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

Abstract

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.

The Development of Late‐Stage Continental Breakup: Seismic Reflection and Borehole Evidence from the Danakil Depression, Ethiopia

“…This distribution in seismicity implies that faulting in the Danakil Depression accommodates a larger proportion of extension than elsewhere in Afar. Additionally, local seismicity recorded in the Danakil during 2011-2013 shows that the in-rift fault system near Dallol is seismically active (Illsley-Kemp et al, 2017), consistent with the view that the role of magmatism in achieving extension here is reduced. To facilitate mechanical upper crustal extension and rapid basin subsidence, it is likely that contemporaneous ductile stretching of the lower crust and mantle lithosphere has occurred; this would explain the marked thinning of the crust in Danakil (~15 km) compared to elsewhere in Afar (~25 km; (Eagles et al, 2002).…”

Fault‐magma interactions during early continental rifting: Seismicity of the Magadi‐Natron‐Manyara basins, Africa

“…The b ‐value of 0.87 ± 0.03 estimated for the Magadi‐Natron‐Manyara rift zone is identical to the b ‐value of 0.87 for the central Kenya rift [ Tongue et al ., ], and similar to the b ‐value of 0.84 for Tanzania, which includes the Archaean craton [ Langston et al ., ], and 0.9 for the intensely intruded and faulted Danakil Depression [ Illsley‐Kemp et al ., ] (supporting information Figure SM1). A large number of earthquakes occurred south of our array in the Manyara basin where Albaric et al .…”

“…[] estimate an unusually low b ‐value of 0.77 for a 7 month time period of observation in the Magadi basin, which included an intense swarm interpreted as a dike intrusion. Inclusion of events from the 1989 Magadi swarm may explain the apparent temporal variations between the two studies, as has been observed in other magmatically active zones [e.g., Wiemer et al ., ; Illsley‐Kemp et al ., ].…”

Initiation of a Proto‐transform Fault Prior to Seafloor Spreading

“…A total of 4,971 earthquakes of magnitude 0.4–5.8 was recorded during the experiment and was located with a 2‐D velocity model (Hammond et al, ; Lomax et al, ; Makris & Ginzburg, ). These earthquakes have average location errors of ±1.9 km and ±4.1 km in the horizontal and vertical directions, respectively, and the catalogue is complete above magnitude 2.0 (Illsley‐Kemp et al, ). Generally, earthquakes are focused at the western rift margin, which separates the Ethiopian plateau from the Afar depression, or in the vicinity of volcanic centers (Illsley‐Kemp et al, ; Figure ).…”