Yunnan Area Research Articles

Virtual ChuanDian — A parallel numerical modeling of Sichuan-Yunnan regional strong earthquake activities: Model construction and parallel simulation

Reasonable use of mathematical modeling and numerical simulation of regional earthquake activities has attracted more and more attentions in realistic seismicity studies in recent years. The inherent nature of their attendant multi-scale properties, from either the temporal or the spatial direction, necessitates the use of the cutting-edge massive parallel data processing techniques. Our study here is focused on the model construction and numerical simulation of the regional stress evolution after large earthquake sequences along the fault zones in Sichuan and Yunnan areas, using large-scale parallel finite element approximations with tens of millions of unstructured meshes on the distributed systems. The objective of our study is to provide a state-of-the-art model and a large-scale numerical simulation platform, on which estimation of the risk of large earthquake triggering sequences can be carried out in a middle or long-term time scale with realistic data and constrains inputted from field observation and geological survey.

Crustal and upper mantle velocity structure in Yunnan, Southwest China

Four wide-angle seismic profiles acquired in analogue form in the 1980s from Yunnan, Southwest China, are re-digitized and re-interpreted for a detailed P-wave velocity structure of the crust and lithospheric mantle. The results reveal that the crust can be divided roughly into upper, middle and lower crusts with bottom depths of 17–25, 26–33, and 33–56 km, respectively. The Moho depth varies from 56 km in the north of Yunnan to about 33 km in the southern part, and from 38–40 km in the west of Yunnan to 42 km in the eastern zone. The velocity anisotropy in the lower crust is of 4.4%, in contrast with the very low values estimated for the upper-middle crust, which are close to 1.6%. This suggests that the surface deformation in the Yunnan area is decoupled from the lower crustal flow. The seismic velocity discrepancy along different azimuths in the lower crust and upper mantle beneath the profiling cross points leads to a flow regime in the north-south direction that agrees with contemporary tectonic deformation, and also provides evidence for the rotation of the lower crust-upper mantle medium around the east Himalayan foreland. High-velocity anomalies in the lower crust might be explained as intrusions of mantle material or remaining patches of ancient oceanic crust.

Life crises on land across the Permian–Triassic boundary in South China

The western Guizhou and eastern Yunnan area of southwest China commands a unique and significant position globally in the study of Permian–Triassic boundary (PTB) events as it contains well and continuously exposed PTB sections of marine, non-marine and marginal-marine origin in the same area. By using a range of high-resolution stratigraphic methods including biostratigraphy, eventostratigraphy, chronostratigraphy and chemostratigraphy, not only are the non-marine PTB sections correlated with their marine counterparts in the study area with high-resolution, the non-marine PTB sections of the study area can also be aligned with the PTB Global Stratotype Section and Point (GSSP) at Meishan in eastern China. Plant megafossils (“megaplants”) in the study area indicate a major loss in abundance and diversity across the PTB, and no coal beds and/or seams have been found in the non-marine Lower Triassic although they are very common in the non-marine Upper Permian. The megaplants, however, did not disappear consistently across the whole area, with some elements of the Late Permian Cathaysian Gigantopteris flora surviving the PTB mass extinction and locally even extending up to the Lower Triassic. Palynomorphs exhibit a similar temporal pattern characterized by a protracted stepwise decrease from fern-dominated spores in the Late Permian to pteridosperm and gymnosperm-dominated pollen in the Early Triassic, which was however punctuated by an accelerated loss in both abundance and diversity across the PTB. Contemporaneous with the PTB crisis in the study area was the peculiar prevalence and dominance of some fungi and/or algae species. The temporal patterns of megaplants and palynomorphs across the PTB in the study area are consistent with the regional trends of plant changes in South China, which also show a long-term decrease in species diversity from the Late Permian Wuchiapingian through the Changhsingian to the earliest Triassic, with about 48% and 77% losses of species occurring respectively in the end-Wuchiapingian and end-Changhsingian. Such consistent patterns, at both local and regional scales, contradict the hypothesis of a regional isochronous extinction of vegetation across the PTB, and hence call into question the notion that the end-Permian mass extinction was a one-hit disaster. Instead, the data from the study area and South China appears more consistent with a scenario that invokes climate change as the main driver for the observed land vegetation changes across the PTB in South China.

Application of ultrasonics to enhance the efficiency of cleaning Thelephora ganbajun

The Thelephora ganbajun is a rare edible mushroom which is distributed only in Yunnan area of China. It grows generally in the red soil under Burma pine. Therefore, there is a lot of dirt on the mushroom. The dirt adheres and embeds on the root firmly and deeply, thus to clean them is very difficult. The paper reports an efficient method to clean T. ganbajun using ultrasonic cleaner. The optimum parameters for ultrasonic clean are: detergent: 0.67%, sample:water = 1:100, T = 21.5–25.0 °C, and cleaning time: 20 min. Under the optimum condition, the cleaning efficiency can be increased 55.5% than that without sonication. Higher cleaning efficiency and more edible parts of the rare mushroom was got.

Structure and Significance of S-wave Velocity and Poisson’s Ratio in the Crust beneath the Eastern Side of the Qinghai-Tibet Plateau

The receiver functions of body waves of distant earthquakes obtained for the regions beneath 41 digital stations (Lhasa and GANZ in Tibet, Mandalay and Rangoon in Myanmar, SHIO in India, CHTO in Thailand, and station network in Sichuan and Yunnan) were used to invert for S-wave structure in the crust and upper mantle in Sichuan, Yunnan, and their surrounding areas. Meanwhile the distribution characteristics of the Poisson’s ration and the crustal thickness in Sichuan and Yunnan areas were also obtained. Results indicate that the depth of Moho beneath the eastern side of Qinghai-Tibetan plateau varies strikingly. It is obvious that the greatest changes in crustal thickness occur in a north-south direction. The crustal thickness decreases from north to south, being as thick as 70 km in eastern Tibet, the northern portion of our area of interest, and less than 30 km in Chaing Mai and Rangoon, the southern portion of our area. There are, however, exceptions regarding the trend. The thickness exhibits an east-west variation trend in the area from Ma’erkong-Kongding in Sichuan to Lijiang in Yunnan. In general the Jinpingshan-Longmenshan fault and Anninghe fault can be taken as the boundaries of this exception area. The thickness in Kongding in the west is 68 km, while it is only 39 km in Yongchuan in the east. Moreover the Poisson’s ratio values in the blocks of central Sichuan and Sichuan-Yunnan Diamond are high, and a low velocity layer in the crust of this area can be obviously detected. The distribution characteristics of the high Poisson’s ratio and the low velocity of the crust in this block correspond to the tectonic structure, being in contrast with the surrounding areas. Combining with the distribution features of the modern tectonic stress field, it is deduced that the Sichuan-Yunnan area is probably the channel through which the materials of the lithosphere flow eastward.

Three-dimensional mechanical modeling of large-scale crustal deformation in China constrained by the GPS velocity field

We present a quantitative model for the crustal movement in China with respect to the Eurasia plate by using the three-dimensional finite element code ADELI. The model consists of an elastoplastic upper lithosphere and a viscoelastic lower lithosphere. The lithosphere is supported by the hydrostatic pressure at its base. The India–Eurasia collision is modeled as a velocity boundary condition. Ten large-scale faults are introduced as Coulomb-type frictional zones in the modeling. The values for the root mean square (RMS) of the east and north velocity components differences (RMS(Ue) and RMS(Un)), which are between the observation and the prediction, are regarded as the measurements to evaluate our simulations. We model the long-term crustal deformation in China by adjusting the faults frictions ranged from 0.01 to 0.5 and considering the effects resulted from lithospheric viscosity variation and topographic loading. Our results suggest most of the large-scale faults frictions are not larger than 0.1, which is consistent with other large-scale faults such as the North Anatolian fault (Provost, A.S., Chery, J., Hassani, R., 2003. Three-dimensional mechanical modeling of the GPS velocity field along the North Anatolian fault. Earth Planet. Sci. Lett. 209, 361–377) and the San Andreas fault (Mount, V.S., Suppe, J., 1987. State of stress near the San Andreas fault: implications for wrench tectonics. Geology, 15, 1143–1146). Further, we examine the effects on the long-term crustal deformation in China of three causes: the large-scale faults, lithospheric viscosity structure and topographic loading. Results indicate that the lithospheric viscosity structure and the topographic loading have important influences on the crustal deformation in China, while the influences caused by the large-scale faults are small. Although our simulations satisfactorily reproduce the general picture of crustal movement in China, there is a poor agreement between the model and the observed GPS velocity field in Sichuan–Yunnan area. It may result from our simple models such as that the faults are all vertical from model surface to bottom and that the effects caused by the subduction of Burma slab are neglected.

Application of apparent stress in earthquake prediction

On the basis of the assumption that ω 2 model accords with source displacement spectra of moderate-small shocks, we calculate the apparent stress values of 1 020 moderate-small shocks recorded by the Digital Seismic Network of Yunnan Province by using the low-frequency flat level and corner frequencies. The results show that the apparent stress is of good significance in earthquake prediction. The moderate-small shock with apparent stress larger than 0.9 MPa can be used as a referential index to predict moderate-strong earthquakes in Yunnan area. And its relevant predictability evaluation (R value) has a high confidence level.

Subarea characteristics of earthquake types in Yunnan

Studies on the earthquake sequences and the source mechanisms of the strong earthquakes show that Yunnan has more obvious subarea characteristics of earthquake type. Strike-slip seismic fault and mainshock-aftershock earthquake sequences are dominant in whole Yunnan area. Considering the ratio of non strike-slip faults and non mainshock-aftershock, Yunnan area can be divided into four subareas with different characteristics, which are strike-slip mainshock-aftershock in central Yunnan (A1), incline-slip swarm in northwestern Yunnan (A2), strike-slip double shocks in western Yunnan (B1) and quasi-strike-slip mainshock-aftershock in southwestern Yunnan (B2), respectively.

Precursory specialties of apparent stresses in Yunnan earthquake series

On the basis of the assumption that ω2 model accords with source displacement spectra, we have obtained the mathematical expressions for calculating apparent stresses of moderate-small shocks from low-frequency flat level and corner frequency. By using digital seismic records, apparent stress values are calculated for 823 moderate-small shocks of 4 earthquake series in Yunnan area following corrections for instrument response, propagation influence and site effect. The results show that for the 4 earthquake series in Yunnan area, apparent stress hints precursory information, which means that if a moderate-small shock occurs with apparent stress larger than 1 MPa in an earthquake series, a moderate-strong earthquake will occur afterwards; and if there is not moderate-small shock with apparent stress larger than 1 MPa after a moderate-strong event in an earthquake series, strong aftershock will not occur. The research also indicates that the average apparent stress value is 0.8 MPa in Yunnan area, therefore, apparent stress is not obviously related to seismic magnitude.

Spatial distribution features of sequence types of moderate and strong earthquake in Chinese mainland

Based on 294 earthquake sequences with magnitude greater than or equal to 5.0 occurred in Chinese mainland since 1970, the spatial distribution features of sequence types have been studied. In southwestern China, it takes mainshock-aftershock sequence type (MAT) as the major in Chuan-Dian rhombic block and concerned Xianshuihe-Anninghe-Xiaojiang seismic belt, as well as in Jinshajiang-Honghe seismic belt. Multiple mainshock type (MMT) mainly distributes in western Yunnan, and Longlin and Lancang areas in Tengchong-Baoshan block in west of Nujiang-Lancangjiang fault zone. A few isolated earthquake type (IET) mainly occurred in northwestern Sichuan and there is no IET occurred in Yunnan region. In northwestern China, it takes mainshock-aftershock sequence type (MAT) as the major in west segment of South Tianshan in Xinjiang region. Some MMT also occurred in this area in the intersection of Kalpin block and the Puchang fault zone. It takes IET as the major in middle Tianshan in Xinjiang. Along the Qilianshan seismic belt, most of sequences are MAT. In Qinghai region, it takes MAT as the major, but the regional feature of the spatial distribution of sequence types is not very clear. In North China, it takes MAT as the major in Yinshan-Yanshan-Bohai seismic belt, north edge of North China, and in Hebei plain seismic belt, as well as in sub-plate of lower river area of Yangtze River. In intersection of north segment of Shanxi seismic belt and the NW-trending Yinshan-Yanshan-Bohai seismic belt, there are several moderate or strong MMT with magnitude from 5.0 to 6.0 occurred. In south of North China around the latitude line of 35°N, it takes IET as the major. The spatial distribution of sequence types is relevant to the patterns of tectonic movements. MAT is mostly produced by the ruptures of locked units or asperities or the neonatal separating segments inside the fault zones. MMT is generally relevant to the conjugate structures or intersection of many tectonic settings. Further extension of simple fault often produces IET. Spatial distribution of sequence types is also correlative to the regional and deep environment of crustal medium to some extent. MAT mainly distributes in high velocity area in upper crust or in the transition zone between high velocity area and low velocity area, MMT mostly occurred in the low velocity area in upper crust.

On temporal and spatial distribution of seismic apparent stresses in Yunnan area

The method of radiation energy (E R) of the earthquake wave measured by the peak velocity (r·v) of the ground motion is applied to a detailed study on the temporal and spatial distribution of the seismic apparent stresses (σ a) for the moderate and small earthquakes and two aftershock sequences in Yunnan area. The results show that there exists an obvious non-homogeneity for the seismic apparent stresses in the spatial distribution. The concentrated regions of the high apparent stresses are related to the active places of the moderate and small earthquakes. Before the Dayao M=6.2 earthquake, there was a period in which the apparent stresses were high and the value was 5 times of the average value, 0.25 MPa. The relatively high values of apparent stresses distribute around the epicentral area of the major shock and nearby. It indicates that the variation characteristics of the apparent stresses can be taken as a new kind of criterion for the earthquake-risk forecast. Usually the ratio of the apparent stresses of the aftershock sequence σ aA to the ones σ aM of main shock is less than 1.0.

Characteristics of coda wave attenuation in Yunnan area

The characteristic of seismic coda wave attenuation in Yunnan area in 7 frequency-bands range from 1 Hz to 20 Hz was estimated by using the local earthquake’s waveform data recorded from 22 Yunnan digital seismic stations. Coda attenuation Q c −1 of each station was firstly calculated by single scattering method. Then, mean free path L e and seismic albedo B 0 of each station were calculated, and scattering attenuation Q s −1 and intrinsic attenuation Q i −1 were separated from total attenuation Q t −1 by multiple lapse time window analysis based on the multiple scattering model in uniform random isotropic scattering medium. The attenuating characteristics in Yunnan show that most value of L e are in 10–30 km, with maximal within 2–6 Hz; B 0 are about 0.5 at 1–2 Hz, but less than 0.5 at other frequency-bands, which means Q i −1 is comparable with Q s −1 at 1–2 Hz, and after 1–2 Hz, Q i −1 is greater than Q s −1 and dominates the attenuation process. Q c −1 is close to Q i −1 at other frequency bands except 1–2 Hz. Results show that Q −1 especially Q s −1 varies spatially, Q −1 in eastern Yunnan zone is a bit higher than in north-western Yunnan zone; northwestern Yunnan zone higher than southwestern Yunnan zone. Comparing with other results in global, Q s −1 in Yunnan is lower than the global average value among these results, Q i −1 is higher than the global average value, and Q t −1 lies the middle among these results.

Frequency-dependent attenuation of P and S waves in Yunnan region

We analyzed digital seismogram data of 5 668 earthquakes that occurred in Yunnan region between July of 1999 and December of 2003. Among the 22 seismic stations, six ones, namely Baoshan, Yongsheng, Lijiang, Heqing, Yimen, and Luquan, were selected and their attenuations of P and S waves were measured by using the extended coda-normalization method. The six stations were classified into three regions according to their location, that is, Baoshan area, Yongsheng-Lijiang-Heqing (YLH) area and Luquan-Yimen (LY) area. The values of Q S −1 and Q P −1 are expressed as Q S −1 =0.008 67 f −0.86, Q P −1 =0.011 55 f −0.93, Q S −1 =0.018 24 f −0.94, Q P −1 =0.022 88 f −0.92, and Q S −1 =0.016 47 f −0.91, Q P −1 =0.028 26 f −0.97 in Baoshan, YLH, LY areas respectively. The attenuation of YLH, LY are closer to each other, however, Baoshan area is apparently lower. Comparing attenuation in the three areas with other areas of the world using the same method, it is suggested that the attenuations of P and S waves in YLH and LY areas are close to Kanto of Japan, but much higher than southeast of South Korea. The Q S −1 and Q P −1 in Baoshan area are slightly higher than southeastern South Korea. Furthermore, the results indicate that our Q S −1 in Yunnan area is close to others by analyzing the coda attenuation.

Using Surface Wave and Receiver Function to Jointly Inverse the Crust-Mantle Velocity Structure in the West Yunnan Area

Considering the respective sensitivity superiority of surface wave dispersion to the S-wave velocity and receiver function to the interface depth, the data of surface wave and receiver function are combined to realize the joint inversion, and to obtain the crust-mantle velocity structure in the West Yunnan Area. In this paper, the technique of matched-filter frequency-time analysis is used to process the data of surface wave in the West Yunnan Area, the dispersion of group velocity of surface wave in the period range of 10.5~105.0s is acquired, and further, using the method of grid inversion the pure path dispersion is obtained within the grid of 1° × 1°. Based on the teleseismic broadband recordings of three components in the West Yunnan Area, the receiver function of teleseismic P-wave under the station is obtained after deconvolution. Jointing the dispersion information of pure path of surface wave with the data of receiver function, a systematical equation has been set up. Utilizing the differential damped least-squares scheme to realize the joint inversion, and the velocity structure of S-wave of the crust-mantle in the West Yunnan Area has been acquired. The results show that taking the Red River Fault as a boundary there exists an obvious difference of the crust-mantle structure on east and west sides. There exists a low velocity layer with thickness of ~10km at the depth of ~20km on the west side of the fault, but not obvious on the east. Wanding and Cangyuan located in the Yunnan-Myanmar-Thailand block belong to the low velocity area in the upper mantle, and another one is located in the Kongdian Palaeo-upheaval of the Mid-Yunnan block. Both low velocity areas have good corresponding relations with the geothermal distribution and strong earthquakes.

High-resolution terrestrial Permian–Triassic eventostratigraphic boundary in western Guizhou and eastern Yunnan, southwestern China

The adjoining area of western Guizhou and eastern Yunnan Provinces in southwest China is an ideal place to investigate the feasibility of correlating marine and nonmarine Permian–Triassic boundary (PTB) sequences, as it contains outcrop sections of shallow marine, marginal marine (or paralic), and terrestrial PTB sections, all in close geographic proximity. This paper documents for the first time multiple stratigraphic data from several well-preserved terrestrial PTB sections in the area and attempts to use these data to define, locate, and correlate the PTB in the area. A study of the spores and pollen and vegetation types across the terrestrial PTB sections in the study area suggests three distinct evolutionary stages across the boundary: Stage 1 (Xuanwei Formation) is characterised by Late Permian or Paleozoic-type ferns and pteridosperms (85.0%), with a few gymnosperms (15.0%); stage 2 is marked by an abrupt drop of sporopollen elements of Late Permian aspects, coupled with the appearance of fungal spores and limited Early Triassic palynomorphs; stage 3 (top Xuanwei Formation and Kayitou Formation) is dominated by gymnosperm pollen (58.8%) of clearly Early Triassic aspect, although still retaining limited ferns and pteridosperms. The three biotic stages seem to well correspond with the changing trend of the δ 13C org curves from the same sections, which is characterized by a sharp drop just before the PTB, followed by a short term partial recovery across the boundary, and then succeeded by a gradual decline after the PTB in the Early Triassic. Combining evidence from eventostratigraphic (i.e., the succession of boundary clay beds), biostratigraphic (using both macroplants and palynomorphs), and chemostratigraphic (i.e., organic carbon isotope excursion signals), we propose that a high-resolution PTB succession, closely correlatable to its marine counterpart at the Meishan section in eastern China, is recognisable at the terrestrial PTB sections in the western Guizhou–eastern Yunnan area in southwest China.

The upper mantle anisotropy in Yunnan area, China

Shear wave phase SKS of 11 earthquakes, collected from 23 stations of Yunnan Digital Seismic Network, is analyzed by fitting the theoretical transverse component with the observed one for determining the orientation and extent of polarization seismic anisotropy of upper mantle. Shear wave splitting is obviously observed in all stations except Heqing station (HQ). The results show that the polarization of fast split S-wave of upper mantle in Yunnan area is north-northeast in general and the time delay between fast and slow split shear waves is 0.5–2.0 s. It suggests that the influence of faults upon anisotropy analysis could not be neglected in such a geologically complex area. As the transitional zone between Qinghai-Tibetan plateau and the block of southern China, in Yunnan area the orientation of fast shear wave polarization indicating the subduction of Indian plate into Eurasian plate is the fundamental background of earth dynamics. While the southeast or south-southeast movement of Sichuan-Yunnan rhomb block, formed by the uplift of Qinghai-Tibetan plateau, plays an important role in the composition of complicated structural and stress environment of Yunnan area. The divergence between the fast direction and the movement of upper mantle indicates in Yunnan area there exists complex coupling effect between lower velocity layer or asthenosphere and crustal block. The distribution of structure driving force looks like a palm extending to northeast. According to the time delay between fast and slow shear waves, it is deduced that the thickness of anisotropy layer is 60–225 km with variation range roughly equal to that of 104–260 km of the buried depth of lower velocity layer of the earth in Yunnan area. So it suggests the top of anisotropy zone starts from the bottom of crust or from the lower velocity layer varying with specific locations related to the tremendous variation of the Moho discontinuity in Yunnan area. Furthermore, it is deduced that the anisotropy of the upper mantle is mainly from the lithosphere rather than the whole upper mantle.

Earthquake focal mechanisms and stress field in Sichuan-Yunnan area determined using P wave polarity and short period P and S waveform data

Based on waveform data, several methods to determine focal mechanisms of small earthquakes were developed since 1980. Kisslinger (1980) and Julian, Foulger (1996) proposed an approach to determine solution by using amplitude ratio of P and S wave. Schwartz (1995) devised a method to determine solutions by the use of polarity data and amplitudes of seismogram envelopes. Amplitudes of short period seismic waves propagating in an inhomogene-ous medium are sensitive to the variation in velocity and Q structure. Nakamura, et al (1999) took medium inhomo-geneity into account in determining focal mechanisms of small earthquakes using waveform data. If the locations of small earthquakes are concentrated in a small region, we can assume that the raypaths from the events to a given station are almost the same. So P and S wave attenuations are independent of event locations. In this case it is con-venient to determine focal mechanisms of these events by using short period P and S wave dataj. Focal mechanism solutions of small earthquakes in 5 regions, i.e., Rongchang, Mabian-Muchuan, Ya￠an, Baoxing and Mianzhu, which are covered by the Chengdu Telemetered Network, are obtained by analyzing the P polarity and short body wave amplitude data recorded in the network since 1992. According to the method proposed by Gephart and Forsyth (1984), based on well determined focal mechanism solutions in 15 sub-zones of Sichuan and Yunnan area, three principal stress tensors s1, s2, and s3, instead of averages of P, B, and T axis of the solutions, are determined to represent the regional stress field distribution.

Geographical differentiation of Asian taro, Colacasia esculenta (L.) Schott, detected by RAPD and isozyme analyses

Geographical differentiation and phylogenetic relationships of Asian taros, Colocasia esculenta (L.) Schott, and related species were analyzed by random amplified polymorphic DNA (RAPD) and isozymes of 13 enzyme systems with special interest in the accessions from the Yunnan area of China, which supposedly has served the secondary center of taro diversification and dispersal into the temperate Far East Asia. The RAPD analysis was found to be better suited for detecting genetic differences within taros and among its related species. However, both RAPD and isozyme analyses estimated quite similar genetic relationships within taros and between related species. Genetic differentiation was evident in the taro accessions of Nepal, Yunnan and other Asian areas; but, phylogenetic relationships between the differentiated taro groups were not clearly determined. When taro cultivation was introduced to a new area, only a small fraction of genetic variability in heterogeneous taro populations was transferred possibly causing random differentiation among locally adapted taro populations. Some of the Yunnan and Japanese accessions were found to be direct descendants of the common triploid population. Further analysis on taros from the eastern China and Korea is necessary to clarify the Yunnan's role in taro diversification and dispersal. The significant local differentiation in Asian taros was clearly demonstrated by RAPD and isozyme analyses in this study, and the results of this study will serve as a base to establish evolutionary and genetic relationships among Asian taros.

Crustal movement in Chinese mainland observed from 1998 to 1999

In this paper, GPS observations during 1998 to 1999 at fiducial stations in the Crustal Movement Observation Network of China (CMONOC) and GPS observation data processing with Bernese Software are briefly introduced. The processed results of crustal movement in the network during the period from early September 1998 to early July 1999, including the displacements at each fiducial station and strains in different part of the network, are discussed in detail. During the above mentioned period of observations, the eastern part of China was relatively stable. In contrast, the southwest part of China moved northwards about 17 mm with a dominant compression in NNE; the northwestern part of China moved northwestwards about 11 mm; and the Yunnan area moved southwards about 14 mm. The North-South seismic belt in the middle part of China is a belt of strong shear deformation. The maximum principal strain in the network is 2.9×10−8. The observation result shows that the northward push by the Indian Plate has still played the dominant role in the crustal movement in China.

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.