Wang Chun-yong

Crustal structure beneath the Songpan—Garze orogenic belt

The Benzilan-Tangke deep seismic sounding profile in the western Sichuan region passes through the Songpán-Garze orogenic belt with trend of NNE. Based on the travel times and the related amplitudes of phases in the record sections, the 2-D P-wave crustal structure was ascertained in this paper. The velocity structure has quite strong lateral variation along the profile. The crust is divided into 5 layers, where the first, second and third layer belong to the upper crust, the forth and fifth layer belong to the lower crust. The low velocity anomaly zone generally exists in the central part of the upper crust on the profile, and its integrates into the overlying low velocity basement in the area to the north of Ma’erkang. The crustal structure in the section can be divided into 4 parts: in the south of Garze-Litang fault, between Garze-Litang fault and Xianshuihe fault, between Xianshuihe fault and Longriba fault and in the north of Longriba fault, which are basically coincided with the regional tectonics division. The crustal thickness decreases from southwest to northeast along the profile, that is, from 62 km in the region of the Jinshajiang River to 52 km in the region of the Yellow River. The Moho discontinuity does not obviously change across the Xianshuihe fault based on the PmP phase analysis. The crustal average velocity along the profile is lower, about 6.30 km/s. The Benzilan-Tangke profile reveals that the crust in the study area is orogenic. The Xianshuihe fault belt is located in the central part of the profile, and the velocity is positive anomaly on the upper crust, and negative anomaly on the lower crust and upper mantle. It is considered as a deep tectonic setting in favor of strong earthquake’s accumulation and occurrence.

Source mechanism of small-moderate earthquakes and tectonic stress field in Yunnan Province

In the paper, source mechanisms of 33 small-moderate earthquakes occurred in Yunnan are determined by modeling of regional waveforms from Yunnan digital seismic network. The result shows that most earthquakes occurred within or near the Chuandian rhombic block have strike-slip mechanism. The orientations of maximum compressive stresses obtained from source mechanism are changed from NNW-SSN to NS in the areas from north to south of the block, and tensile stresses are mainly in ENE-WSW or NE-SE. In the eastern Tibetan Plateau, the orientations of maximum compressive stress radiate toward outside from the plateau, and the tensile stress orientations mostly parallel to arc structures. Near 28°N the orientations of both maximum compressive stress and tensile stress changed greatly, and the boundary seems to correspond to the southwestern extended line of Longmenshan fault. Outside of the Chuandian rhombic block, the orientations of P and T axes are some different from those within the block. The comparison shows that the source mechanism of small-moderate events presented in the paper is consistence with that of moderate-strong earthquakes determined by Harvard University, which means the source mechanism of small-moderate events can be used to study the tectonic stress field in this region.

Crustal structure of Dabieshan orogenic belt

The crustal structures ofP velocity and density on the deep seismic sounding profile across the Ilabieshan orogenic belt are presented. There is a 5-km-thick crustal “root” between the Yuexi and Xiaotian where the elevation is highest on the profile. An apparent Moho offset of 4. 5 km beneath the Xiaotian-Mozitan fault marks the paleo-suture of the Triassic collision. A high-velocity anomaly zone at the depth below 3 km beneath the ultra-high pressure (UHP) zone may be correlated to the higher content of UHP metamorphic rocks.

S-wave velocity structure inferred from re-ceiver function inversion in Tengchong volcanic area

Tengchong volcanic area is located near the impinging and underthrust margin of India and Eurasia plates. The volcanic activity is closely related to the tectonic environment. The deep structure characteristics are inferred from the receiver function inversion with the teleseismic records in the paper. The results show that the low velocity zone is influenced by the NE-trending Dayingjiang fault. The S-wave low velocity structure occurs obviously in the southern part of the fault, but unobviously in its northern part. There are low velocity zones in the shallow position, which coincides with the seismicity. It also demonstrates that the low velocity zone is directly related to the thermal activity in the volcanic area. Therefore, we consider that the volcano may be alive again.

Crustal structure in northern margin of Tianshan mountains and seismotectonics of the 1906 manas earthquake

The thin-skinned structure in the crust of the northern Tianshan piedmont is explored by an 86-km-long, NS-trending deep seismic reflection profile through the Ürümqi depression of the north margin of the Tianshan mountains. On the CDP stacking section, the first- and second-row parallel to anticlines in the north margin of the Tianshan mountains are shown on the segment to the south of the Shihezi. The detachments, corresponding to the reflection events at TWT 2.5–3.0 s and 5.5–6.0 s respectively, join the crustal deep structure to the reverse fault-fold zone. The Manas thrust extends downwards in listric shape, merges into the detachment at about TWT 2.5 s and joins to the Qingshuihe thrust. The reflection events on 5.5–6.0 s are corresponding to the main detachment, which joins the lower Manas anticline, and finally converge to the Junggar Southern Marginal Fault. A 12–14 km-thick sedimentary basin exists on the region in the north to the Shihezi. The depth of the Moho discontinuity beneath the Junggar basin is about 45 km, and increases southwards to 50 km. The crustal structure inferred from the deep seismic sounding profile and the Bouguer anomaly in the same region is consistent with the image from the deep seismic reflection profile. The seismogenic model of the 1906 Manas earthquake is related to the fault system, which consists of the Qingshuihe thrust, the detachments and the shallow Manas ramp.The thin-skinned structure in the crust of the northern Tianshan piedmont is explored by an 86-km-long, NS-trending deep seismic reflection profile through the Urumqi depression of the north margin of the Tianshan mountains. On the CDP stacking section, the first- and second-row parallel to anticlines in the north margin of the Tianshan mountains are shown on the segment to the south of the Shihezi. The detachments, corresponding to the reflection events at TWT 2.5–3.0 s and 5.5–6.0 s respectively, join the crustal deep structure to the reverse fault-fold zone. The Manas thrust extends downwards in listric shape, merges into the detachment at about TWT 2.5 s and joins to the Qingshuihe thrust. The reflection events on 5.5–6.0 s are corresponding to the main detachment, which joins the lower Manas anticline, and finally converge to the Junggar Southern Marginal Fault. A 12–14 km-thick sedimentary basin exists on the region in the north to the Shihezi. The depth of the Moho discontinuity beneath the Junggar basin is about 45 km, and increases southwards to 50 km. The crustal structure inferred from the deep seismic sounding profile and the Bouguer anomaly in the same region is consistent with the image from the deep seismic reflection profile. The seismogenic model of the 1906 Manas earthquake is related to the fault system, which consists of the Qingshuihe thrust, the detachments and the shallow Manas ramp.

Variations of shear wave splitting in the 2013 Lushan Ms7.0 earthquake region

In this paper, variations of shear wave splitting in the 2013 Lushan Ms7.0 earthquake sequence were studied. By analyzing shear wave particle motion of local events in the shear wave window, the fast polarization directions and the delay time between fast and slow shear waves were derived from seismic recordings at eight stations on the southern segment of the Longmenshan fault zone. In the study region, the fast polarization directions show partition characteristics from south to north. And the systematic changes of the time delays between two split shear waves were also observed. As for spatial distribution, the NE fast polarization directions are consistent with the Longmenshan fault strike in the south of focal region, whereas the NW fast direction is parallel to the direction of regional principal compressive stress in the north of focal region. Stations BAX and TQU are respectively located on the Central and Front-range faults, and because of the direct influence of these faults, the fast directions at both stations show particularity. In time domain, after the main shock, the delay times at stations increased rapidly, and decreased after a period of time. Shear-wave splitting was caused mostly by stress-aligned microcracks in rock below the stations. The results demonstrate changes of local stress field during the main shock and the aftershocks. The stress on the Lushan Ms7.0 earthquake region increased after the main shock, with the stress release caused by the aftershocks and the stress reduced in the late stage.

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.

Lg coda Q 0 value and its relation with the tectonics in chinese mainland and adjacent regions

We have collected 432 vertical component records from 45 stations of new CENC (China Earthquake Network Center) in Chinese mainland and adjacent regions. These records were used to calculate Q0 (Q at 1Hz) and η values of Lg coda from each station by the stack spectral ratio (SSR) method. Then the tomography method was applied to obtaining lateral variation of Q0 and η values in Chinese mainland and adjacent regions. The result indicates that Q0 value varies between 150 and 600 in the studied areas. Yunnan, southwest Sichuan, and northwest Myanmar show the lowest Q0 value (Q0<240) and the crust of these regions is characterized by complicated crack and strong hydrothermal activity. The highest Q0 value (Q0>510) exists in the border of southern Mongolia, Alxa and Ordos block. The η value varies between 0.45 and 0.75 in Chinese mainland and its adjacent regions.

A study on deep structure using teleseismic receiver function in Western Yunnan

AbstractsWestern Yunnan is located at the boundary of collision or underthrusting zone of Eurasian plate and is influenced by many times tectonic movements. With very complex geological environment and tectonic background, it is one of the seismically active areas. In the paper, the teleseismic records were selected from 16 national, local and mobile stations, including 4 very-wide-band mobile stations of PASSCAL. And nearly 2 000 receiver functions were extracted. Two measuring lines are 650 km and 450 km, respectively and across some major tectonic units in Western Yunnan. It is indicated that Nujiang might be a seam characterized by underthrusting. The western and eastern boundaries of Sichuan-Yunnan rhombus block, i.e., Honghe and Xiaojiang faults, might be an erection seam or collision belt. Panxi tectonic zone still has the characteristics of continental rift valley, that is, the surface is hollow and the upper mantle is upwarping. The tectonic situation in Western Yunnan is of certain regulation with the interlacing distribution of orogenic zone and seam. The crustal thickness decreases gradually from the north to the south and the S wave velocity is globally lower here.

Issues on crustal and upper-mantle structuresassociated with geodynamics in the northeastern Tibetan Plateau

The northeastern Tibetan Plateau is one of the key regions to explore the geodynamics of the Tibetan Plateau. In 1958, a research team led by Prof. Zeng Rongsheng did low-frequency seismic exploration in the Qaidam Basin, which was a prelude to deep geophysical research in the northeastern Tibetan Plateau. Since 1960s, a series of scientific projects have been carried out, including 27 deep seismic sounding profiles, which provided good coverage and fundamental constraints on velocity structure of the crust and upper mantle of tectonic units in the region. Among them, a number of deep seismic reflection profilings were used to reveal fine crustal structures of key tectonic areas. Moreover, crustal electrical structure and density structure were inferred from magnetotelluric sounding profiling and Bouguer gravity anomaly data, respectively. 3-D P-wave and S-wave velocity structures were determined using body-wave travel time tomography, surface-wave group velocity and phase velocity tomography based on seismic data. Since 2000, with the dramatically increased number of broadband seismic stations, application of modern seismology methods, such as teleseismic receiver function inversion, ambient noise imaging, as well as shear-wave splitting and seismic anisotropy, led to further understanding of structure and deformation of crust and upper mantle in the northeastern Tibetan Plateau. Among published research papers, most results on the deep structure are compatible. For example, the ambient noise imaging, surface wave tomography and receiver function inversion jointly show a wide distribution of low shear wave velocity in the middle and lower crust. Deep geophysical explorations show coexistence of crustal thickening, low P-wave velocities, low resistivity and high heat flow values. H- κ stacking analysis of receiver functions shows low-to- moderate crustal Poisson’s ratios in the Qilian fold system, the northern Songpan-Garze block and the west Qinling orogenic belt. The crustal Poisson’s ratios in the northeastern Tibetan Plateau are obviously lower than those in the central plateau. These compatibilities are crucial to understanding basic features of the deep structure. However, several issues related to the dynamics of the region remain in debate: (1) low velocity-high conductivity layer in the upper-middle crust; (2) crustal thickening mode; (3) crustal and mantle anisotropy; (4) lower crustal channel flow; (5) southward subduction of the Eurasian lithosphere. No consensus has been reached for these issues yet at present. Although several deep seismic sounding profiles and magnetotelluric sounding profiles collectively displayed evidence of existence of a low velocity-high conductive layer in the upper-middle crust, results of some other profiles did not show such layer. Mechanism of crustal thickening in the northeastern Tibetan Plateau can be summarized in the following three end-member hypotheses: (1) uniform crustal thickening; (2) lower crustal thickening; (3) upper crustal thickening. A prevailing view is that the crustal shortening generates folding and deformation of the upper crust, and fragment stacking is the main mode of crustal thickening. However, this is inconsistent with the crustal model from deep seismic sounding profiles. Different deep tectonic models and interpretations are causes of the on-going debate on issues such as the “lower crustal channel flow” and “southward subduction of Eurasian continent”. One of the reasons for lack of consensus might be that the resolution power of the existing seismic data is still not high enough to identify the details in deep crust and upper mantle. The national and regional seismic networks, and large-scale temporary seismic array observation, which is currently being implemented, will greatly improve the reliability and resolution of the target model. This is an effective way to enhance the knowledge of crustal and upper mantle structures and geodynamics in the northeastern Tibetan Plateau.