The 1988 Tennant Creek,northern territory,earthquakes: A synthesis

Three large earthquakes with surface‐wave magnitudes 6.3–6.7 on 22 January 1988 were associated with 32 km of surface faulting on two main scarps 30 km southwest of Tennant Creek in the Northern Territory. These events provide an excellent opportunity to study the mechanics of midplate earthquakes because of the abundance of geological and geophysical data in the area, the proximity of the Warramunga seismic array and the ease of access to the fault zone. The 1988 earthquakes were located in the North Australian Craton in an area that had no history of moderate or large earthquakes before 1986. Additionally, no smaller earthquakes from the fault zone were identified at the Warramunga array, which is situated only 30 km from the nearest scarp, between the 1965 installation of the array and 1986. The main shocks were preceded by a swarm of moderatesized (magnitude 4–5) earthquakes in January 1987 and many smaller aftershocks throughout 1987. Careful relocation of all teleseismically recorded earthquakes from the fault zone shows that the 1987 activity was concentrated in an area only 6 km across in the gap between the two main fault scarps. The main shocks also nucleated in the centre of the fault zone near the 1987 activity. Field observations of scarp morphology indicate that the scarp is divided into three segments, each showing primarily reverse faulting. However, whereas the western and eastern segments show movement of the southern block over the northern, the central scarp segment shows the opposite, with the northern block thrust over the southern block.

Analysis of the first arrival times at Warramunga suggests that the three main shocks were associated with the western, central and eastern scarp segments, respectively. The locations of aftershocks determined using data from temporary seismograph arrays in the epicentral area define three inclined zones of activity that are interpreted as fault planes. In the western and eastern portions of the aftershock zone, these concentrations of activity dip to the south at 45° and 35°, respectively, but in the central section the aftershock zone dips to the north at 55°. Focal mechanisms derived from modelling broadband teleseismic data show thrust and oblique thrust faulting for the three main shocks. The first event ruptured unilaterally up and to the northwest on the westernmost fault segment, while the third main shock ruptured horizontally to the southeast. Modelling of repeat levelling data from the epicentral area requires at least three distinct fault planes, with the eastern and western planes dipping to the south and the central plane dipping to the north. The combination of scarp morphology, aftershock distribution and elevation data makes a strong case for rupture of fault planes in conjugate orientation during the 22 January 1988 Tennant Creek earthquakes. More than 20000 aftershocks have been recorded at Warramunga and activity continues to the present‐day with occasional shocks felt in the town of Tennant Creek and some recent off‐fault aftershocks located directly under the Warramunga seismic array. Stratigraphic relationships exposed in trenches excavated across the scarps suggest that during the Quaternary, a large earthquake ruptured the surface along one segment of the 1988 scarps.