Abstract
SUMMARYObservations from different disciplines have shown that our planet is highly heterogeneous at multiple scale lengths. Still, many seismological Earth models tend not to include any small-scale heterogeneity or lateral velocity variations, which can affect measurements and predictions based on these homogeneous models. In this study, we describe the lithospheric small-scale isotropic heterogeneity structure in terms of the intrinsic, diffusion and scattering quality factors, as well as an autocorrelation function, associated with a characteristic scale length (a) and RMS fractional velocity fluctuations (ε). To obtain this characterization, we combined a single-layer and a multilayer energy flux models with a new Bayesian inference algorithm. Our synthetic tests show that this technique can successfully retrieve the input parameter values for 1- or 2-layer models and that our Bayesian algorithm can resolve whether the data can be fitted by a single set of parameters or a range of models is required instead, even for very complex posterior probability distributions. We applied this technique to three seismic arrays in Australia: Alice Springs array (ASAR), Warramunga Array (WRA) and Pilbara Seismic Array (PSAR). Our single-layer model results suggest intrinsic and diffusion attenuation are strongest for ASAR, while scattering and total attenuation are similarly strong for ASAR and WRA. All quality factors take higher values for PSAR than for the other two arrays, implying that the structure beneath this array is less attenuating and heterogeneous than for ASAR or WRA. The multilayer model results show the crust is more heterogeneous than the lithospheric mantle for all arrays. Crustal correlation lengths and RMS velocity fluctuations for these arrays range from ∼0.2 to 1.5 km and ∼2.3 to 3.9 per cent, respectively. Parameter values for the upper mantle are not unique, with combinations of low values of the parameters (a < 2 km and ε < ∼2.5 per cent) being as likely as those with high correlation length and velocity variations (a > 5 km and ε > ∼2.5 per cent, respectively). We attribute the similarities in the attenuation and heterogeneity structure beneath ASAR and WRA to their location on the proterozoic North Australian Craton, as opposed to PSAR, which lies on the archaean West Australian Craton. Differences in the small-scale structure beneath ASAR and WRA can be ascribed to the different tectonic histories of these two regions of the same craton. Overall, our results highlight the suitability of the combination of an energy flux model and a Bayesian inference algorithm for future scattering and small-scale heterogeneity studies, since our approach allows us to obtain and compare the different quality factors, while also giving us detailed information about the trade-offs and uncertainties in the determination of the scattering parameters.
Highlights
Alice Springs array (ASAR) and Warramunga Array (WRA) are located on the North Australian Craton (NAC), one of the Proterozoic cratons in the Precambrian westernmost two-thirds of the Australian continent (e.g. Myers, 1990; Simons et al, 1999; Cawood and Korsch, 2008; Wellman, 1998) (Fig. 6)
We applied the linear leastsquares fit of the squared stacked envelopes at the free surface (Fig. S4) to a time window starting tN s after the theoretical P wave arrival, since the Energy Flux Model (EFM) is only applicable after the direct wave has left the scattering layer (Korn, 1990; Hock and Korn, 2000)
WRA and ASAR lie on the proterozoic North Australian Craton (NAC), but while WRA is situated near its center, ASAR is on its southern border, a margin with more complex and recent tectonic history than the interior of the craton, which correlates with the generally lower quality factor values we observe for ASAR
Summary
Background lithosphericP-wave velocities (Fig. 5) and Moho depths for each seismic array were obtained from the Australian Seismological Reference Model (AuSREM, Kennett and Salmon, 2012; Salmon et al, 2013b; Kennett et al, 2013; Salmon et al, 2013a) and AusMoho model (Kennett et al, 2011) respectively.443 TECTONIC SETTINGASAR and WRA are located on the North Australian Craton (NAC), one of the Proterozoic cratons in the Precambrian westernmost two-thirds of the Australian continent (e.g. Myers, 1990; Simons et al, 1999; Cawood and Korsch, 2008; Wellman, 1998) (Fig. 6). Thick crust (Lc > 40 km) with a wide 457 and smooth Moho transition has generally been found in the Proterozoic shields of Central Australia while the Archaean regions of western Australia have thinner crust (Lc < 40 km) and sharper crust-upper mantle transitions (e.g. Clitheroe 460 et al, 2000; Sippl, 2016; Salmon et al, 2013a; Kennett et al, 2011; Kennett and Saygin, 2015).
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