A passive seismic experiment in the Perth Basin, Western Australia

To assess the feasibility of using passive seismic techniques in difficult environments, a passive seismic experiment was undertaken in the onshore Perth Basin. The Perth Basin is a long, thin geological basin next to a transcrustal fault scarp in an urban environment. Data was collected from an array of 29 stations. These stations comprised a north–south transect from near Moora in the north to near Harvey in the south, with a shorter east–west transect near Moora and extending on to the adjacent Yilgarn Craton. Additional data were also acquired in the Perth metropolitan area from 10 more installations. The project also aimed to image the Moho in an attempt to learn more about the mechanism of the creation of the Perth Basin.Data analysis using common conversion point stacking of teleseismic arrivals and ambient noise tomographic methods allows crustal velocity structure to be mapped at a regional scale. In general, signal-to-noise levels were poor, largely due to shorter than expected deployment times. This resulted in poor resolution of the deep geology. Consequently, the Moho is only poorly constrained to be at a depth of more than about 45 km due to it being at the limit of data resolution. It is possibly represented by a velocity transition rather than a sharp discontinuity. The thickness of the Perth Basin, at around 10 km, is consistent with previous estimates based on interpretation of seismic reflection data. Beneath the basin, the crust has a largely consistent shear wave-velocity structure with laterally continuous zones. Shear velocities reach 4 km/s immediately below the Perth Basin within the basement rocks which are assumed to be part of the Pinjarra Orogen, typically extending to about 25 km depth. Beneath this is a layer of lower velocity, typically as low as 3.5 km/s which is particularly evident between 31.5°S and 32.3°S. Here, the different structure may be due to a different crustal component of the Pinjarra Orogen, perhaps reflecting the presence of a basement block with different affinities to elsewhere in the survey area.