Spatial and temporal variations of seismicity in the Horn of Africa from 1960 to 1993

Ayele, Atalay; Kulhánek, Ota

doi:10.1111/j.1365-246x.1997.tb01875.x

Cited by 42 publications

(37 citation statements)

References 30 publications

(28 reference statements)

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“…Earthquakes were predominantly 8-14 km deep . Records of seismicity over the past ϳ150 years show a similar pattern with seismicity concentrated near Ankober and within the en echelon magmatic segments along the axis of the MER (e.g., Gouin, 1979;Asfaw, 1982;Kebede and Kulhánek, 1994;Ayele and Kulhánek, 1997;Hofstetter and Beyth, 2003). The spatial correlation between seismicity and aligned cones in magmatic segments suggests that earthquakes are induced by dike injection, and the temporal patterns of seismicity suggest that magmatic segments are fed from discrete sources of magma and that extension occurs mainly in episodic dike-injection events .…”

Section: Tectonic Settingmentioning

confidence: 81%

“…Attenuation curves for southern California (Richter, 1935, Hutton andBoore, 1987), for Tanzania (Langston et al, 1998), and for Ethiopia (from our study). and local catalogs has meant that reliable earthquakemagnitude statistics for the MER have not been calculated (Ayele and Kulhánek, 1997). Earthquake magnitude is also important in integrated seismic and geodetic studies that aim to understand lithospheric deformation processes in rift systems by quantifying relative amounts of seismic and aseismic strain (e.g., Hofstetter and Beyth, 2003;Bendick et al, 2006).…”

Section: Introductionmentioning

confidence: 99%

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Local Earthquake Magnitude Scale and Seismicity Rate for the Ethiopian Rift

Keir¹,

Stuart²,

Jackson³

et al. 2006

Bulletin of the Seismological Society of America

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A calibrated local earthquake magnitude scale is essential for quantitative analyses of seismicity. In Ethiopia, effective monitoring of earthquakes and resulting assessment of seismic hazard are especially important as regions with seismic and volcanic activity coincide with regions of economic significance and population growth. We have developed a local magnitude (M L ) scale for the northern Main Ethiopian rift (MER) using earthquake data collected during 2001-2003 on 122 three-component broadband seismic stations. Waveform data from 2139 local earthquakes were corrected for instrument response and convolved with the nominal Wood-Anderson torsion seismograph response appropriate for the original definition of local magnitude. The hypocentral distances considered are 5 to 800 km, with the best represented range from 5 to 150 km. A total of 30,908 maximum zero-to-peak amplitudes (A WA ) were incorporated into a direct linear inversion for individual earthquake local magnitudes (M L ), 244 station factors (C), and 2 linear distance-dependent factors (n, K) in the distance correction term, log (A o ), of the equation for local magnitude: M L ‫ס‬ log(A WA ) ‫מ‬ log(A o ) ‫ם‬ C. The resulting distance correction is given by ‫מ‬log(A o ) ‫ס‬ 1.196997log(r/17) ‫ם‬ 0.001066(r ‫מ‬ 17) ‫ם‬ 2, which implies that ground-motion attenuation is relatively high, consistent with ongoing magma intrusion and the presence of shallow magma reservoirs beneath the MER. Station corrections significantly reduce M L residuals and range between ‫24.0ע‬ M L units. The catalog of earthquakes is complete above M L ϳ2.1 and the annual cumulative seismicity rate follows the relation log N ‫ס‬ 4.5 ‫מ‬ 1.13 M L . Our results are critical for accurate routine quantitative analysis of past, current and, future seismicity in Ethiopia.

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Section: Tectonic Settingmentioning

confidence: 81%

Section: Introductionmentioning

confidence: 99%

Local Earthquake Magnitude Scale and Seismicity Rate for the Ethiopian Rift

Keir¹,

Stuart²,

Jackson³

et al. 2006

Bulletin of the Seismological Society of America

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“…South Sudan, Tanzania, and Kenya, which are all regions of continental rifting, have bvalues that range from 0.7 to 0.9 (Tongue et al, 1992;Ayele and Kulhanek, 1997;Langston et al, 1998). The Danakil region b-value is also significantly higher than that in other continental rift systems, such as the Rio Grande and Eger rifts, both of which exhibit b-values of 0.8 (Ibs-von Seht et al, 2008).…”

Section: Discussionmentioning

confidence: 99%

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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“…The spatial distribution of seismicity is representative of longer-term patterns from catalogs of earthquakes recorded on global and regional networks (Gouin 1979, Kebede & Kulhánek 1994, Ayele & Kulhánek 1997, Jacques et al 1999, Hofstetter & Beyth 2003, Ayele et al 2007b. Temporary arrays around the regional capital, Semara (Gresta et al 1997), the Erta'Ale lava lake ( Jones et al 2006), and a sparse permanent network in Eritrea (Ogubazghi et al 2004;Hagos et al 2006aHagos et al , 2006b) showed frequent, small magnitude events, but much of the rest of the region was unmonitored.…”

Section: Tectonic Settingmentioning

confidence: 99%

Length and Timescales of Rift Faulting and Magma Intrusion: The Afar Rifting Cycle from 2005 to Present

Ebinger

Ayele

Keir

et al. 2010

Annu. Rev. Earth Planet. Sci.

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172

163

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Although fault and magmatic processes have achieved plate spreading at mid-ocean ridges throughout Earth's history, discrete rifting episodes have rarely been observed. This paper synthesizes ongoing seismic, structural, space-based geodetic, and petrologic studies from the subaerial Red Sea rift in Ethiopia where a major rifting episode commenced in September 2005. Our aims are to determine the length and timescales of magmatism and faulting, the partitioning of strain between faulting and magmatism, and their implications for the maintenance of along-axis segmentation. Most of the magma for the initial and subsequent 12 intrusions was sourced from the center of the Dabbahu-Manda Hararo rift segment. Strain is accommodated primarily by axial dike intrusions fed from mid-segment magma chamber(s). These findings show that episodic (approximate century interval), rapid opening of discrete rift segments is the primary mechanism of plate boundary deformation. The scale (∼65 km × 8 km) and intensity of crustal deformation (∼6 m), as well as the volume of intrusive and extrusive magmatism (>3 km 3 ), provokes a re-evaluation of seismic and volcanic hazards in subaerial rift zones.

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Spatial and temporal variations of seismicity in the Horn of Africa from 1960 to 1993

Cited by 42 publications

References 30 publications

Local Earthquake Magnitude Scale and Seismicity Rate for the Ethiopian Rift

Local Earthquake Magnitude Scale and Seismicity Rate for the Ethiopian Rift

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

Length and Timescales of Rift Faulting and Magma Intrusion: The Afar Rifting Cycle from 2005 to Present

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