S11A- Rift initiation in cratonic lithosphere: Seismicity patterns in the Manyara-Natron-Magadi basins and Oldoinyo Lengai volcano

C. Lambert1, C. Ebinger1, A. Rodzianko2, S. Roecker2, M. Msabi3, J. D. Muirhead6, N. Rasendra5, C. Tiberi5, R. Ferdinand-Wambura3, K. Mtelela3, A. Muzuka4, G. Mulibo4, E. Witkin1 1. University of Rochester, 2. Rensselaer Polytechnic Institute, 3. University of Dar es Salaam, 4. The Nelson Mandela African Institute of Science and Technology, 5. Université de Montpellier II, 6. University of Idaho

Conclusions

• Our results show that seismicity in Natron and Magadi basins peaks at a depth of 11 km, deeper than but comparable to the 2007 fault-dike sequence, indicating that shallow earthquakes are typical of background seismicity in these basins.• There is a strong north-south difference in hypocentral depths between Manyara

basin and Natron/Magadi.• The site of the 2007 fault-dike sequence remains active.• In Natron basin, there are some deeper earthquakes which occur near Lengai at 20-25 km depth. The

question remains whether these earthquakes are an expression of the border fault, and what the relationship

is between the border fault and magmatism at Lengai.

Support from NSF, IRIS/PASSCAL gratefully acknowledged. Many thanks to Fortes Car Hire. Fig 6. Summit of Oldoinyo Lengai

Fig 2. Cross sections show situations analogous to Manyara basin (A) and Natron/Magadi basins (B). Deep seismicity in Manyara has been constrained by Albaric et al., 2009, whose studies of Manyara and Natron indicate north-south differences in seismogenic layer thickness. Seismicity in Natron has previously been determined only from the 2007 intrusion-eruption sequence.

Main questions to be considered:•Are shallow earthquakes typical of background activity in the northern basins?•Is there deep seismicity in the Natron and Magadi basins? •What is the relation between volcanism at Oldoinyo Lengai and the border fault?

Depth Distribution of Earthquakes

Fig 5. Depth distribution results for: a. 2007 Gelai fault-dike eruption sequence, Albaric et al., 2009. b. Manyara basin, Albaric et al., 2009, and c. CRAFTI results in Natron and Magadi. CRAFTI hypocentral depths peak at 11 km, deeper than during the Gelai sequence, but with a strong contrast to activity in the south, which peaks at 28 km depth.

Introduction The multi-disciplinary CRAFTI project aims

to quantify the partitioning of strain between faulting and magmatism during the early stages of continental rifting in Archaean and Proterozoic lithosphere. The goals of the project include the determination of the relative contributions of diking and faulting along and across the rift, the spatial and temporal relationships between magmatism and faulting, and the geometry and kinematics of active fault systems, dike intrusions, and active volcanoes. We present preliminary results of seismicity analyses from 38 broadband seismometers deployed in January 2013. We present time-space relations of seismicity for the first 3 weeks of data, and focal mechanisms for the largest events during that time period. Comparisons are made with earlier results from the 2007 fault-dike intrusion-volcanic eruption sequence, and results from the Manyara basin to the south.

Fig 1. The <7 My Eastern Rift System in northern Tanzania and southern Kenya comprises several rift segments at different stages of the rifting cycle.

Fig 3. Epicentral earthquake locations and focal mechanisms. Orange circles are from the 2008 temporary array; CRAFTI locations are in red and represent the first three weeks of the array. Focal mechanisms were determined using FOCMEC (Snoke, 2003), and allowed for no more than two errors. Some errors were retained due to errors in takeoff angles caused by shallow earthquake depths. Focal mechanisms from CRAFTI, in black, trend both north-south, parallel to young faults, and north-east, parallel to the craton’s Archaean basement fabric. Focal mechanisms from the 2007 teleseisms, in green, are north-east trending. In Manyara basin, only 150 km south of Natron, a swarm of seismic activity has been long lasting and unlinked to present active faults (Mulibo and Nyblade, 2009, Albaric et al., 2010). We examine differences in hypocentral depths between this southern seismicity and activity in Natron and Magadi basins.

Manyara

Natron

Magadi

a.

Fig 4. Enlarged view of seismicity in the vicinity of Natron basin and Gelai volcano. Green circles are the double-difference epicentral locations from the 2008 temporary array. Red are epicentral locations from CRAFTI (stations indicated in orange), and were located with hypoinverse using the 1-D velocity model of Albaric et al. 2010. The site of the 2007 dike intrusion and faulting sequence near Gelai volcano remains seismically active.

b.

c.