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宁杰远
电话: 62765852
类别: 科研

标题 3月16日(星期五)下午3:00优赛科技北大地球物理学术报告会  发布时间 2018年3月12日14时25分 星期一
The 2017 Jiuzhaigou Earthquake:
1. rupture process of a young fault system
2. Localify refined aftershock monitoring network and signal enhancement algorithm

Dr. Han YUE
Peking University

The Minshan Uplift Zone (MUZ) is located at the eastern margin of the Tibetan Plateau, which is the junction of three tectonic terranes. The observed discrepancy between a high uplifting and low shortening rate over the MUZ is attributed to the intrusion of a viscous lower-crust. In the last 50 years, several significant earthquakes occurred at the boundaries of the MUZ, i.e. the Huya and Mingjiang fault. On August 8, 2017, the Jiuzhaigou earthquake (Mw 6.5) occurred on the northern extension of the Huya fault. We adopt a joint inversion of the Interferometric Synthetic Aperture Radar and teleseismic body-wave data to investigate the rupture process of this event. The obtained slip model is dominated by left-lateral strike-slips on a sub-vertical fault presenting significant shallow slip deficit. The rupture initiation is composed of both thrust and strike-slip mechanisms producing a none-double-couple solution. We also resolve a secondary fault branch forming an obtuse angle with the main fault plane at its northern end. These phenomena indicate that the northern Huya fault is a young (less mature) fault system. Focal mechanisms of the regional earthquakes demonstrate that the northern and southern Huya fault presents different combinations of strike-slip and reversed motion. We attribute such discrepancy to the lateral extension of the viscous lower-crust, which appears to extrude to the east beyond the northern Huya fault, in comparison with that confined under the MUZ near the southern Huya fault. This conceptual model is also supported by geomorphological and magneto-telluric observations.

Detection of weak seismic signal is important to investigate seismicity and/or aseismic behavior of a fault system, thus in the last decades, people adopted specially designed networks to detect such weak signals. High sensitivity networks generally require dense station coverage, long deployment period, special field environment or high funding cost, which are not suitable for immediate monitoring of aftershocks. To investigate aftershock activities of the 2017 Mw = 6.5, Jiuzhaigou earthquake, we designed and deployed an experimental seismic network (Sept - Dec 2017) near the source region. This network is composed of 9 refined sub-arrays, which is composed of 8-16 short period seismic stations with lateral coverage of several hundred meters. Such design utilizes the waveform coherency within each sub-array to enhance seismic signals and to suppress random and impulsive noises. We conduct a theoretical analysis of two signal enhancement algorithms, e.g. linearly stacking and Geometric Mean Envelope (GME) weighting, and found the GME algorithm could significantly suppress impulsive noise presented at a single station. We test the performance of two algorithms with short time average over long time average (STA/LTA) criterion and found the GME algorithm can significantly reduce the occurrence of earthquake over-counting. Such array geometry and the GME weighting algorithm serve as a promising network design and signal enhancement algorithm to improve the monitoring of micro-seismic signals.

时间: 2018年3月16日星期五3:00PM
地点: 北京大学理科2号楼2829N

欢迎各位参加!
宁杰远
电话: 62765852
类别: 科研

标题 3月9日(星期五)下午4:00优赛科技北大地球物理学术报告会   发布时间 2018年2月28日22时10分 星期三
RESOLVING THE ORIGIN OF INTRA-PLATE VOLCANISM IN CAMEROON, WEST AFRICA

Mr. Adebayo Oluwaseun Ojo

School of Earth and Space Sciences, University of Science and Technology of China, Hefei, 230026, China
Department of Geosciences, Faculty of Science, University of Lagos, Akoka-Yaba, Lagos, Nigeria
(ojo@mail.ustc.edu.cn, aoojo@unilag.edu.ng)

  Although about 95% of volcanoes are generated at plate boundaries (e.g., mid-ocean ridge, subduction zone or arcs), the remaining 5% are located within the tectonic plate. Hence, the origin of volcanism along plate boundaries have been well studied over the years and are relatively well understood compared to intra-plate volcanism which are still enigmatic and often debated without reaching a consensus. Notwithstanding, the origin of Intra-plate volcanism are generally thought to be associated with large plumes of anomalously hot and less dense mantle materials that are hypothesized to be sourced in the lower mantle and rises by convection through the lithosphere before erupting to form volcanoes on the surface of the earth. Based on this hypothesis, the plume that feeds the hot spot volcanoes remains stationary while the overlying plates moves, thus forming a linear, age-progressive hotspot track that shows the direction of plate movement like we have in Hawaiian Islands, Yellowstone and other hotspots locations worldwide. Whether or not such mantle plumes exists and if they do, how deep they are, the mechanism by which they maintain their position for millions of years and how they produce volcanic activities is still a subject of much debate amongst Earth scientists. Moreover, the abovementioned hypothesis cannot explain the origin of all known intra-plate volcanism (e.g., in Central and South Pacific region, African rift, Cameroon Volcanic Line (CVL) etc.) and scientists are seeking alternative hypotheses that explain the origin of intra-plate volcanism at these different locations. Out of these anomalous hotspots, the CVL is somewhat unique because it occurs in a geologically unusual fashion having both an oceanic and a continental section without a clear age progression along its line. Previous studies in this region have revealed contrasting evidences such as relatively slow velocities along the CVL, low seismicity, low asthenospheric shear, remarkably low Vp/Vs ratio, nearly uniform transition zone thickness, limited evidence for magmatism-induced crustal modification, absence of melt, cooled mafic crustal intrusions or lower-crustal intrusions often termed underplate. So far, a number of mechanisms have been proposed for the origin of the CVL and this includes multiple mantle plumes, reactivation of previously existing shear zones, lithospheric delamination, thinning through basal erosion and mantle upwellings driven by convection currents of various sources. Therefore, in this talk, I will focus on resolving the competing mechanisms for the origin of the CVL using new results from seismic anisotropic studies of the crust and uppermost mantle in Cameroon, West Africa.

时间: 2018年3月9日星期五4:00PM
地点: 北京大学理科2号楼2829N

欢迎各位参加!
宁杰远
电话: 62765852
类别: 科研

标题 3月1日(星期四)下午4:00优赛科技北大地球物理学术报告会  发布时间 2018年2月25日14时14分 星期日
High-resolution 3D displacement field and stories behind: Case studies in earthquake, volcano and underground nuclear test

Dr. Teng Wang
Senior Research Fellow in Nanyang Technological University
http://www.earthobservatory.sg/people/wang-teng

  Synthetic aperture radar (SAR) imagery is a powerful remote-sensing technology that allows for mapping the surface displacement with the resolution of a few meters. Although Interferometric SAR (InSAR) has been widely used in earth science studies, applications often suffer from decorrelation in the area of large displacement gradient, causing data gap near the ruptured crust. Pixel offsets between SAR amplitude images can provide unambiguous ground displacement measurements in both the radar line-of-sight (range) direction and the along-track (azimuth) flight direction, allowing for deriving complete (3D) displacement in the near field. Here I will present how SAR amplitude images can help us better understand geodynamic processes via three independent cases:

1) The 2014-15 Holuhraun fissure eruption at Bárearbunga volcano, Iceland. In this case, we show the detailed 3D deformation of a forming graben. We also show that the near-field 3D displacement renewed our knowledge of how the curst was opened during an oblique rift event.

2) The 2016 Kaikoura (New Zealand) earthquake. In this case, we combine SAR images from the ESA’s Sentinal-1A and the JAXA’s ALOS-2 for identifying surface ruptures, and for deriving 3D coseismic displacements. We show that the vertical displacements are positively correlated with topography along the coast but are negatively correlated with topography farther inland, suggesting opposite coseismic contributions to the shaping of the Earth’s surface in a single seismic event.

3) The 2017 North Korea Nuclear test. In this case, we determine the first-ever 3D displacement field associated with underground nuclear test using TerraSAR-X SAR amplitude images. Elastic dislocation modeling of the surface displacements and focal mechanism solutions consistently reveal compaction and collapse processes following the initial explosion.

  The three cases presented here demonstrate that SAR amplitude information is important for revealing unprecedented displacement field near the ruptured crust. Such detailed geodetic measurements can bring new knowledges about the deformed crust.

时间: 2018年3月1日星期四4:00PM
地点: 北京大学理科2号楼2829N

欢迎各位参加!
盖增喜
电话: 62758176
类别: 教务

标题 Why deep earthquakes have so many non-double-couple components?  发布时间 2017年11月30日9时19分 星期四
报告题目:Why deep earthquakes have so many non-double-couple components?
报告人:Dr. Yingcai ZHENG (UNIVERSTY OF HOUSTON)
时间: 2017年12月6日(星期三)下午1:00
地点:北京大学理科二号楼 2829N会议室

报告人简介:
Yingcai Zheng is an Assistant Professor in seismic imaging and reservoir characterization, in the Department of Earth and Atmospheric Sciences, at University of Houston. Prior to his joining University of Houston in 2014, he was a Postdoctoral Fellow and Research Scientist at Earth Resources Laboratory at Massachusetts Institute of Technology. He got his Ph.D. in Geophysics from University of California Santa Cruz in 2007 and B.S. degree (2001) also in Geophysics from Peking University, Beijing. He is a recipient of the SEG's 2015 Karcher Award for his contribution in exploration geophysics.

宁杰远
电话: 62765852
类别: 科研

标题 Detection of small earthquakes and air-traffic events with dense array data  发布时间 2017年11月19日20时0分 星期日
报告人:孟浩然( University of Southern California)
时  间: 2017年11月20日(星期一),13:00
地  点:北京大学理科二号楼 2829N会议室

Abstract:
We present a technique to detect small earthquakes not included in standard catalogs using data from a dense seismic array. The technique is illustrated with continuous waveforms recorded in a test day by 1108 vertical geophones in a tight array on the San Jacinto fault zone. Waveforms are first stacked without time shift in 9 non-overlapping sub-arrays to increase the signal to noise ratio. The 9 envelope functions of the stacked records are then multiplied with each other to suppress signals associated with sources affecting only some of the 9 sub-arrays. Running an STA/LTA detection algorithm on the product leads to 723 triggers in the test day. Using a local P-wave velocity model derived for the surface layer from Betsy gunshot data, 5 s long waveforms of all sensors around each STA/LTA trigger are beamformed for various incident directions. Of the 723 triggers, 220 are found to have localized energy sources and 103 of these are confirmed as earthquakes by verifying their observation at 4 or more stations of the regional seismic network. This demonstrates the general validity of the method and allows processing further the validated events using standard techniques. The number of validated events in the test day is > 5 times larger than that in the standard catalog. Using these events as templates can lead to additional detections of many more earthquakes.
We also observe frequent air-traffic events with strong amplitudes recorded the tight array on the San Jacinto fault zone. These events have clear frequency changing characteristics in the spectrograms converted from the waveforms using short time Fourier Transform. Most of these characteristics can be well modeled by Doppler effect with an aircraft radiating constant frequency acoustic waves and flying-over the array with constant velocity along a straight trace and negligible wind speed. By determining the flying velocities and flying heights, these events are identified either as airplanes or helicopters flying-overs. The flying trace can then be inverted by fitting the parameters determined by Doppler effect at each station of the entire array. From Julian day 129 to 157, 2014, we manually detect 892 events in total, which are about 31 events per day. There are significant fewer events at night form 23:00 to 4:00 local time. These air-traffic events, with average duration of ~ 200 s, contribute to > 7 percent of the amount of time per day.
盖增喜
电话: 62758176
类别: 其他

标题 11月3日 优赛科技北大地球物理报告会  发布时间 2017年11月2日20时14分 星期四
报告人:  高永新 研究员(合肥工业大学)
时间: 2017年11月3日(星期五), 下午4:00
地点: 北京大学 理科二号楼 2829N会议室

摘要:
观测资料表明,在地震发生后,远处的台站可以观测到两种电磁信号,一种是与地震波同步出现的同震电磁场,另一种是提前于地震波到达的电磁波。这些电磁信号携带着震源参数、传播路径以及近地表介质特性等有用信息,对地震预警有重要意义。但目前对地震电磁场的认识并不充分,对其产生的机制仍存有争议。与岩石孔隙渗流有关的双电层动电效应是一种可能的机制,我们基于动电效应模拟了地震(包括点源和有限断层模型)发生过程中所产生的电磁场,计算结果很好地解释了实际观测的电磁信号,为证实动电效应是产生地震电磁场的有效机制提供了有力证据,同时也为电磁资料的解释提供了理论依据。

简介:
高永新,1982年生, 2001-2010年在哈尔滨工业大学工程力学专业就读本、硕、博,2011年—2014年在中国科学技术大学分别作为博士后和特聘副研究员从事科研工作。2014年-2015年在斯坦福大学地球物理系从事博士后研究,2015年进入合肥工业大学,现为土木学院工程力学系研究员,博士生导师。主要从事地震诱导电磁场的耦合机理和数值模拟,基于动电效应和地震发电机效应两种机制实现了对地震诱导电磁场的定量计算,揭示了地震电磁场的特征,为阐明地震电磁场产生机制、解决地震电磁学研究中一些有争议的问题提供了重要的证据。在地球物理学报、GJI、JGR、GRL等著名地球物理期刊上发表SCI论文9篇,其中2010年发表在GJI上关于双力偶点源激发电磁场全空间格林函数的论文获优秀学生论文奖。
盖增喜
电话: 62758176
类别: 科研

标题 11月7日北京大学学术讲座  发布时间 2017年11月2日20时12分 星期四
Title: 
A deep thermal plume in the mantle beneath Hawaii? Constraints from tomography and reflection seismology

Speaker: Robert Von der Hilst
时间:11月7日上午10:00
地点:北京大学中关新园学术报告厅(一号楼2楼)
 
Abstract:
The topic of today’s lecture is mantle convection in broad sense and, specifically, the question as to whether or not a hot thermal plume exists in the deep mantle beneath Hawaii.  Mantle transition zone (MTZ) discontinuities due to phase transitions in silicate minerals (e.g., olivine, garnet) near 410 and 660 km depth play an important role in modulating mantle flow.  Convection is foremost a thermally driven system and most MTZ studies use discontinuity topography to estimate in situ temperature anomalies. Compositional heterogeneity is also expected but direct observational evidence is scarce and often qualitative.  We study the base of the MTZ with ajoint seismological and mineral physics analysis of the amplitudes of so-called SS precursors (S waves that bounce off MTZ discontinuities). The study area includes Hawaii, where a hot upwelling has long been proposed. The data are not consistent with a simple thermal plume but provide evidence for lateral variation in composition at the base of the MTZ as expected in high temperature, low viscosity environments near lower mantle upwellings.  Combined with tomographic models the results suggest that the MTZ acts as a partial barrier for mantle flow.
盖增喜
电话: 62758176
类别: 其他

标题 10月30日 优赛科技北大地球物理报告会  发布时间 2017年10月25日14时36分 星期三
报告题目: Response of the valley of Mexico, The earthquakes of September 19th 1985 and 2017

报告人:  Professor Francisco José Sánchez-Sesma(Full time level C Professor. Institute of Engineering, UNAM, Mexico)
时间: 2017年10月30日(星期一), 上午10:00-12:00
地点: 北京大学 理科二号楼 2829N会议室


Francisco J. Sanchez-Sesma is a seismologist with some experience in developing analytic solutions and numerical methods to study wave propagation in elastic media to compute site effects. He has contributed to the passive seismic technique.

FIELD OF RESEARCH AND WORK GROUPS:
Elastic waves propagation modeling in irregular geometrical configurations.
Development of numerical and analytical methods:
· Application of Statistical Physics for the interpretation of ambient seismic noise within the framework of Diffuse Fields.
· Workgroup of the Administration of Seismological Engineering at the Institute of Engineering at the National Autonomous University of Mexico (UNAM), dedicated to seismic wave propagation and site effects due to surficial geology.
盖增喜
电话: 62758176
类别: 其他

标题 10月27日 优赛科技北大地球物理报告会  发布时间 2017年10月24日11时22分 星期二
报告题目:Formation of the Lunar Fossil Bulges and Its Implication for The Early Earth and Moon

报告人:  钟时杰 教授(Department of Physics, University of Colorado at Boulder, USA)
时间: 2017年10月27日(星期五), 中午12:00-13:30
地点: 北京大学 理科二号楼 2829N会议室

摘要:
First recognized by Laplace over two centuries ago, the lunar gravitational anomalies associated with tidal-rotational bulges are significantly larger than hydrostatic values. They are likely relics of a former hydrostatic state when the Moon was closer to the Earth and had larger bulges, and they were established when stresses in a thickening lunar lithosphere could maintain the bulges against hydrostatic adjustment. We formulate the first quantitative model of this process and show that bulge formation is controlled by the relative timing of lithosphere thickening and lunar orbit recession. Viable solutions indicate (i) that the early Earth was significantly less dissipative to lunar tides than at present, implying a paucity of oceans, (ii) that lunar bulge formation was a geologically slow process lasting several hundred million years, and (iii) that the process was complete about four billion years ago when the Moon-Earth distance was less than ~32 Earth radii.
报告人基本信息:
钟时杰教授,1985年本科毕业于中国科学技术大学地球和空间科学系,1988年在中国科学院地球物理研究所获得硕士学位,1994年在美国密西根大学取得博士学位,之后先后在加州理工学院和麻省理工学院从事博士后研究,2000年起在美国科罗拉多大学任助理教授、副教授、教授至今。钟教授在包括Science、Nature、Nature Geoscience等国际一流刊物上已经发表或合作发表专业学术论文一百余篇,先后获得Sloan Fellowship, NSF Career Award, Packard Fellowship等多项荣誉,并于2014年当选为美国地球物理联合会会士(AGU Fellow)。
盖增喜
电话: 62758176
类别: 其他

标题 10月20日 优赛科技北大地球物理报告会  发布时间 2017年10月24日11时21分 星期二
报告题目:川滇地区三维电性结构及动力学
报告人:  陈小斌 研究员(中国地震局地质研究所)
时间: 2017年10月20日(星期五), 下午4:00
地点: 北京大学 理科二号楼 2829会议室

报告人基本信息:
        陈小斌,男,1972年生,中国地震局地质研究所研究员,博士生导师。1995、2000年于江汉石油学院分别获得学士学位、硕士学位,2003年在中国地震局地质研究所获固体地球物理学博士学位,2003~2005年在北京大学地空学院做博士后研究。现为中国地震局地质研究所固体地球物理与深部构造研究室副主任。
        主要研究方向为电磁测深方法与应用,包括数据处理、正反演算法、软件研发、电性结构探测及其动力学解释等多个侧面。先后主持或参与多项国家级项目研究,发表论文70余篇。

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