1-D P- and S-wave velocity models for the collision zone between the northern Tianshan mountain and the Junggar basin based on local earthquake data

We have developed crustal minimum 1-D P- and S-wave velocity models of the collision zone between the northern Tianshan mountain and the Junggar basin (86°E–89°E, 43°N–44.5°N). These two models were created through inversion of 1 370 P- and 1 396 S-wave travel times from 173 well-constrained local earthquakes recorded by the Ürümqi sparse local seismic network and temporary seismic arrays. In contrast to previous models, our results indicate relatively low velocity at both shallow (<10 km) and deep (30–45 km) depths. The shallow zone is interpreted to be the result of thick surficial sedimentary deposits, whereas the deeper anomaly is interpreted to result from ductile shearing and lower crustal flow. Additionally, we detected several transition layers under the lower crust, which may imply structural complexity of the uppermost mantle in this region. The improved models reduce the RMS residual of earthquake locations by 41.7% from 1.2 to 0.5 seconds. The more accurately located hypocenters appear to correlate with prominent local over-thrusts, which underlie an anticlinal fold belt and several blind faults. Positive station corrections are observed near the Junggar basin, which likely reflects low wave velocity; negative corrections near the Tianshan mountain and Bogda mountain suggest high wave velocity.National Natural Science Foundation (China) (Grant 41204037)China Earthquake Administration (Basic Research Project Grant 2012IES010103

Determination and interpretation of earthquake source locations in Sichuan Province, China

Thesis (S.M. in Earth and Planetary Sciences)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2001.Includes bibliographical references (leaves 149-153).This thesis involves locating and interpreting earthquakes from the Sichuan Province, China. The main contributions of this research are: successfully fitting the travel time data of three explosions to a two-layer crust model; and the improvement in locating earthquakes. To achieve these objectives, the Gauss-Newton method is applied iteratively to find the nonlinear least squares solution. The Monte Carlo method and the Gauss- Newton method were jointly used to locate events and simultaneously optimize the crust model. The iterative station correction method is adopted to compensate the incorrectness of the velocity model and to improve the event locations. The joint master event method can improve the location of the events near the master event. The modified Hypoinverse not only can locate events based on the spherical crust model, but can also jointly improve the crust model used. A three-dimensional crust model is next to be optimized and to improve event locations further. We also present geological interpretation about earthquake locations in Sichuan and their tectonic implications.by Youshun Sun.S.M.in Earth and Planetary Science

Crustal structure of the central Tibetan plateau and geological interpretation

Based on teleseismic data obtained from 225 stations from two networks in the central Tibetan plateau, we have generated detailed crustal structure images using P-wave receiver function techniques with more accurate piercing-depth-correction and time-depth-correction than what have previously been available. Our images indicate an undulatory Moho beneath the Tibetan plateau with a steep jump beneath the northern Himalaya, and obviously different structures in proximity to the Bangong-Nujiang suture. In several sections of the Tibetan plateau, the lower crust is characterized by pervasive high-velocity regions, which are consistent with the preservation of eclogite bodies beneath the plateau, whose presence affects the dynamics of the Tibetan plateau.China Earthquake Administration (Grant 201308013)National Natural Science Foundation (China) (Grants 40974034, 41174086, 41074052 and 41021003

Depth determination of the Moho interface beneath the Tibetan plateau and other areas of China

We apply the adaptive moving window method of Sun et al. to the most recent catalog data and the data recorded by portable stations to construct the velocity structure of the crust and upper mantle, and to determine the depth of the Moho interface beneath the Tibetan plateau and other areas of China. We first select 2 600 locations in the study region with 1° intervals, then at each location invert for a five-layer 1-D P-wave velocity model from the surface down to the uppermost mantle by performing a Monte Carlo random search. The Moho depth at each location is then determined, and the Moho interface beneath the study region is obtained through proper interpolation with certain smoothing. Compared to depths obtained by previous studies, our results show more accurate Moho depths in the Tibetan plateau, Tianshan region and other areas of the study region.United States. Defense Threat Reduction Agency (Contract DTRA01-00-C-0024)Chinese Academy of Sciences (Fund KJCX2-EW-121

Crustal and uppermost mantle structure of Caucasus and surrounding regions

A 3-D P-wave velocity model is developed for the crust and uppermost mantle of Caucasus and the surrounding area by applying the tomographic method of Zhao et al. using 300 000 high-quality P-wave first arrivals from 43 000 events between 1964 and 2005. This tomographic method can accommodate velocity discontinuities such as the Moho in addition to smooth velocity variations. The spatial resolution is 1°×1° in the horizontal direction and 10 km in depth. The velocity images of the upper crust correspond well with the surface geology. Beneath the southern Caucasus high velocity anomalies are found in the middle crust and low velocity anomalies are found in the uppermost mantle. Relatively low Pn velocities are located under the Lesser Caucasus, eastern Turkey, and northern Iran. Higher Pn velocities occur under the eastern portion of the Black Sea and the southern Caspian Sea, and also extend into the eastern edge of Azerbaijan. Tomographic model significantly reduces the travel-time residuals.United States. Defense Threat Reduction Agency (Contracts DE-AC-52-04NA25612, NNSA-03-2S2 and W-7405-ENG-483)Chinese Academy of Sciences (Fund KJCX2-EW-121

Caucasus Seismic Information Network: Data and Analysis Final Report

The geology and tectonics of the Caucasus region (Armenia, Azerbaijan, and Georgia) are highly variable. Consequently, generating a structural model and characterizing seismic wave propagation in the region require data from local seismic networks. As of eight years ago, there was only one broadband digital station operating in the region – an IRIS station at Garni, Armenia – and few analog stations. The Caucasus Seismic Information Network (CauSIN) project is part of a nulti-national effort to build a knowledge base of seismicity and tectonics in the region. During this project, three major tasks were completed: 1) collection of seismic data, both in event catalogus and phase arrival time picks; 2) development of a 3-D P-wave velocity model of the region obtained through crustal tomography; 3) advances in geological and tectonic models of the region. The first two tasks are interrelated. A large suite of historical and recent seismic data were collected for the Caucasus. These data were mainly analog prior to 2000, and more recently, in Georgia and Azerbaijan, the data are digital. Based on the most reliable data from regional networks, a crustal model was developed using 3-D tomographic inversion. The results of the inversion are presented, and the supporting seismic data are reported. The third task was carried out on several fronts. Geologically, the goal of obtaining an integrated geological map of the Caucasus on a scale of 1:500,000 was initiated. The map for Georgia has been completed. This map serves as a guide for the final incorporation of the data from Armenia and Azerbaijan. Description of the geological units across borders has been worked out and formation boundaries across borders have been agreed upon. Currently, Armenia and Azerbaijan are working with scientists in Georgia to complete this task. The successful integration of the geologic data also required addressing and mapping active faults throughout the greater Caucasus. Each of the major faults in the region were identified and the probability of motion were assessed. Using field data and seismicity, the relative activity on each of these faults was determined. Furthermore, the sense of motion along the faults was refined using GPS, fault plane solutions, and detailed field studies. During the course of the integration of the active fault data, the existence of the proposed strike slip Borjomi-Kazbeki fault was brought into question. Although it had been incorporated in many active tectonic models over the past decade, field geologists and geophysicists in Georgia questioned its existence. Detailed field studies were carried out to determine the existence of the fault and estimate the slip along it; and it was found that the fault zone did not exist. Therefore, the convergence rate in the greater Caucasus must be reinterpreted in terms of thrust mechanisms, instead of strike-slip on the Borjomi-Kazbeki fault zone

Processing of randomly acquired seismic data

Thesis (S.M. in Geosystems)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1998.Includes bibliographical references (leaves 62-64).by Youshun Sun.S.M.in Geosystem

P-wave and S-wave tomography of the crust and uppermost mantle in China and surrounding areas

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2005.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references.This thesis involves inverting the seismic structure of the crust and uppermost mantle in China from the P- and S-wave travel-time tomography. The main contributions of this research are: 1) introducing the adaptive moving window method to obtain 2338 1D P and S models in China; 2) introducing a tomographic method to perform the 3D body wave travel-time tomography with the Moho discontinuity included. Both horizontal and vertical resolutions are highly controlled and smooth transitions among adjacent locations are guaranteed in the final models. To achieve these objectives, the Monte-Carlo (random search) method and the Gauss-Newton method are applied iteratively to find the nonlinear least square solutions and to optimize the models in the crust and uppermost mantle. The models we obtained provide accurate travel-time calculation, ground-truth event relocation and seismogram fittings. These models can therefore be applied to reliable earthquake location. Geological, geodynamic, and volcanic implications of our models are discussed in this thesis. Our tomographic models provide new insights into the geological structure and tectonics of the region, such as lithological variations and large fault zones across the major geological terranes.(cont.) Compared with previous tomographic studies, we have used a larger, higher quality data set and applied an updated tomographic method to take into account the effects of the complex Moho geometry in this region. Our results cast a new light over the complex structure and seismotectonics of China and surrounding areas.by Youshun Sun.Ph.D

Shock-Tube Study of the Autoignition of <i>n</i>‑Butane/Hydrogen Mixtures

Shock-tube measurements and a kinetic study on the autoignition of hydrogen/<i>n</i>-butane blends were carried out. The Aramco2.0 model was employed in numerical simulation; this model can well capture the autoignitions of the hydrogen/<i>n</i>-butane blends under all test conditions. The pressure dependence, equivalence dependence, and influence of blending of the autoignitions for pure hydrogen, pure <i>n</i>-butane, and the hydrogen/<i>n</i>-butane binary mixture have been studied. A negative pressure dependence of autoignition delay is obtained at the intermediate and low temperatures for the hydrogen and lean <i>X</i>_H₂ = 98% mixture. The autoignition of <i>n</i>-butane can be nonlinearly enhanced by hydrogen addition. The autoignition of hydrogen was insensitive to the equivalence ratio, but ignitions of the <i>n</i>-butane and binary blends became longer with the rising fuel concentration. The ignition chemistry of hydrogen and <i>n</i>-butane was interpreted

Comparative Study of the Effects of Nitrous Oxide and Oxygen on Ethylene Ignition

To explore the effects of N₂O and O₂ on C₂H₄ ignition, ignition delay times of stoichiometric C₂H₄/O₂/N₂O/Ar mixtures with mole blending ratios of N₂O/(N₂O + O₂) = 0, 50, 80, and 100% were measured in a high-pressure shock tube. Reflected shock conditions cover a range of pressures from 1.2 to10 atm and temperatures from 1090 to 1760 K. In addition, ignition delay times of C₂H₄/N₂O/Ar mixtures are measured at pressures of 1.2–10 atm, equivalence ratios of 0.5–2.0, and temperatures of 1214–1817 K. The results indicate that, in the studied conditions, the ignition delay times of C₂H₄ greatly increase as the N₂O concentration increases at a given pressure and temperature. Five recent literature models are tested against the new measured ignition delay times and show very small discrepancies among each other for the C₂H₄/N₂O/Ar mixtures but exhibit significant discrepancies for the C₂H₄/N₂O/O₂/Ar mixtures. Moreover, the kinetic analysis is performed to reveal the reason for the discrepancies among the five models and to investigate the different effects of N₂O and O₂ on the C₂H₄ ignition