Crustal surface wave velocity structure of the east Albany-Fraser Orogen, Western Australia, from ambient noise recordings

Abstract

S U M M A R YGroup and phase velocity maps in the period range 2-20 s for the Proterozoic east AlbanyFraser Orogen, Western Australia, are extracted from ambient seismic noise recorded with the 70-station ALFREX array. This 2 yr temporary installation provided detailed coverage across the orogen and the edge of the Neoarchean Yilgarn Craton, a region where no passive seismic studies of this scale have occurred to date. The surface wave velocities are rather high overall (>3 km s −1 nearly everywhere), as expected for exposed Proterozoic basement rocks. No clear signature of the transition between Yilgarn Craton and Albany-Fraser Orogen is observed, but several strong anomalies corresponding to more local geological features were obtained. A prominent, NE-elongated high-velocity anomaly in the northern part of the array is coincident with a Bouguer gravity high caused by the upper crustal metamorphic rocks of the Fraser Zone. This feature disappears towards longer periods, which hints at an exclusively upper crustal origin for this anomaly. Further east, the limestones of the Cenozoic Eucla Basin are clearly imaged as a pronounced low-velocity zone at short periods, but the prevalence of low velocities to periods of ≥5 s implies that the uppermost basement in this area is likewise slow. At longer periods, slightly above-average surface wave velocities are imaged below the Eucla Basin.

Empirical Green's Function Retrieval Using Ambient Noise Source‐Receiver Interferometry

“…and a smaller low-velocity zone (L2) to the south (Figure 8a), consistent with the observations from the study by Sippl et al (2017). The inversion of synchronous C 2 functions largely confirms the velocity pattern observed in the C 1 result, with an average velocity difference of 0.02 km/s across the study region.…”

“…the C 1 functions (Figure 6a). The range of group velocity measurements (i.e., standard deviations) are similar between the two sets of EGFs and is also consistent with the measurements from an earlier study (Sippl et al, 2017).…”

“…In terms of amplitude, C 1 and C 2 functions show a reversal of the dominating energy on the causal and acausal branches. The energy on C 1 functions is predominately controlled by the distribution of noise sources in the nearby oceans (Sippl et al, 2017). Consequently, the ocean noises to the east and west of the network contribute to the near-offset C 1 with primarily E-W orientated raypaths, leading to a more symmetric waveform pattern.…”

“…A constant velocity of 3.24 km/s is assigned to each node location as the initial value. We follow the damping and smoothing criteria from Sippl et al (2017) for the inversions of C 1 , synchronous C 2 , and the joint C 1 and asynchronous C 2 functions considering a similar raypath coverage (Figure 7). Lower values are adopted for the inversion of asynchronous C 2 functions to account for the intrinsic smoothing effect imposed by the long-distance raypaths (Figure 7c).…”

Green’s function retrieval from deterministic seismic wavefield using higher-order cross-correlation

“…Another promising application of C 2 is its ability to tie temporary arrays deployed at different times (e.g., the campaign mode arrays in eastern Australia) to constrain the inter-array structure, which remains largely undersampled due to the current network topology. With much improved data sampling, the existing regional (e.g., Sippl et al 2017) or continental scale crustal models (e.g., Saygin and Kennett 2012) can be further refined and eventually lead to a better understanding of the Australian continent.…”