Why does near ridge extensional seismicity occur primarily in the Indian Ocean?

The possibility that thermoelastic stresses due to plate cooling contribute significantly to the stress field and seismicity in young oceanic lithosphere has been a subject of considerable recent interest. This effect is suggested by three key observations: a decrease in seismicity with lithospheric age, the fact that focal mechanisms show extension perpendicular to the spreading direction, and a depth stratification of mechanism types. A difficulty with this idea is that although thermoelastic stresses should be comparable in different regions, the intraplate seismicity seems to occur in local concentrations. In particular, the ridge-parallel extensional seismicity occurs preferentially in the Central Indian Ocean region.We explore the possibility that much of the data favoring thermoelastic stresses can be interpreted in terms of stresses resulting from individual plate geometry and local boundary effects. In particular, the dramatic concentration of extensional seismicity in the Central Indian Ocean region is consistent with finite element results for the intraplate stress incorporating the effects of the Himalayan collision and the various subduction zones. The ridge parallel extensional stresses show a decrease with age similar to that of the seismicity. As earthquakes in this area provide a major portion of the data for both ridge-parallel extension and depth stratification, these effects may be due more to the regional stress. We thus propose that thermoelastic stresses provide a low level “background” in all plates, but that the dominant effect is that of individual plate geometry and local boundary effects.     

Intraplate stresses along a section of the Kavali-Udipi DSS profile in the south Indian shield induced by density heterogeneities and topography

Occurrence of intraplate seismicity has been attributed to several causes. The perturbation in the local stress regime, either due to local strength weakening of rock mass or surface and subsurface loading, is considered as a plausible mechanism of intraplate earthquake occurrences. The present work is aimed at analysing the state of stress in a part of south Indian shield induced by existing topography and undulations in the subsurface interfaces. The stresses are computed along a section of well studied Kavali-Udipi profile. The general nature of the stresses is compressive in the upper lithosphere except for a small region of extensional regime at both the ends of the profile. The magnitude of these stresses in comparison to the plate tectonic stresses shows that it also forms a significant component of the prevailing stresses in the region. The computed results also show a lateral variation in the stresses along the profile section. Thus, the role of stresses due to density heterogeneities and topography should be taken into consideration in explaining the microseismicity of the region.     

中国地块的地震应力场

印度板块往北,菲律宾板块往北西,太平洋板块往南西与欧亚板块碰撞的格局是中国地震构造的决定性因素。根据实地应力测量、地震断层面解、河谷走向和节理产状等不同资料,计算出了由于板块相互作用所形成应力场的方向。计算结果表明:上述板块的碰撞形成了中国境内板内的三大地震构造区,即西部区、华北区、东南区。区内应力是连续的。由于不同区内应力方向有显著不同,这种应力的连续性就必然导致了在其结合部出现应力的不连续,这就是大致沿N40°E的南北地震带。所以南北地震带的出现可能是中国及邻近地区诸板块相互作用的结果。     

新马德里地震带的岩石圈强度与板内形变

提出一个简单的假说来解释为什么在相对稳定的板块内部地区会存在高地震活动区与高构造形变区．首先,对于大多数板内地区而言,特别是大陆地盾地区与老的海洋盆地,下地壳与上地幔的温度相当低,那里的岩石相对坚硬在这些地区不可能发生明显的岩石圈变形,因为岩石图累积强度大大超过板块驱动力．相反,如果下地壳与上地幔温度相对较高,板块驱动力则主要由上地壳承受,因为下地壳与上地幔相对软弱在这种地区,由于岩石圈累积强度与板块驱动力大小相当,构造形变相对较快．本文将这种假说应用在位于美国中部的新马德里地震带与周围地区．地震带内部热流密度值约为60mw／m2,略高于本区背景热流密度值45mW／m2．计算得到的地温梯度与实验室结果所揭示的延性流动定律表明,在地震带内下地壳与上地幔相当软弱,板内应力主要由上地壳传递．那里的形变速率相对较高．与此相反,在周围地区热流值相对较低,岩石四累积强度大大超过板块驱动力,构造应力由地壳与上地幔共同承受热流值的大小和下地壳上地幔的受力状态是决定地震活动性在地震带内与周围地区强烈对比的主要因素．     

Earthquake source mechanisms from body-waveform inversion and intraplate tectonics in the northern Indian Ocean

Double-couple point-source parameters for 11 of the largest intraplate earthquakes in the northern Indian Ocean during the last 20 y were determined from a formal inversion of long-period P and SH waveforms. Nine of the events have centroid depths at least 17 km below the seafloor, well into the upper mantle; two have centroid depths as great as 39 km. Using the source mechanisms of these earthquakes, we distinguish two major intraplate tectonic provinces in the northern Indian Ocean. To the west of the Ninetyeast Ridge, in the southern Bay of Bengal, intraplate earthquakes have thrust-faulting mechanisms with P axes oriented N-S. The centroid depths of these earthquakes range from 27 to 39 km below the seafloor. Lithospheric shortening in this region is thus accomplished by thrust faulting in the strong core of the oceanic upper mantle, while other geophysical evidence suggests that shallow sedimentary and crustal layers apparently deform predominantly by folding. In the immediate vicinity of the Ninetyeast Ridge, earthquakes display strike-slip mechanisms with left-lateral motion on planes parallel to the ridge. This type of faulting occurs from at least 10°S to the northern end of the Ninetyeast Ridge near 10°N, where the ridge meets the Sunda Arc. Seismic activity diminishes to the east of the Ninetyeast Ridge, but is also characterized by strike-slip faulting. Despite these variations in deformational style, the inferred orientation of greatest compressive stress in the northern Indian Ocean displays a consistent long-wavelength pattern over a large portion of the Indian plate, varying smoothly from nearly N-S in the Bay of Bengal to NW-SE in the northeastern Indian Ocean. This plate-wide stress pattern and the high level of intraplate seismicity in the northern Indian Ocean are likely the results of substantial resistance, along the Himalayan continental collision zone, to the continued northward motion of the western portion of the Indian plate. Oceanic intraplate earthquakes in other regions, where the level of deviatoric stress associated with the long-wavelength part of the stress field is likely to be smaller, need not be comparably reliable indicators of the plate-wide stress field.     

The craton: Its effect on the distribution of seismicity and stress in North America

The gross distribution of seismicity in North America suggests the interior platform and shield provinces are relatively stable and significantly affect the intraplate stress field. Focal mechanisms and in-situ stress measurements indicate the cratonic areas of North America must be relatively immobile with respect to the eastern and western portions of the continent. Simple kinematic models are presented to illustrate the importance of the craton in any attempt to explain the general pattern of intraplate stress and seismicity in North America.     

Spatial variation of crustal strain in the Kachchh region,India: Implication on the Bhuj earthquake of 2001

The Kachchh province of Western India is a major seismic domain in an intraplate set-up. This seismic zone is located in a rift basin, which was developed during the early Jurassic break-up of the Gondwanaland. The crustal strain determined from the GPS velocity data of post-seismic time period following the 2001 Bhuj earthquake indicates a maximum strain rate of ∼266 × 10⁻⁹ per year along N013°. Focal mechanism solutions of the main event of 26 January 2001 and the aftershocks show that the maximum principal stress axis is close to this high strain direction. Maximum shear strain rate determined from the GPS data of the area has similar orientation. The unusually high strain rate is comparable in magnitude to the continental rift systems. The partitioning of the regional NE–SW horizontal stress (S_Hmax) by the pre-existing EW-striking boundary fault developed the strike–slip components parallel to the regional faults, the normal components perpendicular to the faults, NE-striking conjugate Riedel shear fractures and tension fractures. The partitioned normal component of the stress is considered to be the major cause for compression across the regional EW faults and development of the second-order conjugate shear fractures striking NE–SW and NW–SE. The NE-striking transverse faults parallel to the anti-Riedel shear planes have become critical under these conditions. These anti-Riedel planes are interpreted to be critical for the seismicity of the Kachchh region. The high strain rate in this area of low to moderate surface heat flow is responsible for deeper position of the brittle–ductile transition and development of deep seated seismic events in this intraplate region.     

On the origin of compressional intraplate stresses in South America

Intraplate stresses in middle South America are not negligible. We report thrust-faulting mechanisms for five intraplate earthquakes, which indicate a dominant horizontal deviatoric compressional stress oriented in a NW-SE direction. We conclude that this state of stress is due to forces connected with spreading on the Mid Atlantic Ridge and resistive forces exerted by the Caribbean plate to the north and the Nazca plate to the west. The existence and nature of the resistive forces is inferred from earthquake mechanisms and geological evidence presented in other studies. All the available intraplate stress data for Nazca and South America indicate that both plates are under deviatoric compression generated at spreading centers. The absence of tensional earthquake focal mechanisms, particularly in the Nazca plate near the trench, suggests that the forces associated with the gravitational sinking of subducted lithosphere are locally compensated. We present a simple numerical calculation of a non-subducting plate to show how the compressional deviatoric stresses in middle South America can be used to estimate an upper bound of about 10²¹ P for the viscosity of the mantle.     

On the tectonics of Asia

This report describes an interpretation of the tectonics of central Asia made from seismic and geologic data. It is suggested that central Asia is not a tectonically passive unit, as previously proposed by others, responding solely to the convergence of the Indian plate with Asia. We postulate that the tectonics of central Asia can be represented by the motion of a few continental blocks which are influenced by spreading from the Baikal rift zone as well as compression due to the collision of the Indian plate. Here, a block is defined as a tectonic unit, within a continental plate, with boundaries delineated by broad zones of high seismicity with respect to the interior of the unit. Five tectonic units are postulated for central Asia. These are: the Siberian block, the East and West China blocks, the Southeast Asian block; and the Indian plate. An unusual phenomenon is noted along the boundary between the Siberian and West China blocks. There is general horizontal crustal compression along this boundary from the Hindu Kush north-eastward to the southern tip of Lake Baikal; however, there is general horizontal extension eastward from Lake Baikal through the Stanovoy range. Thus the West China block, to the south of this boundary, seems to be turning clockwise about a point near the southern tip of Lake Baikal. The major known faults within this block, which strike mainly northwest-southeast, may be interpreted as shear zones where interior stresses, due to the block rotation, are released. We cannot support this suggestion with an analytical model because of the uncertainties in various model parameters and geometries. The suggested model gives a possible explanation of why India, to the south of the Himalayas, is almost completely aseismic while the regions to the north and northeast have higher seismicity.     

Derived gravity field of the seismogenic upper crust of SE Germany and West Bohemia and its comparison with seismicity

The gravity field of the seismogenic upper crust was derived from the Bouguer gravity map by applying the Butterworth high-pass filter in the wave-number domain. The cutoff wavelength of the filter was 110 km, to pass the gravity signals of structures within the 18 km thick seismogenic layer. The derived residual gravity map reveals potential stress concentrating structures, which may cause seismicity provided they lie within the existing zones of weakness. Furthermore we derived a shaded relief map of the horizontal gravity gradient, which highlighted the tectonic lines accompanied by density contrast. The directional analysis of this map shows three dominant strike directions. The most prominent one is “the Hercynian” NW-SE strike direction represented by the Franconian Line, the Gera-Jáchymov Fault Zone and the Elbe Zone. The second dominant strike is the Rhenisch NNE-SSW trending represented by the Upper Rhine Graben Zone, Rheinsberg-Heldburg Line and several Proterozoic volcanic belts in the Teplá-Barrandien Unit. The third pronounced trending of the ENE-WSW direction is represented by the Erzgebirge and Eger Graben gravity low. The N-S trending Rostock-Leipzig-Regensburg Zone (Pritzwalk-Naab Lineament) is not distinctly reflected in the derived gravity maps, although many fault segments have a meridian direction. The relative reactivation potential of some pre-existing fault systems identified in the gravity map was studied with respect to the wide range of the recent stress configuration determined in the West Bohemia/Vogtland region. The resulting diagrams show that the steep NNW-SSE to N-S faults (represented by some segments of the Mariánské Lázně Fault Zone) are oriented favourably for reactivation. On the contrary, the orientation of the ENE-WSW faults limiting the Eger Graben (Litoměřice Fault, etc.) is unfavourable for reactivation for all dip values.     

云南强震活动的多层次动力源分析

对云南地区多层次动力过程作了分析研究，结果表明，若以两大板块之间在边界上的相互作用为最高层次的动力作用，云南地区现代构造运动至少包括三个层次的动力作用过程：(1)印度板块和亚欧板块两大地壳板块在喜马拉雅碰撞带东部弧顶和东翼相互作用产生的边界动力源对云南地区产生的直接影响和间接作用；还有菲律宾海板块对亚欧板块的北西西向的推挤，通过华南地区对云南东部的间接作用，构成了云南地区现代构造运动第一层次动力作用；(2)以康滇菱形断块为主体，包括川青断块、滇西南断块带等板内断块的整体向南南东—南东方向的相对移动产生的动力作用，是第二层次的动力作用；(3)由于板内断块边界断裂运动速率的差异，主要是水平滑动速率差异造成的板内断块内部次级断块移动产生的动力作用，是第三层次的动力作用。对印度板块和亚欧板块两大地壳板块碰撞挤压带东部弧顶和东翼相互作用产生的边界动力源与云南及邻区构造运动、构造应力场分布格局和强震活动关系作了分析研究，认为云南及邻区多层次动力作用过程，是强震活动时空分布的主要原因。     

Historical seismicity near Chagos: a complex deformation zone in the equatorial Indian Ocean

Over the last twenty years, Chagos Bank has a seismicity rate disproportionate to its supposed intraplate location. Earthquake relocation also shows a high seismicity rate in pre-WWSSN time (1912–1963), with seven events located off of the Central Indian Ridge, including large events in 1912 (M = 6.8) and 1944 (M = 7.2). This study uses the moment variance technique, a systematic search for the mechanism which best fits P, PP, SH, Love and Rayleigh amplitudes, to determine the focal mechanisms of two pre-WWSSN earthquakes. A test with a recent event of known mechanism demonstrates that accurate focal parameter determination is possible even when only a few good records are available. Moment variance analysis shows a thrust faulting mechanism for the 1944 event, northeast of Chagos Bank near the Chagos-Laccadive ridge, and a strike-slip focal mechanism for a smaller 1957 event west of Chagos Bank. The 1944 event, one of the largest oceanic “intraplate” earthquakes known (moment 1.4 × 10²⁷ dyne-cm), indicates that the Chagos seismicity reflects not an isolated occurrence of normal faulting as previously thought, but rather regional tectonic deformation extending northeast of Chagos Bank and including thrust, normal and strike-slip events. This seismicity and previously studied seismicity near the Ninetyeast Ridge and Central Indian Basin suggest a broad zone of deformation stretching across the equatorial Indian Ocean. This zone contains all known magnitude seven oceanic “intraplate” earthquakes not associated with subduction zones or continental margins, suggesting that elsewhere such extensive deformation occurs only along plate boundaries. This study proposes that a slow, diffuse plate boundary extends east from the Central Indian Ridge to the Ninetyeast Ridge and north to the Sumatra Trench. A recent plate motion study confirms this boundary and suggests that it separates the Australian plate from a single Indo-Arabian plate.     

Electrical signatures due to thermal anomalies along mobile belts reactivated by the trail and outburst of mantle plume: Evidences from the Indian subcontinent

In this study the geodynamical scenario along with concepts of mantle plume and mobile belts is utilized to show that most of the existing and potential high thermal regions fall along the (mobile arms affected by the outburst and) traces of mantle plumes. Effects of channeling and partitioning of thermomagmatic flux (TMF) due to these mantle plumes along the mobile belts, particularly near the triple junctions, can be seen in the form of high heat flow and presence of hot springs. Triple junctions manifest over the Indian lithosphere are: Kutch-Cambay, Narmada Son-Godavari, Tapi-Mahanadi, Tapi-Damodar, Pondicherry region, Gulf of Mannar and SW corner of the subcontinent (off-shore), etc. Apart from mobile belts, the deltaic regions of Krishna, Godavari, Ganga, Cauvery, Narmada-Tapi and Indus, etc., are also posses higher level of thermal anomalies as these regions seem to have been substantially influenced by outbursts and traces of Reunion, Kerguelen, Marion and Crozet hotspots. This is reflected from the correlation between plume affected mobile belts and high heat flow regions, large number of hot springs, anomalous electrical conductivity and also deformation or seismicity. Such correlation can be seen along Cambay, west coast trend, Narmada-Son lineament zone, Godavari-Damodar grabens and Bengal basin. Himalayan belt being ongoing collision zone, thermal anomalies are identified in the form of hot springs along the Himalayan arc. At some locations, which might be junction of tectonic trends, there exist significantly large thermal outputs. Puga in Himalayan region is one such example, as seen from high heat flow (max. 468 mW m^− 2) and geothermal gradient (234 °C/km max.). Similarly, Tatapani in Narmada Son Lineament (NSL) region is another such example. The present study discusses the correlation between thermal reservoirs identified by magnetotelluric (MT) study results and plume activity and suggests the need for systematic and detailed MT investigations along plume activated mobile strips in other regions to search for geodynamical history and geothermal resources.     

Probabilistic Assessment of Earthquake Hazards in the Indian Subcontinent

—The Indian subcontinent is one of the most seismic prone areas of the world. The Himalayan mountains in the north, mid-oceanic ridges in the south and earthquake belts surrounding the Indian plate all show that the subcontinent has undergone extensive geological and tectonic processes in the past. The probability of the occurrence of earthquakes with magnitude 6<Mb<7 during a specified interval of time has been estimated on the basis of four probabilistic models namely Lognormal, Weibull, Gamma and Exponential distribution for the Indian subcontinent. The seismicity map has been prepared using the earthquake catalogue from the period 1963–1994, and six different zones have been identified on the basis of clustering of events. The model parameters have been estimated by the method of maximum likelihood estimates (MLE) and method of moments (MOM). A computer program package has been developed for all four models, which represents the distributions of time intervals fairly well. The logarithmic of likelihood (ln L) is estimated for testing the models and different models have been found to be plausible. The probability of different magnitude thresholds has been evaluated using the Gutenberg–Richter formula Log N = a - bM for magnitude distribution. The constants a and b have been computed for each region and found to be varying between 5.46–8.53 and 0.87–1.34, respectively.     

The mantle convection pattern and force source mechanism of recent tectonic movement in China

The Runcorn stress equations and 2–30° harmonic coefficients of the geopotential have been applied to determine the mantle convection pattern beneath China. The pattern is compared with geophysical and geological observations and it is found that the directional change belts of mantle flows coincide with the major fault belts between tectonic units of China. The stress field generated by mantle flows, except in the Tian Shan region, also coincide with the stress field of recent tectonic movement in China. The Tarim and Junggar basins are formed by tensional stresses due to divergent mantle convection currents under northwest China. The formation of the Qinhai-Xizang (Tibet) plateau is due mainly to the compression of the Tarim block and Indian plate, caused by convergent mantle convection currents. The shear-fault belts in central China (100–105°E) are generated by the running change belt of mantle flows, a well-known N-S seismic zone. In eastern China, tensional faults, grabens, lake and sea depressions are related to the eastward displacement of continental lithosphere exerted by eastward dispersal mantle flows under this region.This paper provides new material for further study of the force source mechanism of recent tectonic movement from the viewpoint of mantle convection currents.     

The source rupture feature of the southern Taiwan Straits earthquake of September 16, 1994 (M S7.3) & the analysis of earthquake circumstance in southeastern coast of China

According to the source mechanism of the main shock and the distribution feature of the aftershocks occurring in the southern Taiwan Straits on Sept. 16, 1994, in this paper the authors analysed the source rupture feature of the major earthquake, demonstrated that this seismic sequence possessed the characteristics of a large intraplate earthquake. And according to the seismotectonic background and the historical seismicity in the area, the authors clarified the active characteristics of the seismically active belts along northwestern direction and analysed preliminarily the earthquake circumstance in the southeastern coast of China.     

The source rupture feature of the southern Taiwan Straits earthquake of September 16，1994 （Ms7．3） ＆ the analysis of earthquake circumstance in southeastern coast of China

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The source rupture feature of the southern Taiwan Straits earthquake of September 16, 1994 ( M S7.3) & the analysis of earthquake circumstance in southeastern coast of China

According to the source mechanism of the main shock and the distribution feature of the aftershocks occurring in the southern Taiwan Straits on Sept. 16, 1994, in this paper the authors analysed the source rupture feature of the major earthquake, demonstrated that this seismic sequence possessed the characteristics of a large intraplate earthquake. And according to the seismotectonic background and the historical seismicity in the area, the authors clarified the active characteristics of the seismically active belts along northwestern direction and analysed preliminarily the earthquake circumstance in the southeastern coast of China.     

Intraplate seismicity of the Pacific Basin, 1913–1988

We establish here a comprehensive database of intraplate seismicity in the Pacific Basin. Relocation and analysis of 894 earthquakes yield 403 reliable intraplate earthquakes during 1913–1988. These numbers do not include earthquake swarms, which account for another 838 events. Most of the remainder (304 events) are actually plate boundary earthquakes that have been erroneously located in intraplate regions. A significant number occur in recent years when location capabilities should have guarded against this situation. Relocations involve a careful linear inversion ofP andS arrivals, accompanied by a Monte Carlo statistical analysis. We have also attentively removed the high number of clerical errors and nuclear tests that exist in epicenter bulletins.A geographical examination of the relocated epicenters reveals several striking features. There are three NW-SE lineaments north of the Fiji Plateau and in Micronesia; diffuse seismicity and incompatible focal mechanisms argue against the southernmost, discussed byOkal et al. (1986) andKroenke andWalker (1986), as the simple relocation of the Solomon trench to the North. Besides another striking lineament, along the 130°W meridian, there is also a strong correlation between seismicity and bathymetry in certain parts of the Basin. In the Eastcentral Pacific and Nazca plates there are many epicenters on fracture zones and fossil spreading ridges, and hot spot traces like the Louisville, Nazca and Cocos Ridges also display seismicity.     

中国周边板块的相互作用及其对中国应力场的影响——（Ⅱ）印度板块的影响

本文利用地震资料并结合地质资料,讨论了印度板块与欧亚板块在中国周边的相互作用及其对中国应力场的影响,指出两板块在喜马拉雅山前断裂地区碰撞,碰撞边界向西延续到35°N,74°E附近,其主要挤压方向为NNE,并形成SE方向的物质流动.帕米尔地区有强烈的构造运动,并存在俯冲带形态的构造.在26.5°N,97°E附近,板块边界的走向发生突变,并形成东倾的缅甸山弧俯冲带,但印度板块挤压造成的主压应力方向为NNE向.在安达曼-尼科巴-苏门答腊-爪哇岛弧,印度板块俯冲于欧亚板块之下,在中国南海一带形成NNW向或近Ns向的主压应力.