ALBERT

Notes: On 1988 January 22 three earthquakes of Ms 6.3-6.7 occurred in a 12-hr period near Tennant Creek (Northern Territory) in the Proterozoic North Australian Craton and produced 32 km of surface rupture on two main scarps. the Lake Surprise scarp is shaped like a boomerang with the east arm trending ESE and the west arm, WSW. the Kunayungku scarp lies 7 km to the northwest and trends ESE. Locations of 211 aftershocks determined using data from portable seismograph arrays define a 10-km by 40-km aftershock zone elongated parallel to the trend of the surface ruptures. In the western and eastern part of the fault zone, aftershocks occur only south of the Kunayungku and eastern Lake Surprise scarps, respectively. In the central section, in contrast, aftershocks lie primarily to the north of the western arm of the Lake Surprise scarp. Focal depths range from the surface to 8km. In all sections the shallowest earthquakes lie closest to the scarps, whereas deeper events lie further away. In cross-section, the aftershocks lie in three distinct inclined zones, which delineate fault surfaces ruptured by the main shocks. Aftershocks are concentrated near the edges of the inferred fault surfaces suggesting that stress was relieved in the centre of the faults by rupture during the main shocks. the inferred fault-plane dips are: 45° to the SSW for the Kunayungku fault, 55° to the NNW for the western Lake Surprise fault and 35° to the SSW for the eastern Lake Surprise fault. These dip directions are consistent with the sense of thrust movement inferred from surface deformation. the reversal of fault dip and sense of movement between the central and end segments requires a complicated fault geometry. the aftershock data would allow a second, south-dipping plane in the central section that does not intersect the surface. This blind fault may be responsible for much of the moment release interpreted from the modelling of broad-band teleseismic waveforms (Choy & Bowman 1989). Rupture of the north-dipping plane may have been controlled by a pre-existing fault marked at the surface by a quartz ridge 5-km long and may be of secondary importance. Although the quartz ridge suggests previous faulting, the absence of significant topographic relief together with estimates of erosion rates suggest an average repeat time in excess of 104yr. Preliminary interpretation of water withdrawal histories and water-level data from a borefield adjacent to the eastern Lake Surprise scarp does not support a causal relationship between the borefield and the occurrence of the earthquakes.The nearby Warramunga (WRA) seismic array provides a record of local earthquake activity from 1965 to the present. the fault area was essentially aseismic until two small earthquakes in 1986 February and a series of six magnitude 4-5 earthquakes in 1987 January, which were followed by more than 1000 aftershocks. the frequency of occurrence of aftershocks declined very slowly in 1987, with p= 0.2 in the modified Omori formula, whereas for the 1988 aftershock sequence, p= 1. the low p during 1987 may have been diagnostic of high stress in the source and Water Authority provided partial financial support for J.R.B. and G.G.