Shi Yaolin

Three-dimensional thermal structure of the Chinese continental crust and upper mantle

We invert S-wave velocities for the 3D upper-mantle temperatures, in which the position with a temperature crossing the 1300°C adiabat is corresponding to the top of the seismic low velocity zone. The temperatures down to the depth of 80 km are then calculated by solving steady-state thermal conduction equation with the constraints of the inverted upper-mantle temperatures and the surface temperatures, and then surface heat flows are calculated from the crustal temperatures. The misfit between the calculated and observed surface heat flow is smaller than 20% for most regions. The result shows that, at a depth of 25 km, the crustal temperature of eastern China (500–600°C) is higher than that of western China (<500°C). At a depth of 100 km, temperatures beneath eastern and southeastern China are higher than the adiabatic temperature of 1300°C, while that beneath west China is lower. The Tarim craton and the Sichuan basin show generally low temperature. At a depth of 150 km, temperatures beneath south China, eastern Yangtze craton, North China craton and around the Qiangtang terrane are higher than the adiabatic temperature of 1300°C, but is the lowest beneath the Sichuan basin and the regions near the Indian-Eurasian collision zone. At a depth of 200 km, very low temperature occurs beneath the Qinghai-Tibet Plateau and the south to the Tarim craton.

Tele-seismic coseismic well temperature changes and their interpretation

Coseismic water level oscillation and correlated deep water temperature changes have been observed in a water well at Tangshan City by high sensitivity measurement. Amount of water temperature changes depend on amplitude of water level oscillation. Coseismic water temperatures always decrease as water level oscillates, drop of temperature ranges from 0.001 °C to 0.01 °C corresponding to amplitude of water level oscillation from several centimeters to about one meter. Temperatures usually recover one to several hours after the oscillation. We suggest that the temperature drop is produced by dispersive transfer of heat as the water oscillates, and follow-up thermal conduction makes temperature recovery. Our finite element calculations support quantitatively the idea. High accuracy measurements of water temperature at different depths in the future may test our interpretation.

Deep geodynamics of far field intercontinental back-arc extension: Formation of Cenozoic volcanoes in northeastern China

There are three cases of variation of trench location possible to occur during subduction: trench fixed, trench advancing, and trench retreating. Retreat of trench may lead to back-arc extension. The Pacific plate subducts at low angle beneath the Eurasia plate, tomographic results indicate that the subducted Pacific slab does not penetrate the 670 km discontinuity, instead, it is lying flat above the interface. The flattening occurred about 28 Ma ago. Geodynamic computation suggests: when the frontier of the subducted slab reaches the phase boundary of lower and upper mantle, it may be hindered and turn flat lying above the boundary, facilitates the retreat of trench and back-arc extension. Volcanism in northeastern China is likely a product of such retreat of subduction, far field back-arc extension, and melting due to reduce of pressure while mantle upwelling.

Origin of tectonic stresses in the Chinese continent and adjacent areas

Based on data of principal stress orientation from focal mechanism and of geological features in China, we made pseudo-3D genetic algorithm finite element (GA-FEM) inversion to investigate the main forces acting on the Chinese continent and adjacent areas which form the Chinese tectonic stress field. The results confirm that plate boundary forces play the dominant role in forming the stress field in China, as noticed by many previous researchers. However, we also find that topographic spreading forces, as well as basal drag forces of the lower crust to the upper crust, make significant contribution to stresses in regional scale. Forces acting on the Chinese continent can be outlined as follows: the collision of the India plate to the NNE is the most important action, whereby forces oriented to the NW by the Philippine plate and forces oriented to the SWW by the Pacific plate are also important. Topographic spreading forces are not negligible at high topographic gradient zones, these forces are perpendicular to edges of the Tibetan Plateau and a topographic gradient belt running in the NNE direction across Eastern China. Basal drag forces applied by the ductile flow of the lower crust to the base of upper crust affect the regional stress field in the Tibetan Plateau remarkably, producing the clockwise rotation around the eastern Himalaya syntax.

Lithospheric structure and continental geodynamics

This paper briefly reviews main progress in the research on lithospheric structure and continental geodynamics made by Chinese geophysicists during last 4 years since 22nd IUGG general assembly in July 1999. The research mainly covers the following fields: investigations on regional lithospheric structure, DSS survey of crust and upper mantle velocity structure, study on present-day inner movement and deformation of Chinese mainland by analyzing GPS observations, geodynamics of Qingzang plateau, geophysical survey of the Dabie-Sulu ultra-high pressure metamorphic belt and probing into its formation mechanism, geophysical observations in sedimentary basins and study on their evolution process, and plate dynamics,etc.

Check of Earth’s free oscillations excited by Sumatra-Andaman Large Earthquake and discussions on the anisotropy of inner core

Sumatra-Andaman Large Earthquake on Dec. 26, 2004 generated not only the Indian Ocean Tsunami but also the Earth’s free oscillations (EFO). The signals of Earth’s free oscillations were perfectly recorded by the superconducting gravimeter C0-32 at Wuhan station in China. After the pre-treatment and spectral analysis on the observational data from Wuhan station, we obtained more than ninety EFO modes including 42 fundamental modes, 2 radial modes and 49 harmonic modes. On the basis of the discussions on some observed harmonic modes and abnormal splitting phenomena, we considered that the real rigidity might be lower than the theoretical prediction of PREM model in the inner core and however the anisotropy of compressive wave was brightly higher than the present estimations in the inner core. This suggested that the anisotropy of the inner core could be much more complicated than our present understanding, and there might be some new geophysical phenomena in the formation process of the inner core.

Study on excitation of the two-tone acoustic characteristic of the chime bell of Marquis Yi of Zeng by finite element method

The bronze chime bells of Marquis Yi of Zeng are precious cultural relics of China. The bells have a two-tone acoustic characteristic, i.e., striking different parts of the bell generates different tones. In this study, we use a finite element method to simulate the propagation of elastic waves and the generation of free oscillations of the bell. Vibrations with different spectral characteristics can be obtained by striking different parts of the chime bell of Marquis Yi of Zeng. By the observation of the vibration design and through spectral analysis, we further help the understanding of the two-tone acoustic characteristic of the chime bell of Marquis Yi of Zeng.

Summation and decomposition of principal stresses in the crust

In the compilation ofWorld Stress Map, 9% of data comes from overcoring and hydraulic fracturing measurement, 23% from borehole breaking off, 63% from earthquake focal mechanism, and 5% from young geological investigation (Zoback,et al, 1989). Only overcoring and hydraulic fracturing can provide both the orientation and magnitude of the horizontal stress, all other methods can only provide the orientation, but no the magnitude of the stresses. Although some researchers tried to estimate magnitudes of stresses in earthquake mechanism research based on some additional assumptions (CHEN, Duda, 1996; ZHAO,et al, 2002). This method, however, has not been widely applied. What kind of analysis can be done with orientation-only data? What kind of incorrect operations should be avoided? These are basic important problems. However, some confusions and misunderstandings exist. For example, a simple operation is to use the average of measured orientation to represent the principal stress orientation in a specific area; or decompose a stress into a summation of long wavelength and short wave length components. Is it correct to do in this way? Some fundamental ideas are hidden in these seemingly simple problems. We will discuss these questions in this note.

An scientific evaluation of annual earthquake prediction ability

The scientific idea of earthquake prediction in China is introduced in this paper. The various problems on evaluation of earthquake prediction ability are analyzed. The practical effect of prediction on annual seismic risk areas in 1990–2000 in China is discussed based on R-value evaluation method, and the ability of present earthquake prediction in China is reviewed.

The electrical conductivity of eclogite in Tibet and its geophysical implications

The electrical conductivity of Tibetan eclogite was investigated at pressures of 1.5–3.5 GPa and temperatures of 500–803 K using impedance spectroscopy within a frequency range of 10−1–106 Hz. The electrical conductivity of eclogite increases with increasing temperature (which can be approximated by the Arrhenius equation), and is weakly affected by pressure. At each tested pressure, the electrical conductivity is weakly temperature dependent below ∼650 K and more strongly temperature dependent above ∼650 K. The calculated activation energies and volumes are 44±1 kJ/mol and −0.6±0.1 cm3/mol for low temperatures and 97±3 kJ/mol and −1.2±0.2 cm3/mol for high temperatures, respectively. When applied to the depth range of 45–100 km in Tibet, the laboratory data give conductivities on the order of 10−1.5–10−4.5 S/m, within the range of geophysical conductivity profiles.