Yinshuang Ai

A complex 660 km discontinuity beneath northeast China

Abstract We present a detailed seismic study of the 660 km discontinuity beneath northeast China using the receiver function technique. We use seismic data collected from 24 broadband stations in northeast China. Analysis of these data shows that the 660 km discontinuity is locally depressed in the region from longitude 128.0°E to 130.5°E and latitude 40.0°N to 44.0°N, and then it splits into multiple discontinuities in the surrounding regions. The complexity of the 660 km discontinuity beneath the subduction zones is probably attributable to the interaction between the upper mantle and subducted slab. We speculate that the flattening of the subducted lithosphere near the bottom of the upper mantle causes the multiple discontinuity structure, and that the slab penetrating the 660 km discontinuity into the lower mantle causes the narrow 660 km depression area.

Discontinuity structure of the mantle transition zone beneath the North China Craton from receiver function migration

[1] A better understanding of the significant Phanerozoic tectonic reactivation and destruction of the North China Craton (NCC) demands a detailed knowledge of the deep structural features of the region. We applied the wave equation-based poststack migration technique to a combined receiver function data set from more than 250 broadband seismic stations to construct the structural image of the mantle transition zone beneath the NCC. Our imaging results reveal a relatively simple and flat 410-km discontinuity but a structurally complicated 660-km discontinuity beneath the region. Double discontinuities and a 30-km depression of the 660-km discontinuity are observed locally in the southern part of the eastern NCC, in contrast to the smoothly varying structure to the north and in the central and western parts of the craton. Distinctly rapid variations in both the 660-km discontinuity structure and mantle transition zone thickness were found across the northsouth gravity lineament (NSGL) near the boundary between the eastern and central NCC, which probably reflects different thermal and probably chemical properties on the two sides of the NSGL. These differences are possibly associated with the Pacific slab, which is imaged tomographically as a flat-lying structure in the mantle transition zone under the region east of the NSGL. The structural variation in the deep upper mantle appears to coincide with the sudden changes in surface topography, gravity field, and crustal and lithospheric structures as well, indicating that the two domains may have tectonically deformed differently throughout the whole upper mantle during the Phanerozoic cratonic destruction. The mantle transition zone on the eastern side of the NSGL is up to 30 40 km thicker than the global average; this thickness and the complex structure of the 660-km discontinuity in this region may reflect the strong influence that the deep subduction and stagnancy of the Pacific slab, and its possible sporadic penetration into the lower mantle, have had on mantle dynamics and lithospheric reactivation in the eastern NCC since the Mesozoic time. On the other hand, the less variable structure and normal-to-thin mantle transition zone imaged beneath the central and western NCC may indicate that the IndiaEurasia collision has had a relatively weak effect on the Cenozoic tectonics of these regions.

Small scale hot upwelling near the North Yellow Sea of eastern China

[1] In order to image the structure of the upper-mantle discontinuity beneath Eastern China, we have applied a common conversion point (CCP) stacking method of receiver function (RF). Both the 410-km and the 660-km discontinuities (hereafter called the 410 and the 660) clearly show continuous positive phases along the selected profile. The 410 shows depression, whereas the 660 shows uplift, in the eastern section of the study profile. The transition zone (TZ) to the west of longitude 122 is thicker than the global average, though the TZ is thinner to the east of that longitude. The thinnest part of the TZ, with 10� 15 km of thinning (an increase in temperature of up to 100C), is located at the North Yellow Sea. We suggest that either the small-scale convection associated with the deep penetration of the sinking slab into the lower mantle, or a small plume from lower mantle, has generated hot upwelling in this region. Citation: Ai, Y., T. Zheng, W. Xu, and Q. Li (2008), Small scale hot upwelling near the North Yellow Sea of eastern China, Geophys. Res. Lett., 35, L20305, doi:10.1029/ 2008GL035269.

The 16 September 1994 Taiwan Strait earthquake: a simple rupture event starting as a break of asperity

Abstract In this paper we give a detailed study of the 16 September 1994 Taiwan Strait earthquake using a broad-band body-waveform inversion. An adaptive hybrid global search algorithm was used to solve the nonlinear problems of inverting the spatial distributions of slip amplitude, rake, slip duration and rupture time on a finite fault. The best-fitting solution reveals that the largest slip of 14 m took place in the hypocentre region, in which the rise time is short. The slip amplitude decreases and the rise time increases from the initiation point outward. A suggested mechanism is that the rupture initiated as a breaking of a strong asperity with low dynamic friction and was arrested by large friction around the asperity. The hypocentre is a region of high frequency radiation. We examined the sensitivity by comparing the inversion solutions utilizing varied parameterizations and constraints. These numerical tests show that the major features of the rupture model are better resolved.

A seismic model for crustal structure in North China Craton

We present a digital crustal model in North China Craton (NCC). The construction of crustal model is based on digitization of original seismic sounding profiles, and new results of three‐dimensional structure images of receiver functions. The crustal model includes seismic velocity and thickness of crustal layers. The depths to Moho indicate a thinning crust ~30 km in the east areas and a general westward deepening to more than 40 km in the west. The P wave velocity varies from 2.0 to 5.6 km/s in the sedimentary cover, from 5.8 to 6.4 km/s in the upper crust, and from 6.5 to 7.0 km/s in the lower crust. By analyzing regional trends in crustal structure and links to tectonic evolution illustrated by typical profiles, we conclude that: (1) The delimited area by the shallowing Moho in the eastern NCC represents the spatial range of the craton destruction. The present structure of the eastern NCC crust retains the tectonic information about craton destruction by extension and magmatism; (2) The tectonic activities of the craton destruction have modified the crustal structure of the convergence boundaries at the northern and southern margin of the NCC; (3) The Ordos terrene may represent a relatively stable tectonic feature in the NCC, but with the tectonic remnant of the continental collision during the assembly of the NCC in the north‐east area and the response to the lateral expansion of the Tibetan Plateau during the Cenozoic in the south‐west.