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1.
The Vienna Basin Transfer Fault (VBTF) is a slow active fault with moderate seismicity (I max~8–9, M max~5.7) passing through the most vulnerable regions of Austria and Slovakia. We use different data to constrain the seismic potential of the VBTF including slip values computed from the seismic energy release during the 20th century, geological data on fault segmentation and a depth-extrapolated 3-D model of a generalized fault surface, which is used to define potential rupture zones. The seismic slip of the VBTF as a whole is in the range of 0.22–0.31 mm/year for a seismogenic fault thickness of 8 km. Seismic slip rates for individual segments vary from 0.00 to 0.77 mm/year. Comparing these data to geologically and GPS-derived slip velocities (>1 mm/year) proofs that the fault yields a significant seismic slip deficit. Segments of the fault with high seismic slip contrast from segments with no slip representing locked segments. Fault surfaces of segments within the seismogenic zone (4–14 km depth) vary from 55 to 400 km2. Empirical scaling relations show that these segments are sufficiently large to explain both, earthquakes observed in the last centuries, and the 4th century Carnuntum earthquake, for which archeo-seismological data suggest a magnitude of M ≥ 6. Based on the combination of all data (incomplete earthquake catalog, seismic slip deficits, locked segments, potential rupture areas, indications of strong pre-catalog earthquakes) we argue, that the maximum credible earthquake for the VBTF is in the range M max = 6.0–6.8, significantly larger than the magnitude of the strongest recorded events (M = 5.7).  相似文献   

2.
Remote sensing data and Geographical Information System (GIS) has been integrated with the weighted index overlay (WIO) method and E 30 model for the identification and delineation of soil erosion susceptibility zones and the assessment of rate of soil erosion in the mountainous sub-watershed of River Manimala in Kerala (India). Soil erosion is identified as the one of the most serious environmental problems in the human altered mountainous environment. The reliability of estimated soil erosion susceptibility and soil loss is based on how accurately the different factors were estimated or prepared. In the present analysis, factors that are considered to be influence the soil erosion are: land use/land cover, NDVI, landform, drainage density, drainage frequency, lineament frequency, slope, and relative relief. By the WIO analysis, the area is divided into zones representing low (33.30%), moderate (33.70%), and high (33%) erosion proneness. The annual soil erosion rate of the area under investigation was calculated by carefully determining its various parameters and erosion for each of the pixels were estimated individually. The spatial pattern thus created for the area indicates that the average annual rate of soil erosion in the area was ranging from 0.04 mm yr−1 to 61.80 mm yr−1. The high soil erosion probability and maximum erosion rate was observed in areas with high terrain alteration, high relief and slopes with the intensity and duration of heavy precipitation during the monsoons.  相似文献   

3.
Soil erosion is a growing problem in southern Greece and particularly in the island of Crete, the biggest Greek island with great agricultural activity. Soil erosion not only decreases agricultural productivity, but also reduces the water availability. In the current study, an effort to predict potential annual soil loss has been conducted. For the prediction, the Revised Universal Soil Loss Equation (RUSLE) has been adopted in a Geographical Information System framework. The RUSLE factors were calculated (in the form of raster layers) for the nine major watersheds which cover the northern part of the Chania Prefecture. The R-factor was calculated from monthly and annual precipitation data. The K-factor was estimated using soil maps available from the Soil Geographical Data Base of Europe at a scale of 1:1,000,000. The LS-factor was calculated from a 30-m digital elevation model. The C-factor was calculated using Remote Sensing techniques. The P-factor in absence of data was set to 1. The results show that an extended part of the area is undergoing severe erosion. The mean annual soil loss is predicted up to ∼200 (t/ha year−1) for some watersheds showing extended erosion and demanding the attention of local administrators.  相似文献   

4.
In this article the implementation and potential of the Seismotectonic Information System of the Campania Region (SISCam) are described, in particular an application of this Web-based GIS system to the seismotectonic analysis of the Sannio area (Southern Apennines) is performed. WEB-GIS technologies greatly contribute to both the environmental monitoring and the disaster management of areas affected by high natural risks. Specifically the SISCam system has been developed with the aim of providing easy access and fast diffusion, through Internet technology, of the most significant geological, geophysical, and territorial data relative to the Campania Region. The Sannio area has been selected as our application example because it is among the most active seismic regions in Italy. This portion of the Southern Apennines which was hit by the June 5, 1688 strong earthquake (M W = 6.7, CPTI 1999) and by some low- and moderate-energy seismic sequences (1990–1992, 1997), is characterized by a complex inherited tectonic setting and low-tectonic deformation rates that hide the seismogenic sources position. Since this case study turned out to be complicated, the use of the SIScam WEB-GIS has become indispensable because it allowed us to visualize, integrate and analyze all the data available, in order to obtain an accurate and direct picture of the seismotectonic setting of the area. Moreover, a different approach of data analysis was necessary, due to the lack of up-to-date neotectonic and structural data; therefore, the operation of this GIS system enabled us to process and generate some original informative layers, through image analysis, such as new structural lineaments represented on a map of the potential active faults of the area, which has been the final result of our application, as a contribution to new knowledge about the local seismic risk parameters.  相似文献   

5.
A geotechnical information system (GTIS) was constructed within a spatial geographic information system (GIS) framework to reliably predict geotechnical information and accurately estimate site effects at Gyeongju, an urban area in South Korea. The system was built based on both collected and performed site investigation data in addition to acquired geo-knowledge data. Seismic zoning maps were constructed using the site period (T G) and mean shear wave velocity to a depth of 30 m (V S30), and these maps were presented as a regional strategy to mitigate earthquake-induced risks in the study area. In particular, the T G distribution map indicated the susceptibility to ground motion resonance in periods ranging from 0.2 to 0.5 s and the corresponding seismic vulnerability of buildings with two to five stories. Seismic zonation of site classification according to V S30 values was also performed to determine the site amplification coefficients for seismic design and seismic performance evaluation at any site and administrative subunit in the study area. In addition, we investigated the site effects according to subsurface and surface ground irregularities at Gyeongju by seismic response analyses in time domains based on both two- and three-dimensional spatial finite element models, which were generated using spatial interface coordinates between geotechnical subsurface layers predicted by the GTIS. This practical study verified that spatial GIS-based geotechnical information can be a very useful resource in determining how to best mitigate seismic hazards, particularly in urban areas.  相似文献   

6.
The catchment basin of the River Hunte (Lower Saxony, NW-German Basin) was studied on a mesoscale (length of ~90 km) to investigate the influence of the geological subground on modern morphology. A Geo Information System (GIS) was used to calculate linear correlation coefficients between the depth of geological strata (Base Zechstein to Base Quaternary) and the height of the modern landscape (Holocene Alluvial Plain, Lower Weichselian Terrace, catchment basin and watershed). High linear correlation coefficients between the Base of Tertiary and the height of the modern topography (catchment basin [r2=0.87], Lower Weichselian Terrace [r2=0.95] and Holocene Alluvial Plain [r2=0.95]) indicate control of the modern topography by the depth of the geological subsurface via tilting of the entire basin. Most likely northward tilting of the NW-German Basin forces the River Hunte to flow in a northerly direction by relative uplift of the hinterland (Wiehengebirge, Rhenish Massif) and subsidence of the North Sea area.  相似文献   

7.
Assessment and inventory of landslide susceptibility are essential for the formulation of successful disaster mitigation plans. The objective of this study was to assess landslide susceptibility in relation to geo-diversity and its hydrological response in the Lesser Himalaya with a case study using Geographic Information System (GIS) technology. The Dabka watershed, which constitutes a part of the Kosi Basin in the Lesser Himalaya, India, in the district of Nainital, has been selected for the case illustration. The study constitutes three GIS modules: geo-diversity informatics, hydro informatics and landslide informatics. Through the integration and superimposing of spatial data and attribute data of all three GIS modules, Landslide Susceptibility Index (LSI) has been prepared to identify the level of susceptibility for landslide hazards. This resonance study, carried out over a period of five years (2007–2011), found that areas of most stressed geo-diversity (comprising very steep slopes above 30°, geology of Lower Krol and Lariakanta formation, geomorphology of moist areas and debris sites, land use of barren land with a very high drainage frequency and spring density) have a high landslide susceptibility because of high rate of average runoff (33 l/s/km2), flood magnitude (307.28 l/s/km2), erosion (398 tons/km2) and landslide density (5–10 landslides/km2). The areas of least stressed geo-diversity (comprising gentle slopes below 10°, geology of Kailakhan and Siwalik formation, geomorphology of depositional terraces, land use of dense forest with low drainage frequency and spring density) have the lowest landslide susceptibility because of the low rate of average runoff (6.27 l/s/km2), flood magnitude (20.49 l/s/km2), erosion (65.80 tons/km2) and landslide density (1–2 landslides/km2).  相似文献   

8.
Seismic risk assessment of the 3rd Azerbaijan gas pipeline in Iran   总被引：1，自引：1，他引：0  
A comprehensive seismic risk assessment has been performed for the existing route of the 3rd Azerbaijan natural gas buried pipeline in Iran. The major active seismic sources along the pipeline were identified and the geometrical parameters as well as the seismicity rates were determined. The seismic hazard assessment of the ground vibrations along the pipeline was performed in the framework of the Probabilistic Seismic Hazard Analysis using the CRISIS 2007 software. All of the components of the gas pipeline along the route were identified and the corresponding fragility functions are established through the methodology described in the HAZUS guideline (HAZUS MH MR4 Technical manual 2007 Department of homeland security emergency. Preparedness and Response Directorate, FEMA). A detailed cost analyses was taken into consideration based on the expert opinions in the National Iranian Gas Company, in order to provide more practical loss model for the pipeline route. Also, a simple method is suggested in order to account for the vent gas in the total loss estimation. The spatial analysis of the hazard function layer in combination with the loss model layer, in Geographical Information System  (GIS) platform, reveal the financial consequences of different earthquake scenarios.  相似文献   

9.
Late Quaternary sequence stratigraphy of Lake Malawi (Nyasa), Africa   总被引：1，自引：0，他引：1  
High resolution seismic data, multichannel seismic data and sediment cores were used to examine the Songwe Sequence, the uppermost of four depositional sequences identifiable on multichannel seismic data from Lake Malawi (Nyasa). The sequence has a maximum thickness of about 115 m in two areas of the lake, but is typically less than 70 m thick over most of the basin. The sequence is distributed along the entire length of the 560 km long lake, and is concentrated in three main depocentres. 14C age dates from sediment piston cores are extrapolated to provide an age estimate of about 78 000 yr bp for the oldest sediments within the Songwe Sequence. In the North and Central bathymetric basins of the lake, high resolution seismic data indicate a dynamic depositional environment, dominated by turbidity and mass flow deposits. Seismic data from the southern basin show acoustically transparent sediments with relatively low amplitude internal reflections, indicative of pelagic and hemipelagic sedimentation. In many areas the Songwe Sequence is underlain by a pronounced angular unconformity, suggestive of a significant, prolonged, low lake stage prior to deposition of the sequence. Seismic reflectors within the Songwe Sequence can be correlated to younger low lake stages identified from sediment core data. Major late Quaternary low lake level stages in Lake Malawi, interpreted from features identified in the seismic data and sediment core analyses, are tentatively interpreted at 6000 to 10 000 yr bp , 28 000 to >40 000 yr bp , and prior to 78 000 yr bp . Budget calculations indicate mean sediment concentrations from catchment runoff during the period of deposition of the Songwe Sequence to be about 190 mg 1−1, comparable to estimates of modern rainy season discharges from the major river systems. Erosion rates within the drainage basin are estimated to be higher than the African average by a factor of three or more, probably due to the high relief within the Lake Malawi catchment.  相似文献   

10.

The detrimental effects of an earthquake are strongly influenced by the response of soils subjected to dynamic loading. The behavior of soils under dynamic loading is governed by the dynamic soil properties such as shear wave velocity, damping characteristics and shear modulus. Worldwide, it is a common practice to obtain shear wave velocity (V s in m/s) using the correlation with field standard penetration test (SPT) N values in the absence of sophisticated dynamic field test data. In this paper, a similar but modified advanced approach has been proposed for a major metro city of eastern India, i.e., Kolkata city (latitudes 22°20′N–23°00′N and longitudes 88°04′E–88°33′E), to obtain shear wave velocity profile and soil site classification using regression and sensitivity analyses. Extensive geotechnical borehole data from 434 boreholes located across 75 sites in the city area of 185 km2 and laboratory test data providing information on the thickness of subsoil strata, SPT N values, consistency indices and percentage of fines are collected and analyzed thoroughly. A correlation between shear wave velocity (V s) and SPT N value for various soil profiles of Kolkata city has been established by using power model of nonlinear regression analysis and compared with existing correlations for other Indian cities. The present correlations, having regression coefficients (R 2) in excess of 0.96, indicated good prediction capability. Sensitivity analysis predicts that significant influence of soil type exists in determining V s values, for example, typical silty sand shows 30.4 % increase in magnitude of V s as compared to silt of Kolkata city. Moreover, the soil site classification shows Class D and Class E category of soil that exists typically in Kolkata city as per NEHRP (Recommended provisions for seismic regulations for new buildings and other structures—Part 1: Provisions. Prepared by the Building Seismic Safety Council for the Federal Emergency Management Agency (Report FEMA 450), Washington, DC, 2003) guidelines and thereby highlighting the seismic vulnerability of the city. The results presented in this study can be utilized for seismic microzonation, ground response analysis and hazard assessment for Kolkata city.

  相似文献   

11.
Tunnicliffe, J., Church, M. & Enkin, R. J. 2012 (January): Postglacial sediment yield to Chilliwack Lake, British Columbia, Canada. Boreas, Vol. 41, pp. 84–101. 10.1111/j.1502‐3885.2011.00219.x. ISSN 0300‐9483. Seismic records and evidence from sediment cores at Chilliwack Lake provide the basis for a long‐term (postglacial) sediment budget for a 324‐km2 Cordilleran catchment. Chilliwack Lake (11.8 km2 surface area), situated in the North Cascade Mountains, near Chilliwack, British Columbia, was formed behind a valley‐wide recessional moraine in the final phase of post‐Fraser alpine glaciation. Seismic surveys highlight the postglacial lacustrine record, which is underlain by a thick layer of sediments related to deglacial sedimentation. Sediment cores provide details of grain‐size fining from the delta to the distal lake basin. The cores also show a record of intermittent fire and debris flows. Magnetic measurements of lake sediments provide information on grain size, as well as a dating framework. The total postglacial lake‐floor deposit volume is estimated to be 397 ± 27 × 106 m3. Including estimates of fan and delta deposition, the specific postglacial yield to the lake is calculated to be ～86 ± 13 Mg km2 a?1. The sediment volume in the uppermost (Holocene) lacustrine layer is 128 ± 9 × 106 m3, representing ～41 ± 4 Mg km2 a?1 in the Holocene. Compared with other Cordilleran lakes of similar size, particularly those with glacial cover in the watershed, Chilliwack Lake has experienced relatively modest rates of sediment accumulation. This study provides an important contribution to a growing database of long‐term (postglacial) sediment yield data for major Cordilleran lakes, essential for advancing our understanding of the pace of landscape evolution in formerly glaciated mountainous regions.  相似文献   

12.
In‐situ 40Ar/39Ar laser microprobe dating was carried out on the Hoping pseudotachylite from a mylonite‐fault zone in the metamorphosed basement complex of the active Taiwan Mountain Belt to determine the timing of the responsible earthquake(s). The dating results, distributed from 3.2 to 1.6 Ma with errors ranging from 0.2 to 1.1 Ma, were derived from a combination of two Ar isotopic system end‐members with inverse isochron ages of 1.55 ± 0.05 and 2.87 ± 0.07 Ma, respectively. Petrographical observations reveal fault melt containing ultracataclasites, therefore the older inverse isochron end‐member may be attributed to the relic wall rock Ar isotopic system contained in micro‐breccia. Without significant Ar loss expected, the ~1.6 Ma for the young end‐member defines the exact time of the pseudotachylite formation. Seismic faulting therefore occurred during basement rock exhumation in the Taiwanese hinterland.  相似文献   

13.
The selection of landfill sites for municipal solid waste (MSW) disposal involves consideration of geological, hydrological and environmental parameters which exhibit large spatial variability. Therefore, it is necessary to define, to what extent the chosen sites are reliable such that the probability of environmental pollution and health risks to population is minimal. In the present study, groundwater vulnerability to contamination has been assessed using the standard DRASTIC method. The results showed that the study region has 9.45% of very less, 32.94% of less, 25.47% of moderate, 22.79% of high and 9.35% of very high vulnerable zones. The study also revealed that none of the landfills are located in safe zones. This suggests that it requires proper remedial measures to avoid environmental pollution. A landfill site selection process has been carried out using the Analytical Hierarchy Process integrated with Geographical Information System tools. The obtained results showed that only 3.59?km2 (0.08%) of the total area is suitable for landfills. The reliability analysis of the site suitability revealed that landfills are located at unreliable locations where the probability of risk to environmental pollution is high. The presented approach assists decision-makers in selecting reliable locations for the safe disposal of MSW.  相似文献   

14.
Soil loss evaluation by means of radioisotopic content measurements represents a promising technique, half-way between field surveys and theoretical models, which still suffers from its practical limits when applied at basin scale. A main limit is represented by the costs of field investigations and laboratory investigations on the high number of soil samples required by a traditional sampling scheme, such as regular grid or transects. A non-conventional soil sampling scheme was tested with the aim to consider a relative scanty number of soil samples and to verify the feasibility of this technique on large areas. This scheme was based on the hypothesis that land analysis and classification could point out areas characterised by homogeneous behaviour with respect to the 137Cs deposition and transmission model. A Geographic Information System (GIS)-aided procedure allowed to classify the selected basin area in pedo-morphological units, representative of the different pedologic, morphologic and land-use conditions, to locate few sampling points for each unit. Outcomes pointed out a low correlation between 137Cs contents and soil physical and compositional characteristics. Nevertheless, the isotopic methodology allowed to estimate a total soil loss value at basin scale almost consistent with both observed data, given by reservoir sedimentation measurements and estimates from the application of the RUSLE model. Thus, the results can be considered encouraging and they allow to deem that the isotopic methodology can be refined in order to account for erosion and deposition processes even at river basin scale and with a limited number of soil samples.  相似文献   

15.
On the morning of January 4, 2010, a massive landslide swept the Attabad and Sarat villages into the Hunza River. The debris from the landslide blocked the low-lying river, creating a barrier lake in the area and poses a major threat to the villages located downstream. The aim of the current study was to evaluate the environmental advantages and disadvantages created by the formation of the artificial lake. For this purpose, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data were used to create the contours and triangulated irregular network (TIN) of the region. Data from ??Google Earth?? image on March 19, 2010 were used as the reference and to determine the river bed elevation of the study area. Landsat satellite data of Enhanced Thematic Mapper Plus (ETM+) sensor on January 10, March 15, April 24 and May 2, 2010, were used for constructing the Geographic Information System (GIS) layers of the river banks, land use area, overbank flow area and water area estimation. Our results show that the area covered by the water in the lake has increased from 1.28?km2 on January 10, 2010 to 6.25?km2 on May 2, 2010. The total upstream urban area affected by the river blockage is 13.99?km2. We also applied the Hydrologic Engineering Center River Analysis System (HEC-RAS) model to estimate the potential catastrophes due to dam burst for different peak outflow scenarios with conclusions and recommendations.  相似文献   

16.
Seismic reflection data reveal prominent bottom-simulating reflections (BSRs) within the relatively young (<0.78 Ma) sediments along the West Svalbard continental margin. The potential hydrate occurrence zone covers an area of c. 1600 km2. The hydrate accumulation zone is bound by structural/tectonic features (Knipovich Ridge, Molloy Transform Fault, Vestnesa Ridge) and the presence of glacigenic debris lobes inhibiting hydrate formation upslope. The thickness of the gas-zone underneath the BSR varies laterally, and reaches a maximum of c. 150 ms. Using the BSR as an in-situ temperature proxy, geothermal gradients increase gradually from 70 to 115 °C km−1 towards the Molloy Transform Fault. Anomalies only occur in the immediate vicinity of normal faults, where the BSR shoals, indicating near-vertical heat/fluid flow within the fault zones. Amplitude analyses suggest that sub-horizontal fluid migration also takes place along the stratigraphy. As the faults are related to the northwards propagation of the Knipovich Ridge, long-term disturbance of hydrate stability appears related to incipient rifting processes.  相似文献   

17.
Following wildfires, emergency-response and public-safety agencies can be faced with evacuation and resource-deployment decisions well in advance of coming winter storms and during storms themselves. Information critical to these decisions is provided for recently burned areas in the San Gabriel Mountains of southern California. A compilation of information on the hydrologic response to winter storms from recently burned areas in southern California steeplands is used to develop a system for classifying magnitudes of hydrologic response. The four-class system describes combinations of reported volumes of individual debris flows, consequences of debris flows and floods in an urban setting, and spatial extents of the hydrologic response. The range of rainfall conditions associated with different magnitude classes is defined by integrating local rainfall data with the response magnitude information. Magnitude I events can be expected when within-storm rainfall accumulations (A) of given durations (D) fall above the threshold A = 0.4D 0.5 and below A = 0.5D 0.6 for durations greater than 1 h. Magnitude II events will be generated in response to rainfall accumulations and durations between A = 0.4D 0.5 and A = 0.9D 0.5 for durations less than 1 h, and between A = 0.5D 0.6 and A = 0.9D 0.5 or durations greater than 1 h. Magnitude III events can be expected in response to rainfall conditions above the threshold A = 0.9D 0.5. Rainfall threshold-magnitude relations are linked with potential emergency-response actions as an emergency-response decision chart, which leads a user through steps to determine potential event magnitudes and identify possible evacuation and resource-deployment levels. Use of this information in planning and response decision-making process could result in increased safety for both the public and emergency responders.  相似文献   

18.
Discordant zebra dolomite bodies occur locally in the Middle Cambrian Cathedral and Eldon Formations of the Main Ranges of the Canadian Rocky Mountains Fold and Thrust Belt. They are characterized by alternating dark grey (a) and white (b) bands, forming an ‘abba’ diagenetic cyclicity. These bands developed parallel to both bedding and cleavage. Dark grey (a) bands consist of fine (< 300 μm) non-planar crystalline impure dolomite. The white (b) bands are composed of coarse (up to several millimetres) milky-white pure saddle dolomites (b1) which are often covered by pore-lining zoned dolomite (b2). The b phases often possess a saddle-shaped morphology. In contrast to the replacement origin of the a dolomite, the zoned b2 dolomite rims are interpreted as a cement formed in open cavities. The b1 dolomite is interpreted as the result of recrystallization with diagenetic leaching of non-carbonate components. All the zebra dolomites studied are (nearly) stoichiometric and are characterized by enriched Na and depleted Sr concentrations. Fe and Mn concentrations in these dolomites differ depending on the sample locality. Fluid inclusion data indicate that the dolomites formed from relatively hot (TH = 130–200 °C), saline (20–23 wt% CaCl2 eq.) fluids. A diagenetic high temperature origin is also supported by depleted δ18O values (−20 to −14‰ VPDB). A contribution of 87Sr-enriched fluids is reflected in the 87Sr/86Sr values (0·7091–0·7123). Zebra dolomite development is explained by focused fluid flow, which exploited areas of structural weaknesses (e.g. basin-platform, rim areas, faults, etc.). Expulsion of hot basinal brines in a tectonically active regime generated overpressures, which explains the development of secondary porosity during zebra dolomitization as well as the intra-zebra fracturing at decimetre to micrometre scale.  相似文献   

19.
Seismic hazard in terms of peak ground acceleration (PGA) has been evaluated in northern Algeria using spatially smoothed seismicity data. We present here a preliminary seismic zoning in northern Algeria as derived from the obtained results.Initially, we have compiled an earthquake catalog of the region taking data from several agencies. Afterwards, we have delimited seismic areas where the b and mmax parameters are different. Finally, by applying the methodology proposed by Frankel [Seismol. Res. Lett. 66 (1995) 8], and using four complete and Poissonian seismicity models, we are able to compute the seismic hazard maps in terms of PGA with 39.3% and 10% probability of exceedance in 50 years.A significant result of this work is the observation of mean PGA values of the order of 0.20 and 0.45 g, for return periods of 100 and 475 years, respectively, in the central area of the Tell Atlas.  相似文献   

20.
The heterogeneous upper mantle low velocity zone   总被引：2，自引：1，他引：2  
The upper mantle low velocity zone (LVZ) is a depth interval with slightly reduced seismic velocity compared to the surrounding depth intervals. The zone is present below a relatively constant depth of 100 km in most continental parts of the world, both in cratonic areas with high average velocity and tectonically active areas with low average velocity. Evidence for the low velocity zone arises from controlled and natural source seismology, including studies of surface waves and of primary and multiple reflections of body waves from the bounding interfaces, calculations of receiver functions, and absolute velocity tomography. The available data indicates a more pronounced reduction in seismic velocity and Q-value for S-waves than P-waves as well as high electrical conductivity in the LVZ. Seismic waves are strongly scattered by the zone, which demonstrates the existence of small-scale heterogeneity. The depth to the base of the LVZ is systematically shallower in cold, stable cratonic areas than in hot, active regions of the world. Because of its global occurrence below a relative constant depth of 100 km, the LVZ cannot be explained by metamorphic or compositional variation and rheological changes. Calculated upper mantle temperatures indicate that the rocks are close to the solidus in an interval with variable thickness below 100 km depth, provided that the rocks contain water and carbon dioxide. The presence of, even small amounts of such fluids in the mantle rocks will lower the solidus by several hundred degrees and introduce a characteristic kink on the solidus curve around 80–100 km depth. The seismic velocities and Q-values are significantly reduced of rocks, which are close to the solidus or contain small amounts of partial melt. Hence, the LVZ may be explained by upper mantle temperatures being close to the solidus in a depth interval below 100 km. Assuming that the rocks contain only limited amounts of fluids, this mechanism may explain the low velocities, Q-values, and resistivity, as well as the intrinsic scattering, and the characteristic variation in thickness of the low velocity zone.  相似文献   

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