大地震对龙门山构造带剥蚀-风化-碳循环的深远影响

构造运动对地表剥蚀、硅酸盐风化、长尺度碳循环以及气候系统的影响一直存在争论和不确定性。其主要原因在于,已有关于"构造-风化-碳循环-气候变化"耦合的证据主要基于河水化学计算的和海洋沉积物记录的硅酸盐风化以及有机碳输移通量,一直缺乏对构造事件前后剥蚀、风化通量的直接观测和对比研究,更不清楚构造运动对流域侵蚀和风化过程影响的幅度、范围及持续时间。龙门山位于构造活动活跃的青藏高原东缘,是学术界长期关注的热点区域,拥有大量的基础资料与背景数据。发生于龙门山地区的2008年汶川地震和2013年芦山地震引发了近8万个山体滑坡;这些滑坡体快速向河流系统输送大量碎屑物质,极大地影响了地表过程,为评价构造事件对流域剥蚀和风化的影响提供了非常珍贵的机会。文章通过定量评估地震滑坡对流域侵蚀、河流颗粒有机碳（POC）输移和河水溶质的影响及持续时间,结合滑坡体积与地震震级和复发频次关系,提出了地震滑坡对以龙门山为代表的活跃造山带长时间尺度剥蚀速率控制的新机制。结果表明,在2008年汶川地震之后的5年里,龙门山地区主要河流的悬浮物通量较地震前增加了3~7倍,指示了地震滑坡对流域侵蚀的直接贡献;清空单次地震产生的龙门山地区细颗粒滑坡物质需要几十年至数百年、粗颗粒则可能需要数千年,而滑坡密度和高强度径流是控制滑坡物质搬运时间的主要因素。通过模拟建立了龙门山地区地震震级-滑坡体积与复发频次的关系,计算得到该地区理论上的长时间尺度平均地震滑坡剥蚀速率,约为0.5~1.0 mm/a。这一理论值与宇宙成因核素和低温热年代学获得的长尺度剥蚀速率大致相当,并且汶川滑坡的空间分布与测得的长尺度剥蚀速率的空间分布相似,这表明在构造时间尺度上,龙门山构造带的剥蚀通量很可能主要是由地震引发的滑坡碎屑物质持续提供的,为解释构造活跃区的高剥蚀通量提供了一个全新的机制。与地震滑坡剥蚀同步的是河流现代POC和河水溶质的成倍增加。汶川地震造成岷江上游杂谷脑河现代POC供给增加了2~4倍,且只有少部分会被氧化降解;同时,震后岷江河水碱度和硅酸盐风化通量则增加了4倍左右,由此造成的CO2消耗通量和87Sr/86Sr比值分别增加了4.3±0.4倍和0.000644±0.000146。本研究将构造活动与长时间尺度山脉剥蚀、流域风化和碳循环直接地联系到一起,为"构造-风化-碳循环-气候变化"耦合提供了直接有力的观测证据和基于滑坡的全新机制解释。如果其他的地震也有类似汶川地震引发的有机碳埋藏和硅酸盐碳消耗成倍增加的双重碳汇效应,那么地震及其引发的滑坡可能在调节长时间尺度大气CO2和全球气候中的确起着重要的作用。     

Upwarped high velocity mafic crust, subsurface tectonics and causes of intraplate Latur-Killari (M 6.2) and Koyna (M 6.3) earthquakes, India – A comparative study

A unique attempt is made to understand the genesis of intraplate seismicity in the Latur-Killari and Koyna seismogenic regions of India, through derived crustal structure by synthesizing active and passive seismic, magnetotelluric, gravity and heat flow data. It has indicated presence of relatively high velocity/density intermediate granulite (and amphibolite) facies rocks underneath the Deccan volcanic cover caused mainly due to a continuous geodynamic process of uplift and erosion since Precambrian times. These findings have been independently confirmed by detailed borehole geological, geochemical and mineralogical investigations. The crystalline basement rock is found to contain 2 wt% of carbon-di-oxide fluid components. The presence of geodynamic process, associated with thermal anomalies at subcrustal depths, is supported by a high mantle heat flow (29–36 mW/m²) beneath both regions, although some structural and compositional variations may exist as evidenced by P- and S-wave seismic velocities. We suggest that the stress, caused by ongoing uplift and a high mantle heat flow is continuously accumulating in this denser and rheologically stronger mafic crust within which earthquakes tend to nucleate. These stresses appear to dominate over and above those generated by the India–Eurasia collision. The role of fluids in stress generation, as advocated through earlier studies, appears limited.     

Finite element modelling of elastic intraplate stresses due to heterogeneities in crustal density and mechanical properties for the Jabalpur earthquake region,central India

Deep lower crustal intraplate earthquakes are infrequent and the mechanism of their occurrence is not well understood. The Narmada-Son-lineament region in central India has experienced two such events, the 1938 Satpura earthquake and the 1997 Jabalpur earthquake, having a focal depth of more than 35 km. We have estimated elastic stresses due to the crustal density and mechanical properties heterogeneities along the Hirapur-Mandla profile passing through the Jabalpur earthquake region to analyse conditions suitable for the concentration of shear stresses in the hypocentral region of this earthquake. Elastic stresses have been computed by a finite element method for a range of material parameters. The results indicate that the shear stresses generated by the density heterogeneities alone are not able to locally enhance the stress concentration in the hypocentral region. The role of mechanical properties of various crustal layers is important in achieving this localization of stresses. Among a range of material parameters analysed, the model with a mechanically strong lower crust overlying a relatively weak sub-Moho layer is able to enhance the stress concentration in the hypocentral region, implying a weaker mantle in comparison to the lower crust for this region of central India.     

Tidal triggering of earthquakes in the subducting Philippine Sea plate beneath the locked zone of the plate interface in the Tokai region, Japan

We found a characteristic space–time pattern of the tidal triggering effect on earthquake occurrence in the subducting Philippine Sea plate beneath the locked zone of the plate interface in the Tokai region, central Japan, where a large interplate earthquake may be impending. We measured the correlation between the Earth tide and earthquake occurrence using microearthquakes that took place in the Philippine Sea plate for about two decades. For each event, we assigned the tidal phase angle at the origin time by theoretically calculating the tidal shear stress on the fault plane. Based on the distribution of the tidal phase angles, we statistically tested whether they concentrate near some particular angle or not by using Schuster's test. In this test, the result is evaluated by p-value, which represents the significance level to reject the null hypothesis that earthquakes occur randomly irrespective of the tidal phase angle. As a result of analysis, no correlation was found for the data set including all the earthquakes. However, we found a systematic pattern in the temporal variation of the tidal effect; the p-value significantly decreased preceding the occurrence of M ≥ 4.5 earthquakes, and it recovered a high level afterwards. We note that those M ≥ 4.5 earthquakes were considerably larger than the normal background seismicity in the study area. The frequency distribution of tidal phase angles in the pre-event period exhibited a peak at the phase angle where the tidal shear stress is at its maximum to accelerate the fault slip. This indicates that the observed small p-value is a physical consequence of the tidal effect. We also found a distinctive feature in the spatial distribution of p-values. The small p-values appeared just beneath the strongly coupled portion of the plate interface, as inferred from the seismicity rate change in the past few years.     

Re-evaluation of minor events: The examples of the 1895 and 1909 Rome earthquakes

The city of Rome is subjected to moderate seismic risk due to both local and external seismicity. Up to now, the maximum intensity felt has never exceeded VIII MCS. The 1 November 1895 (I _o = VII) and 31 August 1909 (I _o = VI) earthquakes demonstrate that small local events can also cause damage in a large old city. In the present work, we have re-evaluated the intensity values of those two events by means of automatic processing. A comparison between the present results with geological evidence and previous studies is shown, especially for the historical centre of Rome. For the first time, the 1909 earthquake instrumental magnitudeM _L = 3.6 has been calculated from original recordings.     

大陆板内构造变形及其动力学机制

典型大陆板内变形发生在克拉通化的大陆岩石圈内部,距离同变形期活动板块构造边界数百至2000km以上。收缩变形主要表现为区域尺度的盆地构造反转、结晶基底与上覆盖层共同卷入变形的厚皮式逆冲构造,具有变形局部化特征。因为流变学分层特征不同,大陆板内变形可以发生在中上部地壳、整个地壳乃至岩石圈尺度上,表现为不同波长的地壳或岩石圈尺度纵弯弯曲。大陆岩石圈板块内部物质组成与结构的不均一性、流体活动、热作用、克拉通内盆地巨厚沉积产生的覆盖效应、地壳加厚等导致的岩石圈强度的局部降低等,是导致大陆板内变形以及应变局部化的原因。构造活化是大陆板内变形的重要方式。板块俯冲或碰撞远程效应被认为是大陆板内变形的主导动力学模型,但是放射性元素积累导致的岩石圈强度热弱化,或大陆冰川消退触发板内应力状态变化等导致大陆板内变形的动力学模型也应该引起关注。     

The large-scale distribution of great shallow intraplate earthquakes in mainland China and adjacent areas and seismic coupling along plate margins

In mainland China and its surroundings the large-scale distribution of great, shallow, intraplate earthquakes shows that there are four main areas of high intraplate seismicity which are (a) the Northern China Seismic Area in east China (30°–42°N); (b) the Southeast-coast Seismic Area in eastern China (19°–25°N); (c) the North-south trending Seismic Area in western China and its surroundings (Burma–China–Mongolia); (d) the Central Asian Seismic Area in west China and its surroundings (Pamirs–Tianshan Mts–Baikal). These four intraplate seismic areas are approximately perpendicular to those sections of the Eurasian plate boundary where the Eurasian plate has a strong seismic coupling with the North American–Pacific Ocean–Philippine Sea plates, and with the Indian plate. The large-scale uneven distribution of intraplate seismicity in China and its surroundings may be controlled by heterogeneity in the stress state on different sections of the plate boundary.     

Lithosphere,crust and basement ridges across Ganga and Indus basins and seismicity along the Himalayan front,India and Western Fold Belt,Pakistan

Spectral analysis of the digital data of the Bouguer anomaly of North India including Ganga basin suggest a four layer model with approximate depths of 140, 38, 16 and 7 km. They apparently represent lithosphere–asthenosphere boundary (LAB), Moho, lower crust, and maximum depth to the basement in foredeeps, respectively. The Airy’s root model of Moho from the topographic data and modeling of Bouguer anomaly constrained from the available seismic information suggest changes in the lithospheric and crustal thicknesses from ∼126–134 and ∼32–35 km under the Central Ganga basin to ∼132 and ∼38 km towards the south and 163 and ∼40 km towards the north, respectively. It has clearly brought out the lithospheric flexure and related crustal bulge under the Ganga basin due to the Himalaya. Airy’s root model and modeling along a profile (SE–NW) across the Indus basin and the Western Fold Belt (WFB), (Sibi Syntaxis, Pakistan) also suggest similar crustal bulge related to lithospheric flexure due to the WFB with crustal thickness of 33 km in the central part and 38 and 56 km towards the SE and the NW, respectively. It has also shown the high density lower crust and Bela ophiolite along the Chamman fault. The two flexures interact along the Western Syntaxis and Hazara seismic zone where several large/great earthquakes including 2005 Kashmir earthquake was reported.The residual Bouguer anomaly maps of the Indus and the Ganga basins have delineated several basement ridges whose interaction with the Himalaya and the WFB, respectively have caused seismic activity including some large/great earthquakes. Some significant ridges across the Indus basin are (i) Delhi–Lahore–Sargodha, (ii) Jaisalmer–Sibi Syntaxis which is highly seismogenic. and (iii) Kachchh–Karachi arc–Kirthar thrust leading to Sibi Syntaxis. Most of the basement ridges of the Ganga basin are oriented NE–SW that are as follows (i) Jaisalmer–Ganganagar and Jodhpur–Chandigarh ridges across the Ganga basin intersect Himalaya in the Kangra reentrant where the great Kangra earthquake of 1905 was located. (ii) The Aravalli Delhi Mobile Belt (ADMB) and its margin faults extend to the Western Himalayan front via Delhi where it interacts with the Delhi–Lahore ridge and further north with the Himalayan front causing seismic activity. (iii) The Shahjahanpur and Faizabad ridges strike the Himalayan front in Central Nepal that do not show any enhanced seismicity which may be due to their being parts of the Bundelkhand craton as simple basement highs. (iv) The west and the east Patna faults are parts of transcontinental lineaments, such as Narmada–Son lineament. (v) The Munghyr–Saharsa ridge is fault controlled and interacts with the Himalayan front in the Eastern Nepal where Bihar–Nepal earthquakes of 1934 has been reported. Some of these faults/lineaments of the Indian continent find reflection in seismogenic lineaments of Himalaya like Everest, Arun, Kanchenjunga lineaments. A set of NW–SE oriented gravity highs along the Himalayan front and the Ganga and the Indus basins represents the folding of the basement due to compression as anticlines caused by collision of the Indian and the Asian plates. This study has also delineated several depressions like Saharanpur, Patna, and Purnia depressions.     

Probabilities of occurrence of great earthquakes in the Himalaya

Long-term conditional probabilities of occurrence of great earthquakes along the Himalaya plate boundary seismic zone have been estimated. The chance of occurrence of at least one great earthquake along this seismic zone over a period of 100 years (beginning the year 1999) is estimated to be about 0.89. The 100-year probability of such an earthquake occurring in the Kashmir seismic gap is about 0.27, in the central seismic gap about 0.52 and in the Assam gap about 0.21. The 25-year probabilities of their occurrence in these gaps are 0.07, 0.17, and 0.05 respectively. These probability estimates may be used profitably to assess the seismic hazard in the Himalaya and the adjoining Ganga plains.     

Seismic structure and origin of active intraplate volcanoes in Northeast Asia

Three-dimensional P-wave velocity structure beneath the Changbai and other intraplate volcanic areas in Northeast Asia is determined by inverting 1378 high-quality P-wave arrival times from 186 teleseismic events recorded by 61 broadband seismic stations. Low-velocity (low-V) anomalies are revealed beneath the Changbai, Longgan, Xianjindao volcanoes. High-velocity (high-V) anomalies are found in the mantle transition zone, where deep-focus earthquakes under Hunchun occur at depths of 500–600 km. The high-V anomaly reflects the deep subduction of the Pacific slab under NE Asia which may have contributed to the formation of the Changbai, Longgang, Xianjindao and Jingpohu intraplate volcanoes. A low-V anomaly is also revealed in the mantle transition zone, which may have a close relationship with the occurrence of deep earthquakes under the Hunchun area. Our results support the Big Mantle Wedge (BMW) model by Zhao et al. [Zhao, D., Lei, J., Tang, Y., 2004. Origin of the Changbai volcano in northeast China: evidence from seismic tomography, Chin. Sci. Bull. 49, 1401–1408; Zhao, D., Maruyama, S., Omori, S., 2007. Mantle dynamics of western Pacific and East Asia: insight from seismic tomography and mineral physics. Gondwana Res. 11, 120–131.] who proposed that the intraplate volcanoes in NE Asia are caused by the back-arc magmatism associated with the deep dehydration process of the subducting slab and convective circulation process in the BMW above the stagnant Pacific slab.     

Statistical Models of the Seismicity of the Vrancea Region, Romania

A parameterization derived from the Weibull distribution is used to model the seismic activity of the Vrancea region.The analysis of 498 crustal earthquakes with local magnitudes greater than 2.0, and 1377 subcrustal events with local magnitudes greater than 2.5 emphasizes that the shallow sequences show a strong clustering tendency, while the intermediate depth mainshock sequences are modeled by a completely random pattern in space and time. These results are not influenced by the magnitude threshold and the width of the time window.The difference between the seismicity patterns in the crust and in the subcrustal zone correlates with the difference between the stress field within these two regions.     

The role of Pan-African structures in intraplate seismicity near the termination of the Romanche fracture zone, West Africa

The distribution of epicenters of both historic earthquakes and recent seismic events in southeastern Ghana, compiled from local and teleseismic networks, show strong correlation with the Pan-African structures onshore and indicate an alignment with disruptions on seismic sections offshore. The seismic reflection sections reveal basement structures of the external zone of the Pan-African Dahomeyide orogen and these structures can be traced to offsets of shelf strata and seabottom reflectors, providing direct evidence, for the first time, for neotectonic activity that may be responsible for seismicity in the area. The deep structure of the external zone consists of moderately-dipping reflectors inferred to represent high-strain zones in the variably deformed margin of the West African craton. Taken together, the available data suggest that active tectonics in this intraplate environment may involve inversion of the Pan-African thrust structures but that this activity is apparently not related to reactivation of the nearby Romanche Fracture Zone.     

亚洲大陆逃逸构造与现今中国地震活动

2008年5月12日汶川地震让中国地学界强烈感受到深入研究地震地质与构造变形的重要性和肩负防震减灾巨大的社会责任。本文作者从构造地质学家的角度对中国大陆地震分布、成因规律以及发展趋势做了一些讨论。按地震分布,中国大陆可以粗分为两个区域,其交界是一条过渡带。该过渡带的东界是郯庐断裂及其和海南岛的连线,西界是齐齐哈尔—北京—邯郸—郑州—宜昌—贵阳—(越南)河内连成的线,后者其实就是松辽盆地的西界(大兴安岭的东界、太行山的东界、大娄山的东界）。我们不妨将上述两线所夹过渡带称之为“地震区分界线”。分界线以西的广大地区,活动断裂、活动褶皱、活动盆地都与印度板块楔入欧亚大陆造成的青藏高原隆升、快速侧向扩展、亚洲大陆逃逸构造活动有关。流变性较好的造山带(如青藏高原和天山)和流变性较差的古老地块(如塔里木、准噶尔、阿拉善、鄂尔多斯、四川盆地等)在其边界强烈对抗,形成强震。地震区分界线以东的中国沿海地区受太平洋和菲律宾海板块运动的影响也会发生地震,但其强度和频度与该线以西的青藏高原周边、天山、鄂尔多斯地块周缘以及张家口渤海断裂带上地震低得多。由太平洋板块在日本海沟向西深俯冲形成的地震在中国仅分布在吉林省珲春—汪清一带,这些深源地震对地面工程建筑破坏性不大。处于欧亚、菲律宾海和南海3个板块的交汇部位的我国台湾地震不断。受我国台湾地震的影响,闽粤沿海NW和NE向断裂往往被激活,形成地震。总之,虽然中国大陆的现代地震受太平洋、欧亚、印度和菲律宾海四大板块联合作用控制,但最主要、最直接、影响最大的还是印度板块楔入欧亚大陆造成的青藏高原隆升、快速侧向扩展和大陆逃逸。因此,对中国的地震研究不能仅局限于某区域或某条断裂,而应把整个亚洲大陆逃逸构造作为整体的、统一的“一盘棋”看待。     

A simulation of upper crustal stresses for great and moderate thrust earthquakes of the Himalaya

We assume that great and moderate Himalayan earthquakes occur through reactivation of subhorizontal thrust faults by frictional failure under the action of stresses induced by Himalayan topography, isostasy related buoyancy forces, crustal overburden and plate tectonic causes. Estimates of stresses are based on two dimensional plane strain calculations using analytical formulae of elasticity theory and rock mechanics under suitable simplifying assumptions. Considerable attention is focussed on a point on the detachment at a depth of 17 km below mean sea level under the surface trace of the Main Central Thrust (MCT). According to recent views, great Himalayan earthquakes should nucleate in the detachment in the vicinity of such a point. Also many moderate earthquakes occur on the detachment similarly under the MCT. Vertical and horizontal normal stresses of 622 and 262 MPa and a corresponding shear stress of 26 MPa are estimated for this point due to topography, buoyancy and overburden. For fault friction coefficient varying between 0.3 to 1.0, estimates of plate tectonic stress required are in the range of 386 to 434 MPa, when the cumulative principal stresses are oriented favourably for reactivation of the detachment. Estimates of shear stress mobilized at the same point would be from 27 to 32 MPa for the identical range of fault friction coefficient. Our calculations suggest that presence of pore water in the fault zones is essential for reactivation. Pore pressure required is between 535 to 595 MPa for friction coefficient in the range of 0.3 to 1.0 and it is less than lithostatic stress of 603 MPa at the above point. For the specific nominal value of 0.65 for fault friction coefficient, the estimated values of plate tectonic stress, shear stress and pore pressure at the above point on the detachment are 410 MPa, 30 MPa and 580 MPa respectively. Similar estimates are obtained also for shallower points on the detachment up to the southern limit of the Outer Himalaya. Our estimates of the plate tectonic stress, shear stress and pore pressure for reactivation of upper crustal thrust faults compare favourably with those quoted in the literature.     

A statistical comparison of various source formulations for explosions and earthquakes

Stochastic models are derived for two source formulations for explosions. Using this kind of model, a comparison is made between source time functions due to Blake, Haskell, Mueller and Murphy, and Von Seggern and Blandford, for explosions, and between -square (Aki, Brune)and -cube (Aki, Haskell)models for earthquakes. When seeking a stochastic model for records of Rayleigh waves from atmospheric explosions, the k-model corresponding to Haskell's time function was found to be an appropriate choice.     

Regional fracture analysis south Latitude 29° N of Egypt and their influence on earthquakes

Egypt had been subjected to earthquakes of various degrees but earthquake observation in this country started only in 1899. Earthquakes were found in the locations where fractures have relatives high densities.The Fracture pattern of Egypt south Latitude at 29° N was studied regionally based on Landsat images and aerial photomosaics. Fractures in the Eastern Desert have the prominent trends, NW, EW, NNW and ENE, showing high density in the northern and southern parts. These fractures control the distribution of mineral deposits and radioactivity in the basement rocks.Fractures in Western Desert are less remarkable with the main trend NNW, E-W, N-S and NW, showing high density in the central part. The NNW trending fractures have the same direction of sand dunes which cover larger areas in the northwestern part of Western Desert.Based on the geographical distribution of earthquakes in the seismic maps and centers of high fractures density on the structural contour maps, the area south of latitude 29° N of Egypt was divided into three regions: The Red Sea, Western Desert and Aswan Environs. This correlation led to the conclusion that the fractures have an effect on earthquake activity, are trending ENE and WNW in the Red Sea, NW and N-S in the Western Desert and E-W and NNW in Aswan Environs.It should be emphasized that fractures with higher density are more susceptible to earthquakes in the locations characterized by two dominant sets of fractures especially at their intersections.     

Estimation of the waiting time distributions of earthquakes

Whether the earthquake occurrences follow a Poisson process model is a widely debated issue. The Poisson process model has great conceptual appeal and those who rejected it under pressure of empirical evidence have tried to restore it by trying to identify main events and suppressing foreshocks and aftershocks. The approach here is to estimate the density functions for the waiting times of the future earthquakes. For this purpose, the notion of Gram-Charlier series which is a standard method for the estimation of density functions has been extended based on the orthogonality properties of certain polynomials such as Laguerre and Legendre. It is argued that it is best to estimate density functions in the context of a particular null hypothesis. Using the results of estimation a simple test has been designed to establish that earthquakes do not occur as independent events, thus violating one of the postulates of a Poisson process model. Both methodological and utilitarian aspects are dealt with.     

Tectonic implications of the earthquakes in the Indian subcontinent

The Bhuj earthquake (26 January 2001) in India and the Ghori earthquake (8 October 2005) in Pakistan, both occurred close to the Indian-Iranian plate boundary related to the activity along the intercontinental Chaman transform fault. It is suggested that the seismic activity along NNW — NNE trending weak zones or faults is more intense in the sub-continent than along the WNW trending zones. Since the stress along the former is less compressive but more of the shear or translational type. The devastative Koyna (1967) and Latur (1993) earthquakes both occurred along faults or weak zones that were close to the meridional rather than the equatorial trend. The Indian plate is moving to the north or NNE or NNW, along a rotational trajectory and hence the force tends to be more compressive along the equatorial weak zones. In contrast, it tends to be less compressive and more of the shear or translational along the weak zones that are close to meridional trend. The seismic activity is therefore more intense along the weak zones with NNW to NNE trend than along the ENE to EW trending zones.     

Rheological thickness and strength of the Indian continental lithosphere

The estimates of rheological thickness and total lithospheric strength for the Indian continental lithosphere have been obtained based on the representative rheological properties of upper crust, lower crust and upper mantle, and some of the available heat flow and heat generation data. The rheological thickness, computed at different locations in the Indian shield, shows lateral variation ranging from 79km in the southern part to 65 km in the northern part for a strain rate of 10^-14 s^-1. The total strength of the continental lithosphere is of the order of 10¹³ Nm^-1 for the same value of strain rate and decreases northward. The computations carried out for a range of strain rates show an increase in the rheological thickness and strength of the lithosphere with increasing strain rate. These results would be important in understanding the flexural response of the Indian continental lithosphere to surface and subsurface loading, and response to tectonic forces acting on it.