Relocation of M >= 2 events of the 1989 Dobi seismic sequence in Afar: evidence for earthquake migration

The Ethiopian side of central Afar was struck in August 1989 by the largest seismic sequence (three 6.1 less than or equal to M-s less than or equal t o 6.3 events, 15 with M-s or m(b) greater than or equal to 5.0) since that of Serdo in 1969. Using the Djibouti seismological network, we relocated 29 7 of the events of that sequence. As most of the large events took place ou tside the network, we assessed the accuracy and stability of earthquake rel ocations by using three different velocity models and two relocation codes to try to relate individual shocks to distinct faults and surface breaks. A majority of the events apparently occurred underneath the floor of the Dob i graben, an area about 45 km long and 15 km wide, rupturing boundary and i nner floor faults, in agreement with the surface cracks and scarps that we mapped in the area. The relocation shows that the principal events propagat ed about 50 km northwestwards along the graben in the first 40 hr. A day an d a half after the beginning of the sequence, smaller events (M less than o r equal to 4) started to propagate more than 55 km eastwards, towards Asal Lake. Using two three-component stations installed near the Ethiopian borde r, we could determine reliable depths for 21 events. The depths are compati ble with a seismogenic crust about 14 km thick in the Dobi and Hanle graben area. Although the Dobi sequence ruptured about 50 km of the fault array e xtending from Serdo to Asal, where the regional stress was released by eart hquakes in 1969 and 1978, respectively, a seismic gap about 50 km long stil l subsists along the northern part of the Gaggade region (Der'ela half-grab en).