Significant changes in physical properties produce conversions between compressional (P) and shear (S) waves. The Earth’s surface also returns energy to depth that can be reflected back to the surface. Analysis of this chain of conversions and reverberations yields information about the structure beneath the station.
The most common approach is the use of receiver functions that exploit the conversions and so can map out major interfaces. The receiver functions exploit the properties of the same time segment of different components of ground motion, suppressing the source contributions to enhance conversions.
A powerful alternative is to exploit the autocorrelation of a single component of ground motion, and thereby enhance consistent features equivalent to reflections from depth. The use of the vertical component of ground motion extracts an estimate of the reflectivity associated with P waves at higher frequencies than used for receiver functions.
With many closely spaced seismic stations it is therefore possible to build a reflection profiles for the entire lithosphere from the recordings of many different earthquakes (e.g., Sun & Kennett, 2020). The frequencies employed (1– 4 Hz) are much lower than in conventional reflection profiling using vibrator sources at the surface, but provide penetration of both the crust and the lithospheric mantle.
With support from AuScope, a line of 68 seismic recorders at approximately 3.5 km spacing was deployed in northern South Australia spanning a zone where previous results indicated a major change in crustal thickness from around 50 km to less than 30 km over a 200 km horizontal distance. This MAL experiment (Figure 1) extends from Marla on the sturt Highway across to the north of Oodnadatta and somewhat to the east. From nearly a year of recordings, in 2018–2019, Liang & Kennett (2020) have shown it is possible to construct an image of the P reflectivity through the entire lithosphere, as displayed in Figure 2.
The change in crustal thickness art the eastern end of the profile is clearly evident and links to the estimates of the position of the edge of the Gawler craton from other geophysical evidence (particularly magnetic anomalies). An intriguing feature is a transition zone at the base of the rust that wedges out to the east at the craton margin, this is indicated by the dashed light green line in Figure 2.
In conventional reflection profiles, with frequencies of 20 Hz and above, the base of the crust marks the end of P reflectivity. With the longer wavelengths from the illumination by distant earthquakes it becomes clear that the apparently bland lithospheric mantle has significant structure, but at larger scales than in the crust. Such behaviour had been inferred from a variety of other lines of evidence (Kennett et al., 2017) and now is seen in a direct image.
The MAL experiment has demonstrated that valuable information on lithospheric architecture can be extracted from distant earthquakes. Such reflection profiles exploiting distant earthquakes can be carried out in places where it is uneconomic or logistically difficult to use conventional equipment, though the timescale for data acquisition is much longer.
Kennett, B.L.N., Yoshizawa, K., Furumura, T., 2017. Interactions of multi-scale heterogeneity: Australia. Tectonophysics,717, 193–213. doi: 10.1016/j.tecto.2017.07.009
Liang S. & Kennett B.L.N. (2020) Passive seismic imaging of a craton edge – Central Australia, Tectonophysics, 797, 228662. doi: 10.1016/j.tecto.2020.228662
Sun, W., Kennett, B.L.N., 2020. Common-reflection-point-based prestack depth migration for imaging lithosphere in Python: Application to the dense Warramunga array in northern Australia, Seismological Research Letters, 91, 2890–2899. doi: 10.1785/0220200078.