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This paper describes a sequence of small earthquakes ( m b≤3.3) that occurred at the Dallas–Fort Worth Airport, Texas, between 30 October 2008 and 31 May 2009. Analysis of records at regional station WMOK identified more than 180 earthquakes in the sequence; about 90 percent occurred in four clusters on 30 October–1 November, 20 November, 26 December and 15–17 May. After the sequence began, a six-station temporary local network obtained high-quality three-component records for 11 earthquakes occurring between 20 November and 1 December. Analysis of these data demonstrated that all 11 earthquakes originated from a focus near 32.855° N, 97.051° W, with an estimated depth ∼4.4 km. This location is less than 0.5 km from a well completed in August 2008 that extends to a depth of 4.2 km, drilled to dispose of brines collected during flowback of hydraulic fracturing fluids associated with the production of natural gas. Brine disposal commenced at the well on 12 September 2008. Seismograms and ( S - P ) intervals for the earthquakes are similar though not identical, and relative locations indicate they occurred along a north-northeast–south-southwest trend with horizontal and vertical dimensions of ∼1.1 km and 0.2 km, respectively. This trend is approximately coincident with that of a mapped normal fault in the subsurface, and consistent with the maximum horizontal in situ stress direction. Because of the absence of previous historical earthquakes, the proximity of the brine disposal well, and the similarity with other documented cases of induced seismicity, it seems likely that fluid injection induced the 2008–2009 sequence.

A swarm of dikes forms the core of the Sagatu Ridge, a 70-km-long topographic feature elevated to more than 4000 m above sea level and 1500 m above the level of the Eastern (Somalian) plateau. The ridge trends NNE and lies about 50 km east of the northeasterly trending rift-valley margin. Intrusion of the dikes and buildup of the flood-lava pile, largely hawaiitic but with trachyte preponderant in the final stages, occurred during the late Pliocene-early Pleistocene and may have been contemporaneous with downwarping of the protorift trough to the west. The ensuing faulting that formed the present rift margin, however, bypassed the ridge. The peculiar situation and orientation of the Sagatu Ridge, and its temporary existence as a line of crustal extension and voluminous magmatism, are considered related to a powerful structural control by a major line of Precambrian crustal weakness, well exposed further south. Transverse rift structures of unknown type appear to have limited the development of the ridge to the north and south.

Geomorphological evigernce indicates that Mount Badda, Ethiopia, had a Pleistocene ice cap of at least 140 km2. Altituger of the snow line was probably about 4 000 m, only 350 m below the summit, suggesting that glaciation occurred during the Würm glacial maximum.