Multiparameter adjoint tomography of the crust and upper mantle beneath East Asia: 1. Model construction and comparisons

Abstract

We present a 3-D radially anisotropic model of the crust and mantle beneath East Asia down to 900 km depth. Adjoint tomography based on a spectral element method is applied to a phenomenal data set comprising 1.7 million frequency-dependent traveltime measurements from waveforms of 227 earthquakes recorded by 1869 stations. Compressional wave speeds are independently constrained and simultaneously inverted along with shear wave speeds (V SH and V SV ) using the same waveform data set with comparable resolution. After 20 iterations, the new model (named EARA2014) exhibits sharp and detailed wave speed anomalies with improved correlations with surface tectonic units compared to previous models. In the upper 100 km, high wave speed (high-V) anomalies correlate very well with the Junggar and Tarim Basins, the Ordos Block, and the Yangtze Platform, while strong low wave speed (low-V) anomalies coincide with the Qiangtang Block, the Songpan Ganzi Fold Belt, the Chuandian Block, the Altay-Sayan Mountain Range, and the back-arc basins along the Pacific and Philippine Sea Plate margins. At greater depths, narrow high-V anomalies correspond to major subduction zones and broad high-V anomalies to cratonic roots in the upper mantle and fragmented slabs in the mantle transition zone. In particular, EARA2014 reveals a strong high-V structure beneath Tibet, appearing below 100 km depth and extending to the bottom of the mantle transition zone, and laterally spanning across the Lhasa and Qiangtang Blocks. In this paper we emphasize technical aspects of the model construction and provide a general discussion through comparisons.

The imprint of crustal density heterogeneities on regional seismic wave propagation

“…Gaite et al, 2015), or East Asia (e.g. Chen et al, 2015). This suggests that the rms variations of S velocity variations with 200 km lateral and 20 km vertical correlation length are representative of real crustal structure at least in some regions.…”

The latest geodynamics in Central Asia: primary and secondary mantle melting anomalies in the context of orogenesis, rifting, and lithospheric plate motions and interactions

“…Часть моделей [Huang, Zhao, 2006;Li et al, 2006;Chen et al, 2015b] построена с целью полу-чения обзорной картины под большой террито-рией Азии с глубинами, захватывающими нижнюю мантию. В таких генерализованных моделях мелкого масштаба связь кайнозойских вулка-нических полей с низкоскоростными аномалиями не очевидна.…”

“…Сейсмические скоростные модели, построенные разными авторами для территории Центральной Монголии Mordvinova et al, 2007;Chen et al, 2015aChen et al, , 2015b, свидетель-ствуют о низких скоростях под Хангаем преимуще-ственно в диапазоне глубин от 40 до 200 км. Ниж-немантийное происхождение расплавной анома-лии непосредственно под этим горным сооружени-ем сомнительно.…”

Adjoint tomography of the crust and upper mantle structure beneath the Kanto region using broadband seismograms

“…For each earthquake, numerical simulations need to be conducted only twice: once for the construction of the forward and adjoint wavefields and once to obtain the gradients of the misfit function. Based on a combination of the spectral-element and adjoint methods, various wave speed models have been obtained in previous studies, including models of the Southern California crust (Tape et al 2007(Tape et al , 2010, the western Pyrenees (Wang et al 2016), the Australasian upper mantle (Fichtner et al 2009), the European upper mantle (Zhu et al 2012(Zhu et al , 2015Zhu and Tromp 2013), the East Asian crust and upper mantle (Chen et al 2015), and the crust and upper mantle around the Japanese archipelago (Simutė et al 2016). Bozdag et al (2016) conducted the first study in which a model of the entire crust and mantle of the earth was constructed using the iterative fullwaveform inversion technique.…”