Curie Point Depth Estimates from Aeromagnetic Data from Death Valley and Surrounding Regions, California

Aeromagnetic data were analyzed to determine the Curie point depth (CPD) by power density spectral and three-dimensional inversion methods within and surrounding Death Valley in southern California. We calculated the CPD for 0.5° regions using 2D power density spectral methods and found that the CPDs varied between 8 and 17 km. However, the 0.5° region may average areas that include shallow and deep CPDs, and because of this limitation, we used the 3D inversion method to determine if this method may provide better resolution of the CPDs. The final 3D model indicates that the depth to the bottom of the magnetic susceptible bodies varies between 5 and 23 km. Even though both methods produced roughly similar results, the 3D inversion method produced a higher lateral resolution of the CPDs. The shallowest CPDs occur within the central and southern Death Valley, Panamint Valley, Coso geothermal field and the Tecopa hot springs region. Deeper (>15 km) CPDs occur over outcropping granitic and Precambrian lithologies in the Panamint Range, Grapevine Mountains, Black Mountains and the Argus Range. The shallowest CPD occurs within the central Death Valley over a possible seismically imaged magma body and slightly deeper values occur within the Panamint Valley, southern Death Valley and Tecopa Hot Springs. The shallow CPD values suggest that partially molten material may also be found in these latter regions. The CPD computed heat flow values for the region suggest that the entire area has high heat flow values (>100 mW m^?2), on the other hand, locally extremely high values (>200 mW m^?2) occur within the Panamint Valley, the southern and central Death Valley and Tecopa Hot Springs region. These locally high heat flow values may be related to midcrustal magma bodies; but additional geophysical experiments are needed to determine if the magma bodies exist.     

Curie Point Depth Investigation of Central Anatolia,Turkey

The residual aeromagnetic total field intensity anomalies in central Anatolia were calculated from the regional aeromagnetic anomalies surveyed by the Mineral Research and Exploration (MTA) of Turkey. The residual aeromagnetic data were analyzed to produce Curie point estimates by the method of OKUBO et al. (1985). The Curie point depth of central Anatolia varies from 7.9 km and 22.6 km. The shallowest Curie point depths were observed around the Cappadocia and Erciyes Volcanic complexes in central Anatolia. A good correlation was deduced between the Curie point depths and the heat-flow data measured previously, which is most certainly important for the geothermal resources of the region. The shallow Curie point depths also correlate well with the hot spring locations in central Anatolia.     

Curie Point Depth of the Island of Crete (Greece)

—The aeromagnetic data of the island of Crete were inverted to produce Curie point estimates. The data were high-pass filtered to remove components arising from topography and magnetic core fields which were not adequately modeled by IGRF. The depth to the centroid, z ₀?, of the deepest distribution of the magnetic dipoles, was obtained by computing a least-squares fit to the lowest-fre quency segment of the azimuthally averaged log power spectrum. The depth to the top of the deepest crustal block was computed as the depth, z _t ?, to the centroid of the second deepest distribution, using the second lowest-frequency segment of the spectrum. The depth to the bottom of the deepest dipoles, the inferred Curie point depth, is then z _b = 2z ₀?z _t ?. The Curie depth estimates range between 24 and 28 km. This is in accordance with the depths inferred by extrapolating heat-flow values measured in boreholes.     

Estimating Curie Point Depth and Heat Flow Map for Northern Red Sea Rift of Egypt and Its Surroundings, from Aeromagnetic Data

In this study, we aim to map the Curie point depth surface for the northern Red Sea rift region and its surroundings based on the spectral analysis of aeromagnetic data. Spectral analysis technique was used to estimate the boundaries (top and bottom) of the magnetized crust. The Curie point depth (CPD) estimates of the Red Sea rift from 112 overlapping blocks vary from 5 to 20 km. The depths obtained for the bottom of the magnetized crust are assumed to correspond to Curie point depths where the magnetic layer loses its magnetization. Intermediate to deep Curie point depth anomalies (10–16 km) were observed in southern and central Sinai and the Gulf of Suez (intermediate heat flow) due to the uplifted basement rocks. The shallowest CPD of 5 km (associated with very high heat flow, ~235 mW m^?2) is located at/around the axial trough of the Red Sea rift region especially at Brothers Island and Conrad Deep due to its association with both the concentration of rifting to the axial depression and the magmatic activity, whereas, beneath the Gulf of Aqaba, three Curie point depth anomalies belonging to three major basins vary from 10 km in the north to about 14 km in the south (with a mean heat flow of about 85 mW m^?2). Moreover, low CPD anomalies (high heat flow) were also observed beneath some localities in the northern part of the Gulf of Suez at Hammam Fraun, at Esna city along River Nile, at west Ras Gharib in the eastern desert and at Safaga along the western shore line of the Red Sea rift. These resulted from deviatoric tensional stresses developing in the lithosphere which contribute to its further extension and may be due to the opening of the Gulf of Suez and/or the Red Sea rift. Furthermore, low CPD (with high heat flow anomaly) was observed in the eastern border of the study area, beneath northern Arabia, due to the quasi-vertical low-velocity anomaly which extends into the lower mantle and may be related to volcanism in northern Arabia. Dense microearthquakes seem to occur in areas where the lateral gradients of the CPD are steep (e.g. entrance of the Gulf of Suez and Brothers Island in the Red Sea). These areas may correspond to the boundaries between high and low thermal regions of the crust. Thus, the variations in the microseismic activity may be closely related to thermal structures of the crust. Indeed, shallow cutoff depths of seismicity can also be found in some geothermal areas (e.g. western area of Safaga city along the Red Sea coastal region and at Esna city along the River Nile). These facts indicate that the changes in the thickness of the seismogenic layer strongly depend on temperature. Generally, the shallow Curie point depth indicates that some regions in our study area are promising regions for further geothermal exploration particularly in some localities along the River Nile, Red Sea and Gulf of Suez coastal regions.     

Curie Point Depths of Macedonia and Thrace, N. Greece

—?The aeromagnetic data of Macedonia and Thrace were used to produce Curie point estimates. The data were high pass filtered to remove components arising from topography and magnetic core fields which were not adequately modeled by a DGRF. The depth to the centroid, z ₀, of the deepest distribution of the magnetic dipoles was obtained by computing a least-squares fit to the lowest-frequency segment of the azimuthally averaged log power spectrum. The average depth to the top of the deepest crustal block was computed as the depth to the top, z _t , of the second lowest-frequency segment of the spectrum. The depth to the bottom of the deepest magnetic dipoles, the inferred Curie point depth, was then calculated from z _b =2z _0???z _t . The Curie depth estimates for Macedonia and Thrace range between 11.2 and 17.3?km. These results are consistent with the depths inferred by extrapolating known geothermal gradient and heat-flow values.     

全球主要俯冲带处板块运动与地震各向异性及应力场的相关性讨论

在全球板块的很多地方,包括俯冲带、大洋中脊、甚至大陆板块的内部等,地震各向异性都与板块绝对运动图像存在一定的相关性,或者与板内应力场的优势取向一致。本文统计分析了全球9个包含主要俯冲带的板块边界内板块运动与地震各向异性及应力场的相关性,结果表明,板块的绝对或相对运动控制着板块边界的地震各向异性和应力场特征,尤其是板块的相对运动,在板块边界带处的影响十分明显;从计算结果还可以看出,板块的相对运动方向与地震各向异性及应力场的相关性要好于板块绝对运动。在包含俯冲带的板块边界处,由于俯冲机理的复杂性和控制因素的多样性,使得俯冲带处两者的相关性较为复杂,不同深度来源的各向异性表现出不同特征,且应力状态受多种因素的控制。     

日本西南部与俯冲带相关的非火山成因的深部颤动

在日本西南部非火山区发现长周期深部颤动。颤动震中沿菲律宾俯冲板块走向分布,长600km。颤动的平均深度约30km,位于莫霍面附近。每次颤动最多持续几周。俯冲带内颤动分布位置表明,颤动可能是由板块脱水过程产生的流体造成的。     

下扬子地区与西太平洋俯冲带地震相关性研究及动力学背景探讨

对下扬子地区与琉球岛弧、日本岛弧、喜马拉雅碰撞带的地震活动性特征和不同区块之间地震活动的相关性进行分析．并结合地质构造、岩石圈结构、运动学特征等探讨了下扬子地区中强震发生的动力学背景。研究结果表明：下扬子地块与琉球岛弧带应变释放过程趋势一致;下扬子地区中强震与琉球岛弧带强震相关性较好：受琉球岛弧带发生强震影响,在6-8年时间范围内下扬子地区发生中强震概率较高：从更长远时间来看。下扬子地区中强震和构造应变主要受西太平洋板缘俯冲构造运动影响。     

Subduction zone earthquakes and stress in slabs

Summary The pattern of seismicity as a function of depth in the world, and the orientation of stress axes of deep and intermediate earthquakes, are explained using viscous fluid models of subducting slabs, with a barrier in the mantle at 670 km. 670 km is the depth of a seismic discontinuity, and also the depth below which earthquakes do not occur. The barrier in the models can be a viscosity increase of an order of magnitude or more, or a chemical discontinuity where vertical velocity is zero. LongN versus depth, whereN is the number of earthquakes, shows (1) a linear decrease to about 250–300 km depth, (2) a minimum near that depth, and (3) an increase thereafter. Stress magnitude in a subducting slab versus depth, for a wide variety of models, shows the same pattern. Since there is some experimental evidence thatN is proportional toe , where is a constant and is the stress magnitude, the agreement is encouraging. In addition, the models predict down-dip compression in the slab at depths below 400 km. This has been observed in earlier studies of earthquake stress axes, and we have confirmed it via a survey of events occurring since 1977 which have been analysed by moment tensor inversion. At intermediate depths, the models predict an approximate but not precise state of down-dip tension when the slab is dipping. The observations do not show an unambiguous state of down-dip tension at intermediate depths, but in the majority of regions the state of stress is decidedly closer to down-dip tension than it is to down-dip compression. Chemical discontinuities above 670 km, or phase transitions with an elevation of the boundary in the slab, predict, when incorporated into the models, stress peaks which are not mirrored in the profile of seismicity versus depth. Models with an asthenosphere and mesosphere of appropriate viscosity can not only explain the state of stress observed in double Benioff zones, but also yield stress magnitude profiles consistent with observed seismicity. Models where a nonlinear rheology is used are qualitatively consistent with the linear models.     

Anisotropy and Flow in Pacific Subduction Zone Back-arcs

—We have obtained constraints on the strength and orientation of anisotropy in the mantle beneath the Tonga, southern Kuril, Japan, and Izu-Bonin subduction zones using shear-wave splitting in S phases from local earthquakes and in teleseismic core phases such as SKS. The observed splitting in all four subduction zones is consistent with a model in which the lower transition zone (520–660 km) and lower mantle are isotropic, and in which significant anisotropy occurs in the back-arc upper mantle. The upper transition zone (410–520 km) beneath the southern Kurils appears to contain weak anisotropy. The observed fast directions indicate that the geometry of back-arc strain in the upper mantle varies systematically across the western Pacific rim. Beneath Izu-Bonin and Tonga, fast directions are aligned with the azimuth of subducting Pacific plate motion and are parallel or sub-parallel to overriding plate extension. However, fast directions beneath the Japan Sea, western Honshu, and Sakhalin Island are highly oblique to subducting plate motion and parallel to present or past overriding plate shearing. Models of back-arc mantle flow that are driven by viscous coupling to local plate motions can reproduce the splitting observed in Tonga and Izu-Bonin, but further three-dimensional flow modeling is required to ascertain whether viscous plate coupling can explain the splitting observed in the southern Kurils and Japan. The fast directions in the southern Kurils and Japan may require strain in the back-arc mantle that is driven by regional or global patterns of mantle flow.     

Sources of Tsunami and Tsunamigenic Earthquakes in Subduction Zones

—We classified tsunamigenic earthquakes in subduction zones into three types earth quakes at the plate interface (typical interplate events), earthquakes at the outer rise, within the subducting slab or overlying crust (intraplate events), and "tsunami earthquakes" that generate considerably larger tsunamis than expected from seismic waves. The depth range of a typical interplate earthquake source is 10–40km, controlled by temperature and other geological parameters. The slip distribution varies both with depth and along-strike. Recent examples show very different temporal change of slip distribution in the Aleutians and the Japan trench. The tsunamigenic coseismic slip of the 1957 Aleutian earthquake was concentrated on an asperity located in the western half of an aftershock zone 1200km long. This asperity ruptured again in the 1986 Andreanof Islands and 1996 Delarof Islands earthquakes. By contrast, the source of the 1994 Sanriku-oki earthquake corresponds to the low slip region of the previous interplate event, the 1968 Tokachi-oki earthquake. Tsunamis from intraplate earthquakes within the subducting slab can be at least as large as those from interplate earthquakes; tsunami hazard assessments must include such events. Similarity in macroseismic data from two southern Kuril earthquakes illustrates difficulty in distinguishing interplate and slab events on the basis of historical data such as felt reports and tsunami heights. Most moment release of tsunami earthquakes occurs in a narrow region near the trench, and the concentrated slip is responsible for the large tsunami. Numerical modeling of the 1996 Peru earthquake confirms this model, which has been proposed for other tsunami earthquakes, including 1896 Sanriku, 1946 Aleutian and 1992 Nicaragua.     

Correlation between microbiological and chemical parameters of some hydrothermal springs in New Mexico, USA

Discharge areas of hydrothermal springs are known to be inhabited by diverse types of microorganisms including archaea, prokaryotes and eukaryotes. A total of 11 hydrothermal samples from the Rio Grande rift and the Valles caldera in New Mexico were analyzed to investigate the correlation between chemical and microbiological parameters of hydrothermal waters. The sampled fluids are categorized into three chemical groups: (I) steam-condensing acid sulfate waters, (II) deep geothermal and derivative waters and (III) thermal meteoric waters. Analyses of the microbial phospholipid fatty acids and denaturing gradient gel electrophoresis of DNA show that acid sulfate waters were populated by thermoacidophilic organisms and had high biomass content. Mineralized deep geothermal and derivative waters exhibited a high degree of microbial diversity, but had low biomass content. Thermal meteoric waters are low in total dissolved solids, and exhibit very low biomass content and microbial diversity. DNA sequences from several previously unknown microbial species were detected. The results of this study support the hypothesis that microbes can be used as tracers for specific types of subsurface environments.     

Subduction zone geometry and stress patterns in the tyrrhenian sea

Joint hypocenter determination is performed for intermediate and deep earthquakes of the Tyrrhenian Sea region.This analysis allowed us to obtain a catalogue of 70 well-located events in this peculiar Benioff zone, which is characterized by quite low seismic activity, compared to the Pacific deep earthquake regions. The method used for the analysis is that ofFrohlich (1979), a variant of the successive approximation technique, which allows use of a great number of events and stations but saves computer memory. The results show a spoon-shaped Benioff zone, dipping NW in the Tyrrhenian Sea to 500km depth. 32 reliable fault-plane solutions have been determined using these new earthquake locations, confirming the predominance of down-dip compression in the central part of the slab and more complex motion along the borders of the zone, as previously suggested byGasparini et al. (1982).     

Electrical and Thermal Anomalies in the Central Andean Subduction Zone

—An enhancement of the electrical conductivity in the upper layers in the subduction zones of southern Peru and northern Argentina is found by analyzing the solar quiet geomagnetic variations. This feature appears in a zone associated with large subduction angles. In this region high values of heat flow have been measured, therefore a one-dimensional thermal model is proposed using regional characteristic parameters to reproduce these values. It is found that thermal convection necessarily plays a contributing role. This effect may be produced by the ascending magma and fluids, which can explain the enhancement of the electrical conductivity.     

Spatial Scaling of Effective Modulus and Correlation of Deformation Near the Critical Point of Fracturing

Many observations point to the lithosphere being metastable and close to a critical mechanical point. Exercises in modelling deformation, past or present, across subsurface reservoirs need to take account of this criticality in an efficient way. Using a renormalization technique, the spatial scaling of effective elastic modulus is derived for 2-D and 3-D bodies close to the critical point of through-going fracturing. The resulting exponent, d_μ, of spatial scaling of effective modulus with size, , takes the values ~ −2.5 and −4.2 in two- and three-dimensional space, respectively. The exponents are compatible with those for scaling of effective modulus with fracture density near the percolation threshold determined by other workers from numerical experiments; the high absolute values are also approximately consistent with empirical data from a) fluctuations in depth of a seismic surface; b) `1/k' scaling of heterogeneities observed in one-dimensional well-log samples; c) spatial correlation of slip displacements induced by water injection. The effective modulus scaling modifies the spatial correlation of components of displacement or strain for a domain close to the critical point of fracturing. This correlation function has been used to geostatistically interpolate components of the strain tensor across subsurface reservoirs with the prime purpose of predicting fracture densities between drilled wells. Simulations of strain distributions appear realistic and can be conditioned to surface depths and observations at wells of fracture-related information such as densities and orientations, welltest permeabilities, changes in well-test permeabilities, etc.     

Pressure Variations with Depth in Aquifers with or without Groundwater Leakage

Hydrodynamic analysis allowed establishing elevated hydrodynamic pressure in aquifers in the case of ascending groundwater movement from one layer to another as compared with the pressure in the case, where there is no leakage. In the case of descending movement of groundwater with leakage from layer to layer, the hydrodynamic pressure was found to be lower. Equations are given that allow calculating water leakage and the part of the hydrodynamic pressure that is caused by water leakage from layer to layer. For concrete hydrogeological conditions, estimates are given for the hydrodynamic pressure and its part determined by water leakage between aquifers and from layer to layer in the case of ascending and descending groundwater flows.     

三维数值模拟研究俯冲区火山的时空活动性

俯冲板块的深部脱水使得上覆地幔含水, 从而降低含水地幔的熔点, 导致上覆地幔部分熔融。 部分熔融的地幔柱一旦喷发到地表就是俯冲带火山, 也形成新的地壳。 相对于周围的地幔来讲, 具有较小密度和黏度的部分熔融地幔的时空活动性就控制着俯冲带火山的时空分布特征。 本文主要回顾近年来运用三维热力学岩石力学模型数值模拟研究与板片脱水相关的俯冲带火山活动的时空分布特性。 结果表明, 部分熔融地幔的有效黏度和密度是影响俯冲板片之上的三维地幔柱横向分布特征的主要因素。 高黏度的部分熔融地幔(10²⁰~10²¹ Pa·s )易于形成近平行于海沟的、 长波长(70~100 km)的、 薄的波状地幔柱; 低黏度(10¹⁸~10¹⁹ Pa·s )的熔融地幔易于形成平行于海沟的, 短波长(30~50 km)的蘑菇状地幔柱和垂直于海沟的山脊状地幔柱。 当部分熔融地幔和周围地幔的密度相差小于50 kg/m³时, 在俯冲板片之上只能形成长波长低幅度(宽50~100 km, 高10~15 km)的地幔山丘。 岩浆产率随着时间的变化反映了火山活动的生命周期性。 板块俯冲速度会影响地幔柱形成的深度和范围大小。 高效率熔融提取会增加新地壳增长总量。 低的板块俯冲速度和低的熔融提取效率会增加上地壳(花岗岩质)和中地壳(英安岩质)化学成分的比例。 数值模拟结果可以很好地解释如日本东北、 新西兰、 南阿拉斯加俯冲区火山的横向分布特征。