低纬度环球带和环太平洋带大地震的幕式活动分析

统计分析低纬度环球带(E带)和环太平洋带(P带)在1900～2008年间MW≥7.5地震的时空分布,进一步证实两者本身和两者之间都存在15～18年的幕式交替活动. 将这两大地震带分别划分为5个地震区,发现这些分区地震幕式活动互不相同, 但E带各区奇数幕与偶数幕年均地震数之比OER均大于1.58, 而P带各区的OER均小于0.93, 显示出前者以奇数幕活跃为特征, 后者以偶数幕活跃为特征. 对这些地震区的组合分析进一步证明,从幕式活动的共性看, 中太平洋地区宜纳入E带. 本文推论:低纬度环球带是另一条全球尺度地震带;大地震幕式活动的成因应归因于板块运动有几十年时间尺度的非平稳幕式运动;未来5～8年, E带的大地震还会比P带多.     

Reconstructing the setting for deposition of distal tephra in the Sea of Japan Basin: A catastrophic eruption of Baitoushan Volcano

This paper presents a detailed analysis of the attitude and accumulation of fine-grain ash material from the Baegdusan-Tomakomai bed (B-Tm) on the seafloor; this material was deposited after a powerful explosion of the ultraplinian type that occurred around 938 AD. It was found that, provided one has a large digital array of grain-size analyses that characterizes both areal and vertical variability in the bed and makes use of advanced mathematical data-processing techniques (division of the grain-size distribution into subpopulations, a trend analysis of transport pathways, and so on), one can derive new information concerning the character of the eruption, its location, the pathways of aerial and marine transport, and the condition of the atmosphere and water at the time of the eruption. It was found that the grain-size distributions for the tephra in the bed in question contain information on two explosive episodes that lasted 10 days each that took place under the conditions of atmospheric circulation typical of spring (the earlier episode) and of autumn (the later episode). For both of these episodes we determined the most likely areas of ash deposition on the marine water surface. We also identified a complex structure of the areas of concentrated particle distribution on the basin bottom that reflected a vortical water structure in the Sea of Japan during the eruption.     

Eruptive activity at Mount St Helens,Washington, USA, 1984–1988: a gas geochemistry perspective

The results from two different types of gas measurement, telemetered in situ monitoring of reducing gases on the dome and airborne measurements of sulfur dioxide emission rates in the plume by correlation spectrometry, suggest that the combination of these two methods is particularly effective in detecting periods of enhanced degassing that intermittently punctuate the normal background leakage of gaseous effluent from Mount St Helens to the atmosphere. Gas events were recorded before lava extrusion for each of the four dome-building episodes at Mount St Helens since mid-1984. For two of the episodes, precursory reducing gas peaks were detected, whereas during three of the episodes, COSPEC measurements recorded precursory degassing of sulfur dioxide. During one episode (October 1986), both reducing gas monitoring and SO₂ emission rate measurements simultaneously detected a large gas release several hours before lava extrusion. Had both types of gas measurements been operational during each of the dome-building episodes, it is thought that both would have recorded precursory signals for all four episodes. Evidence from the data presented herein suggests that increased degassing at Mount St Helens becomes detectable when fresh upward-moving magma is between 2 km and a few hundred meters below the base of the dome and between about 60 and 12 hours before the surface extrusion of lava.     

Prediction of episodic acidification in North‐eastern USA: an empirical/mechanistic approach

Observations from the US Environmental Protection Agency's Episodic Response Project (ERP) in the North‐eastern United States are used to develop an empirical/mechanistic scheme for prediction of the minimum values of acid neutralizing capacity (ANC) during episodes. An acidification episode is defined as a hydrological event during which ANC decreases. The pre‐episode ANC is used to index the antecedent condition, and the stream flow increase reflects how much the relative contributions of sources of waters change during the episode. As much as 92% of the total variation in the minimum ANC in individual catchments can be explained (with levels of explanation >70% for nine of the 13 streams) by a multiple linear regression model that includes pre‐episode ANC and change in discharge as independent variables. The predictive scheme is demonstrated to be regionally robust, with the regional variance explained ranging from 77 to 83%. The scheme is not successful for each ERP stream, and reasons are suggested for the individual failures. The potential for applying the predictive scheme to other watersheds is demonstrated by testing the model with data from the Panola Mountain Research Watershed in the South‐eastern United States, where the variance explained by the model was 74%. The model can also be utilized to assess ‘chemically new’ and ‘chemically old’ water sources during acidification episodes. Copyright © 1999 John Wiley & Sons, Ltd.     

Discrete‐Storm Water‐Table Fluctuation Method to Estimate Episodic Recharge

We have developed a method to identify and quantify recharge episodes, along with their associated infiltration‐related inputs, by a consistent, systematic procedure. Our algorithm partitions a time series of water levels into discrete recharge episodes and intervals of no episodic recharge. It correlates each recharge episode with a specific interval of rainfall, so storm characteristics such as intensity and duration can be associated with the amount of recharge that results. To be useful in humid climates, the algorithm evaluates the separability of events, so that those whose recharge cannot be associated with a single storm can be appropriately lumped together. Elements of this method that are subject to subjectivity in the application of hydrologic judgment are values of lag time, fluctuation tolerance, and master recession parameters. Because these are determined once for a given site, they do not contribute subjective influences affecting episode‐to‐episode comparisons. By centralizing the elements requiring scientific judgment, our method facilitates such comparisons by keeping the most subjective elements openly apparent, making it easy to maintain consistency. If applied to a period of data long enough to include recharge episodes with broadly diverse characteristics, the method has value for predicting how climatic alterations in the distribution of storm intensities and seasonal duration may affect recharge.     

东南沿海地震形势分析

研究１９００年以来东南沿海地震活动序特征，并进行了地震活动分幕，结果表明，１９８６年开始进入了本世纪以来的第Ｖ地震活跃幕，本活跃幕将延至２０００年左右，期间有发生６级左右地震的危险，最大可能的发震区是闽粤交界及其沿海地区。     

我国大陆强震活动的韵律性特征

本文应用1895年以来我国大陆地区7级和大于7级的强震资料,讨论了我国大陆强震活动的轮回特性,指出了我国大陆地区强震高潮的到来有一个发展过程,即经历强震活动的平静、增强及出现强震高潮三个幕。文中讨论了各幕的持续时间及其强震活动的频度和能量分布,同时还分析了各轮回中强震的地区分布,探讨了每个强震高潮的主体活动区的形成特点。这些结果可为研究我国大陆强震高潮到来和结束的标志及判断未来强震高潮的主体活动地区等强震预测问题提供线索。此外,本文还分析了我国大陆强震高潮与东北深震的相关关系等问题     

Multiple ruptures for Long Valley microearthquakes: a link to volcanic tremor?

Despite several episodes of ground deformation and intense seismic activity starting in 1978, the Long Valley, California, volcanic area has not produced clearly recognized volcanic tremor. Instead, a variety of atypical microearthquakes have been recorded during these episodes, including events dominated by low-frequency (long-period) or mixed high and low-frequency (hybrid) signals. During a 1997 episode, a number of unusual microearthquakes occurred within a temporary 40-station seismic network surrounding the Casa Diablo area, allowing the events to be precisely located and analyzed as a function of azimuth, offset, and source characteristics. Eight prime examples lie within two, 7 km-deep clusters of seismicity separated by about 1 km, with four events in each cluster. Empirical Green's function deconvolution shows that these events are composed of two to three sub-events, the sub-events consisting of ordinary (single rupture, double-couple) microearthquakes. The delay times between the sub-events are constant within each cluster, equaling 0.092 s in one and 0.06 s in the other. Events from other clusters show similar delays. The signal interference produced by the closely spaced sub-events gives rise to modulated, delay-dependent source spectra. The regularity of the delays suggests that the sub-events are being triggered by a fixed length and/or time scale process, an example being the length/inflation rate of a magmatic or hydrothermal flow structure. With continued action of the triggering process, the sub-events could proliferate and evolve into observable volcanic tremors at Mammoth.     

Contrasts in storm event hydrochemistry in an acidic afforested catchment in upland Wales

The hydrochemistry of stream water in an acidic afforested catchment in the Welsh uplands was monitored routinely between 1985 and 1990. Nineteen storm episodes were sampled intensively during this period. Although the general storm response of the stream can be characterised by increased concentrations of H⁺, Al and dissolved organic carbon (DOC), and a dilution of Ca and SiO₂, the detailed hydrochemistry of individual acid episodes exhibited marked contrasts. The minimum pH reached during specific episodes ranged from 4.1 to 5.0, and peak dissolved Al concentrations varied from 9 to 44 μmol l⁻¹. The reasons for such differences in the hydrochemical response can be identified for each individual episode by examining the complex interactions between (1) the quantity and quality of event precipitation, (2) antecedent patterns of weather and atmospheric deposition and (3) the hydrological processes which dominate the storm runoff response. The dynamic nature of catchment hydrology was found to exert a particularly strong influence on the hydrochemistry of specific acid episodes.     

华北地区第三、 四活动期平静幕后期中强地震活动与未来强震

本文通过分析、 研究华北地区第三、 四活动期各平静幕后期的中强地震活动特征, 认为: ① 华北地区第三、 四活动期9个活跃幕中有7个在首发强震前3年内发生中强地震, 其中第四活动期1～4活跃幕首发强震前2年内都发生了中强地震, 未来第5活跃幕首发强震前, 以发生中强地震作为序幕的可能性较大; ② 华北地区平静幕后期的中强地震活动, 对其后活跃幕首发强震在时间和地点上有一定指示作用。 由于华北地区可能即将进入新的强震活跃幕, 该研究对判断华北地区未来强震趋势具有实用意义。     

对长江宜昌站月平均径流量突变的分析

本文简要叙述了一阶矩突变扫描式t检验和二阶矩突变扫描式F检验算法,并应用于长江宜昌水文站1882-2002年间历年月平均径流量的实测序列,划分出24个年际至十年际时间尺度的相对5级旱、涝时段及其变率特征,与有关长江上游洪水、干旱灾害的历史记载相吻合.     

Late stage thermal history of the Songliao Basin and its tectonic implications: Evidence from apatite fission track (AFT) analyses

Apatite Fission Track (AFT) data from the Songliao Basin indicates that the late stage tectonic movements in the Songliao Basin have zoning in space and episodes in time. The late stage tectonic movements started from the east part of the basin and migrated westward. AFT ages in the east part of the basin are older than those in the west part of the basin, suggesting that the uplift occurred earlier in the east than in the west. The denudation thickness in the east part of the basin is significantly greater than that in the centre and west. The thermal history evolved two episodes of rapid cooling and subsequent slow cooling processes. Age-depth relationship derived from the AFT data indicates a four-episode denudation history. Further Monte Carlo random simulation of the AFT data reveals the four changing points of the thermal evolution at 65 Ma, 43.5 Ma, 28 Ma and 15 Ma, respectively. The uplifting and denudation rates from different episodes of evolution are proportional to the plate convergence rate. Based on the above analyses and the regional geologic background, it is concluded that the late stage thermal events in the Songliao Basin are the far field response to the subduction of the Pacific Plate under the Eurasian Plate. The first episode of the rapid cooling probably started at the end of the Nenjiang Formation, climaxed at the end of the Cretaceous and ceased at the Late Eocene. The subsequent slow cooling lasts another 15 Ma. The first episode of the evolution is the far field response to the major episode of the Yanshan Movement and subsequent series of the tectonic reorganization, especially the directional change of the Pacific Movement and also the subduction of the Indian Plate underneath the Eurasian Plate. While the second episode of the evolution is the far field response to the extension and closure of the Sea of Japan. Extension led to the migration and converging of the mantle heat flow to the Sea of Japan and resulted in the rapid cooling of the Songliao Basin.     

Possible effect of ENSO on annual sediment discharge of debris flows in the Jiangjia Ravine based on Morlet wavelet transforms

The multi-time-scale structures of an annual sediment discharge series of debris flow in the Jiangjia Ravine and the Southern Oscillation index are analysed using the method of Morlet wavelet transformations. The possible effects of E1 Nirio episodes on the annual sediment discharge are discussed by comparing the period variations of ENSO and the discharge. The results show that the annual sediment discharge series of debris flow is related to E1 Nifio episodes. Generally, the annual sediment discharge of debris flow is less than usual during an E1 Nifio episode and debris flow is less active. On the contrary, the annual sediment discharge of debris flows is greater than usual during a La Nifia episode and debris flows are more frequent. There is a relationship between the annual sediment discharges of debris flow in the Jiangjia Ravine and the summer Southern Oscillation index, with both having quasi-periodic variations of 2 and 5-6 years.     

Late Pleistocene to Holocene eruptive activity of Pico de Orizaba,Eastern Mexico

The Late Pleistocene to Holocene eruptive history of Pico de Orizaba can be divided into 11 eurptive episodes. Each eruptive episode lasted several hundred years, the longest recorded being about 1000 years (the Xilomich episode). Intervals of dormancy range from millenia during the late Pleistocene to about 500 years, the shortest interval recorded in the Holocene. This difference could reflect either changes in the volcano's activity or that the older stratigraphic record is less complete than the younger. Eruptive mechanisms during the late Pleistocene were characterized by dome extrusions, lava flows and ash-and-scoria-flow generating eruptive columns. However, in Holocene time plinian activity became increasingly important. The increase in dacitic plinian eruptions over time is related to increased volumes of dacitic magma beneath Pico de Orizaba. We suggest that the magma reservoir under Pico de Orizaba is stratified. The last eruptive episode, which lasted from about 690 years bp until ad 1687, was initiated by a dacitic plinian eruption and was followed by effusive lava-forming eruptions. For the last 5,000 years the activity of the volcano has been gradually evolving towards such a trend, underlining the increasing importance of dacitic magma and stratification of the magma reservoir. Independent observations of Pico de Orizaba's glacier early this century indicate that some increase in volcanic activity occurred between 1906 and 1947, and that it was probably fumarolic.     

The Occurrence of Haze in Malaysia: A Case Study in an Urban Industrial Area

— Klang Valley, a heavily industrialized urban area in Malaysia, has experienced severe haze episodes since the early 1980s. Total Suspended Particulate matter (TSP) is used in studying this phenomenon. Three severe haze episodes during the early 1990s are reviewed; August 1990, October 1991, and August–October 1994. The nature of these episodes, their possible causes, and their major features are discussed. Meteorological conditions associated with these episodes were analyzed. Results of the study indicate that stability and trapping of particles are the main factors affecting the pollution during haze periods. Maximum total suspended matter (TSP) was recorded in October 1991. The August–October 1994 episode was the most persistent and least affected by meteorological variables. Analysis of wind direction data showed that southerly and southwesterly winds coincided with the worst haze periods.     

Multi-episode fluid boiling in the Shizishan copper-gold deposit at Tongling, Anhui Province: its bearing on ore formation

The Shizishan copper-gold deposit at Tongling, Anhui Province consists of two magmato-hydrothermal mineralization types: the crypto-explosive breccia type and the skarn type. At least four episodes of boiling occurred to the ore-forming fluids in this deposit. The first episode took place in accompany with the formation of the crypto-explosive breccias. The melt-fluid inclusions giving temperatures above 600℃ and salinities higher than 42% NaCl equiv represent a residual magma related to this episode. The second episode occurred during skarnization, giving fluid temperatures of 422℃-472℃, averaging 458℃, and salinities of 10.2%-45.1% NaCl equiv. The third episode corresponds to the main mineralization stage, i.e., the quartz-sulphide stage. Fluid temperatures of this episode vary in a range of 337℃-439℃ with an average of 390℃, and salinities in a range of 3%-30% NaCl equiv. The forth episode happened at the waning stage of mineralization, giving fluid temperatures below 350℃ with an average of 265℃ and salinities of 2.1%-40.4% NaCl equiv.     

New insights into the 1999 eruption of Shishaldin volcano,Alaska, based on acoustic data

Data collected by a pressure sensor provide new insights into the 1999 eruption of Shishaldin volcano, Unimak Island, Alaska. On 19 April 1999, after 3 months of unrest and an extended period of low-level Strombolian activity, Shishaldin experienced a Subplinian eruption (ash plume to >16 km), followed by several episodes of strong Strombolian explosions. Acoustic data from the pressure sensor allow us to investigate the details of an eruption which was instrumentally well recorded, but with few visual observations. In the 12 h prior to the Subplinian phase, the pressure sensor detected a series of small, repeated pulses with a constant spectral peak at 2–3 Hz. The amplitude and occurrence rate of the pulses both grew such that the signal became a nearly continuous hum just before the Subplinian eruption. This humming signal may represent gas release from rising magma. The main Subplinian phase was heralded by (1) the abrupt end of the humming signal, (2) several pulses of low-frequency sound interpreted as ash bursts, and (3) a dramatic increase in seismic tremor amplitude. The change in acoustic signature at this time allows us to precisely time the start of the Subplinian eruption, previously approximated as the time of strongest tremor increase. The 50-min Subplinian phase actually contained several bursts of signal, each of which may represent a discrete volume of magma passing through the system. Following the Subplinian event, the pressure sensor recorded four discrete episodes of Strombolian gas explosions on 19–20 April and another on 22–23 April. Four of the five episodes were accompanied by strong seismic tremor; the fifth has not been previously recognized and was not associated with anomalous tremor amplitudes. In time series these events are similar to explosions recorded at other volcanoes but in general they are much larger, with maximum amplitudes of >65 Pa at 6.5 km from the vent, and they have low (0.7–1.5 Hz) peak frequencies. These large explosions occurred at rates of 3–20 per minute for 1–5 h in each episode. The explosions were accompanied by a small (<5 km above sea level) ash plume and only minor amounts of ejecta were produced. Thus, the explosion activity was dominated by gas release.     

Thermal history of the Rio Muni (West Africa)–NE Brazil margins during continental breakup

We document the thermal record of breakup of the conjugate Rio Muni (West Africa) and NE Brazil margins using apatite fission track analysis, vitrinite reflectance data and stratigraphic observations from both margins. These results permit determination of the timing of four cooling episodes, and the temperature of samples at the onset of each episode. All samples are interpreted to have experienced higher temperatures in the geological past due to i) elevated basal heatflow (palaeogeothermal gradient in Rio Muni-1 well decaying from 58 °C/km during the Mid Cretaceous to 21.5 °C/km in the Late Cenozoic) and ii) progressive exhumation from formerly greater burial depth. A well constrained history of changing palaeogeothermal gradient allows for much more precise quantification of the thickness of eroded section (exhumation) than if a constant heatflow is assumed. Cooling episodes identified from the palaeotemperature data at 110–95 Ma (both margins) and 85–70 Ma (Rio Muni only) coincide with major unconformities signifying, respectively, the cessation of rifting (breakup) and compressional shortening that affected the African continent following the establishment of post-rift sedimentation (drift). The interval between these separate unconformities is occupied by allochthonous rafts of shallow-water carbonates recording gravitational collapse of a marginal platform. The rift shoulder uplift that triggered this collapse was enhanced by local transpression associated with the obliquely divergent Ascension Fracture Zone, and thermal doming due to the coeval St Helena and Ascension Plumes. The data also reveal a c.45–35 Ma cooling episode, attributed to deep sea erosion at the onset of Eo-Oligocene ice growth, and a c.15–10 Ma episode interpreted as the record of Miocene exhumation of the West African continental margin related to continent-wide plume development. Integration of thermal history methods with traditional seismic- and stratigraphy-based observations yields a dynamic picture of kilometre-scale fluctuations in base level through the breakup and early drift phases of development of these margins. Major unconformities at ocean margins are likely to represent composite surfaces recording not only eustasy, but also regional plate margin-generated deformation, local ‘intra-basinal’ reorganization, and the amplifying effect of negative feedbacks between these processes.     

琼东南盆地构造沉降史及其主控因素

以28口钻井、3条主干地震剖面最新解释成果资料为依据的构造沉降史恢复结果表明：琼东南盆地新生代存在三期快速沉降过程和一期缓慢沉降过程：第一期为始新世,第二期为渐新世—早中新世,第三期存在时空差异：在盆地西部乐东凹陷为晚中新世以来,在盆地中东部陵水凹陷和松南-宝岛凹陷为上新世以来;缓慢沉降过程亦存在时空差异,盆地西部中中新世沉降缓慢,盆地中东部中—晚中新世沉降缓慢.第一期快速沉降过程受东亚陆缘扩张和红河断裂左旋走滑共同影响,第二期快速沉降受南海海底扩张和红河断裂左旋走滑联合作用,第三期快速沉降主要受红河断裂右旋走滑控制,缓慢沉降过程与南海海底扩张停止以及红河断裂构造活动处于宁静阶段相耦合.