“Noise interferometry from crust to core”
Xiaodong SONG
University of Illinois at Urbana-Champaign and Wuhan University
Seismic imaging of the Earth’s interior had relied almost exclusively on the illumination fromenergetic sources, such as earthquakes or controlled sources. However, the ability to image theearth with noise, including ambient noise from the Oceans or the reverberations fromearthquakes, has revolutionized the field of seismology in the recent decade. The basic idea isthat a random field contains coherent signals traveling between the receivers, which can bestacked and amplified while all other arrivals are canceled out in the cross-correlation or theautocorrelation. In particular, surface waves have been found to be most easily retrievable fromnoise correlations. The extraction of body waves is much more difficult, but recent successes
have allowed imaging of the deep Earth. Here I discuss our recent efforts in imaging the Asianlithosphere, particularly the Tibetan Plateau, and the Earth’s inner core using noiseinterferometry methods.
Models for the growth and deformation of the Tibetan Plateau have been a subject of great
debate for decades. A critical information is structure at depth. We shed lights on the debatewith high-resolution imaging from surface-wave tomography and joint inversions. Beneath theTibetan Plateau (TP), pronounced S-wave mid-crustal low velocity zone (LVZ) is observed underthe margins (Himalaya and Qiangtang blocks), but less so under its interior (the Lhasa Block).
The mantle lithosphere beneath the TP shows strong lateral variation with evidence for the“tearing” of Indian lithosphere beneath two zones in the central and western TP. The mantlevariations correlation with crustal velocity variations as well as Vp/Vs ratios. The observations
support the existence of a proto-Tibetan Plateau core and the outward growth of the margins at
a later stage after the India-Eurasian collision. The crust and mantle lithosphere act mostly as acoherent unit. The LVZ in northern TP shows strong correlation with the region of the midMiocene to Quaternary potassic magmatism, suggesting that delamination of lithosphere may
have played an important role in the rise of the TP.
The Earth’s inner core poses strong seismic anisotropy, which shows a complex pattern laterally
and with depth. All the previous inner core anisotropy models have assumed a cylindricalanisotropy with the symmetry axis parallel (or nearly parallel) to the Earth’s spin axis. However,
we have recently found that the fast axis in the inner part of the inner core is close to theequator from inner-core waves extracted from earthquake coda. We obtained inner corephases, PKIIKP2 and PKIKP2 (round-trip phases between the station and its antipode that passes
straight through the center of the Earth and that is reflected from the inner core boundary,respectively), from stackings of autocorrelations of the coda of large earthquakes at seismicstation clusters around the world. We observed large variation of up to 10 s along equatorial
paths in the differential travel times PKIIKP2 – PKIKP2. The observations can be explained by acylindrical anisotropy in the inner-inner core (IIC) (with a radius of slightly less than half theinner core radius) that has a fast axis aligned near the equator and a cylindrical anisotropy in theouter-inner core (OIC) that has a fast axis along the north-south direction. More recent
observations confirm the early results. The results may suggest a major shift in the tectonic
regime of the inner-core formation and growth.