琉球沟弧盆系莫氏面深度的三维广义反演

本文利用三维广义反演技术对重力资料进行反演，计算出了琉球沟弧盆系地区的莫氏面深度。由于考虑了分辨率和方差之间的最佳折衷问题，反演结果不论是在精度上还是在分辨能力方面都有了明显改进。分析结果也表明，重力反演得到的莫氏面深度同李昭兴等人用地震折射波法得到的莫氏面深度有较大的差别。文中地产生这种差别的可能原因提出了自己的见解。     

Super-deep CTD measurements in the Izu-Ogasawara trench and a comparison of geostrophic shears with direct measurements

CTD (Conductivity-Temperature-Depth) data at five stations across the Izu-Ogasawara Trench at 34°N were examined. Geostrophic velocity was in accordance with the directly measured currents. Above the trench floor, potential temperature increased at a rate of 0.6 m°C/1000 db from 8000 db to 9417 db, and salinity increased from 8300 db to the bottom. Potential density was almost constant at 7100–8700 db, and it increased to the bottom. Above the eastern and western flanks, inversion of potential density was indicated in the bottom layers with an increase of potential temperature and a decrease of salinity, suggesting geothermal heating and outflow of ground water.     

Deep CTD Casts in the Challenger Deep,Mariana Trench

On 1 December 1992, CTD (conductivity-temperature-depth profiler) casts were made at three stations in a north-south section of the Challenger Deep to examine temperature and salinity profiles. The station in the Challenger Deep was located at 11°22.78′ N and 142°34.95′ E, and the CTD cast was made down to 11197 db or 10877 m, 7 m above the bottom by reeling out titanium cable of 10980 m length. The southern station was located at 11° 14.19′ N and 142°34.79′ E, 16.1 km from the central station, where water depth is 9012 m. CTD was lowered to 7014 db or 6872 m. The northern station was located at 11°31.47′ N and 142° 35.30′ E, 15.9 km from the central station, and CTD was lowered to 8536 db or 8336 m, 10 m above the bottom. Below the thermocline, potential temperature decreased monotonously down to 7300–7500 db beyond a sill depth between 5500 m and 6000 m, or between 5597 db and 6112 db, of the trench. Potential temperature increased from 7500 db to the bottom at a constant rate of 0.9 m°C/1000 db. Salinity increased down to 6020–6320 db, and then stayed almost constant down to around 9000 db. From 9500 db to the bottom, salinity increased up to 34.703 psu at 11197 db. Potential density referred to 8000 db increased monotonously down to about 6200 db, and it was almost constant from 6500 db to 9500 db. Potential density increased from 9500 db in accordance with the salinity increase. Geostrophic flows were calculated from the CTD data at three stations. Below an adopted reference level of 3000 db, the flow was westward in the north of Challenger Deep and eastward in the south, which suggests a cyclonic circulation over the Challenger Deep. Sound speed in Challenger Deep was estimated from the CTD data, and a relation among readout depth of the sonic depth recorder, true depth, and pressure was examined.     

冻融作用对不同初始状态基坑稳定性的影响研究

为研究季节冻土区冻融作用对基坑工程的影响,基于考虑冰-水相变的热传导理论、考虑黏聚力的修正剑桥模型和强度折减法,建立了适用于季节冻土区基坑稳定性分析的计算平台。在此基础上,结合具体的试验数据对不同初始状态的基坑算例进行计算,研究季节冻土区基坑冻融前后稳定性和位移的变化情况,并根据土体冻融前后的物理力学参数对上述变化规律作出分析。研究结果表明,冻融作用加剧季节冻土区基坑的局部变形,且土体的初始干重度对变形规律的影响较大,但基坑的整体安全系数并未受到冻融作用的显著影响。存在一个临界干重度γ_d0,基坑土体的初始干重度接近临界干重度时,围护桩水平位移在冻融前后的变化较小,随着土体初始干重度与临界干重度的差距增大,围护桩水平位移在冻融前后的变化量也随之增大。土体的初始干重度越小,基坑围护桩的竖向位移和坑底隆起位移在冻融前后的变化量越大,即土体初始干重度较小的基坑在经历冻融后,围护桩的竖向位移产生显著沉降,但坑底隆起情况有明显缓解。研究成果对季节冻土区基坑工程的设计和施工具有重要意义。     

Space–time distribution of interplate moment release including slow earthquakes and the seismo-geodetic coupling in the Sanriku-oki region along the Japan trench

Along the Japan trench where some Mw8 class interplate earthquakes occurred in the past century such as the 1896 Sanriku tsunami earthquake (M6.8, Mt8.6, 12×10²⁰ N m) and the 1968 Tokachi-oki earthquake (Mw8.2, 28×10²⁰ N m), the Pacific plate is subducting under northeast Japan at a rate of around 8 cm/year. The seismic coupling coefficient in this region has been estimated to be 20–40%. In the past decade, three ultra-slow earthquakes have occurred in the Sanriku-oki region (39°N–42°N): the 1989 Sanriku-oki (Mw7.4), the 1992 Sanriku-oki (Mw6.9), and the 1994 Sanriku-oki (Mw7.7) earthquakes. Integrating their interplate moments released both seismically and aseismically, we have the following conclusions. (1) The sum of the seismic moments of the three ultra-slow earthquakes was (4.8–6.6)×10²⁰ N m, which was 20–35% of the accumulated moment (18.6–23.0)×10²⁰ N m, in the region (39°N–40.6°N, 142°E–144°E) for the 21–26 years since the 1968 Mw8.2 Tokachi-oki earthquake. This is consistent with the previous estimates of the seismic coupling coefficient of 20–40%. On the other hand, the sum of the interplate moments including aseismic faulting is (11–16)×10²⁰ N m, leading to a “seismo-geodetic coupling coefficient” of 50–85%, which is an extension of the seismic coupling coefficient to include slow events. (2) The time constants showed a large range from 1 min (10² s) for the 1968 Tokachi-oki earthquake to 10–20 min (10³ s) for the 1896 Sanriku tsunami earthquake, to one day (10⁵ s) for the 1992 Sanriku-oki ultra-slow earthquake, to on the order of one year (10⁷ s) for the 1994 Sanriku-oki ultra-slow earthquakes. (3) Based on the space–time distribution, three “gaps of moment release,” (40.6°N–42°N, 142°E–144°E) 39°N–40°N, 142°E–143°E) and (39°N–40°N, 142°E–144°E), are identified, instead of the gaps of seismicity.     

Cooling and inferred exhumation history of the Ryoke metamorphic belt in the Yanai district, south-west Japan: Constraints from Rb–Sr and fission-track ages of gneissose granitoid and numerical modeling

Abstract The Ryoke metamorphic belt in south-west Japan consists mainly of I-type granitoids and associated low-pressure/high-temperature metamorphic rocks. In the Yanai district, it has been divided into three structural units: northern, central and southern units. In this study, we measured the Rb–Sr whole-rock–mineral isochron ages and fission-track ages of the gneissose granodiorite in the central structural unit. Four Rb–Sr ages fall in a range of ca 89–87 Ma. The fission-track ages of zircon and apatite are 68.9 ± 2.6 Ma and 57.4 ± 2.5 Ma (1σ error), respectively. Combining the newly obtained ages with previously reported (Th–)U–Pb ages from the same unit, thermochronologic study revealed two distinctive cooling stages; 1) a rapid cooling (> 40°C/Myr) for a period (~7 Myr) soon after the peak metamorphism (~ 95 Ma) and 2) the subsequent slow cooling stage (~ 5°C/Myr) after ca 88 Ma. The first rapid cooling stage corresponds to thermal relaxation of the intruded granodiorite magma and its associated metamorphic rocks, and to the uplift by a displacement along low-angle faults which initiated soon after the intrusion of the magma. Uplift by the later stage deformation having formed large-scale upright folds resulted in progress of the exhumation during the first stage. The average exhumation velocity of the stage is ≥ 2 mm/yr. During the second stage, the rocks were not accompanied by ductile deformation and were exhumed with the rate of 0.1–0.2 mm/yr. The difference in the exhumation velocity between the first and second cooling stages resulted from the difference in the thickness of the crust and in the activity of ductile deformation between the early and later stages of the orogenesis.     

1994年世界灾害地震综述

本文综合叙述了１９９４年世界地震灾害的概况和特征；不同国情的地震灾害特点；成灾主要原因：地震预测、预报和抗震减灾的一些问题。     

罕见的“强烈震群”活动

本文介绍了全世界范围内的“强烈震群”的地区分布、活动频率和强度等特征，并指出“强烈震群”主要分布在太平洋西半部的岛弧－海沟地带，这些“强烈震群”是依据近百年来的现代地震仪观测数据获得的。