Constraints on the Moho depth beneath the Australian continent with teleseismic converted and reflected waves

The Moho or the crust-mantle boundary is the first major discontinuity of the Earth's interior. It is generally believed to mark the changes in seismic wave speed, chemical composition, density, and rheological properties. Despite more than 100 yrs' exploration, the nature of the Moho, however, is still poorly understood. Among varies techniques used for detecting the Moho, seismic waves provide the most accurate image of the subsurface structures. Although seismic reflection and refraction with active sources provide fine structures of the crust, their cost is usually expensive and the Moho can be elusive. On the contrary, passive seismic surveys using broadband seismometers are deep penetrating and the cost is modest. Since late 1970s, receiver functions (RF) have been extensively used to image the crust as well as upper mantle structures. By deconvolving source signatures, different events can be stacked to enhance weak mode conversions (such as P-to-s) near the receiver, which provides constraints on the interface depth and crustal properties. On the other hand, the interpretation of RF is nonunique. It is most sensitive to velocity jumps across interfaces while with loose constraint on the absolute velocities. In cases of a gradual Moho, the mode conversion on RFs is weak and it is difficult to distinguish it from noise. In this study, we introduce a new method of virtual deep seismic sounding (VDSS) using teleseismic reflected phases (SsPmp) near the receiver to image the Moho (Tseng et al., 2009; Yu et al., 2012). The high amplitude of the SsPmp phase bypasses the process of deconvolution and further stacking. In most cases, one high quality event is enough to constrain the Moho depth. We combine RF and VDSS methods to study the crustal structure beneath the Australian continent. For most stations with high quality RF and VDSS, the Moho depths obtained from these two methods are in general consistent. Significant differences are observed, indicating strong lateral variations in Moho geometry. In the Northern Australia, for example, the Moho beneath the Warramunga seismic array offsets ~10 km in depth within less than 50 km laterally. Gradual Moho is detected from RF in the southeastern Australia, and the large amplitude of SsPmp phase of VDSS gives an average crustal thickness. We also show that even for stations underlain by thick sediments, such as FORT in the Eucla Basin where RF fails to detect the Moho, VDSS is still a powerful tool to estimate the Moho depth. Reference: Tseng, T.-L., Chen, W.-P., Nowack, R.L., 2009. Northward thinning of Tibetan crust revealed by virtual seismic profiles. Geophys. Res. Lett. 36(24), L24304 Yu Chun-Quan, Wang-Ping Chen, Jie-Yuan Ning, Kai Tao, Tai-Lin Tseng, Xin-Ding Fang, Yong-Shun Chen, and Robert D. van der Hilst, 2012. Thick Crust beneath the Ordos Plateau: Implications for Instability of the North China Craton. (manuscript under review at Earth Planet. Sci. Lett.).