Yumei He

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.

Static Deformation Due to Shear and Tensile Faults in a Layered Half-Space

Based on the generalized reflection and transmission coefficient matrix method, formulations for surface static displacements in a layered half-space are extended to include tensile and inflation point sources from a point pure shear dis- location source. Equations for calculating internal displacement fields from these sources are also derived. The validity of the formula and precision of the new method are illustrated by comparing the consistency of our results and the analytical solutions given by Okadas (1985, 1992) code in a homogenous half-space and Wang et al.s (2003) numerical solutions in a multilayered half-space. We also study the effect of a layered half-space on the surface displacement created by various finite faults. Several typical velocity structures in reality are selected. For strike-slip, reverse dip- slip, and tensile finite-fault models, the focal depth is very sensitive to the presence of the layered model. The slip displacement is more sensitive to the layered model in the case of the normal dip-slip sources. More numerical tests show that the sen- sitive slip is mainly due to the ultralow-velocity topsoil. For inflations, the source depth and volume change also altered due to the layered model.

Seismological constraints on the crustal structures generated by continental rejuvenation in northeastern China.

Crustal rejuvenation is a key process that has shaped the characteristics of current continental structures and components in tectonic active continental regions. Geological and geochemical observations have provided insights into crustal rejuvenation, although the crustal structural fabrics have not been well constrained. Here, we present a seismic image across the North China Craton (NCC) and Central Asian Orogenic Belt (CAOB) using a velocity structure imaging technique for receiver functions from a dense array. The crustal evolution of the eastern NCC was delineated during the Mesozoic by a dominant low seismic wave velocity with velocity inversion, a relatively shallow Moho discontinuity, and a Moho offset beneath the Tanlu Fault Zone. The imaged structures and geochemical evidence, including changes in the components and ages of continental crusts and significant continental crustal growth during the Mesozoic, provide insight into the rejuvenation processes of the evolving crust in the eastern NCC caused by structural, magmatic and metamorphic processes in an extensional setting. The fossil structural fabric of the convergent boundary in the eastern CAOB indicates that the back-arc action of the Paleo-Pacific Plate subduction did not reach the hinterland of Asia.

Seismic evidence for an 850 km thick low‐velocity structure in the Earth's lowermost mantle beneath Kamchatka

We detect an 850 km thick low-velocity structure in the Earths lowermost mantle beneath Kamchatka surrounded by and overlying a 210 km thick high-velocity D′′ structure. The velocity structure is constrained by modeling the observed anomalously broadened waveforms for seismic shear waves sampling the lowermost mantle recorded at large distances from 90° to 100°. Waveform modeling analyses reveal that the low-velocity anomaly has a stem with a diameter of about 550 km in the lowermost 210 km of the mantle and a cap with a diameter of about 1600 km. The low-velocity structure of the cap decreases from 0% at the top to −1.5% at about 400 km above the core-mantle boundary (CMB) and to −1.2% at 210 km above the CMB. We suggest that the geometrical and velocity features of the low-velocity anomaly indicate that it may represent a localized mantle plume undetected before in the lower mantle.

Source Finiteness and Rupture Propagation Using Higher-Degree Moment Tensors

Higher-degree moment tensor representation of seismic sources is obtained for a horizontally layered homogeneous medium. A Taylor series expansion of Green’s function around a reference source position and time is made, and this enables us to approximate the seismic radiation in both the regional and teleseismic distances through a sequence of terms representing increasingly detailed aspects of the source behavior. Source coefficients and orientation factors of the first- and second-degree moment tensors are obtained. The representation is applied to a unilateral rupture Haskell fault model, and the synthetic seismograms of different models calculated by the higher-degree moment tensors are compared with the theoretical solutions for a propagating source. Our results show that, the representation of higher- degree moment tensors up to degree 2 can describe the response of a moving source well enough, and it’s possible to use the moment series as a tool for calculating seismograms from finite and propagating faults in the forward sense. The computation takes much less time than the method of summing point sources over the fault surface. The information yielded by the higher-degree moments may solve problems such as the fault-plane ambiguity and the space–time evolution of the rupture propagation of an earthquake.