Browsing by Author "Ferdinand, Richard W."
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Item Active Fault Mapping in Karonga-Malawi after the December 19, 2009 Ms 6.2 Seismic Event(Elsevier, 2015) Macheyeki, Athanas S.; Mdala, Hassan; Chapola, Lostina S.; Manhiça, V. J.; Chisambi, Joshua; Feitio, Paulino; Ayele, Atalay; Barongo, J.; Ferdinand, Richard W.; Ogubazghi, G.; Goitom, B.; Hlatywayo, Dumisani J.; Kianji, Gladys; Marobhe, I.; Mulowezi, A.; Mutamina, Daniel; Mwano, J. M.; Shumba, Blessing; Tumwikirize, I.The East African Rift System (EARS) has natural hazards – earthquakes, volcanic eruptions, and landslides along the faulted margins, and in response to ground shaking. Strong damaging earthquakes have been occurring in the region along the EARS throughout historical time, example being the 7.4 (Ms) of December 1910. The most recent damaging earthquake is the Karonga earthquake in Malawi, which occurred on 19th December, 2009 with a magnitude of 6.2 (Ms). The earthquake claimed four lives and destroyed over 5000 houses. In its effort to improve seismic hazard assessment in the region, Eastern and Southern Africa Seismological Working Group (ESARSWG) under the sponsorship of the International Program on Physical Sciences (IPPS) carried out a study on active fault mapping in the region. The fieldwork employed geological and geophysical techniques. The geophysical techniques employed are ground magnetic, seismic refraction and resistivity surveys but are reported elsewhere. This article gives findings from geological techniques. The geological techniques aimed primarily at mapping of active faults in the area in order to delineate presence or absence of fault segments. Results show that the Karonga fault (the Karonga fault here referred to as the fault that ruptured to the surface following the 6th–19th December 2009 earthquake events in the Karonga area) is about 9 km long and dominated by dip slip faulting with dextral and insignificant sinistral components and it is made up of 3–4 segments of length 2–3 km. The segments are characterized by both left and right steps. Although field mapping show only 9 km of surface rupture, maximum vertical offset of about 43 cm imply that the surface rupture was in little excess of 14 km that corresponds with Mw = 6.4. We recommend the use or integration of multidisciplinary techniques in order to better understand the fault history, mechanism and other behavior of the fault/s for better urban planning in the area.Item Average Attenuation of 0.7–5.0 Hz Lg Waves and Magnitude Scale Determination for the Region Bounding the Western Branch of the East African Rift(1998) Ferdinand, Richard W.The investigation of L G attenuation characteristics in the region bounding the western branch of the East African rift system using digital recordings from a seismic network located along the rift between Lake Rukwa and Lake Malawi is reported. A set of 24 recordings of L G waves from 12 regional earthquakes has been used for the determination of anelastic attenuation, QLg , and regional body‐wave magnitude, MbLg , scale. The events used have body‐wave magnitudes, Mb , between 4.6 and 5.5, which have been determined teleseismically and listed in ISC bulletins. The data were time‐domain displacement amplitudes measured at 10 different frequencies (0.7–5.0 Hz). QLg and its frequency dependence, Η, in the region can be represented in the form QLg = (186.2 ± 6.5) F (0.78±0.05). This model is in agreement with models established in other active tectonic regions. The L G‐wave‐based magnitude formula for the region is given by MbLg = log A + (3.76 ± 0.38) log D − (5.72 ± 1.06), where A is a half‐peak‐to‐peak maximum amplitude of the 1 s L G wave amplitude in microns and D is the epicentral distance in kilometres. Magnitude results for the 12 regional earthquakes tested are in good agreement with the ISC body‐wave magnitude scale.Item Contrasted Seismogenic and Rheological Behaviours from Shallow and Deep Earthquake Sequences in the North Tanzanian Divergence, East Africa(Elsevier, 2010) Albaric, Julie; Perrot, Julie; Déverchère, Jacques; Deschamps, Anne; Le Gall, Bernard; Ferdinand, Richard W.; Petit, Carole; Tiberi, Caterina; Sue, Christian; Songo, Majura A. M.We report preliminary results of a seismological experiment, SEISMO-TANZ’ 07, which consisted in the deployment of a local network (35 stations) in the East African Rift System (EARS), North Tanzania, during 6 months in 2007. We compare two earthquake sequences (Gelai and Manyara) occurring, respectively, in the southern end of the Kenya rift and in the North Tanzanian Divergence (NTD). Only distant of ∼150 km, their triggering mechanisms are different. None of the sequences depicts typical swarm or mainshock–aftershock patterns. They highlight the change in the magmatic/tectonic nature of the rift where the eastern branch of the EARS enters the Tanzanian craton. The similar shape and long-axis of the elongate sequences emphasize the preferred locus of active strain release along NE–SW discontinuities which probably root at depth into steep Proterozoic shear zones. At Gelai, the deformation is dominated by aseismic process involving slow slip on normal fault and dyke intrusion within the upper crust (Calais et al., 2008). The spatial and temporal earthquake distribution indicates a possible correlation between the Gelai crisis and the eruption of the nearby Oldoinyo Lengai volcano. At Manyara, the sequence is more uncommon, revealing a long-lasting seismic activity deeply rooted (∼20–35 km depth) possibly related to stress loading transmitted laterally. The yield strength envelope modelled from the depth frequency distribution of earthquakes in the NTD is consistent with the presence of a mafic lower crust and further supports the strength increase of the rifted crust from south Kenya to the NTD.Item The Determination of Source Mechanisms of Small Earthquakes and Revised Models of Local Crustal Structure by Moment Tensor Inversion(2002) Ferdinand, Richard W.; Arvidsson, RonaldWe describe a procedure for retrieving the source mechanism of small earthquakes and a revised model of crustal structure in heterogeneous media based on interactive inversion of high frequency seismograms and demonstrate its application to three local earthquakes. The earthquakes are located along the Rukwa rift, south-western Tanzania, and were recorded by a local network in the rift. Due to strong lateral crustal heterogeneity in the area, the procedure involves a single-station interactive inversion for moment tensor solution and modelling of the crustal structure along the propagation path between the source and the station. Based on this source solution and the a priori crustal structure, synthetic seismograms are generated and compared with observed displacement seismograms at other stations where the event has been recorded. Deviations between the calculated and observed seismograms are indicated by time shifts and amplitude mismatches on prominent phases; these are employed to correct the phase arrivals used to invert for the partial multistation source solution. The moment tensor inversion of high frequency seismogram data in this study is based on a point source approach. The process includes generation of synthetic seismograms using the frequency-wavenumber integration technique for an elastic, horizontally-layered, medium. Moment tensor inversion is carried out by minimizing the L2 norm of the difference between synthetic and observed displacement data under a deviatoric condition. Results for the three small earthquakes show good agreement between single station and multistation source solutions. The inversions for these events indicate dominantly dip-slip mechanisms with seismic moment magnitudes in the range 2.0 <=Mw<= 3.3.Item Focal Mechanisms, Stress Field and Crustal Rheology in the North Tanzanian Divergence (East African Rift) Inferred from Local Seismicity Analysis(2010) Albaric, Julie; Godano, Maxime; Déverchère, Jacques; Perrot, Julie; Deschamps, Anne; Sue, Christian; Le Gall, Bernard; Ferdinand, Richard W.; Petit, Carole; Tiberi, CaterinaWe deployed a temporary local seismic network in the North Tanzanian Divergence (NTD) for 6 months in 2007 (35 stations, SEISMOTANZ'07 experiment). The region is characterized by major changes in the magmatic/tectonic nature of the rift, at the place where the eastern branch of the East African Rift enters the Tanzanian craton. More than 200 earthquakes were accurately located south of Lake ManyaraItem Insights into Initial Stages of Rifting from Seismotectonics and SKS Splitting in the North Tanzanian Divergence(2010-12) Albaric, Julie; Barruol, Guilhem; Déverchère, Jacques; Deschamps, Anne; Perrot, Julie; Tiberi, Caterina; Ferdinand, Richard W.; Sue, Christian; Le Gall, Bernard; Petit, CaroleMagmatism and faulting are preponderant processes involved in continental rifting. Their interaction, relative importance, and dependence to the rheological properties of the lithosphere and to the timing of rifting, remain poorly known. To address this question, we have used the results from a seismological experiment, called SEISMO-TANZ (35 stations, broadband and enlarged-band), launched in the North Tanzanian Divergence (NTD) for 6 months in 2007. The region encompasses one of the youngest parts of the East African rift (EAR) and is characterized by the development of the rift into the Tanzanian craton. The NTD is often considered as non-volcanic compared to other places in EAR and the lithosphere is highly resistant. More than 2000 local earthquakes were recorded, highlighting active faults and one magmatic intrusion. Inherited structures play a key role as guides for dykes and slips. 26 Focal mechanisms (double-couple hypothesis) were obtained from P-wave polarities and indicate a transtensive deformation in the southern part of the region (Manyara rift). The stress inversion performed indicates a stable, well-determined σ3 axis striking ESE-WNW. From 25 teleseismic events recorded during the experiment, we have measured seismic anisotropy (SKS splitting) and present here our last results. Fast polarization directions are quite homogeneously NE-SW and delays times increase from the craton (W) to the Mozambique belt (E). Fossilized anisotropy and dykes or melt-filled lenses alignments would both explain the majority of these observations. We finally compare these results with other seismic anisotropy measurements made in EAR and with geodetic and seismotectonic analyses in order to better assess the origin of the strain pattern in this part of the rift, and to discuss the respective role of magmatism, faulting and fabrics in the extending lithosphere.Item Qualitative Microzonation of Earthquake Susceptibility - A Case Study for Dodoma Urban Area, Central Tanzania(2013-01) Lupogo, Keneth; Ferdinand, Richard W.; Bujulu, Pancras M. S.Item Regional Phases from Earthquakes along the Western Branch of the East African Rift and Relocation of Epicentres between 1N and 12S(2000) Ferdinand, Richard W.; Camelbeeck, ThierryTo improve earthquake locations, regional seismic phases, i.e., Pn, Pg, Sn and Sg (Lg) have been studied in the region bounding the western branch of the East African Rift System between 1N and 12S. We analysed arrival time data from 56 regional earthquakes recorded by a local network (Mbeya network) in south-western Tanzania, consisting of five three-component digital stations. The earthquakes were located by adding data from 17 stations of the East and Southern Africa Seismic Network (ESASN). Calculation of phase parameters and earthquake locations result from an interactive inversion of data. The resulting crustal model is characterised by an average velocity of 6.32 +- 0.26 km/s and 3.61 +- 0.02 km/s for P and S waves, respectively. Upper mantle velocities corresponding to Pn and Sn phases are 8.11 +- 0.05 km/s and 4.67 +- 0.05 km/s, respectively. Achieved velocities are comparable to those obtained for the African Shield. The crustal thickness is estimated to be 41.3 +- 7.5 km assuming that the avItem Seismic Waves Attenuation in Continental Lithosphere under Extensional Condition: Comparison of the East African and Baikal Rift Systems(2012) Dobrynina, A. A.; Albaric, Julie; Deschamps, Anne; Perrot, Julie; Ferdinand, Richard W.; Déverchère, Jacques; San'kov, Vladimir; Chechelnitsky, V. V.Item Strain Accommodation by Slow Slip and Dyking in a Youthful Continental Rift, East Africa(2008) Calais, Eric; d’Oreye, Nicolas; Albaric, Julie; Deschamps, Anne; Delvaux, Damien; Déverchère, Jacques; Ebinger, Cynthia; Ferdinand, Richard W.; Kervyn, François; Macheyeki, Athanas S.; Oyen, Anneleen; Perrot, Julie; Saria, Elifuraha; Smets, Benoit; Stamps, Sarah D.; Wauthier, ChristelleContinental rifts begin and develop through repeated episodes of faulting and magmatism, but strain partitioning between faulting and magmatism during discrete rifting episodes remains poorly documented. In highly evolved rifts, tensile stresses from far-field plate motions accumulate over decades before being released during relatively short time intervals by faulting and magmatic intrusions. These rifting crises are rarely observed in thick lithosphere during the initial stages of rifting. Here we show that most of the strain during the July-August 2007 seismic crisis in the weakly extended Natron rift, Tanzania, was released aseismically. Deformation was achieved by slow slip on a normal fault that promoted subsequent dyke intrusion by stress unclamping. This event provides compelling evidence for strain accommodation by magma intrusion, in addition to slip along normal faults, during the initial stages of continental rifting and before significant crustal thinning.Item Surface-wave imaging of the weakly-extended Malawi Rift from ambient-noise and teleseismic Rayleigh waves from onshore and lake-bottom seismometers.(Geophysical Journal International, 2017) Accardo, N.J; Gaherty, J.B; Shillington, D.J; Ebinger, Cynthia; Nyblade, A.A; Mbogoni, G.J; Chindandali, P. R. N; Ferdinand, Richard W.; Mulibo, G.D; Kamihanda, G; Keir, D; Scholz, C; Selway, K; O'Donnell, J.P; Tepp, G; Gallacher, RLocated at the southernmost sector of the Western Branch of the East African Rift System, the Malawi Rift exemplifies an active, magma-poor, weakly extended continental rift. To investigate the controls on rifting, we image crustal and uppermost mantle structure beneath the region using ambient-noise and teleseismic Rayleigh-wave phase velocities between 9 and 100 s period. Our study includes six lake-bottom seismometers located in Lake Malawi (Nyasa), the first time seismometers have been deployed in any of the African rift lakes. Noise levels in the lake are lower than that of shallow oceanic environments and allow successful application of compliance corrections and instrument orientation determination. Resulting phase-velocity maps reveal slow velocities primarily confined to Lake Malawi at short periods (T <= 12 s), indicating thick sediments in the border-fault bounded rift basin. The slowest velocities occur within the Central Basin where Malawi Rift sedimentary strata may overlie older (Permo-Triassic) Karoo group sediments. At longer periods (T > 25 s), a prominent low-velocity anomaly exists beneath the Rungwe Volcanic Province at the northern terminus of the rift basin. Estimates of phase-velocity sensitivity indicates these low velocities occur within the lithospheric mantle and potentially uppermost asthenosphere, suggesting that mantle processes may control the association of volcanic centres and the localization of magmatism. Beneath the main portion of the Malawi Rift, a modest reduction in velocity is also observed at periods sensitive to the crust and upper mantle, but these velocities are much higher than those observed beneath Rungwe.Item Upper Mantle Structure of the Eastern Africa from Body Wave Tomography(2010-12) Mulibo, G. D.; Nyblade, Andrew A.; Ferdinand, Richard W.; Reusch, Mouse M.; Adams, Aubreya; Tugume, F. A.This study presents preliminary results of the upper mantle structure beneath the east Africa from body wave tomography. This work is part of an on-going study aimed at investigating the origin and structure of the African Superplume. The available global tomographic studies suggest that the African Superplume is a low velocity-anomaly extending from the core-mantle boundary upward into the mid mantle beneath southern Africa and may reach the upper mantle beneath eastern Africa. However, the limited vertical resolution of global tomographic models makes it difficult to confirm a connection from the lower to the upper mantle. Previous regional studies of upper mantle structure in east Africa have found evidence of a low velocity anomaly beneath the region that has been suggested as the upper mantle expression of the Superplume. Models from previous tomographic studies in east Africa have limited resolution below ~400 km beneath the eastern rift and are less well resolved beneath the western part of the rift due to less data coverage. This study uses teleseismic data from a wider region in east Africa than previously used. Data for this study are from a 3-year (2007-2010) deployment of 40 broadband seismic stations in Uganda and Tanzania. The dataset is supplemented by data from the 1994-1995 Tanzania broadband seismic experiment, the 2001-2002 Kenya broadband seismic experiment, the permanent AfricaArray seismic stations and IRIS/GSN stations. The data have been used for body wave tomography by computing relative travel time delays using a multi-channel cross-correlation technique and then inverting them for a 3D wave speed model. Preliminary results from the inversion of the relative delay times show that there is a low wave speed anomaly beneath east Africa extending from shallow upper mantle depths to at least 500 km