Causes of sinks near Tucson, Arizona, USA

 Land subsidence in the form of sinks has occurred on and near farmlands near Tucson, Pima County, Arizona, USA. The sinks occur in alluvial deposits along the flood plain of the Santa Cruz River, and have made farmlands dangerous and unsuitable for farming. More than 1700 sinks are confined to the flood plain of the Santa Cruz River and are grouped along two north-northwestward-trending bands that are approximately parallel to the river and other flood-plain drainages. An estimated 17,000 m³ of sediment have been removed in the formation of the sinks. Thirteen trenches were dug to depths of 4–6 m to characterize near-surface sediments in sink and nonsink areas. Sediments below about 2 m included a large percentage of dispersive clays in sink areas. Sediments in nonsink areas contain a large component of medium- to coarse-grained, moderately to well sorted sand that probably fills a paleochannel. Electromagnetic surveys support the association of silts and clays in sink areas that are highly electrically conductive relative to sand in nonsink areas. Sinks probably are caused by the near-surface process of subsurface erosion of dispersive sediments along pre-existing cracks in predominantly silt and clay sediments. The pre-existing cracks probably result from desiccation or tension that developed during periods of water-table decline and channel incision during the past 100 years or in earlier periods. Submitted, April 1997 · Revised, November 1997 · Accepted, April 1998     

城市地面沉降设防问题

我国地面沉降地质灾害存在长期蔓延之势,主观原因在于对上海地面沉降的治疗认识不足。通过对地面沉降发生的必要条件的探究,对有关城市地面沉降设防的若干问题,作一初步探讨。     

确定大江大河防洪堤高度应考虑地面沉降因素——对1998中国大洪水的反思

世界上抽汲地下水或石油引起地面沉降正越来越普遍。大江大河防洪能力降低是地面沉降灾害现象之一。上海因地面沉降产生严重的洪涝灾害,上海外滩防汛墙高度的主要决定因素是地面沉降。沙市、武汉、哈尔滨和大庆等城市和地区是我国1998年夏特大洪水的重灾地区,这些城市和地区实际上已存在地面沉降问题。几十年来产生的地面沉降很可能已降低了这些地区江河大堤的防洪能力,从而加重了这次洪涝灾害。建议在确定大江大河防洪堤高度应考虑地面沉降因素,并尽快对重点城市和地区进行地面沉降调查与预测研究。     

北京市地面沉降监测网建设

本文主要介绍了北京市地面沉降监测系统组成情况,简要陈述各种监测方法,为初步了解北京市地面沉降监测网提供参考。     

天津市地面沉降的灰色系统—马尔柯夫预测模型应用

地面沉降速率监测值多呈波形动态,数据波动给预测带来了困难。本文以天津地面沉降多个水准点的监测数据为基本序列,介绍了传统灰色系统模型GM（1,1）的预测过程及结果,并在此基础上进行马尔柯夫处理,建立灰色系统-马尔柯夫预测模型。其结果表明,后者预测精度明显提高,67％的样本误差可降至10％以内,92％的样本预测值误差低于30％。     

Karst breakdown mechanisms from observations in the gypsum caves of the Western Ukraine: implications for subsidence hazard assessment

The term karst breakdown is employed in this paper to denote the totality of processes and phenomena of gravitational and/or hydrodynamic destruction of the ceiling of a karst cavity and of the overlying sediments. It refers not only to the existence of a surface subsidence (collapse) feature but, first of all, to the internal (hidden in the subsurface) structures that precede development of a surface form. This study reports and discusses the results of direct mapping and examination of breakdown structures in the gypsum karst of the Western Ukraine, at the level of their origin, i.e., in caves. The accessibility of numerous laterally extensive maze cave systems in the region provided an excellent opportunity for such an approach, which made it possible to examine the relationship between breakdown structures and particular morphogenetic or geological features in caves, and to reveal stages of breakdown development. It is found that breakdown is initiated mainly at specific speleogenetically or geologically weakened localities, which classify into a few distinct types. The majority of breakdowns, which are potent to propagate through the overburden, relate with the outlet cupolas/domepits that represent places where water had discharged out of a cave to the upper aquifer during the period of transverse artesian speleogenesis. Distribution of breakdown structures does not correlate particularly well with the size of the master passages. Several distinct mechanisms of breakdown development are revealed, and most of them proceed in several stages. They are guided by speleogenetic, geological and hydrogeological factors. The study confirms that a speleogenetic approach is indispensable to the understanding of breakdown pre-requisites and mechanisms, as well as for eventual subsidence hazard assessment. Direct observations in caves, aimed both at speleogenetic investigation and breakdown characterization on regional- or site-specific levels, should be employed wherever possible.     

Submarine flank eruption preceding caldera subsidence during the 2000 eruption of Miyakejima Volcano, Japan

During the early part of a seismic swarm preceding eruption and caldera formation at Miyakejima Volcano, discoloured sea surfaces were observed 1.5 km off the western coast of Miyakejima on 27 June 2000. A later survey of the area using a multi-beam side scan sonar and a remotely operated small submarine revealed four craters of 20–30 m diameter aligned east-west in a 100×10–30 m area on the seafloor, with hot water at 140°C being released from one of the centres. Each crater consists of submarine spatter overlain in part by scoria lapilli. Dredged spatter from the craters was fresh, and there was no evidence of activity of marine organisms on the spatter surface, indicating that the discoloured sea surface resulted from magmatic eruption on the seafloor. This eruption occurred when a westward-propagating seismic swarm, initiated beneath Miyakejimas summit, passed through the area. Finding new magma on the seafloor demonstrates that this seismic swarm was associated with intruding magma, moving outward from beneath Miyakejima. Submarine spatter shows flattened shapes with a brittle crust formed by cooling in water, and its composition is aphyric andesite of 54 wt% SiO₂. The spatter is similar in whole rock and mineral composition to spatter erupted in 1983. However, the wide range of Cl in melt inclusions in plagioclase of the 27 June submarine spatter shows that it is not simply a remnant of the 1983 magma, which has only high Cl melt inclusions in plagioclase. The mixed character of melt inclusions suggests involvement of a magma with low Cl melt inclusions. The magma erupted explosively on 18 August from Miyakejimas summit, considered as the second juvenile magma in this eruption, contains low Cl melt inclusions in plagioclase. Based on these observations and the eruption sequence, we present the following model: (1) A shallow magma chamber was filled with a remnant of 1983 magma that had evolved to a composition of 54–55 wt% SiO₂. (2) Injection of the 18 August magma into this chamber generated a mixed magma having a wide range of Cl in melt inclusions contained plagioclase. The magma mixing might have occurred shortly before the submarine eruption and could have been a trigger for the initiation of the removal of magma from the chamber as an extensive dyke, which eventually led to caldera subsidence.Editorial responsibility: S Nakada, T Druitt     

Characterization of land subsidence in the Choshui River alluvial fan,Taiwan

Land subsidence in Taiwan has increased rapidly over the past four decades, owing to heavy withdrawal of groundwater. Leveling surveys, layer compressions and groundwater level in individual layers were continuously monitored in the Choshui alluvial fan, western Taiwan. It was found that ground subsidence and layer compression were consistent with the variation of groundwater level. From multi-level compression monitoring, the layers with major compression deformation were identified. Clayey and sandy layers exhibited significant compression deformation. Conceptual models of compression behavior for clays and sands were studied based on the field data. The compression behavior of clays agreed with the Terzaghi consolidation theory. Notable, irrecoverable volumetric strains were also observed in sandy layers. They exhibited obvious elasto-plastic mechanical behavior. The high compressibility of the sand layer in the alluvial fan was attributed to the flaky sand grains in this geological region.     

Rôle des héritages latéritiques et du niveau des nappes dans l'évolution des modelés et des sols en zone intertropicale forestière humide

Analysis of landscapes and soil covers from the outer to the inner part of a $75\text{km}\times 60\text{km}$ watershed in the South-Cameroon rainforest zone, characterized by important remnants of inherited ferricretes and stable hydrostatic base levels, indicates that the development of present landforms has not involved significant erosion by slope retreat because of the relative preservation of downslope residual ferricrete and that soil covers have vertically decayed under in situ geochemical process. To cite this article: D. Bitom et al., C. R. Geoscience 336 (2004).