Precursors to SS result from underside reflections at elasticity contrasts (roughly) beneath the midpoint of the earthquake source and the receiver. The differential travel time between the SS phase and its precursors has been widely used for mapping upper mantle discontinuities, especially the 410- and 660-km discontinuities ("410" and "660").
With a wave packet-based array processing technique (curvelet transform), we improve the signal-to-noise ratio of SS precursors and remove interfering phases, so that precursors can be identified and measured over a larger distance range (Yu et al., 2018, JGR).
We measure the precursor amplitudes relative to the surface reflections (that is, S410S/SS and S660S/SS) and remove effects of geometrical spreading, intrinsic attenuation, mantle heterogeneity, and interface topography. By matching the corrected amplitude ratios as a function of distances with theoretical predictions (similar to AVO analysis in exploration seismology), we estimate the density and wavespeed contrasts across 410 and 660. We further applied thermodynamic modeling to calculate density and velocity profiles along a range of mantle temperatures for several representative mantle compositions. By comparing our observed density and wavespeed contrasts across transition zone discontinuities with those from thermodynamic modeling, we estimate mantle composition near 410 and 660 (Yu et al., 2018, Nat. Commun.).
VDSS utilizes the prominent SsPmp phase, which undergoes S-to-P conversion at the free surface and P-to-P total reflection off the Moho. The differential timing between the SsPmp phase and the direct S phase, Ss, is closely related to the crustal thickness H and average crustal P-wave velocity VP.
where p is the incident S-wave ray parameter (horizontal slowness).
For the estimation of crustal P wave speed and thickness, and particularly buoyancy, VDSS has several distinct advantages over other methods. First, VDSS returns a robust signal from the crust-mantle boundary even if the Moho is complicated or transitional in nature. Second, VDSS is not particularly susceptible to signal-generated noise such as scatterings from thick sediments or intracrustal discontinuities. Last and most important, as will be shown here, VDSS can be used to put tight constraints on crustal buoyancy in spite of an inherent trade-off between overall crustal thickness and P wave speed (Yu et al., 2016, JGR).
Original applications of VDSS rely on deep earthquakes as sources of illumination to circumvent strong, near-source scattering (e.g. depth phases) and are, therefore, limited by the uneven distribution of deep seismicity. To extend both the applicability and the quality of VDSS, we developed a method to effectively remove earthquake source signatures. It involves two steps. First, based on analyses of particle motion, we separate 'pseudo-P' and 'pseudo-S' wave trains from the vertical and the radial component of ground motion. The latter is then used as the appropriate reference time-series for the deconvolution of the vertical and the radial component of ground motion.
The method is verified from a series of synthetic tests, and is further validated using data recorded by the Hi-CLIMB array from both deep and shallow earthquakes. Since shallow earthquakes are much more abundant (and geographically distributed more widely) than deep seismicity, the approach presented here greatly extends the applicability of VDSS, including many geologically important regions where crustal isostasy and dynamic topography are yet to be constrained (Yu et al., 2013, GJI).
In the late 1960s, plate tectonics was introduced by geophysicists as an internally consistent theory of relative motions of vast rigid plates. While oceanic plates with narrow plate boundaries fit this model remarkably well, numerous examples reveal dispersive continental deformations over thousands of kilometers.
Various models have been proposed to describe intracontinental deformations. These models can generally be divided into two categories: (1) continuum models, and (2) microplate or block models. Continuum models assume that the continental lithosphere deforms as continuous ductile material. They aim to address the problems of both kinematic description of lithospheric deformation and its underlining geodynamic driving forces. Microplate or block models, however, only focus on the kinematic aspects of crustal deformation. They divide the brittle/elastic part of the crust into many blocks and study their relative movements. There is still no consensus on which approach is better.
The Tien shan is the most prominent orogenic belt in Central Asia. Widely spread thrust faulting and significant N-S shortening suggest that active intraplate deformation is still going on. In the last few decades, geodetic, geological, and geophysical surveys have accumulated abundant field dataset. Thus it is an ideal place to study the kinematic and dynamic processes of intracontinental deformation.
GPS Measurements, Data fitting, and Shortening rates
We relocate earthquakes using double-difference method. Focal mechanisms are determined using P-wave polarities.
We relocated 1376 earthquakes with Ms>3.5 and obtained 83 well-determined earthquake focal mechanisms with Ms>4.0 in the Wenchuan earthquake sequence. The results show some distinct features.
i) The compressional axis (P-axis) is subhorizontal for almost all large-magnitude earthquakes.
ii) The major fault is dominated by thrust events but there are two exceptions. Around Xiaoyudong in the southern portion and Qingchuan in the northern portion, focal mechanisms are dominated by strike-slip type. Combined with their spatial distribution, we suggest that the Xiaoyudong cluster is located on a left-lateral NW-SE trending fault, and the Qingchuan cluster is located on a right-lateral NW-SW trending fault.
iii) There are two groups of thrust-type focal mechanisms. The first group, including to the main shock, has a P-axis perpendicular to the strike of the main fault. The second group, on the other hand, has a P-axis parallel to the strike of the main fault