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.

S U M M A R YWe report on a receiver function study of the crust and upper mantle within DESERT, a multidisciplinary geophysical project to study the lithosphere across the Dead Sea Transform (DST). A temporary seismic network was operated on both sides of the DST between 2000 April and 2001 June. The depth of the Moho increases smoothly from about 30 to 34-38 km towards the east across the DST, with significant north-south variations east of the DST. These Moho depth estimates from receiver functions are consistent with results from steepand wide-angle controlled-source techniques. Steep-angle reflections and receiver functions reveal an additional discontinuity in the lower crust, but only east of the DST. This leads to the conclusion that the internal crustal structure east and west of the DST is different. The P to S converted phases from both discontinuities at 410 and 660 km are delayed by 2 s with respect to the IASP91 global reference model. This would indicate that the transition zone is consistent with the global average, but the upper mantle above 410 km is 3-4 per cent slower than the standard earth model.

Consulting the Catalogue of the International Seismological Centre (ISC), for the period 1904–2016 to detect the occurrence of potentially damaging earthquakes we observed that in most cases, when a high magnitude earthquake occurs (magnitude of at least 6.5), there is an increased probability that a similar high magnitude earthquake will occur within a relatively short period of weeks (less than a year). This occurs when the two events are located on latitudes of practically the same absolute value. This Apparent Strong Earthquake Pattern (ASEP) is observed in about 90% of all earthquakes of magnitudes greater than, or equal to 6.5. ASEP is evident in a high percentage of events and remains practically unchanged even after shuffling the dates of occurrence of earthquakes several times. This statistical observation is not surprising when considering the geographic distribution of earthquake epicenters and activity rates in regions of relatively frequent high magnitude earthquakes. Nevertheless, it leads us to consider the possibility of introducing ASEP in earthquake risk management and Operational Earthquake Forecasting (OEF). The main objective of this study is to assess the potential of estimating the location, magnitude, and time of occurrence of a strong earthquake after the occurrence of a similar strong earthquake in a distant area, when subsequent events conform to the apparent event pattern of strong earthquakes. The effectiveness of ASEP is quantified in terms of the ratio between the success rates obtained by applying ASEP and the probability of a randomly occurring earthquake of magnitude M in a given seismic zone within dT weeks. The effectiveness of ASEP was initially evaluated through simulations from the data from earthquake catalogues. The simulations reveal, as expected, that when the earthquakes in catalogue A are independent of the earthquakes in catalogue B, and when the occurrence of earthquakes in each catalogue are random and obey laws of Poisson distribution, the effectiveness is always lower than 1, i.e., the chance of a successful random guess is always higher than the probability of successful forecasting that follows ASEP. The opposite is observed when applying the ASEP schema to real cases. After arbitrarily choosing ten pairs of seismogenic zones, the success rates of the apparent earthquake pattern forecasts always yield a higher forecast probability, sometimes considerably higher than would a random guess. It is also observed that when selecting pairs of active zones which fail to obey ASEP requirement about the locations of events, the success rate of forecasting is, in most tested cases, zero. i.e., similar to what is observed in the simulations. Subsequently, the proposed forecasting scheme based on ASEP may be useful for OEF applications which in turn could be considered in earthquake risk management programs. These observations also suggest that the temporal behavior of strong earthquakes is not purely random.

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S U M M A R YLocal earthquake data from a dense temporary seismological network in the southern Dead Sea area have been analysed within the project DESIRE (Dead Sea Integrated Research Project). Local earthquakes are used for the first precise image of the distribution of the P-wave velocity and the v P /v S ratios. 65 stations registered 655 local events within 18 months of observation time. A subset of 530 well-locatable events with 26 730 P-and S-arrival times was used to calculate a tomographic model for the v P and v P /v S distribution. Since the study area is at first-order 2-D, a gradual approach was chosen, which compromised a 2-D inversion followed by a 3-D inversion. The sedimentary basin fill is clearly imaged through high v P /v S ratios and low v P . The basin fill shows an asymmetric structure with average depth of 7 km at the western boundary and depth between 10 and 14 km at the eastern boundary. This asymmetry is reflected by the vertical strike-slip eastern border fault, and the normal faulting at the western boundary, caused by the transtensional deformation within the last 5 Myr. Within the basin fill the Lisan salt diapir is imaged through low v P /v S ratios, reflecting its low fluid content. The extensions were determined to 12 km in E-W and 17 km in N-S direction while its depth is 5-6 km. The thickness of the pre-basin sediments below the basin fill cannot be derived from the tomography data-it is estimated to less than 3 km from former investigations. Below the basin, down to 18 km depth very low P-wave velocities and low v P /v S ratios are observed-most likely caused by fluids from the surrounding crust or the upper mantle.