Local Earthquake Magnitude Scale and Seismicity Rate for the Ethiopian Rift

Keir, Derek; Stuart, G. W.; Jackson, Andrew; Ayele, Atalay

doi:10.1785/0120060051

Cited by 87 publications

(76 citation statements)

References 50 publications

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“…Our new attenuation curve shows that at hypocentral distances of less than 100 km, the attenuation in Danakil is very similar to that in the Main Ethiopian rift and in southern California (Hutton and Boore, 1987;Keir et al, 2006). High attenuation in southern California is explained by high geothermal gradients beneath the San Gabriel Mountains (Schlotterbeck and Abers, 2001).…”

Section: Discussionmentioning

confidence: 54%

“…The direct inversion method is computationally much faster and has been tested by Pujol (2003) on data from Tanzania that was previously analyzed using the iterative technique (Langston et al, 1998). The method used in this study has been tested on data from the Main Ethiopian rift and yielded identical results to those produced by Keir et al (2006).…”

Section: Methodsmentioning

confidence: 94%

“…The data from Danakil include 4275 events recorded on 11 stations, producing a matrix with dimensions of 32;904 × 4299, with 4299 unknowns to be solved. Then, we solve equation (4) Keir et al, 2006). Our approach differs from many previous studies that have used an iterative technique (Langston et al, 1998;Baumbach et al, 2003).…”

Section: Methodsmentioning

confidence: 99%

“…Seismic energy will attenuate at varying rates depending on factors such as temperature, composition, and fluid content of the crust as well as partial melt and magmatic intrusions (Schlotterbeck and Abers, 2001;Carletti and Gasperini, 2003;Keir et al, 2006;Wang et al, 2009). This rate of attenuation is characterized by an attenuation curve.…”

Section: Introductionmentioning

confidence: 99%

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Local Earthquake Magnitude Scale andb‐Value for the Danakil Region of Northern Afar

Illsley‐Kemp¹,

Keir²,

Bull³

et al. 2017

Bulletin of the Seismological Society of America

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The Danakil region of northern Afar is an area of ongoing seismic and volcanic activity caused by the final stages of continental breakup. To improve the quantification of seismicity, we developed a calibrated local earthquake magnitude scale. The accurate calculation of earthquake magnitudes allows the estimation of b-values and maximum magnitudes, both of which are essential for seismic-hazard analysis. Earthquake data collected between February 2011 and February 2013 on 11 three-component broadband seismometers were analyzed. A total of 4275 earthquakes were recorded over hypocentral distances ranging from 0 to 400 km. A total of 32,904 zero-to-peak amplitude measurements (A) were measured on the seismometer's horizontal components and were incorporated into a direct linear inversion that solved for all individual local earthquake magnitudes (M L ), 22 station correction factors (C), and 2 distance-dependent factors (n, K) in the equation M L logA− logA 0 C. The resultant distance correction term is given by − logA 0 1:274336 logr=17 − 0:000273r − 17 2. This distance correction term suggests that attenuation in the upper and mid-crust of northern Afar is relatively high, consistent with the presence of magmatic intrusions and partial melt. In contrast, attenuation in the lower crust and uppermost mantle is anomalously low, interpreted to be caused by a high melt fraction causing attenuation to occur outside the seismic frequency band. The calculated station corrections serve to reduce the M L residuals significantly but do not show a correlation with regional geology. The cumulative seismicity rate produces a b-value of 0:9 0:06, which is higher than most regions of continental rifting yet lower than values recorded at midocean ridges, further supporting the hypothesis that northern Afar is transitioning to seafloor spreading.Electronic Supplement: List of all local earthquakes used in the study with calculated local magnitudes and associated magnitude error.

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Section: Discussionmentioning

confidence: 54%

Section: Methodsmentioning

confidence: 94%

Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Local Earthquake Magnitude Scale andb‐Value for the Danakil Region of Northern Afar

Illsley‐Kemp¹,

Keir²,

Bull³

et al. 2017

Bulletin of the Seismological Society of America

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“…Along the axis of the Main Ethiopian Rift, earthquakes are predominantly localized to depths of less than ~15 km within 20-km-wide, right-stepping, en-echelon zones of Quaternary volcanism and faulting. Seismicity in these zones is characterized by low-magnitude (M L ~1-4) clusters (Keir et al, 2006a) coincident with Quaternary faults, fi ssures, and chains of eruptive centers. All but three focal mechanisms show normal dip-slip motion; the minimum compressive stress is N103°E, perpendicular to Quaternary faults and aligned volcanic cones (Fig.…”

Section: Seismicitymentioning

confidence: 99%

The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE): Probing the transition from continental rifting to incipient seafloor spreading

Bastow¹,

Keir²,

Daly³

2011

Volcanism and Evolution of the African Lithosphere

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The Miocene-Holocene East African Rift in Ethiopia is unique worldwide because it subaerially exposes the transition between continental rifting and seafl oor spreading within a young continental fl ood basalt province. As such, it is an ideal study locale for continental breakup processes and hotspot tectonism. Here, we review the results of a recent multidisciplinary, multi-institutional effort to understand geological processes in the region: The Ethiopia Afar Geoscientifi c Lithospheric Experiment (EAGLE). In 2001-2003, dense broadband seismological networks probed the structure of the upper mantle, while controlled-source wide-angle profi les illuminated both along-axis and across-rift crustal structure of the Main Ethiopian Rift. These seismic experiments, complemented by gravity and magnetotelluric surveys, provide important constraints on variations in rift structure, deformation mechanisms, and melt distribution prior to breakup. Quaternary magmatic zones at the surface within the rift are underlain by high-velocity, dense gabbroic intrusions that accommodate extension without marked crustal thinning. A magnetotelluric study illuminated partial melt in the Ethiopian crust, consistent with an overarching hypothesis of magmaassisted rifting. Mantle tomographic images reveal an ~500-km-wide low-velocity zone at ≥ ≥75 km depth in the upper mantle that extends from close to the eastern edge of the Main Ethiopian Rift westward beneath the uplifted and fl ood basaltcapped NW Ethiopian Plateau. The low-velocity zone does not interact simply with the Miocene-Holocene (rifting-related) base of lithosphere topography, but it also provides an abundant source of partially molten material that assists extension of the seismically and volcanically active Main Ethiopian Rift to the present day.

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The development of magmatism along the Cameroon Volcanic Line: Evidence from seismicity and seismic anisotropy

et al. 2014

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The Cameroon Volcanic Line (CVL) straddles the continent‐ocean boundary in West Africa but exhibits no clear age progression. This renders it difficult to explain by traditional plume/plate motion hypotheses; thus, there remains no consensus on the processes responsible for its development. To understand better the nature of asthenospheric flow beneath the CVL, and the effects of hotspot tectonism on the overlying lithosphere, we analyze mantle seismic anisotropy and seismicity. Cameroon is relatively aseismic compared to hotspots elsewhere, with little evidence for magmatism‐related crustal deformation away from Mount Cameroon, which last erupted in 2000. Low crustal Vp/Vs ratios (∼1.74) and a lack of evidence for seismically anisotropic aligned melt within the lithosphere both point toward a poorly developed magmatic plumbing system beneath the CVL. Null SKS splitting observations dominate the western continental portion of the CVL; elsewhere, anisotropic fast polarization directions parallel the strike of the Precambrian Central African Shear Zone (CASZ). The nulls may imply that the convecting upper mantle beneath the CVL is isotropic, or characterized by a vertically oriented olivine lattice preferred orientation fabric, perhaps due to a mantle plume or the upward limb of a small‐scale convection cell. Precambrian CASZ fossil lithospheric fabrics along the CVL may have been thermomechanically eroded during Gondwana breakup ∼130 Ma, with an isotropic lower lithosphere subsequently reforming due to cooling of the slow‐moving African plate. Small‐scale lithospheric delamination during the 30 Ma recent development of the line may also have contributed to the erosion of the CASZ lithospheric fossil anisotropy, at the same time as generating the low‐volume alkaline basaltic volcanism along the CVL.

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Local Earthquake Magnitude Scale and Seismicity Rate for the Ethiopian Rift

Cited by 87 publications

References 50 publications

Local Earthquake Magnitude Scale andb‐Value for the Danakil Region of Northern Afar

Local Earthquake Magnitude Scale andb‐Value for the Danakil Region of Northern Afar

The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE): Probing the transition from continental rifting to incipient seafloor spreading

The development of magmatism along the Cameroon Volcanic Line: Evidence from seismicity and seismic anisotropy

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