The Afar Depression: interpretation of the 1960-2000 earthquakes

Abstract

SUMMARY We studied the seismic activity of the Afar Depression (AD) and adjacent regions during the period 1960–2000. We define seven distinct seismogenic regions using geological, tectonic and seismological data. Based on the frequency–magnitude relationships we obtain b‐values of about 1 for the different regions. The pattern of the distribution of the location of epicentres fits with the known active fault zone in the AD and the axial volcanic ridges. The Bab el Mandab area and the Danakil‐Aysha'a blocks are less active. For 125 intermediate to strong earthquakes the seismic moment and source parameters were calculated. The results of the fault plane solutions for the Afar Depression indicate mainly strike‐slip and normal sense of movement originating from fault planes striking NW–SE. These results indicate a clockwise block rotation described previously as a bookshelf model in central AD. There are a few right‐lateral faults east of Massawa with E–W‐striking fault planes. At the southern Red Sea, north of the Danakil block, the mixed focal mechanisms, with axial plane striking NW–SE, comprise several reverse faulting, strike‐slip motion and normal faulting. Right‐lateral movement was also calculated for a cluster of seismic events between the Manda Hararo and Alyata volcanic ridges along NW–SE‐striking faults. Along the N–S‐striking faults in the escarpment, at the western Afar margins, there are two distinct clusters of epicentres. The strong earthquakes at the southern cluster exhibit normal or strike‐slip motions. The intermediate to small earthquakes in the northern cluster exhibit reverse and strike‐slip motions. Mainly normal faults were calculated along NE–SW‐striking faults of the Ethiopian East African Rift. Estimates of the seismic efficiency suggest that the maximal values are about 50 per cent or less, implying that most of the motion is taken aseismically.

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

“…5, and this estimate is within the standard error of the maximum likelihood estimate of 0.67 ± 0.16. Our b-value estimate is lower than 1.05 obtained by Hofstetter and Beyth (2003) for the area from a less reliable data, indicating that the frequency of occurrence of relatively large magnitude earthquakes is higher than average, in turn implying that the Ethiopian plateau margin near the western part of the Danakil microplate is highly stressed.…”

“…1a using the maximum likelihood approach (Aki, 1965) and least squares to give an idea of the stress status of the seismogenic zone (Scholz, 1968). The b-value estimate by Hofstetter and Beyth (2003) for the area is 1.05 but calculated from only seven earthquakes with narrow spread in magnitude.…”

Formation and stability of magmatic segments in the Main Ethiopian and Afar rifts

“…The pattern of strain shown by geodesy is supported by historic and local seismicity patterns that indicate that N45-trending border faults are relatively inactive and that earthquakes are predominantly within the less than 30 km-wide magmatic segments (Ayele and Kulhánek, 2000;Keir et al, 2006). In addition, seismic moment release during 1960-2000 accounts for less than 50% strain predicted from timeaveraged rates of plate separation, indicating that a significant component of strain is accommodated by magma injection (Hofstetter and Beyth, 2003).…”

Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode

“…These short-term observations are also borne out by estimates of seismic efficiency over the past 40 years in Afar. Previous studies estimate that half the strain is accommodated aseismically 23 , probably by episodic dyke intrusion 24,25 . Structural data suggest that the locations of magma source(s) maintaining the Dabbahu segment have been stable for ,2 Myr (ref.…”