Source parameters of large African earthquakes:implications for crustal rheology and regional kinematics

Foster, A.; Jackson, James

doi:10.1046/j.1365-246x.1998.00568.x

Cited by 160 publications

(185 citation statements)

References 102 publications

(228 reference statements)

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“…Sparse constraints suggest that the extension is proceeding relatively slowly at ∼3:5 mm=yr (Stamps et al, 2008). The length scales of border faults, the flexure associated with faulting (Ebinger et al, 1991), the occurrence of deep seismicity (Jackson and Blenkinsop, 1993;Foster and Jackson, 1998), and the inference of mafic lower crust (Nyblade and Langston, 1995) suggest that rifting is occurring in relatively strong lithosphere. Volumetrically minor volcanism is observed in this region, providing a serious test for recent models that require intrusive magmatism to initiate rifting in cold strong continental lithosphere (Buck, 2004).…”

Section: Tectonic Backgroundmentioning

confidence: 99%

Acquisition of a Unique Onshore/Offshore Geophysical and Geochemical Dataset in the Northern Malawi (Nyasa) Rift

Shillington

Gaherty

Ebinger

et al. 2016

Seismological Research Letters

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The Study of Extension and maGmatism in Malawi aNd Tanzania (SEGMeNT) project acquired a comprehensive suite of geophysical and geochemical datasets across the northern Malawi (Nyasa) rift in the East Africa rift system. Onshore/offshore active and passive seismic data, long-period and wideband magnetotelluric data, continuous Global Positioning System data, and geochemical samples were acquired between 2012 and 2016. This combination of data is intended to elucidate the sedimentary, crustal, and upper-mantle architecture of the rift, patterns of active deformation, and the origin and age of rift-related magmatism. A unique component of our program was the acquisition of seismic data in Lake Malawi, including seismic reflection, onshore/offshore wide-angle seismic reflection/refraction, and broadband seismic data from lake-bottom seismometers, a towed streamer, and a large towed air-gun source. SCIENCE MOTIVATION AND EXPERIMENT SUMMARYThe primary scientific goal of the Study of Extension and maGmatism in Malawi aNd Tanzania (SEGMeNT) project is to examine the emergence and early evolution of two fundamental features of all divergent plate boundaries: magmatism and segmentation. Magmatism accommodates a significant percentage of plate separation at most midocean ridges and late-stage continental rifts. Likewise, transform faults demarcate discrete spreading segments in midocean ridges, which are broadly characterized by more robust magmatism at their centers than at their edges (Macdonald et al., 1988;Lin et al., 1990). Well-developed magmatic and tectonic segmentation is also observed in late-stage continental rifts and new ocean basins (Taylor et al., 1995;Ebinger and Casey, 2001;Keir et al., 2009), but little is known about the controls on the initiation and development of magmatism and segmentation in immature rifts. Our project is focused on addressing the following questions:• When, where, and why does magmatism initiate in rifts, and what is its role in accommodating extension? • What controls the development of tectonic segmentation in early-stage rifts? How is it manifested in 4D patterns of magmatism and deformation?To address the questions above, we acquired an integrated geophysical and geochemical dataset across the northern Malawi (Nyasa) rift in the southern part of the East Africa Rift system (EARS). Our team included universities and geological surveys in the United States, Malawi, and Tanzania. The following datasets were acquired as a part of the SEGMeNT project: onshore/offshore active and passive seismic data, long-period and wideband magnetotelluric (MT) data, continuous Global Positioning System (GPS) data, and geochemical samples. This is the first integrated geophysical and geochemical dataset acquired across and within any of the great African rift lakes that can constrain rift architecture and processes at a range of scales. TECTONIC BACKGROUNDThe northern Malawi (Nyasa) rift in the Western Branch of the EARS is an excellent locality to examine the early stages of rifting in str...

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Section: Tectonic Backgroundmentioning

confidence: 99%

Acquisition of a Unique Onshore/Offshore Geophysical and Geochemical Dataset in the Northern Malawi (Nyasa) Rift

Shillington

Gaherty

Ebinger

et al. 2016

Seismological Research Letters

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“…4. Summary of all fault plane solutions in the specified spatial window from this study, Shudofsky (1985), Foster and Jackson (1998), Braunmiller and Nabelek (1990), Ayele et al (2006), Keir et al (2006) and Harvard CMT solutions. The beach-ball size is proportional to earthquake magnitude and other acronyms are as specified in Fig.…”

Section: Moment Tensor Inversionmentioning

confidence: 99%

“…Fig. 4 summarizes all post-1960 seismicity in the region for magnitudes m b P 4.5, from this study, Ayele (1995), the USGS catalogue, Shudofsky (1985), Foster and Jackson (1998), Braunmiller and Nabelek (1990), Ayele et al (2006), Keir et al (2006) and Harvard CMT solutions. The m b P 4.5 seismicity is chosen to minimize location uncertainty in the region.…”

Section: Moment Tensor Inversionmentioning

confidence: 99%

“…It is very likely that the sP phase could be confused with the direct P, especially for seismograms from WWSSN stations, which would result in faulty earthquake source mechanisms. The 1969 Serdo earthquakes are known to have strike-slip mechanisms (Mckenzie et al, 1970;Kebede et al, 1989) but, after thorough waveform analysis, some of them proved to be normal faults (Wagner and Langston, 1988;Foster and Jackson, 1998).…”

Section: Moment Tensor Inversionmentioning

confidence: 99%

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The August 2002 earthquake sequence in north Afar: Insights into the neotectonics of the Danakil microplate

Ayele

Stuart

Bastow

et al. 2007

Journal of African Earth Sciences

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In August 2002, there was high seismic activity in Afar concentrated at the plateau margin of the northern Ethiopian rift east of Mekele, near the western part of the Danakil microplate. The spatial and temporal distributions of this seismic activity over four weeks indicate the NNW propagation of the Gulf of Aden rift across the Afar Depression towards the western Ethiopian plateau. Fault plane solutions for six larger earthquakes from the August 2002 sequence are estimated from moment tensor inversion of local broadband waveform data. The results show only normal faulting on NNW trending and NE dipping faults, which agree with tectonics of the area and distribution of aftershocks. No strike-slip component is observed in any of our fault plane solutions or those of other workers including Harvard CMT solutions in the region. Such motion would be indicative of oblique-slip deformation between the Nubian plate and the Danakil microplate consistent with counter-clockwise rotation of the microplate. Hypocentral depths of well-constrained events are 5-7 km, which is the approximate elastic plate thickness in the Main Ethiopian rift, possibly indicating the depth to the brittle-ductile transition zone in this part of the Afar Depression. The shallowness of the depth estimates agree with the macroseismic reports available from a wide area in northern Ethiopia. Potential future shallow crustal deformation may cause significant loss of human life and damage to property in the densely populated highland region around Mekele unless measures are taken in improving building standards. The b-value for this sequence is estimated to be 0.66 using a least squares fit, while it is 0.67 ± 0.16 from a maximum-likelihood approach. This estimated b-value is low or the frequency of occurrence of relatively larger magnitude events is high indicating that it is a highly stressed region as evidenced by the recent increase of the seismicity in the area.

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“…The stress barrier is maintained only in the elastic layer. Observations of deep seismicity in young rifts suggest a strong midlower crust [17][18][19] that maintains the stresses necessary to create a stress barrier at any crustal depth. If the lower crust and upper mantle are weak, the elastic stresses dissipate and z 2 may coincide with the bottom of the elastic layer.…”

mentioning

confidence: 99%

Off-rift volcanism in rift zones determined by crustal unloading

et al. 2014

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When continents are stretched over a long period of time, deep elongated rift valleys form at Earth's surface and zones of ponded magma, centred beneath the rift, form at the crust-mantle boundary 1,2 . Ascending magma sometimes erupts within the rift valley 3,4 or, counterintuitively, at volcanic fields away from the rift valley that are o set by tens of kilometres from the source of magma at depth 5-8 . The controls on the distribution of this o -rift volcanism are unclear. Here we use a numerical model of magmatic dyke propagation during rifting to investigate why some dykes reach the surface outside the rift valley, whereas others are confined to the valley. We find that the location of magmatism is governed by the competition between tectonic stretching and gravitational unloading pressure, caused by crustal thinning and faulting along the rift borders. When gravitational unloading dominates over tectonic stretching forces, dykes ascending from the ponded magma are steered towards the rift sides, eventually causing o -rift eruptions. Our model also predicts the formation of stacked magma sills in the lower crust above the magma-ponding zone, as well as the along-rift propagation of shallow dykes during rifting events, consistent with observations of magmatism and volcanism in rift zones globally. We conclude that rift topography-induced stress changes provide a fundamental control on the transfer of magma from depth to the surface.Continental rifts are commonly flanked by magmatism during their early stages 5,6 . In the Miocene-Recent Main Ethiopian Rift (MER), for example, many Pliocene volcanoes lie outside the Miocene rift border faults 9,10 ( Fig. 1). As the cumulative extension has increased, magmatism has become focused within the rift axis 3 . A similar pattern is observed during the evolution of the Red Sea Rift, where ∼25-Myr-old rift-parallel dykes focus near the ∼30-Myr-old rift margins and the youngest volcanoes focus along the ridge axis 11 . Even in far less magmatically active settings, such as the Baikal Rift (Siberia) or the Chaine des Puys (CdP) in the Cenozoic rift system of France, off-rift volcanism occurred after the onset of rifting 7,8 .During rift initiation, the lithosphere thins by ductile stretching beneath a rift valley formed from normal faulting 12 . The upwelling asthenosphere melts by adiabatic decompression, with the greatest degree of partial melting beneath the most thinned rift valley 1,2 . Although most studies focus on in-rift magmatism 3,4 , a model is needed to explain how off-rift volcanoes are fed. Progressive rift spreading results in the oldest volcanoes being transported away from the rift axis. However, such a process does not explain off-rift volcanism, with volcanoes located outside the rift border faults. Previous models for off-rift volcanoes include low-angle detachments tapping the asthenosphere to the sides of highly asymmetric rifts 13 , or the flexural response at the base of the footwall of the active rift border faults 5 . These models are unsatisfac...

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Source parameters of large African earthquakes:implications for crustal rheology and regional kinematics

Cited by 160 publications

References 102 publications

Acquisition of a Unique Onshore/Offshore Geophysical and Geochemical Dataset in the Northern Malawi (Nyasa) Rift

Acquisition of a Unique Onshore/Offshore Geophysical and Geochemical Dataset in the Northern Malawi (Nyasa) Rift

The August 2002 earthquake sequence in north Afar: Insights into the neotectonics of the Danakil microplate

Off-rift volcanism in rift zones determined by crustal unloading

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