Damage to the shallow Landers fault from the nearby Hector Mine earthquake

Crustal faults have long been identified as sites where localized sliding motion occurs during earthquakes, which allows for the relative motion between adjacent crustal blocks. Although there is a growing awareness that we must understand the evolution of fault systems on many timescales to relate present-day crustal stresses and fault motions to geological structures formed in the past, fault-zone damage and healing have been documented quantitatively in only a few cases. We have been monitoring the healing of damage on the shallow Johnson Valley fault after its rupture in the 1992 magnitude-7.3 Landers earthquake, and here we report that this healing was interrupted in 1999 by the magnitude-7.1 Hector Mine earthquake rupture, which occurred 20-30 km away. The Hector Mine earthquake both strongly shook and permanently strained the Johnson Valley fault, adding damage discernible as a temporary reversal of the healing process. The fault has since resumed the trend of strength recovery that it showed after the Landers earthquake. These observations lead us to speculate that fault damage caused by strong seismic waves may help to explain earthquake clustering and seismicity triggering by shaking, and may be involved in friction reduction during faulting.     

Delayed triggering of the 1999 Hector Mine earthquake by viscoelastic stress transfer

Stress changes in the crust due to an earthquake can hasten the failure of neighbouring faults and induce earthquake sequences in some cases. The 1999 Hector Mine earthquake in southern California (magnitude 7.1) occurred only 20 km from, and 7 years after, the 1992 Landers earthquake (magnitude 7.3). This suggests that the Hector Mine earthquake was triggered in some fashion by the earlier event. But uncertainties in the slip distribution and rock friction properties associated with the Landers earthquake have led to widely varying estimates of both the magnitude and sign of the resulting stress change that would be induced at the location of the Hector Mine hypocentre-with estimates varying from -1.4 bar (ref. 6) to +0.5 bar (ref. 7). More importantly, coseismic stress changes alone cannot satisfactorily explain the delay of 7 years between the two events. Here we present the results of a three-dimensional viscoelastic model that simulates stress transfer from the ductile lower crust and upper mantle to the brittle upper crust in the 7 years following the Landers earthquake. Using viscoelastic parameters that can reproduce the observed horizontal surface deformation following the Landers earthquake, our calculations suggest that lower-crustal or upper-mantle flow can lead to postseismic stress increases of up to 1-2 bar at the location of the Hector Mine hypocentre during this time period, contributing to the eventual occurrence of the 1999 Hector Mine earthquake. These results attest to the importance of considering viscoelastic processes in the assessment of seismic hazard.     

Remote triggering of deep earthquakes in the 2002 Tonga sequences

It is well established that an earthquake in the Earth's crust can trigger subsequent earthquakes, but such triggering has not been documented for deeper earthquakes. Models for shallow fault interactions suggest that static (permanent) stress changes can trigger nearby earthquakes, within a few fault lengths from the causative earthquake, whereas dynamic (transient) stresses carried by seismic waves may trigger earthquakes both nearby and at remote distances. Here we present a detailed analysis of the 19 August 2002 Tonga deep earthquake sequences and show evidence for both static and dynamic triggering. Seven minutes after a magnitude 7.6 earthquake occurred at a depth of 598 km, a magnitude 7.7 earthquake (664 km depth) occurred 300 km away, in a previously aseismic region. We found that nearby aftershocks of the first mainshock are preferentially located in regions where static stresses are predicted to have been enhanced by the mainshock. But the second mainshock and other triggered events are located at larger distances where static stress increases should be negligible, thus suggesting dynamic triggering. The origin times of the triggered events do not correspond to arrival times of the main seismic waves from the mainshocks and the dynamically triggered earthquakes frequently occur in aseismic regions below or adjacent to the seismic zone. We propose that these events are triggered by transient effects in regions near criticality, but where earthquakes have difficulty nucleating without external influences.     

Annual modulation of triggered seismicity following the 1992 Landers earthquake in California

The mechanism responsible for the triggering of earthquakes remains one of the least-understood aspects of the earthquake process. The magnitude-7.3 Landers, California earthquake of 28 June 1992 was followed for several weeks by triggered seismic activity over a large area, encompassing much of the western United States. Here we show that this triggered seismicity marked the beginning of a five-year trend, consisting of an elevated microearthquake rate that was modulated by an annual cycle, decaying with time. The annual cycle is mainly associated with several hydrothermal or volcanic regions where short-term triggering was also observed. These data indicate that the Landers earthquake produced long-term physical changes in these areas, and that an environmental source of stress--plausibly barometric pressure--might be responsible for the annual variation.     

Mapping the Hawaiian plume conduit with converted seismic waves

The volcanic edifice of the Hawaiian islands and seamounts, as well as the surrounding area of shallow sea floor known as the Hawaiian swell, are believed to result from the passage of the oceanic lithosphere over a mantle hotspot. Although geochemical and gravity observations indicate the existence of a mantle thermal plume beneath Hawaii, no direct seismic evidence for such a plume in the upper mantle has yet been found. Here we present an analysis of compressional-to-shear (P-to-S) converted seismic phases, recorded on seismograph stations on the Hawaiian islands, that indicate a zone of very low shear-wave velocity (< 4 km s(-1)) starting at 130-140 km depth beneath the central part of the island of Hawaii and extending deeper into the upper mantle. We also find that the upper-mantle transition zone (410-660 km depth) appears to be thinned by up to 40-50 km to the south-southwest of the island of Hawaii. We interpret these observations as localized effects of the Hawaiian plume conduit in the asthenosphere and mantle transition zone with excess temperature of approximately 300 degrees C. Large variations in the transition-zone thickness suggest a lower-mantle origin of the Hawaiian plume similar to the Iceland plume, but our results indicate a 100 degrees C higher temperature for the Hawaiian plume.     

多孔岩石中应力波传播特性及震源函数

建立了两种着重描述多孔岩石的塑性压缩(孔隙坍塌)机理的弹塑性本构模型,分别为将孔隙压实过程计入P-V关系的变模量理想塑性包络面模型和将孔隙演化行为计入帽盖的双屈服面帽盖模型.在此基础上,对地下球形药包起爆引起的弹塑性波在多孔凝灰岩中的传播过程进行计算,揭示了多孔岩石中应力波传播的一些基本特性.并采用改进的帽盖模型,研究了多孔凝灰岩中地下爆炸震源函数的特性,着重分析了孔隙度对震源函数的稳态值、拐角频率以及过冲特性的影响.     

Significant isostatic imbalance near the seismic gap between the M8.0 Wenchuan and M7.0 Lushan earthquakes

A gravity network with 302 observation points has been established in the western Sichuan Foreland Basin （SFB） to explore Bouguer gravity anomalies （BGAs）. Our observational results reveal that the BGAs are negative as a whole, with a maximum value of -220 mGal （10^-5m s^-2） at the northwest region of the study area. The real Moho depths beneath the SFB revealed by BGA data change smoothly from 39.5 km in the southeast to 43.7 km in the northwest of the monitoring region. However, the isostatic ones deduced from Airy isostatic model and topographical data vary approximately 39.5-42.0 km. The maximum differences of 2.7 km between the real and isostatic Moho depths are found near the seismic gap between the M8.0 Wenchuan and M7.0 Lushan earthquakes, where the crust is in the greatest isostatic imbalance of the monitoring region. Analysis of the isostatic state indicates that the deep dynamic environment near the seismic gap between these two earthquakes indicates an M ≥ 7.0 earthquake in the future. This study indicates that we can use isostasy as a potential approach to study the dynamic process of crustal material movement and to analyze regional potential seismic risks.     

Effects of acoustic waves on stick-slip in granular media and implications for earthquakes

It remains unknown how the small strains induced by seismic waves can trigger earthquakes at large distances, in some cases thousands of kilometres from the triggering earthquake, with failure often occurring long after the waves have passed. Earthquake nucleation is usually observed to take place at depths of 10-20 km, and so static overburden should be large enough to inhibit triggering by seismic-wave stress perturbations. To understand the physics of dynamic triggering better, as well as the influence of dynamic stressing on earthquake recurrence, we have conducted laboratory studies of stick-slip in granular media with and without applied acoustic vibration. Glass beads were used to simulate granular fault zone material, sheared under constant normal stress, and subject to transient or continuous perturbation by acoustic waves. Here we show that small-magnitude failure events, corresponding to triggered aftershocks, occur when applied sound-wave amplitudes exceed several microstrain. These events are frequently delayed or occur as part of a cascade of small events. Vibrations also cause large slip events to be disrupted in time relative to those without wave perturbation. The effects are observed for many large-event cycles after vibrations cease, indicating a strain memory in the granular material. Dynamic stressing of tectonic faults may play a similar role in determining the complexity of earthquake recurrence.     

采空区上覆岩层诱发矿震及预测技术研究

基于大面积采空区垮落灾害案倒及文献资料,探讨了近年来出现的一种特殊矿山煤岩动力灾害现象：采空区顶板火面积垮落诱发矿震与气体动力灾害。论证了矿震发生前期的动力演变与时空尺度。应用能量转换与守恒理论,详尽地揭示了采空区矿震表征及动力灾害构成要素的时空关联尺度。针对矿震的发展及形成过程提出将声发射监测技术作为矿震预报的基础,并提出运用线定位法和任意平面三角形定位法对震源定位的基本原理。     

多维地震动作用下隔震桥梁结构的地震响应分析

分析了多维地震动作用下隔震桥梁结构的水平向和竖向振动反应.首先建立了隔震桥梁结构的非线性运动方程,根据非线性随机振动理论对运动方程进行了等效线性化处理,并采用实模态法获得了等效线性方程的解析解,提出了结构响应的计算公式.最后给出了多维地震动作用下隔震桥梁结构的地震响应分析算例,计算结果显示了分析方法的有效性.     

汶川地震底层框架砖房震害分析及底部侧移刚度控制

文章基于汶川地震中底层框架砖房的典型震害调查,了解了底层框架砖房的震害现象,分析了震害产生的原因,探讨了第二层与底层的侧移刚度比取值的合理范围.指出底框砖房底框与上部砖房侧移刚度比的合理取值,应使地震时底层既有充分弹塑性变形,上部砖房破坏亦不重,此时层间极限剪力系数分布比较均匀,有利于结构整体抗震.     

拱坝河谷的地震波散射效应及坝基体系的动力响应

研究了局部场地条件对地震波的散射效应,对拱坝－地基体系在不均匀散射场和均匀输入场地震作用下的动力时程响应进行了对比分析。对小湾拱坝的实际分析表明,局部场地条件对地震波的散射效应对于拱坝的应力分布的峰值大小的影响是不容忽视的,甚至是至关重要的。     

地震诱发山洪形成机理与评估指标初探

地震诱发山洪是危害极大的地震次生灾害之一．国内外大量的野外现场调查与研究表明,地震滑坡堵江形成堰塞湖在山区河流广泛发育．滑坡堵江堰塞湖存在坝体的渗透变形、稳定性、不均匀沉降和砂土液化等工程地质问题的同时,还存在水力学与河床演变的科学技术问题．本文从地震诱发山洪形成机理初步分析出发,对堰塞湖诱发山洪评估指标进行初步探索．包括堰塞湖排险泄洪明渠粗化保护层形成过程及“拟混凝土”形成条件研究;堰塞湖排险泄洪明渠粗化保护层破坏条件：堰塞湖排险泄洪明渠动态平衡水力几何形态：堰塞湖排险泄洪明渠从下游出口开始的溯源冲刷过程及动态平衡纵横剖面．建议进一步的研究目标是：制订确保堰塞湖安全平稳泄洪排险的指标体系：确定堰塞湖排险泄洪明渠粗化保护层形成与破坏条件的定量化方法：确定堰塞湖下游人员安全避险撤离的时机与优化方案;因势利导,对堰塞湖进行分类重建,为制订《地震地质灾害堰塞湖排险技术规范》提供成套技术的科学支撑．     

Great Himalayan earthquakes and the Tibetan plateau

It has been assumed that Himalayan earthquakes are driven by the release of compressional strain accumulating close to the Greater Himalaya. However, elastic models of the Indo-Asian collision using recently imaged subsurface interface geometries suggest that a substantial fraction of the southernmost 500 kilometres of the Tibetan plateau participates in driving great ruptures. We show here that this Tibetan reservoir of elastic strain energy is drained in proportion to Himalayan rupture length, and that the consequent growth of slip and magnitude with rupture area, when compared to data from recent earthquakes, can be used to infer a approximately 500-year renewal time for these events. The elastic models also illuminate two puzzling features of plate boundary seismicity: how great earthquakes can re-rupture regions that have already ruptured in recent smaller earthquakes and how mega-earthquakes with greater than 20 metres slip may occur at millennia-long intervals, driven by residual strain following many centuries of smaller earthquakes.     

汶川地震触发崩滑与Arias强度关系研究

利用汶川地震获得的强震记录及诱发的大量崩滑数据,对地震触发滑坡和Arias强度的关系进行了研究,得到汶川地震在龙门山震区Arias强度可以触发崩滑的水平向下限值约为0.4m/s,上限值可以定为4m/s,研究区不同出露地层可以触发崩滑的Arias强度变化范围不同,平均下限不超过1.0m/s.对部署于龙门山断裂带的强震数据拟合,得到汶川地震龙门山震区Arias强度的衰减关系.利用衰减关系对地震触发崩滑影响区域的估计与实际情况比较吻合,表明综合了地震动记录的振幅、持时和频率信息的Arias强度参数与地震地表破坏程度关系更为密切.可以将Arias强度参数应用于震后地震触发崩滑等地质灾害影响区域的评估工作,评估结果较以烈度作为评估因子的结果更为集中,比较适合于震后应急期间灾害快速评估,可为应急救援决策和方案的制定提供参考信息.     

地震与“媒介世界”

针对5·12汶川大地震媒介事件,从事件、受灾、媒体人、灾民等角度,解析了这一具有高度参与性,整合力及感召力的媒介仪式。     

由地震引发的相关民事诉讼程序问题探析

地震作为典型的不可抗拒事件,对民事诉讼程序会产生重要影响。汶川大地震后,最高人民法院针对震后地区先后颁布了一系列关于民事审判的司法解释,体现了人民法院司法为民的本质属性和核心价值。地震对民事诉讼程序的影响主要体现在管辖、立案、期间、送达、审判与执行等方面。明确地震期间和灾后民事诉讼程序的特点,有利于更好地审理震后地区的民事案件。     

汶川地震与芦山地震次生地质灾害特征对比

"5·12"汶川特大地震和"4·20"芦山地震在造成重大人员伤亡和财产损失的同时,还诱发了大量的崩塌、滑坡和泥石流等次生地质灾害。基于龙门山地震地质背景,根据已公开发表的成果,结合野外资料,对比分析了汶川地震与芦山地震次生地质灾害的差异,并探讨影响二者差异的因素,得到以下3点认识:(1)芦山地震与汶川地震发震机理不同,但汶川地震对芦山地震具有诱发作用;(2)汶川地震和芦山地震均触发了一系列次生地质灾害,但汶川地震触发的震后次生地质灾害在规模和持续时间上更广泛和持久;(3)水系对地表物质的侵蚀过程因地震驱动的崩滑而被加速。此外,汶川地震的震滑坡量远大于同震岩石隆升增加的山脉体积,未来几十年里,滑坡泥石流等地质灾害将会持续进行。