Glacio-geophysical implication of an NNSS positioning in and around Syowa Station, East Antarctica

Utilization of the two-wave NNSS receiver drastically improved the positioning accuracy on the ice sheet of Antarctica. The NNSS positioning gives us 3 m three-dimensional convergence with 25 accepted satellite passes, and is most useful for the measurement of ice flow velocity. The flow velocity vectors along Route S-H-Z on Mizuho plateau, East Antarctica were obtained by estimating positional change of glaciological traverse stations after 7 years' interval. The transformation of the coordinate system was necessary before the comparison of the positioning in 1973 by the geodetic traverse method on the Bessel Reference Ellipsoid with that in 1980 by the satellite Doppler method on the NWL-8E Ellipsoid. NNSS receiving experiments at astronomical datum point in Syowa Station enabled us to estimate the translation of the coordinate origin of the Bessel Reference Ellipsoid against the geocentric NWL-8E Ellipsoid and correction terms for coordinate transformation. The obtained flow velocity is 15 m/a at H17 of around 1000 m a.s.l. and 70 m/a at Z2 of around 2000 m a.s.l. with ±6% uncertainty, and is too large to be explained only by the laminar flow of the ice sheet. The obtained velocity vectors are found to be mostly parallel to the maximum slope of the free-air gravity anomaly contours and can be interpreted as the ice sheet sliding down the slope of the subglacial mound of 2400 m relative height from the average subglacial bedrock topography. If such bedrock sliding occurrs over the whole region of Mizuho Plateau, the related thinning of the ice sheet may be detected by the precise measurement of the height change of the same marker station. By dynamically modelling the ice sheet and substituting the observed parameters such as precipitation, principal strain rate, etc., into the equation of ice thickness change the submergence velocity of around −1 m/a is expected and will be detected by carefully designed repetitive NNSS receiving experiments after several year's interval.     

Crustal structure in the Mizuho Plateau, East Antarctica, by a two-dimensional ray approximation

Studies on the crustal and upper-mantle structure in Antarctica have been one of the major contributions to Antarctic research since the International Geophysical Year of 1957–1958. Many refraction surveys with small charge size have been conducted in Antarctica, but long-range experiments were also made in 5 regions on the margin of the Antarctic continent.In 1979–1981, the scientific program of the Japanese Antarctic Research Expedition was focused on the earth sciences, and in particular, an explosion seismic experiment along a long survey line was the major item during these years. An experiment along a 300 km-long line with three shots and 27 observation stations was successfully made in the northern Mizuho Plateau, East Antarctica.From the analysis of travel times and the amplitude study of synthetic seismograms, the crustal structure of the northern Mizuho Plateau was determined. The depths of the Conrad and the Moho discontinuties were determined as 31 km and 42 km, respectively. The P-wave velocity and depth relation was determined as 6.0 km/s on the surface of the bedrock, 6.3 km/s at a depth of 2 km, 6.8 km/s at the Conrad and 7.9 km/s at the Moho. The velocity in the crust increases gradually. The crustal structure obtained is representative of East Antarctica.     

Combined Gravimetric–Seismic Crustal Model for Antarctica

The latest seismic data and improved information about the subglacial bedrock relief are used in this study to estimate the sediment and crustal thickness under the Antarctic continent. Since large parts of Antarctica are not yet covered by seismic surveys, the gravity and crustal structure models are used to interpolate the Moho information where seismic data are missing. The gravity information is also extended offshore to detect the Moho under continental margins and neighboring oceanic crust. The processing strategy involves the solution to the Vening Meinesz-Moritz’s inverse problem of isostasy constrained on seismic data. A comparison of our new results with existing studies indicates a substantial improvement in the sediment and crustal models. The seismic data analysis shows significant sediment accumulations in Antarctica, with broad sedimentary basins. According to our result, the maximum sediment thickness in Antarctica is about 15 km under Filchner-Ronne Ice Shelf. The Moho relief closely resembles major geological and tectonic features. A rather thick continental crust of East Antarctic Craton is separated from a complex geological/tectonic structure of West Antarctica by the Transantarctic Mountains. The average Moho depth of 34.1 km under the Antarctic continent slightly differs from previous estimates. A maximum Moho deepening of 58.2 km under the Gamburtsev Subglacial Mountains in East Antarctica confirmed the presence of deep and compact orogenic roots. Another large Moho depth in East Antarctica is detected under Dronning Maud Land with two orogenic roots under Wohlthat Massif (48–50 km) and the Kottas Mountains (48–50 km) that are separated by a relatively thin crust along Jutulstraumen Rift. The Moho depth under central parts of the Transantarctic Mountains reaches 46 km. The maximum Moho deepening (34–38 km) in West Antarctica is under the Antarctic Peninsula. The Moho depth minima in East Antarctica are found under the Lambert Trench (24–28 km), while in West Antarctica the Moho depth minima are along the West Antarctic Rift System under the Bentley depression (20–22 km) and Ross Sea Ice Shelf (16–24 km). The gravimetric result confirmed a maximum extension of the Antarctic continental margins under the Ross Sea Embayment and the Weddell Sea Embayment with an extremely thin continental crust (10–20 km).     

珍贝—黄岩海山链热-重力均衡动态调节机制

珍贝—黄岩海山链作为我国南海的残留扩张中心, 对其研究具有重要的科学意义. 本文运用均衡学方法, 通过重力异常数据反演了过珍贝—黄岩海山链剖面的地壳界面变化, 同时计算了岩石圈热结构状态, 在此基础上建立了珍贝—黄岩海山链的岩石圈地温结构模型. 通过均衡分析方法, 对剖面上测点的海底地形数据进行了热均衡和重力均衡分析, 得到了热均衡和重力均衡形变量. 结果表明, 在珍贝—黄岩海山链高热流区域, 热均衡作用可以产生最大约0.55 km的形变, 其重力均衡形变范围为0.77—1.89 km. 热均衡通过改变海底地形和地壳物质密度不断作用于重力均衡, 重力又反过来作用于热均衡, 形成了热均衡-重力均衡动态调节机制.      

新疆精河6.6级地震周边地区密度构造、均衡异常以及岩石圈挠曲机理

依据EIGEN-6C4重力模型和ETOPO1高程模型数据,围绕新疆精河6.6级地震展开岩石圈均衡与挠曲机理研究,得到如下结论：（1）震中附近的布格与自由空气重力异常分别为-221和-92mGal(10~(-5 )m·s~(-2)）,震中位于重力异常高梯度带上;（2）震中周边地区地壳厚度约为50km,密度结构总体变化平缓,东西方向地壳厚度变化较小,但自南向北地壳厚度逐渐变薄,精河6.6级地震初始破裂发生在上中地壳分界面附近;（3）震中附近岩石圈承载的垂向构造应力为20MPa左右,震中位于岩石圈垂向构造应力极大值附近的高梯度带上;（4）地震周边地区岩石圈有效弹性厚度最优解为26km,加载比最优解为F_1=1,F_2=F_3=0,表明该区域岩石圈相对坚硬,且导致岩石圈变形的初始加载全部来自地表．     

龙门山断裂带地壳密度结构

研究龙门山及邻区地壳密度结构对于认识该地区地震活动性具有重要意义.根据龙门山及邻区( 100°～105°E,28°～33°N)的布格重力异常资料,选取了跨越龙门山断裂带的6条重力测线,在深地震测深资料约束下,使用Geosoft软件分别反演出了龙门山地区地下的沉积层、康拉德界面和莫霍面的深度分布.研究结果表明:龙门山断裂带两侧的地壳结构明显不同,西面高原地区沉积层较薄,大部分为基岩出露;而东边盆地沉积层明显较厚,多在6km以上.莫霍面和康拉德面在两侧均相对平缓,康拉德面从东部的大约24km增加到青藏高原山区的35km左右;莫霍面深度从东部盆地的大约42km增加到西部青藏高原的67km左右.龙门山断裂带整体表现为一条近SN向的陡变重力梯度带,并在其地壳内各界面均发生错断,莫霍面和康拉德面错断距离分别达6 ～ 7km和3～ 5km.该区地壳的这种陡变和不均匀性是导致地震活动性强烈的主要原因之一.     

Sub-ice geology inland of the Transantarctic Mountains in light of new aerogeophysical data

The Transantarctic Mountains are a major geologic boundary that bisects the Antarctic continent, separating the low-lying, tectonically active terrains of West Antarctica from the East Antarctic craton. A new comprehensive aerogeophysical data set, extending 1150 km from the Ross Sea into the interior of East Antarctica provides insights into the complex structure inland of the Transantarctic Mountains. Geophysical maps, compiled from 21 000 km of gravity, magnetic and subglacial topography data, outline the boundaries of several geologic and tectonic segments within the survey area. The coherent pattern in magnetic data and mesa topography suggests a subglacial extent of the Transantarctic Mountains 400–500 km inland the last exposed rock outcrops. We estimate the maximum thickness of a potential sediment infill in the Wilkes Subglacial Basin to be less than 1 km, based on gravity modeling and source depth estimates from magnetic data. The coherent nature of the potential field and topography data, together with the northwest–southeast trends, define the Adventure Subglacial Trench and the Resolution Subglacial Highlands as a tectonic unit. The crustal structure and the strong similarity of the observed gravity with fold-and-thrust belts suggest a compressional scenario for the origin of the Adventure Subglacial Trench and the Resolution Subglacial Highlands. The complexity and apparent structural control of the Wilkes Subglacial Basin raise the issue of what influence pre-existing structures may have played in the formation of the Transantarctic Mountains system. The previous hypothesis of a thermal boundary beneath the mountains is difficult to reconcile with our new gravity data. The apparent difficulties to match our new data with certain key aspects of previous models suggests that a reassessment of the existing uplift models is necessary. We have modeled the prominent gravity anomaly over the Transantarctic Mountains with thicker crust.     

Long-term underground temperature measurements at Syowa Station, East Antarctica

Underground temperature measurements in two shallow boreholes have been carried out by the Japanese Antarctic Research Expedition at Syowa Station, East Antarctica from April, 1981 to January, 1985. Two quartz thermometers were installed in the first borehole at depths of 2 and 5 m and three were in the second one at depths of 1, 4 and 6.8 m. The mean underground temperatures in the first borehole were −8.181 and −8.843°C at depths of 2 and 5 m, and in the second one −8.242 and −8.220°C at depths of 4 and 6.8 m. As the mean air temperature at Syowa Station was −10.8°C, the underground temperature in the 2 −6.8 m depth range is about two degrees higher than the air temperature. The thermal diffusivities of the observation area are determined by the same principle of the Ångström method, using long-term underground temperature records. The thermal diffusivity around Syowa Station is established to be about two times larger than those of ordinary igneous and metamorphic rocks measured in the laboratory. The thermal conductivity of the drilled cores and surrounding outcropping rocks are also measured by the transient method with temperature conditions at +23°C and −20°C. The thermal conductivities measured in various samples at −20°C are about 7% larger than those at +23°C. Thes tendency is consistent with the results of holocrystalline rock experiments.     

A three-dimensional gravity study of the 95.5°W propagating rift in the Galapagos spreading center

Seafloor at the Galapagos 95.5°W propagating rift (PR) has a varied morphological expression that can be spatially correlated with the predicted kinematic history of the PR. A median valley-like depression occurs near the tip of the growing ridge axis. To test if this bathymetry is a dynamic feature supported by mantle or lithosphere strength or if it is due to isostatically compensated crustal thickness variations, we use three-dimensional gravity modelling to constrain the crustal structure in this region, from data collected by Hey in 1979 and 1982. The gravity anomaly at the PR tip depression suggests that the tip depression is not caused by crustal thinning. The data are consistent with a stress-supported PR tip depression caused by asthenospheric along-axis flow into the growing ridge axis (Phipps Morgan and Parmentier [1]). In contrast to the tip depression, seafloor in the sheared zone of material transferred through transform migration from the Cocos to Nazca plate is anomalously shallow and has a pronounced regional 300–400 m tilt towards the growing ridge axis over the 20 km width of the sheared zone. The gravity data also suggest that the sheared zone is not compensated by crustal thickening.     

应用卫星重力信息对横断山系地区布格重力异常特异分布的纠正

在云南省西部,跨越中、缅两国交界的横断山系地区（97°E~102°E,24°N~30°N）有近一半的面积尚没有重力测点、即重力数据空白区和重力测点稀少的普查级测区.以前的有关文献、图集中所给出对此地区的重力场都是十分模糊的结果与图件.因此应用这些资料无法详细地研究该地区重力场特征与深部地壳结构（构造）.本文应用卫星重力异常资料作为“近似空间重力异常”,经中间层改正后给出的“计算布格重力异常”,其分布特征与该地区的地形高程呈很好的镜像相关.对相应山脉、河谷以及断裂构造都有所反映.特别是在横断山系地区该布格重力异常呈现为近南北的走向.为此,据该“计算布格重力异常”,并选定对该区有代表性的一条重力异常剖面作正反演计算,以得到其地壳深部结构剖面.结果表明,在横断山脉地区的地壳厚度在51~56 km间起伏变化;滇西北云岭山系以及玉龙山区的地壳厚度约在60 km以上. 最后,对所得结果与图件进行了讨论,并提出了几点认识和纠正的建议.     

临汾强震区地壳结构及发震背景研究

以诸城—宜川深地震测深速度剖面为约束, 对沿该剖面得到的高精度重力数据进行拟合, 并对临汾强震区平面布格重力异常进行处理, 得到了该地区地壳密度结构及平面重力异常分布． 利用上述结果分析了临汾强震区的地壳结构及构造环境, 结合前人相关研究成果, 认为临汾强震区地壳中存在塑性相对较强的介质, 洪洞和临汾两次历史地震皆发生在其与周边弹性介质的转换边界上． 另外, 临汾凹陷南北两侧局部构造环境存在差异, 在区域应力场作用下, 导致了洪洞地震和临汾地震的发生. 两次地震在发震时间、 地点和震级等地震要素上有所不同．      

南极洲东部普里兹湾海域重磁场特征及地壳结构

普里兹湾位于南极洲东部大陆边缘,其深部地壳结构特征对认识白垩纪冈瓦纳古陆裂解和新生代大陆边缘形成具有重要意义.本文利用重磁、多道反射地震、声纳浮标折射地震和ODP钻井数据对普里兹湾海域的深部地壳结构进行了研究.研究结果显示,普里兹凹陷表现为典型的盆地负重力异常特征,其沉积基底较深,而在四夫人浅滩为高幅重力正异常,其沉积基底普遍抬升.在大陆架中部存在SW-NE向条带状基底的抬升,且呈朝NE向逐渐变深的趋势.在中大陆架外侧,均衡残余重力异常呈V字形负异常条带状分布,其两翼分别与四夫人浅滩和弗拉姆浅滩外的大陆坡相连.该异常带在大陆架中部向陆的偏移可能是由于古大陆架边缘的地形影响,推测其与普里兹冲积扇同属于洋陆过渡带向陆的部分,在重力模拟剖面表现为地壳向海逐渐减薄.普里兹冲积扇的地壳厚度较薄,平均为6 km,最薄处可达4.6 km,并且根据洋陆过渡带向海端的位置,推测可能属于接近洋壳厚度的过渡壳.重力异常分区的走向与兰伯特地堑在普里兹湾的构造走向基本一致,可能主要反映了二叠纪-三叠纪超级地幔柱对普里兹湾的裂谷作用的影响.该区域的自由空间重力异常和均衡残余异常均表现为超过100×10^-5m/s²的高幅正异常特征,可能由位于大陆架边缘的巨厚沉积体负载在高强度岩石圈之上的区域挠曲均衡作用所导致,可能与该区域第二期裂谷期之后的沉积间断以及快速进积加厚的演化过程有关.普里兹湾磁力异常的走向与重力异常明显不同,大致可分为东北高幅正异常区和西南低幅异常区.重磁异常在走向上的差异反映高磁异常主要来源于岩浆作用形成的铁镁质火成岩的影响,并且岩浆作用的时代不同于基底隆升的时代,而可能形成于前寒武纪或者南极洲和印度板块裂谷期间(白垩纪).     

喜玛拉雅“东构造结”地区特异重力场的探讨

跨越中、印、缅三国交界的青藏高原东南的喜玛拉雅“东构造结”地区（92°E~97°E，26°N~30°N）一半以上的面积尚没有重力测点，是重力数据空白区,故无法直接研究其重力场特征与深部地壳结构（构造）.本文分析了卫星重力异常的特性，提出应用卫星重力异常作为近似空间重力异常，并作布格改正后，得到的布格重力异常具有与该地区地形高程呈镜像相关的特征，可用以研究深部地壳结构.据三条重力剖面计算得到该地区三个地壳深部结构剖面的结果，给出青藏高原地壳厚度>70 km；喜马拉雅造山带为55 km左右；布拉马普特拉河谷盆地为33~35 km；那加山山脉地区为40~45 km，显示出三者为三个不同的构造单元.同时给出布拉马普特拉构造单元为相对高密度的刚性物质构成，随着印度洋板块向北运移，在碰撞、挤压下，插入青藏高原东南缘一带.导致该地带的强烈构造运动，和频发大、小地震.最后提出了几点认识和建议.     

Structure and development of the passive continental margin across the Princess Astrid Coast, East Antarctica

On the basis of geological and geophysical data, a contribution is made to understanding the structure and evolution of the Earth's crust across the Princess Astrid Coast, East Antarctica, especially in the area of the Schirmacher Oasis and the Wohlthat Massif. The crustal material is mostly composed of medium- to high-grade metamorphics which underwent three important tectogenic events (close of the Archaean, within the Proterozoic and at the close of the Precambrian). The intrusive age of the Eliseev Anorthosite Massif could be determined (2,400 Ma). The dominant structural features in the Schirmacher Oasis show a strike of some 40° to 60° and indicate a NNE-SSW oriented compressional stress regime. It is probably connected with the last major thermotectonic event some 400 to 600 Ma ago. The Phanerozoic history is controlled by the younger Gondwana break-up and N-S oriented tension.The continental crust as a whole is of relatively acid composition. This is also reflected in the negative regional magnetic anomaly. Probably, its origin is linked to late Precambrian thermotectonic activation. A typical feature is the crustal thinning to the north (from some 50 to 20 km) in connection with deep-reaching E-W crustal faults indicating northward down-faulted blocks.     

Gravity anomaly and crustal density structure in Jilantai rift zone and its adjacent region

This paper deals with the interpretation of Bouguer gravity anomalies measured along a 250 km long Suhaitu-Etuokeqi gravity profile located at the transitional zone of the Alxa and Ordos blocks where geophysical characteristics are very complex. The analysis is carried out in terms of the ratio of elevation and Bouguer gravity anomaly, the normalized full gradient of a section of the Bouguer gravity anomaly (G ^h ) and the crustal density structure reveal that (1) the ratio of highs and lows of elevation and Bouguer gravity anomaly is large between Zhengyiguan fault (F4) and Helandonglu fault (F6), which can be explained due to crustal inhomogeneities related to the uplift of the Qinghai-Tibet block in the northeast; (2) the main active faults correspond to the G ^h contour strip or cut the local region, and generally show strong deformation characteristics, for example the Bayanwulashan mountain front fault (F1) or the southeast boundary of Alxa block is in accord with the western change belt of G ^h , a belt about 10 km wide that extends to about 30 km; (3) Yinchuan-Pingluo fault (F8) is the seismogenic structure of the Pingluo M earthquake, and its focal depth is about 15 km; (4) the Moho depth trend and Bouguer gravity anomaly variation indicates that the regional gravity field is strongly correlated with the Moho discontinuity.     

3D upper crustal density structure of the Deccan Syneclise,Central India

A constrained 3D density model of the upper crust along a part of the Deccan Syneclise is carried out based on the complete Bouguer anomaly data. Spectral analysis of the complete Bouguer gravity anomaly map of the study region suggests two major sources: short wavelength anomalies (<100 km) caused primarily due to the density inhomogeneities at shallow crustal level and long wavelength anomalies (>100 km) produced due to the sources deeper than the upper crust. A residual map of the short wavelength anomalies is prepared from the complete Bouguer anomaly using Butterworth high‐pass filter (100 km cut‐off wavelength). Utilizing the constraints from deep resistivity sounding, magnetotellurics and deep seismic sounding studies, 2.5D density models have been generated along 39 profiles of this region. The mismatch between the calculated response of the a priori 2.5D model with the residual (short wavelength) gravity anomalies is minimized by introducing high‐density intrusive bodies (≥2.81 g/cm³) in the basement. With these 2.5D density models, the initial geometry of our 3D density model, which includes alluvium, Deccan trap, Mesozoic sediment and high‐density intrusive bodies in the basement up to a depth of 7 km (upper crust), is generated. In the final 3D model, Deccan trap extends from 200 m to nearly 1700 m below the 90–150 m thick Quaternary sediment. Further down, the sub‐trappean Mesozoic sediment is present at a depth range of 600–3000 m followed by the basement. The derived 3D density model also indicates six intrusive bodies of density 2.83 g/cm³ in the basement at an average depth of about 4–7 km that best fits the residual gravity anomaly of the study area.     

Topography of the Moho and earth crust structure beneath the East Vietnam Sea from 3D inversion of gravity field data

The Moho depth, crustal thickness and fault systems of the East Vietnam Sea (EVS) are determined by 3D interpretation of satellite gravity. The Moho depth is calculated by 3D Parker inversion from residual gravity anomaly that is obtained by removing the gravity effects of seafloor and Pre-Cenozoic sediment basement topographies from the free air anomaly. The 3D inversion solution is constrained by power density spectrum of gravity anomaly and seismic data. The calculated Moho depths in the EVS vary from 30–31 km near the coast to 9 km in the Central Basin. A map of the lithosphere extension factor in the Cenozoic is constructed from Moho and Pre-Cenozoic sediment basement depths. The fault systems constructed by the maximum horizontal gradient approach include NE-SW, NW-SE, and N-S oriented faults. Based on the interpretation results, the EVS is sub-divided into five structural zones which demonstrated the different characteristics of the crustal structure.     

Crustal structure in the Falcón Basin area, northwestern Venezuela, from seismic and gravimetric evidence

The Falcón Basin in northwestern Venezuela has a complex geological history driven by the interactions between the South American and Caribbean plates. Igneous intrusive bodies that outcrop along the axis of the basin have been associated with crustal thinning, and gravity modeling has shown evidence for a significantly thinned crust beneath the basin. In this study, crustal scale seismic refraction/wide-angle reflection data derived from onshore/offshore active seismic experiments are interpreted and forward-modeled to generate a P-wave velocity model for a 450 km long profile. The final model shows thinning of the crust beneath the Falcón Basin where depth to Moho decreases to 27 km from a value of 40 km about 100 km to the south. A deeper reflected phase on the offshore section is interpreted to be derived from the downgoing Caribbean slab. Velocity values were converted to density and the resulting gravimetric response was shown to be consistent with the regional gravity anomaly. The crustal thinning proposed here supports a rift origin for the Falcón Basin.     

Geophysical models for the tectonic framework of the Lake Vostok region, East Antarctica

Aerogeophysical and seismological data from a geophysical survey in the interior of East Antarctica were used to develop a conceptual tectonic model for the Lake Vostok region. The model is constrained using three independent data sets: magnetic, seismic, and gravimetric. A distinct change in the aeromagnetic anomaly character across Lake Vostok defines a crustal boundary. Depth to magnetic basement estimates image a 400-km-wide and more than 10-km-deep sedimentary basin west of the lake. Analysis of teleseismic earthquakes suggests a relatively thin crust beneath Lake Vostok consistent with predictions from kinematic and flexural gravity modelling. Magnetic, gravity, and subglacial topography data reveal a tectonic boundary within East Antarctica. Based on our kinematic and flexural gravity modelling, this tectonic boundary appears to be the result of thrust sheet emplacement onto an earlier passive continental margin. No data presently exist to date directly either the timing of passive margin formation or the subsequent shortening phase. The preserved thrust sheet thickness is related to the thickness of the passive margin crust. Because a significant amount of time is required to erode the thrust sheet topography, we suggest that these tectonic events are Proterozoic in age. Minor normal reactivation of the thrust sheet offers a simple mechanism to explain the formation of the Lake Vostok Basin. A low level of seismicity exists in the vicinity of this tectonic boundary. The existence of a crustal boundary in the Antarctic interior provides new constraints on the Proterozoic architecture of the East Antarctic craton.     

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. Project supported by the National Natural Science Foundation of China and the Joint Earthquake Science Foundation.