The structure of the crust and uppermost mantle beneath South China from ambient noise and earthquake tomography

Abstract

SUMMARY Two years of continuous recordings of ambient seismic noise observed at 354 stations in South China from 2009 to 2010 are used to estimate Rayleigh wave group and phase velocity maps from 6 to 40 s period. These results are merged with Rayleigh wave phase velocity maps from 25 to 70 s period derived from earthquakes in the same time frame. Eikonal tomography generates the dispersion maps, which, by Monte–Carlo inversion, are used to estimate a 3‐D Vsv model of the crust and upper mantle down to a depth of 150 km across all of South China with attendant uncertainties. A clear image emerges of the ‘West Yangtze Block’, a region of the western Yangtze Craton characterized by relatively thick crust (∼40 km) overlying a seismic mantle lithosphere that extends to at least 150 km that may have been the nucleus for the formation of the Yangtze craton in the Archean and may present a present‐day obstacle to the eastward expansion of Tibet. The West Yangtze Block contrasts with the thinner crust (∼30 km) and mantle lithosphere (∼70–80 km) beneath the eastern Yangtze Craton and South China Foldbelt. These observed differences are consistent with processes associated with flat slab subduction in the Mesozoic that may have eroded the lithosphere of the eastern Yangtze Craton and the South China Foldbelt.

Two-receiver measurements of phase velocity: cross-validation of ambient-noise and earthquake-based observations

“…Average faster velocities from the EQM in comparison to the ANM have also been reported by Zhou et al (2012) curves) -all taken around 30 s. Without stating whether the discrepancy is positive or negative, Köhler et al (2012) also find residuals of up to 1.5 per cent at 25 s. To our knowledge, there are no studies that show faster measurements from the ANM, which leads us to believe that this discrepancy is systematic. There are several possible reasons for the bias between ambient-noise and earthquake results: In Fig.…”

“…In this approach, the zero crossings of the real part of the cross-correlation spectrum are evaluated to derive the phase-velocity dispersion. Importantly, this means that the Green's function's amplitude does not have to be reconstructed, which is the case when phase is to be determined by analysis of time-domain traces (Bensen et al 2007;Lin et al 2008;Zhou et al 2012;Verbeke et al 2012). We verify that our algorithm can successfully measure the phase velocity of both Love and Rayleigh waves from ambient noise.…”

“…Several studies similarly compare Rayleigh phase-velocity maps: Zhou et al (2012) report maximum phase-velocity differences of 0.2 km s −1 , but with a smaller mean of −0.012 km s −1 . Yang & Ritzwoller (2008) find deviations of 0.15 km s −1 .…”

“…A similar approach was followed before by Harmon et al (2008), Yang et al (2008a,b), Yao et al (2008), Ritzwoller et al (2011), Köhler et al (2012), Zhou et al (2012) and Shen et al (2013). Yao et al (2008) present a direct comparison of phase-velocity dispersion curves.…”

“…Udías 1999), making ambient-noise studies ideal to assess the structure of the crust and upper mantle. Recordings of earthquake-generated teleseismic surface waves are dominated by lowerfrequency signal and thus sensitive to larger depths; their combination with ambient-noise data results in a data set that is sensitive to a broader depth range than ever previously achieved (Yao et al 2006(Yao et al , 2008Ritzwoller et al 2011;Köhler et al 2012;Zhou et al 2012). We only treat surface-wave phase velocity in this work, which has the advantage that group velocity can be derived from it, while the opposite is not possible.…”