ff + ∆ = 1.67, dist = 570 569 × 7 % (Jakovlev, 2010). The average deviation d 0.3. Obviously, there is no systematic distance + c ± + ) dist log ( 0.00087, and × = b 3.8). Records are from stations KASS ( c ¨ + oller, 1999), including an implementation of HYPOCENTER, a lo- = L M 0.91, amp = is the maximum horizontal ground amplitude (zero-peak) in nm of a Wood- log ( b × amp 2.3 with a standard deviation of a 49.3 km). 1.0, = = = In general it was possible to determine P-arrival times with a precision of To give an impression of the data quality we present seismograms of two earth- L L ∆ = stations as aM function of epicentral distance (circles). The average of all stations is where Anderson simulation of the recordedin seismogram, and km. So far, thereregion. are We no use calibration the parameters presets of the SEISAN softwarea package: which were derived2010). by Figure Alsaker 4 et presents al. the (1991) magnitude (Ottem of an event calculated at twelve di 2.2 Magnitude determination Local magnitudes were determined using the formula ofM Hutton and Boore (1987): the lower frequency contentonset of is the blurred S-phases by the andprovides P-wave because coda. estimates in During the of many location cases locationnetwork process the the geometry. errors true SEISAN The based software S- RMS onThe residuals travel majority range time of between the residuals 0.1the s events and following and (ca. analysis the 0.2 60 s we %) station all in use were located only most located events. events cases. with with an location error errors better than 3 km. In reliable arrival time pickingsatively (dashed strong lines). ( The( second event (bottom row) was rel- We assume a slightly reduced accuracy for S-wave arrivals of Both stations show clear P and S-onsets with good signal/noise ratios that enable quakes (Fig. 3). Theseismograms first were event recorded (top at row) stations KARU was ( relatively weak ( velocity anomalies between –7of % the and relocated hypocentres comparedis to smaller the than 1-D the locationsvelocity estimated was about anomalies 1-D 1.30 location from km, error which travel-timetre ( tomography of are the only seismic wellal., constrained station 2011). in For network these the withimprove reasons cen- relatively the we location poor think precision vertical that of resolution the the Bram use (Jakovlev model. of et a 3-D velocity model would not the unfiltered seismograms to avoid phasenent shift and e S on onevelocity of model the UVI-N74 horizontal components. of ForBram Bram earthquake model (1975) location with was we a derived usedparallel the fixed from to vp/vs local the ratio and of Albertine regionalmodel 1.74 rift. P we (Table 1). To relocated test The the the hypocentres limits using of a such synthetic a 3D relatively model simple with 1-D several crustal velocity algorithm to the continuous datawith streams. a Prior Butterworth to this high procedure passset the at of data 1 Hz. were trigger filtered Tests parameters, weretoo performed i.e. much to to false determine identify trigger theLTA) were a events. optimum set Time maximum to windows number 3 s for ofger and short- an earthquakes 30 s event. and without respectively, The a long-term routine(Havskov ratio average and data of (STA, Ottem analysis STA/LTA was donecation with algorithm by the Lienert SEISAN and software Havskov package (1995). Arrival times were manually picked on (Fig. 2). 2.1 Hypocentre location In order to detect and extract the numerous local events we applied a trigger STA/LTA moved to new places so that the network in total consisted of 35 station locations 5 5 25 20 15 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ◦ and –1.0 ◦ ◦ N seismic activity ◦ ects only events with mag- ff use into the Lake Albert Rift. In the ff 572 571 c, which starts shortly after sunrise (ca. 06:00) and ffi longitude, we recorded high seismicity aligned with the N-E ◦ 5 km (light and dark bars, respectively). We are not able to identify –30.5 ◦ ≤ 5 km. Only few earthquakes are located within the Rwenzori block. The ≤ latitude, we detected a narrow band of seismicity, separating the main Rwenzori ◦ the whole Rwenzori regionthe including Democratic the Republic western of Congo. and The south-western analysis parts of situated the data in will answer the question if trending eastern rift flank. Theseof events the are probably Toro-Bunyoro located fault atof (Fig. the the 2), southern Lake which segment Albert runson Basin the in (e.g. fault NE Ring, ends directionsouth-western 2008). abruptly along parts However, and of the at continues the about eastern more Rwenzoriwill region 1.0 di side be we shown observed in only Sect. weak seismicthat 3.3., area, activity. this As which is decreases probably the an sensitivity artefacttion of due of the to only station the the network absence and stronger of enables events. stations the Currently in detec- we are operating a new network that covers segments. The highest seismiczoris activity between is Lake observed Albert withinthe a and eastern region Lake rift east George, flank. of wherein The the the planar majority Rwen- mountains of structures. these are Onlybution events connected in with are to few known grouped cases faults. in At itlatitude clusters the is and rather north-eastern possible than 30.2 tip to of associate the the mountains, epicentre between distri- 0.8 cation errors majority of the epicentresrange. lie This within may fault possibly zonespled indicate to rigid that the the block that mountains eastdeformations is can and as surrounded be west proposed considered by of by two as the0.6 Koehn rift a mountain et decou- segments al. and (2008,block resistant 2010). in to However, the seismogenic between south 0.5 bly from an its indication of northern stresses ridge, and relative see movements dotted acting ellipse between the in two Fig. mountain 2. This is proba- 3.1 Hypocentre distribution In total we located almostin 14 000 the earthquakes Rwenzori from area. February 2006 The to seismicity September map 2007 (Fig. 2) includes ca. 10 600 events with lo- 3 Results and discussion of located events decreasesthis to less observation than with 25 an %sensitivity increased of of noise the the level night during seismometer timeactivity, day such network values. as time. is We field During associate considerably work this reducedends and period tra a as the few a hours result afternitudes of sunset smaller (ca. human than 18:00). However, 1.5constant this over a whereas the the whole day. number of larger earthquakes is more or less finished. Between August 2006in and September a 2007, more when orthem the less with network steady errors was state, running weany located clear on seasonal average variationsdependence 766 on of events the per the time of month, seismicity daytivity 613 (Fig. rate. between 6). of 04:00 However, There and we is an 17:00 observe UT apparent breakdown which a in corresponds distinct seismic to ac- 07:00–21:00 LT. The number rameters of SEISAN. 2.3 Temporal variations of seismic activity The analysis of the recordedregion. data Figure revealed 5 high presents microseismiclates activity the fairly in monthly well the numbers with Rwenzori number of the of located number events earthquakes increases of until which operating July corre- stations. when Starting the in expansion of February the 2006 station the network was eters of Alsaker etRwenzori al. area. (1991) For give comparison an werameters appropriate re-calculated that description the were magnitudes derived of for using thefor Tanzania calibration (Langston attenuation Southern pa- et in California al., (Hutton the 1998) and andand Boore, the show 1987). original no The values results significant are deviation also from plotted the in magnitudes Fig. calculated 4 with the default pa- dependence of the calculated magnitudes. This suggests that the calibration param- 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent vertical ff erent places and direc- ff ` ere et al. (2001) find a seis- ´ everch ¨ olbern et al., 2010). The locations of 574 573 ¨ olbern, personal communication, 2011)). We observe this erence in the hypocentral depth distribution: west of the mountains ff : This profile crosses the Albertine Rift north of the Rwenzori mountains : This section is crossing the northern part of the Rwenzoris. Again, there 27 km (BURO, SEMP, ITOJ, KARU)33 km and (KMTW, beneath KAGO, theearthquakes MIRA). rift in A shoulder this area. from remarkable A 29 feature small km group is to of seven the earthquakes detection is located of at depths mantle pattern of hypocentral depth distributiontures. in (1) all profiles. Beneath There the arenected two rift to general shoulders the fea- or shouldersdepth. in seismicity (2) areas extends Beneath from where thepermost the the rift 10 surface Rwenzoris km valley down are together we toseismicity with con- observe beneath ca. reduced the no 30 depth km rift seismic extension. graben1991) is activity The and also absence within Lake observed Magadi of the at (Ibs-vonend shallow Lake up- of Seht Bogoria the et (Young Kenya et al., rift. al., 2001),in They attributed both the the basement, located activity buried at deeper beneath the than the southern 10 rift. kmProfile to B deep faults is a distinct di (towards the rift valley,eastern 5–25 side km) (40–55 km), focal where depthsshoulder, the seismicity Rwenzori extends range Mountains from from the are surface connected 10 downobserve to to only the to 30 km weak rift depth. 20 seismic km. In activitydepths between On we which show the is the typical same for characteristics. the In Rwenzori the block. rift Moho valley they range from 21 km to Profile A and ends up athorizontal the profile rift axis) shoulder to seismicityreaching the is from east. concentrated Beneath the along the surface atermined shoulder down vertical at (35–45 to oriented 34 km km 30 on plane (station km.On KABE) In the indicating this that western area all side eventsrestricted the however occur (5–30 between Moho km), within 10 the depth beneath km crust. and the wasdepth 25 rift de- at km valley, SEML. depth. hypocentres (Moho The are depthreinterpretation Moho at is of SEML also the was raised receiver originallythis functions up published the to site with authors 25 to km 34 modified km. 25 the After km, Moho (W depth at – – For a more detailed analysis of the hypocentral depth distribution we compiled sev- sections: tions (Fig. 8). Fouraxis, profiles whereas profile (A–D) E aremark runs oriented parallel the more to background or the seismicity;projected less rift coloured from onto perpendicular symbols NNE the to to representAdditionally the respective SSW. hypocentres to Black rift planes. that the dots The hypocentres in were verticaleral we Fig. stations have 8 sections marked by are teleseismic the receiver Mohothese presented stations functions depths in are (W determined also Fig. at indicatedFig. 9. in sev- Fig. 7 8. we In now contrast observe to the a overall highly depth complex distribution and of individual picture at the di mogenic lower crust withfew earthquakes significant below earthquake the activity MohoFrom in modelling down the of to Baikal the 35–40 Rift brittle-ductile kmat system, 0–10 transition and km very in depth only similar the and to crust a our they diabase results. rheology infer at a 10–45 quartzeral km rheology depth. vertical sections of seismicity transecting the area in di Rwenzori area. Seismicity extendsclear from maximum the of surface activity down atto to 15 the km. a conclusion This depth that agrees ofdeeper well in 32 than with km East earlier the with Africa observations Mohorovicic a Foster earthquakes leading discontinuity and occur (Albaric Jackson, throughout 1998). the etstricted crust, While al., to but in 2009; the not most Brazier uppercrust other et are crust, continental al., also similar regions 2005; reported focal2006; seismicity for depth Jackson is the et distributions re- Baikal al., throughout Rift 2008; (Chen the Maggi and complete et Molnar, al., 1983; 2000). Emmerson D et al., the north-eastern parts of the Rwenzori area. 3.2 Depth distribution In Fig. 7 we present a histogram of the general hypocentral depth distribution in the seismic activity in this region is actually weak or has a similar high level as observed in 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | usive ff ¨ umpker erent lo- 1.1 and a ff ∼ 1.5 (green and blue symbols) ≤ ¨ 5.1 with a b-value of olbern et al. (2010) interpret this L = M L M 576 575 –0.5 up to = 1.5 are located more evenly over the whole region. 1.3, as derived from the magnitude-frequency distri- L M > = L C ¨ olbern et al., 2010). From modelling the authors derive M M This depth section starts beneath the central peaks of the Rwenzori This profile runs east of the Rwenzoris, roughly in N-S direction, parallel : This profile traverses the Rwenzoris in an area where the main block is temperature could explain the observed decrease of seismic activity in this area. character towards the southernwe profile have end. to This expect isnetwork. less The certainly location seismicity due seems accuracy to to for tracezone the the events (LVZ) that upper fact at was and that the lower identified edge bylow border of receiver 15 of km a function low depth the inversion velocity and at station but can station be MWEY less seen be- pronounced also beneath (W a KASE strong and velocity NYAN at degrease similar whichthe depth is crustal rock unlikely (see caused S-velocity by graphchannel in compositional as Fig. changes a 9). of possible W zonefields of partial in melt within the the area. crust related Reduced to brittleness local volcanic of the lower crust caused by increased the seismic activity splits uprespectively. There into is two a horizontal remarkable levelsfocal coincidence at between depth 15 Moho km positions extension, and and although 25 the km seismicity depth, fades out and has a more di Profile C again extends from the surfaceare down 25 km to and 25 km. 24 km, Moho respectively. depths atProfile NGIT D: and KASS block and crosses the Edwardis no rift seismic towards activity the inalready eastern the rift observed uppermost shoulder. beneath 10 Again, km thethe there beneath mountain Albert the (KILE, rift rift RUBO) graben are inthan – approximately profiles the 22 similar km maximum as A–C. which depth Moho is ofdeepens about depths seismicity 5 systematically km close in down deeper this to to area.neath more Further the than to eastern 25 the km. rift east(NYAN, At RWEB). shoulder seismicity the the end Moho of discontinuity is the located profile,Profile E: at be- 32 km depth to the northward penetrating Edwardthe rift. uppermost Again, there 10 km is beneathprofile no seismic leaves the the activity rift tip within valley. of At25 the km its rift, depth. northern seismicity extends Further end from (E2), south the where surface the down the hypocentres to about concentrate at 15 km depth. Then between 53 km and 60 kmevents (dashed occurred circle) which during is a clearlyand below period 2.4. the Moho. of We These are 14EARS. not days A aware with detailed of study local similarly of(2011). magnitudes deep these between earthquakes events can in 1.4 be other found regions in of Lindenfeld and the R connected with the northerning ridge. the Here mountains wehypocentres observe in a between E–W strip 10 direction km of (30–40 seismicity and km cross- 20 on km horizontal depth. profile On axis) the with eastern side seismicity – – – The heterogeneous spacing of the station network implies also that the complete- presents the cumulative magnitude-frequencycations. relations Area calculated 1 for is two situated di in a region north of the Rwenzoris with high station density could only be located insity. the In north-eastern contrast, area events where with theThis network implies has that its the highestis den- apparently probably reduced due seismic to activityseismic network southwest the west of lack of the of thehere Rwenzoris mountains stations as will in in prove the this if north-eastern there area. region. is The indeed similar data highness of activity of a the recently recorded deployed data set strongly depends on the respective region. Figure 11 Local magnitudes were calculatedplete as data catalogue specified range in from Sect.magnitude of 2.2. completeness The resultsbution. for However, the the map in com- Fig. 10of presents the a calculated highly heterogeneous magnitudes, spatial which distribution tion is network. obviously the The result smallest of events the with unevenly magnitudes spaced sta- 3.3 Magnitude distribution 5 5 25 15 20 10 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | is approximately 1.5 (tri- erent sensor bandwidths ff C M ects of di ff ´ anek (1994) for the western branch of the 578 577 ers clear ambiguity (Fig. 12a). The nodal planes are consistent with ff . The results reveal a remarkable uniform picture for the orientation of ◦ 0.8. The second area lies southeast of the mountains within the Edward rift. = C M Figure 14 shows a map with all derived fault-plane solutions. The events are ar- A classification of the derived fault plane solutions on the basis of P- and T-axis kinematic rift models (Stamps et al., 2008, 2010). The two southernmost groups, 5 and 6 % of allcomponent. analysed 8 % events of have the pure events have reverse pure strike-slip faults mechanisms. orranged into reverse six faults groups withdirections in strike-slip of order the to tension identify axestor (T-axis any trends) widths systematic are of plotted pattern. 15 in For athe each rose-histogram extensional group using stress sec- the field,and particularly east in of groups the 1,groups Rwenzori 3, is Mountains. and WNW-ESE The 4 – average which perpendicular orientation are to of located west the the T-axes rift in axis these – and in good agreement with reverse faulting (green symbols).slip Additionally, we component classified (magenta normalsymbols), symbols), and faults reverse odd with mechanisms faults strike- (grayextensional with symbols). stress As strike-slip is regime component expected thepure for (cyan majority a normal rift of faults structure the or with derived normal source faults mechanisms with represent strike slip component (72 %), whereas only sible source mechanism toWith normal this faulting method with we NNE-SSW substantially70 % increased strike the of direction number the (Fig. of analysed 12b). in reliable events, solutions many compared to cases to almost we 40solutions. % were using able only to P-polarities. improve Furthermore, the quality and significanceplunge angles of is the presented in P-polarity Fig.the 13. graph Data represent points pure that normal are faulting close (red to symbols), the strike-slip three vertices (blue of symbols), and compressional (black symbols) and 2the dilatational grid onsets search (open o symbols). Thetwo result normal of faulting mechanismsmaking it with impossible to nearly allocate perpendicular aa single oriented grid mechanism to search strike this with directions event. However, P performing polarities together with SV/P amplitude-ratios restricts the pos- by 15 stations in total. At 14 stations we were able to determine the P-polarities: 12 with SV/P ratios enabledpolarities us alone. to Fig. derive 12 gives more an reliable illustration fault of plane the solutions procedure. The than event with was P- recorded However, in many cases thepossible to nodal derive planes a covered unique solutionwere large able for angle to the determine ranges respective reliable event. and With sourcequakes. it mechanisms P-polarities To for alone was further only we im- 40 constrain % of thetios the into nodal selected the planes, earth- grid we search.and integrated In order also local to SV/P receiver minimize amplitude conditions thenals ra- e with on a the Butterworth recorded bandthen pass amplitudes in picked we the on 0.1–5.0 pre-processed Hz the frequency the vertical range. sig- Amplitudes seismogram were component. The combination of P-polarities Fault plane solutions wereTo derived achieve from good P-wave polarities dataleast and coverage 12 SV/P we stations. In amplitude processed total ratios. we onlypolarities analysed were events 304 manually events that picked located were on inalgorithm the the recorded to Rwenzori raw by area. determine seismograms. P Then at the wave we1984). orientation used In of a general the grid we nodal search were planes able (FOCMEC to by derive Snoke solutions et without al., admitting P polarity errors. are indicative of tectonicb-values processes (1.0–2.5) in are non-volcanic usuallymicity intraplate observed is areas. in associated Relatively regions with large with1987). eruptions active Our and volcanism magma results where movements mayRwenzori seis- (e.g. suggest Region Karpin has that and its at Thurber, origin least in volcanic-magmatic a processes part within3.4 of the crust. the recorded Source mechanisms seismicity in the of Here, the inter-station distancesangles). are The relatively b-value for large both and magnitudethe distributions values is published 1.1 which byEast is Kebede slightly African and larger Rift than Kulh (0.9–1.0). According to Mogi (1967) b-values between 0.5 and 1.0 and the magnitude-frequency relation (circles) suggests a magnitude of completeness 5 5 25 20 10 15 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | , c M erent ff 0.8 in the northern parts scale in Norway, Bull. Seis- L = M c M ¨ ostlichen Zaire-Becken. Ergebnisse einer Un- 580 579 , 2005. Funding for this study was provided by the Deutsche Forschungsgemein- ` ere, J., Petit, C., Perrot, J., and Le Gall, B.: Crustal rheology and depth 1.5 in the southern parts of the area and ´ everch = c M regime indoi:10.1029/2005GL023156 NE and SWdes Tanzania, ostafrikanischen Grabensystems East und im Africa. Geophys, Res. Lett., 32, L14315, relative motion and dike intrusion2006. in the main Ethiopian rift, Geophys. J. Int., 165, 303–310, distribution of earthquakes: Insights fromTectonophysics, 468, the 28–41, central 2009. and southern East African Rift System, mol. Soc. Am., 81, 379–398, 1991. Fault plane solutions were derived for 304 events using P-polarities and SV/P- The general focal-depth distribution shows that there is considerable seismic energy ¨ umpker (2011). There is no indication for a crustal root beneath the Rwenzori Moun- Bram, K.: Zum Aufbau der Kruste und des oberen Mantels im Bereich des westlichen Grabens Brazier, R., Nyblade, A. A., and Florentin, J.: Focal mechanisms and the stress Bendick, R., McClusky, S., Bilham, R., Asfaw, L., and Klemperer, S.: Distributed Nubia–Somalia Alsaker, A., Kvamme, L. B., Hansen, R. A., and Dahle, A.: The Albaric, J., D and visualization was done with SEISAN and GMT software References is less resolved dueon to microseismic the activity inhomogeneous willnetwork station be which distribution. provided covers Essential also in the information the area future westAcknowledgements. by and southwest a of recently theschaft deployed Rwenzori (DFG) seismic Mountains. in theGeophysical framework Instrumentation of Pool the Potsdam RiftLinkand (GIPP) research GEOFON for for group providing archiving (www.riftlink.org). the the Weand data. seismological thank Technology The equipment and the support of of the the Ugandan Ugandan Wildlife National Authority Council is for Science greatly appreciated. Data processing amplitude ratios. The majorityof of the the analysed source events mechanisms arefor pure are the thrust normal pattern faults. faults, of Thetion, only more T-axis the trends 2 or % extensional give less a stress verticalrift uniform to field. process picture the and They rift also are axis with which aligned kinematic is rift in in models. good WNW-ESE Seismicity agreement direc- southeast with of the the expected Rwenzoris tains, neither from receiver functions nor from focal depth distribution. there is nobeneath seismicity the within rift theindicate shoulders uppermost that seismicity 10 the extends kmHowever, maximum close beneath they focal to correlate the depths the fairly rift varymic surface. well valley, significantly receiver Vertical whereas with functions sections within Moho and theobserved seismicity depths range Rwenzori is that between area. restricted 21 weretion to km of derived the and seven crustal by events 34 part km. at teleseis- R of a This the depth confirms lithosphere, between with that 53 the the and excep- 60 km as reported by Lindenfeld and was derived for severaldensity. subregions and exhibits awhere the significant average dependence inter-station distance on is station considerably smaller. release down to 32 km depth with a pronounced peak of activity at 15 km. Typically, 4 Conclusions From February 2006in until the September Rwenzori 2007 Regiontotal we of with have 13 964 a monitored events networkMagnitude-frequency were the relations located of show local with a 29 local b-value seismicity magnitudes seismicreported of ranging 1.1 stations. by from which During is other –0.5 slightly up this authors above to the period for 5.1. values the a western rift. The magnitude of completeness, entation in WSW-ENE direction inRwenzori, both immediate histograms. at Group the 2 eastern is riftbehaviour. located boundary T-axis northeast trends fault, of and are the reveals predominantly avertical complete oriented to di NEN-SWS, the which T-axis orientation is ofstress more all field or other in less groups. this This area might is be controlled an by indication local that disturbances. the 6, show the same characteristic, but additionally there is a second prevailing T-axis ori- 5 5 25 20 10 15 25 15 20 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ective ff ¨ at Frank- ¨ at Berlin, pub- , 2011. 582 581 scale in southern California, Bull. Seismol. Soc. Am., L M doi:10.1111/j.1365-246X.2011.05048.x , 2008. ¨ umpker, G., Aanyu, K., Haines, S., Passchier, C. W., and Sachau, ¨ oller, L.: Electronic Seismologist. SeisAn Earthquake Analysis Software, ¨ umpker, G.: Detection of mantle earthquakes beneath the East African ´ anek, O.: Spatial and temporal variations of b-values along the East African ¨ umpker, G., Lindenfeld, M., Koulakov, I., Schumann, A., and Ochmann, N.: ` ere, J., Petit, C., Gileva, N., Radziminovitch, N., Melnikova, V., and San’kov, V.: Depth Structural frameworkdoi:10.1029/2007TC002176 and possible role of glaciations, Tectonics, 27, TC4018, East African rift system, Geology, 30, 755–758, 2002. implications for crustal structure, Geophys. J. Int., 121, 49–62, 1995. Geofisica, 42, 1067–1083, 1999. the Rwenzori Mountains from travel-time tomography, Ph.D.furt thesis, Goethe am Universit Main, 2010. Crustal Seismic Velocities of theTomography: Evidence Rwenzori Region, for Low-Velocity East Anomalies African beneathmol. Rift, the from Soc. Mountain Local Am., Range, Travel-Time 101 Bull. ,848–858, Seis- 2011. elastic thickness, and2000. the strength of the continental lithosphere,G., and Geology, Tugume, 28, F. A.: Seismic 495–498, harzard assessment in Eastern and Southern Africa, Ann. di seismotectonics and crustal structureMagadi of area, the Geophys. southern J. Int., Kenya 146, Rift 439–453, – 2001. new datarheology from of the the lithosphere, Lake J. Geol. Soc., 165, 453–465, 2008. globally, Seismol. Res. Lett., 66, 26–36, 1995. Rift, Geophys. J. Int., 186, 1–5, 1496, 1975. Seismol. Res. Lett., 70, 532–534, 1999. 77, 2074–2094, 1987. creation of basement micro-plates within active rifts, Tectonophsics, 458, 105–116,T.: 2008. Active transsection faultsimplications in for rift crustal transfer fragmentationRift, processes zones: Int. J. in evidence Earth the for Sci., western complex (Geol Rundschau) branch stress 99, of fields 1633–1642,Seismicity the 2010. and Rate East for African Tanzania, East Africa, Bull. Seismol. Soc. Am., 88, 712–721, 1998. crustal rheology and regional kinematics, Geophys. J. Int., 134, 422–448, 1998. rift system and the southern Red Sea, Phys. Earth Planet. Inter., 83, 249–264, 1994. Geol. Soc. of Am. Bull. 101, 885–903, 1989. of the lithosphere in the Lake Baikal region, Geophys. J. Int., 167, 1233–1272, 2006. transport: Kilauea Volcano, Hawaii, Pure Appl. Geophys., 125, 971–991, 1987. distribution of earthquakes inthe the lithosphere, Geophys. Baikal J. rift Int., 146, system 714–730, and 2001. its implications for the rheology of their implications for theRes., thermal 88, and 4183–4214, mechanical 1983. properties of the lithosphere, J. Geophys. trough, eastern Africa, J. Geophys. Res., 124, 612–630, 1996. tersuchung der Raumwellen vonlished Nah-Erdbeben, by Ph.D. Dietrich thesis, Reimer Freie Verlag Universit Berlin, 1975. ´ everch Ring, U.: Extreme uplift of the Rwenzori Mountains in the East African Rift, Uganda: Nyblade, A. A. and Langston, C. A.: East African earthquakes below 20 km depth and their Nyblade, A. A. and Brazier, R. A.: Precambrian lithosphere controls on the development of the Mogi, K.: Earthquakes and fractures, Tectonophysics, 5, 35–55, 1967. Midzi, V., Hlatywayo, D., Chapola, L. S., Kebede, F., Atakens, K., Lombe, D. K., Turyomugyendo, Jakovlev, A.: The crustal and upper mantle isotropic and anisotropic velocity structure beneath Jakovlev, A., R Maggi, A., Jackson, J. A., McKenzie, D., and Priestley, K.: Earthquake focal depths, e Jackson, J., McKenzie, D., Priestley, K., and Emmerson, B.: New views on the structure and Maasha, N.: The seismicity of the Rwenzori region in Uganda, J. Geophys. Res., 80, 1485– Ibs-von Seht, M., Blumenstein, S., Wagner, R., Hollnack, D., and Wohlenberg, J.: Seismicity, Lienert, B. R. E. and Havskov, J.: A computer program forLindenfeld, locating earthquakes M. both and locally and R Hutton, L. K. and Boore, D. M.: The Langston, C. A., Brazier, R., Nyblade, A. A., and Owens, T. J.: Local Magnitude Scale and Havskov, J. and Ottem Koehn, D., Lindenfeld, M., R Koehn, D., Aanyu, K., Haines, S., and Sachau, T.: Rift nucleation, rift propagation and the Foster, A. and Jackson, J. A.: Source parameters of large African earthquakes: implications for Kebede, F. and Kulh Emmerson, B., Jackson, J., McKenzie, D., and Priestley, K.: Seismicity, structure and rheology Ebinger, C. J.: Tectonic development and the western branch of the East African rift system, Karpin, T. L. and Thurber, C. H.: The relationship between earthquake swarms and magma D Chorowicz, J.: The East African Rift System, J. Afr. Earth Sci., 43, 379–410, 2005. Chen, W.-P. and Molnar, P.: Focal depths of intracontinental and intraplate earthquakes and Camelbeeck, T. and Iranga, M. D.: Deep crustal earthquakes and active faults along the Rukwa 5 5 30 25 30 20 25 15 20 10 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | doi:10.1007/s00531- ] 1 − [km s S ] v 1 − [km s P 583 584 oley, N., and Evans, J. R.: Implications of the dis- ff ratio of 1.74 as derived by Bram. S /v P v , 2008. depth [km]01632 v 60125 6.00 6.75 8.05 7.75 8.20 3.45 3.88 4.63 4.45 4.71 , 2010. ¨ umpker, G., Schumann, A., and Muwanga, A.: Crustal thinning beneath the Rwen- Velocity model UVI-N74 of Bram (1975) used for earthquake location. S-velocities tribution of seismicity near1991. Lake Bogoria in the Kenya Rift, Geophys. J. Int., 105, 665–674, the Rwenzori mountains: implications forRift heat System flow in in Uganda, the South western Afr. branch J. of Geol., the 112, East 261–271, African 2009. zori region, Albertine rift,Rundschau) 99, Uganda, 1545–1557, from 2010. receiver-function analysis, Int. J. Earth Sci., (Geol 010-0533-2 des, R. M.: Adoi:10.1029/2007GL032781 kinematic model for the East Africansheet Rift, approach, Geophys. Int. Res. J. Lett., Earth 35, Sci., L05304, (Geol Rundschau) 99, 1525–1533, determination by combined use of55, polarity and 51, SV-P 1984. amplitude ratio data, Earthquake Notes, Africa: constraints on the thermo-mechanicalRes. structure Lett., of 14, a 741–744, continental 1987. rift system, Geophys. ¨ olbern, I., R were calculated with a constant Young, P. A. V., Maguire, P. K. H., d’A. La Table 1. W Tugume, F. A. and Nyblade, A. A.: The depth distribution of seismicityTwesigomwe, E. at M.: the Seismic northern hazards in end Uganda, of J. Afr. Earth Sci., 24, 183–195, 1997. Stamps, D. S., Flesch, L. M., and Calais, E.: Lithospheric buoyancy forces in Africa from a thin Stamps, D. S., Calais, E., Saria, E., Hartnady, Ch., Nocquet, J. M., Ebinger, C. J., and Fernan- Snoke, J. A., Munsey, J. W., Teague, A. C., and Bollinger, G. A.: A program for focal mechanism Shudofsky, G., Cloetingh, S., Stein, S., and Wortel, R.: Unusually deep earthquakes in East 5 20 15 10 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 4.5 ≥ 586 585 The East African Rift System. Red dots: global seismicity; 1973–2008, with m The Rwenzori region with seismic station network (triangles) and recorded local seis- Fig. 2. micity (red circles) fromthe February southern 2006 end to of Septemberincreased the 2007. seismicity Toro-Bunyoro The within fault; dashed the the line Rwenzori dotted block in ellipse (see the in Sect. NE 3.1 the marks in centre text). covers the area of (NEIC catalogue). The small(Fig. black 2). square within the Albertine Rift marks the Rwenzori area Fig. 1. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2.3 3.8) = = L L M M erent epicentral dis- ff : Strong event 2 ( (c, d) 0.9), recorded at stations KARU 49.3 km, right). Seismometer chan- = L ∆ = M erent stations with epicentral distances be- ff 0.3 (grey area). For comparison we have also ± 588 587 49.2 km, right). Bottom row 2.8 km, left) and KISA ( ∆ = ∆ = : Relatively week event 1 ( (a, b) Seismogram examples of two events, each recorded at two di Magnitude of an event calculated at twelve di 12.9 km, left) and KISA ( ∆ = computed magnitudes using thenia (plus-signs) calibration and parameters the of(1987). calibration Langston parameters et for al. S. California (1998) derived for by Tanza- Hutton and Boore (horizontal line) with a standard deviation of nels Z, N, and Ecounts. are Picked plotted P- from and top S-phases to are bottom marked of by each dashed record. lines. Amplitudes are plotted in digital tween 44 km and 152 km.software In (circles) based this on study Alsaker we et used al. the (1991). default The calibration average functions of of all stations the comes SEISAN to recorded at stations KASS ( Fig. 4. ( tances. Top row Fig. 3. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 5 km. The number of operating ≤ 590 589 Monthly numbers of located events. Light grey bars indicate all located earthquakes; Number of located events as function of daytime. There is an obvious diurnal variation for earthquakes with magnitudes smallersensitivity than of 1.5 the caused station by network human during activity daytime. which reduces the Fig. 6. stations at the beginning and middle of each month is marked by black symbols. dark grey bars indicate events that could be located with errors Fig. 5. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | erent stations (white triangles) ff 592 591 ¨ olbern et al. (2010) using teleseismic receiver functions. The numbers beside Depth distribution of all located earthquakes. Position of the vertical sections presented in the next Fig. 9. Four profiles (A–D) cross the the station labels are Moho depths in km. were derived by W Fig. 8. Rwenzori area approximately perpendiculardirection to parallel the to rift the axis.northward One rift from profile axis Lake (E) andprojected runs Edward penetrates onto in the to the SSW-NNE respective Lake Lake vertical George George. plane. rift Moho Coloured segment depths symbols at which di denote extends events which are Fig. 7. Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | er- ff erent ff ¨ umpker, 594 593 ¨ olbern et al., 2010). The dashed circle in section B marks the position of Spatial distribution of the computed magnitudes. Coloured symbols represent di Hypocentre depth sections A–E. The position of the profiles is plotted in Fig. 8. Heavy ent magnitude classes, see legend.(green White and triangles blue indicate symbols) the couldtion station only locations. is be Small a located events clear in(cf. areas indication Fig. with that 2) high the is station apparent probablywhere density. This due decrease the illustra- to magnitude-frequency of the relations seismicity lack presented southwest of in of stations Fig. the 11 in Rwenzoris were this calculated. region. Two rectangles mark the areas Fig. 10. Fig. 9. black bars indicateseismic Moho stations depths (W asseven derived mantle from earthquakes which teleseismic2011). were receiver detected The functions in S-velocity 53–60 model at kmat in depth di MWEY. (Lindenfeld the It and lowermost shows R profile a row E. was strong derived low by velocity zone receiver-function inversion (LVZ) that is traced by two seismicity layers in Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | (b) erent magnitudes of ff Result of a grid search using only P- (a) 596 595 erence between the slopes (b-values) of the two distri- ff but now SV/P amplitude ratios were added to perform the grid search. (a) ). C M Determination of a fault plane solution. Cumulative magnitude-frequency distribution for area 1 (circles) and area 2 (triangles), Fig. 12. polarities. Open and closedSame event symbols as represent in dilations and compressions, respectively. The red nodal line representsTension axis the are solution that marked was with finally P assigned and to T this respectively. event. Pressure and completeness ( butions. Both areas exhibit a b-value slightly above 1, but they show di Fig. 11. see Fig. 10. There is no significant di Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 598 597 Classification and frequency of all derived fault plane solutions (304 events) on the Map of all fault-plane solutions. The events are arranged into six groups. The rose- Fig. 14. histograms show the orientation of the T-axes in each group. Fig. 13. basis of P- andwith T-axis plunge strike angles. slip Red:verse component pure faults (2 (23 normal %). %). faults Cyan: Blue: reverse (49 faults(14 %). %). with pure Magenta: strike-slip strike-slip component normal (4 mechanisms %). faults Grey: (8 odd %). mechanisms Green: pure re-