Jiankun He

The subducted slab of Yangtze Continental block beneath the Tethyan orogen in western Yunnan

The western Yunnan area is a natural laboratory with fully developed and best preserved Tethyan orogen in the world. Seismic tomography reveals a slab-like high velocity anomaly down to 250 km beneath the western Yunnan Tethyan orogen, to its west there is a low-velocity column about 300 km wide. In the region from Lancangjiang to Mojiang an obvious low velocity in the lower crust and uppermost mantle overlies on the slab. Synthesizing the available geological and geochemical results, the present paper demonstrates that this slab-like high velocity anomaly is a part of the subducted plate of Yangtze continental segment after the closure of Paleotethys. The collision of India and Eurasia continent starting from 50–60 MaBP might trigger thermal disturbance in the upper mantle and cause the uprising of asthenosphere, in that case the subducted Yangtze plate could be broken off, causing Cenozoic magmatic activities and underplating in the Lancangjiang-Mojiang region.

Study of seismic tomography in Panxi paleorift area of southwestern China

Structural features of the typical continental paleorift in Panxi area are revealed by seismic tomography. (1) In the profile along the minor axis of Panxi paleorift, we found alternating high and low-velocity strips existing at different depths in the crust, presenting itself as a “sandwich” structure. The existence of these high and low-velocity anomaly strips is related to the basal lithology in the rift area. (2) An addition layer with velocity values of 7.1-7.5 km/s and 7.8 km/s exists from the base of lower crust to uppermost mantle and its thickness is about 20 km. Some study results indicate that the addition layer results from the invasion of mantle material. (3) A lens-shaped high-velocity body surrounded by relatively low-velocity material is observed at depths of 110-160 km between Huaping and Huidong in the axis of the paleorift. This is the first time to discover it in the upper mantle of the paleorift. Based on the results of geology, petrology and geochemistry, we infer that the formation of the addition layer and the lens-shaped high-velocity body in the upper mantle are related to the deep geodynamic process of generation, development and termination of the rift. On the one hand, the upwelling of asthenosphere mantle caused partial melting, and then the basaltic magma from the partial melted material further resulted in underplating and formed the crustal addition layer. On the other hand, the high-density content of mineral facies was increased in the residual melted mass of intensely depleted upper mantle, formed by basalt withdrawing. The solid-melt medium in the depleted upper mantle was mainly an accumulation of garnet and peridotite because the heating effect of lithosphere was relatively weakened in the later riftogenesis, so that a lens-shaped high-density and high-velocity zone was produced in the upper mantle. The results indicate that the energy and material exchange between asthenosphere and lithosphere and remarkable underplating would have an important effect on the material state and propagation of seismic wave in the lower crust, crust-mantle interface, asthenosphere and lithosphere. This process possibly is an important mechanism on the growth of continental crust and the evolution of deep mantle.

Rupture process of the M w 7.9 Nepal earthquake April 25, 2015

On April 25, 2015, a magnitude Mw7.9 earthquake occurred in the southern Himalaya, Nepal, at 14:11 local time (UTC 2015-04-25 06:11). Its epicenter was at 28.147°N, 84.708°E with a source depth of 15 km, as determined by the United States Geological Survey (USGS). The earthquake hazard and secondary disasters, including landslides and avalanches, resulted in serious damage to Nepal and surroundings (including Kathmandu and the northern Himalaya of China) and caused huge loss of life and considerable destruction of property. The April 25 earthquake occurred on the active tectonic arc of the Himalaya, which defines the subduction thrust interface between the Indian and the Eurasian plates. Across the Nepalese Himalaya, the northward motion of the Indian Plate relative to the Eurasian Plate is estimated to be about 40 mm/yr, which results from the northward under thrusting of the India Plate beneath the Eurasian Plate. The convergence between India and the Himalaya proceeds at a rate of about 18 mm/yr (Bilham et al., 1997; Bettinelli et al., 2006; Ader et al., 2012). The continental collision of the Indian and the Eurasian plates generates numerous and frequent earthquakes, and makes this area one of the most seismically hazardous regions in the world (Figure 1). The ongoing collision between the Indian and the Eurasian plates has built the Tibetan Plateau (the highest plateau region on Earth) and has induced the imbricate thrust belt as the plate boundary. From south to north, the major faults comprise the Main Frontal Thrust fault (MFT), the Main Boundary Thrust fault (MBT), and the Main Central Thrust fault (MCT). In deeper depth, the fold-thrust belt is connected with the Main Himalaya Thrust fault (MHT) with a low dip angle (Cattin and Avouac, 2000; Lavé and Avouac, 2000; Bettinelli et al., 2006). Research of historical earthquakes and GPS measurements has revealed the high-risk potential of generating great earthquakes in the Nepalese Himalaya (Bilham et al., 1998; Ambraseys and Douglas, 2004; Ader et al., 2012; Sapkota et al., 2013). Based on the convergence rate between the Indian and Eurasian Plates, and on historical seismicity, Ader et al. (2012) warned of the high possibility of the occurrence of a large earthquake in the central Nepal seismic gap; the occurrence of the April 25 earthquake confirmed this concern. Following the earthquake, preliminary results of the source mechanism and rupture process for this earthquake were prepared using the fast source inverse approach with real-time far-field seismograms (http://www.itpcas.ac.cn/ xwzx/zhxw/201504/t20150426_4344080.html). Here, a listric finite fault model is constructed to simulate the earthquake fault according to the tectonic setting. A new source process model is estimated by joint inversion of far-field seismograms and GPS coseismic displacements. The inverted results might help both in disaster mitigation and in research into seismotectonic and dynamic simulations of this region.

Three-Dimensional Velocity Images of the Crust and Upper Mantle beneath the North-South Zone in China

We have determined the three-dimensional P-wave velocity structure beneath the north-south tectonic belt between Tibet and Eastern China by simulta- neously inverting local, regional, and teleseismic data. Our data set is composed of 45,028 P-wave arrival times from 602 local and regional earthquakes and 985 travel times from 102 teleseismic events. With the LSQR (sparse linear equations and least squares) algorithm, the P-wave velocity perturbations were estimated by the simul- taneous inversion of hypocenters and medium parameters from the surface to a depth of 200 km. We tested the stability and the resolution of our inverted results with a checkerboard test and found that the models are well resolved up to a depth of about 50 km for most parts of the studied region. Results show that the north-south tectonic belt is characterized by a significant lateral heterogeneity in velocity both in the crust and in the upper mantle. Correlating these velocity images with the main tectonic features, we find that (1) the shallow velocity distribution above 3 km is consistent with the topographic features and the basin distribution; (2) the middle-lower crustal velocities from 20 km to 50 km characterize a mechanically weak north-south tec- tonic belt, because it bears a relatively lower-velocity perturbation over a large re- gion; (3) the upper mantle velocities from 85 km to 120 km delineate the eastern Tibetan boundary, but changes in some subzones may reflect the effects of several tectonic events, including paleorifting, the Cenozoic convergence between Tibet and southeastern China and other tectonic episodes.

Lower Velocities beneath the Taihang Mountains, Northeastern China

We used regional P -wave arrival times to invert for 3D velocity structures beneath the Taihang Mountains, which are bordered by the intensely active zones of the Shanxi rift to the west and the eastern plain basin to the east in the North China Block. P -wave velocities show that low velocities (−1.0 to −3.0%) are focused at depths of ∼15–20 km beneath the Taihang Mountains, similar to those beneath the active zones of the Shanxi rift and the eastern plain basin, If the lower velocity is representative of weakness in the underlying crust, there our results indicate that the Taihang Mountains might be potentially active, because the structure patterns are similar to their bounding active zones. Future seismic research, therefore, should strongly focus on the Taihang Mountains.

Channel flow of the lower crust and its relation to large-scale tectonic geomorphology of the eastern Tibetan Plateau

The Tibetan Plateau is a large-scale tectonic geomorphologic unit formed by the interactions of plates. It has been commonly believed that convective removal of the thickened Tibetan lithosphere, or lateral flow of the lower crust beneath the Tibetan plateau plays a crucial role in the formation of the large-scale tectonic geomorphologic features. Recent geological and geophysical observations have provided important evidence in support of the lower crustal channel flow model. However, it remains unclear as how the geometry of lower crustal channel and the lateral variation of crustal rheology within the lower crust channel may have affected spatio-temporal evolution of the tectonic geomorphologic unit of the Tibetan Plateau. Here, we use numerical methods to explore the mechanical relations between the lower crustal channel flow and the tectonic geomorphologic formation around the eastern Tibetan plateau, by deriving a series of governing equations from fluid mechanics theory. From numerous tests, our results show that the viscosity of the channeled lower crust is about (1−5)×1018 to (1−4)×1020 Pa s (Pa·s) beneath the margin of the eastern Tibetan Plateau, and increases to about 1022 Pa s beneath the Sichuan Basin and the southern region of Yunnan Province. Numerical tests also indicate that if channel flows of the lower crust exist, the horizontal propagation and the vertical uplifting rate of the eastern Tibetan Plateau margin could be accelerated with the time. Thus, the present results could be useful to constrain the rheological structure of the crust beneath the eastern Tibetan plateau, and to understand the possible mechanics of rapid uplift of the eastern Tibetan Plateau margin, especially since its occurrence at 8Ma as revealed by numerous geological observations.

Two-dimensional finite element modeling on the crustal shortening and the surface erosion-sedimentation process across northern piedmont of the Tianshan Mountains

The Tianshan Mountains, located in the northwestern China, are bounded by the Tarim Basin to south and the Junggar Basin to north. In the north piedmont of this mountain range, ongoing thrusting and folding forms a set of roughly parallel anticlines. Geological observations predicted that averaged over last ~1 Ma time scale, the shortening rates of these anticlines are about 2.1–5.5 mm/a; However by averaged over about 10±2 kyr, their shortening rates reduce to merely about 1.25±0.5 mm/a. The slow shortening of the anticlines in the last ~10±2 kyr is coarsely concurrent in time with the last global deglaciation. Here, we use a two-dimensional finite element model to explore crustal deformation across north piedmont of the Tianshan Mountains under various erosion-sedimentation conditions that are assumed to represent the climate-controlled surface process. Numerical experiments show that with a relatively weak erosion-sedimentation strength, the crustal shortening is accommodated mainly by north piedmont of the Tianshan Mountains, similar to the high shortening rate of anticlines averaged over the last ~1 Ma. By increasing erosion-sedimentation strength, the resultant crustal shortening is transformed gradually toward the Tianshan Mountains, resulting in the shortening rate in its north piedmont being decelerated to what is observed as averaged over the last ~10±2 kyr. This result suggests that erosion and sedimentation could play an important role mechanically on strain localization across an intra-continent active tectonic belt. Hence, if the climate change around the last global deglaciation could be simply representative to the enhancement of surface erosion and sedimentation across the pre-existed Tianshan Mountains and its foreland, our models indicate that the observed shortening-rate variations averaged over ~1 Ma and ~10±2 kyr time scales around north piedmont of the Tianshan Mountains should be resulted from climate changes.

Preliminary result for the rupture process of Nov.13, 2017, Mw7.3 earthquake at Iran‐Iraq border

30◦ < ∆ < 90◦ At UTC 2017-11-12 18:18:17, an Mw7.3 earthquake occurred at the border between Iran and Iraq (location 34.886°N, 45.941°E, depth 23 km according to USGS). We carried out focal mechanism and rupture process studies with the data from IRIS data center, using 26 far-field P-waveforms and 25 SH-waveforms with high S/N ratio and relatively even azimuth coverage (epicentral distance ) in a point source model to invert for the focal mechanism solution; the result (Figure1) was used to construct a finite fault model for rupture process inversion (Yao and Ji,1997; Wang et al., 2008), resulting in a preliminary slip distribution of this earthquake (Figures 2-4). The calculated seismic moment is 1.1×1020 N·m, Mw=7.3. The maximum slip is about 700 cm.