J. Roger Bowman

Geologic investigations of the 1986 Marryat Creek, Australia, earthquake; implications for paleoseismicity in stable continental regions

From introduction: This report summarizes the results of our efforts to document the timing of the last prehistoric movement on the faults that ruptured during the 1988 Tennant Creek earthquake sequence.

High-Frequency Tsunami Signals of the Great Indonesian Earthquakes of 26 December 2004 and 28 March 2005

Tsunamis generated by the great Indonesian earthquakes of 26 December 2004 ( M w 9.3) and 28 March 2005 ( M w 8.6) produced high-frequency (>5 mHz) dispersed signals recorded by hydrophone stations offshore from Diego Garcia and Cape Leeuwin, Australia, and by many seismic stations in the Indian Ocean and on the coast of Antarctica. For the first and greater earthquake, the dispersed energy is commonly observed to 30 mHz and, in one case, to 60 mHz. The high-frequency signals are consistent with being generated at pointlike sources. The earliest arriving tsunami signals originate from a point near 4.3° N, 93.8° E, determined using event- to-station distances estimated by matching predicted dispersion to observations. The location is further constrained with azimuth estimates from an array of hydrophones. Fine structure in the tsunami signal indicates a second high-frequency source just south of Great Nicobar Island near 6.5° N, 93.6° E. The point sources are located close to the maximum slip area determined in several other seismic and tsunami studies. The dispersion of much later-arriving energy is consistent with travel over longer paths, and matches predictions for reflections of the tsunami from bathymetric features in the Indian Ocean basin. For the 28 March 2005 tsunami, the high- frequency dispersion is observed at the Diego Garcia hydrophone station and the AIS seismic station, and tsunami signals without apparent dispersion are seen at four other seismic stations. Phase velocities estimated at hydroacoustic stations agree with linear dispersion theory at frequencies above 12 mHz.

ESR dating of aeolian sand near tennant creek, Northern territory, Australia

Abstract Electron-spin resonance (ESR) dating of aeolian sand in the Northern Territory of Australia was carried out using the Al centres in quartz grains as part of an attempt to date prehistoric faulting associated with the 1988 Tennant Creek earthquakes. Our experiments on optical bleaching of ESR (electron spin resonance) signals show that sunlight does not completely bleach the ESR signal from the Al centre of quartz grains in this aeolian sand. The total doses were determined from the acquired ESR signal intensity, however, a correction to compensate for partial bleaching and the resultant inherited residual ESR signal, has to be carried out. By using this technique, the ESR age estimates an increase with depth, which suggests that nearly complete optical bleaching has occurred through this slice of geologic time. For age estimates younger than 50 ka, the ESR ages are in generally good agreement with independently determined TL (thermoluminescence) data; beyond 50 ka, the ESR ages are systematically older than the TL ages. The oldest ESR age estimate of 160.0 ± 5.9 ka is from a sample at 228 cm depth and ESR results from the vertical samplings yield an apparent sedimentation rate of 2.6 cm/ka.

Relocation of teleseismically recorded earthquakes near Tennant Creek, Australia: Implications for midplate seismogenesis

The three MS > 6 Tennant Creek, Australia, earthquakes of January 22, 1988, were preceded by small and moderate earthquakes in 1986 and 1987. We have used the method of joint hypocenter determination to estimate the positions of teleseismically recorded 1986–1987 earthquakes, the 1988 main shocks, and aftershocks with respect to a system of surface fault scarps. The 1986–1987 shocks and the 1988 main shocks nucleated near the center of the zone of surface scarps, where fault-segment boundaries at depth are implied by complexities in the distribution of scarps at the surface. This suggests that the fault segments that ruptured in 1988 were already in existence in 1986–1987, which is consistent with the hypothesis that strong midplate earthquakes occur on preexisting faults. The redetermined 1986–1987 hypocenters are, however, also consistent with the hypothesis that midplate seismicity is localized by stress concentration due to bulk rheological heterogeneity of the crust, because they are situated on a regionally prominent gravity anomaly. The teleseismically recorded seismicity of the Tennant Creek region prior to the 1988 main shocks has the temporal pattern of a swarm followed by a lull. The concentration of swarm earthquakes between two scarps is consistent with models in which precursory-swarm earthquakes correspond to faulting that is spatially distinct from the site of primary main-shock faulting. The time interval between swarm and main shocks is similar to intervals between intermediate-term precursory swarms and main shocks in regions that have much higher rates of tectonic loading; this similarity suggests that the time intervals between precursory swarms and subsequent main shocks are not strongly influenced by the rate of tectonic loading, but are determined primarily by time dependence of the failure process. The spatial distribution of teleseismically recorded aftershocks is in most respects like that of aftershocks recorded by a local network of portable stations in the half year following the main shocks. The set of teleseismically recorded aftershocks, like the set of locally recorded aftershocks, includes some events that occurred well away from the causative faults of the main shocks. At the length scale of the 1988 main-shock rupture, the distribution of aftershocks occurring more than one year after the main shocks is not representative of the distribution of earlier aftershocks.