首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
We present reults from simultaneous precise levelling and gravity surveys on Mount Etna covering the period August 1980–August 1981. The flank eruption of March 1981 erupted 18–35 × 105m3 of lava. Following it, upward movements of more than 17 cm were observed close to the new fissure and a broad, apparently independent, uplift of 5 cm was observed 4 km to the west. A zone of about 2 cm depression to the east of the fissure is insufficient to account for the volume of magma erupted. Gravity results show positive changes of up to 63 microgal, and display good positive correlation with elevation changes. Both sets of measurements appear to be due to new intrusion of magma rather than subsurface magma drainage. Ground deformation close to the new fissure is well modelled by intrusion of a dyke in the zone 100–500 m below the surface, striking along the fissure and of dip between 75–90°. The gravity changes are modelled as due to a deeper intrusion of magma, along the same line but some 1500 m below the surface. The changes were not present immediately after the eruption but occurred during the ensuing 5 months. It is proposed that this introduction of matter occurred by crack propagation along the fissure in the aftermath of the eruption. Towards the west of the fissure, and some 4 km west of the summit, ground deformation is modelled by intrusion of a dyke in the zone 300–1500 m below the surface and dipping at 80–85°. Again, gravity changes appear to be due to magma intrusion at greater depth, close to sea level. In this case gravity changes are interpreted as due to magma density changes, as a result of pressure increase in a larger scale fissure zone. This same pressure increase may be forcing the new intrusion close to the surface, and makes this part of the volcano a region of especially high risk.  相似文献   

2.
GPS geodetic measurements were conducted around the Askja central volcano located at the divergent plate boundary in north Iceland in 1987, 1990, 1992 and 1993. The accuracy of the 1987 and 1990 measurements is in the range of 10 mm for horizontal components; the accuracy of the 1992 and 1993 measurements is about 4 mm in the horizontal plane. Regional deformation in the Askja region is dominated by extension. Points located outside a 30–45 km wide plate boundary deformation zone indicate a displacement of 2.4±0.5 cm/a in the direction N 99°E±12° of the Eurasian plate relative to the North American plate in the period 1987–1990. Within the plate boundary deformation zone extensional strain accumulates at a rate of 0.8 strain/a. Displacement of control points next to Askja (>7 km from the caldera center) in the periods 1990–1993 and 1992–1993 show deflation and contraction towards the caldera. These results are in accordance with the results obtained by other geodetic methods in the area, which indicate that the deflation at Askja occurs in response to a pressure decrease at about 2.8 km depth, located close to the center of the main Askja caldera. A Mogi point source was fixed at this location and the GPS data used to solve for the source strength. A central subsidence of 11±2.5 cm in the period 1990–1993 is indicated, and 5.5±1.5 cm in the period 1992–1993. The maximum tensional strain rate, according to the point source model, occurs at a horizontal distance of 2.5–6 km from the source, at the same location as the main caldera boundary. Discrepancies between the observed displacements and predicted displacements from the Mogi model near the Askja caldera can be attributed to the regional eastwest extension that occurs at Askja.  相似文献   

3.
This paper presents a new method of analysing lava flow deposits which allows the velocity, discharge rate and rheological properties of channelled moving lavas to be calculated. The theory is applied to a lava flow which was erupted on Kilauea in July 1974. This flow came from a line of fissures on the edge of the caldera and was confined to a pre-existing gully within 50 m of leaving the vent. The lava drained onto the floor of the caldera when the activity stopped, but left wall and floor deposits which showed that the lava banked up as it flowed around each of the bends. Field surveys established the radius of curvature of each bend and the associated lava levels, and these data, together with related field and laboratory measurements, are used to study the rheology of the lava. The results show the flow to have been fast moving but still laminar, with a mean velocity of just over 8 m s–1; the lava had a low or negligible yield strength and viscosities in the range 85–140 Pa s. An extension of the basic method is considered, and the possibility of supercritical flow discussed.  相似文献   

4.
The 2-D crustal velocity model along the Hirapur-Mandla DSS profile across the Narmada-Son lineament in central India (Murty et al., 1998) has been updated based on the analysis of some short and discontinuous seismic wide-angle reflection phases. Three layers, with seismic velocities of 6.5–6.7, 6.35–6.40 and 6.8 km s–1, and upper boundaries located approximately at 8, 17 and 22 km depth respectively, have been identified between the basement (velocity 5.9 km s–1) and the uppermost mantle (velocity 7.8 km s–1). The layer with 6.5–6.7 km s–1 velocity is thin and is less than 2-km deep between the Narmada north (at Katangi) and south (at Jabalpur) faults. The upper crust shows a horst feature between these faults, which indicates that the Narmada zone acts as a ridge between two pockets of mafic intrusion in the upper crust. The Moho boundary, at 40–44 km depth and the intra-crustal layers exhibit an upwarp suggesting that the Narmada faults have deep origins, involving deep-seated tectonics. A smaller intrusive thickness between the Narmada faults, as compared to those beyond these faults, suggests that the intrusive activities on the two sides are independent. This further suggests that the two Narmada faults may have been active at different geological times. The seismic model is constrained by 2-D gravity modeling. The gravity highs on either side of the Narmada zone are due to the effect of the high velocity/high density mafic intrusion at upper crustal level.  相似文献   

5.
The difficulties involved in making geomagnetic measurements from the moving ice shelf at Halley are considered, as are measurements giving information on this movement. These indicate that the observatory has been moving westward, accelerating from 400 m yr–1 in 1969 to 800 m yr–1 in 1980, and rotating at up to 30 min of arc yr–1. The effects of both rotation and tilt on the two types of variometers installed are examined. It is concluded that the geomagnetic data obtained from Halley since 1969 are best treated as variation data.  相似文献   

6.
A very large uplift (about 1.8 m) occurred in the period 1982–1984 at Campi Flegrei caldera, Italy, without culminating in an eruption. A still-standing controversy accompanies the interpretation of deformation and gravity changes recorded during the unrest, which were interpreted to result from the sub-surface magmatic reservoir by some authors and from the hydrothermal system or hybrid sources by others. Here for the first time we take into account crustal layering while inverting leveling, EDM, and gravity data using uniformly-pressurized sources, namely small vertical spheroids and finite horizontal penny-shaped sources. The weight of EDM data in the misfit function is chosen from a trade-off curve in order to balance the compromise between fitting the leveling and the EDM data well. Models using a homogeneous medium cannot give a good simultaneous fit to leveling and EDM deformation data of the 1982–1984 unrest, whereas incorporating a layered structure (determined from seismically derived estimates of the P wave speed for the crust, and not adjusted to improve the fit in any of the inversions) allows a significantly better fit. Also, layering affects the sub-surface mass redistribution effects on gravity changes, and we show that the retrieved intrusion density is in full agreement with densities for highly evolved magmas expected at the Campi Flegrei caldera for depths of 3 to 4 km, ruling out hydrothermal fluids as the primary cause of the 1982–1984 unrest. The source of the 1982–1984 CF unrest was probably a shallow (about 3-km deep) penny-shaped magma intrusion fed by a deeper magma chamber; source overpressure was few MPa.  相似文献   

7.
In the geological record, the intrusion of substantial amounts of magma into circumferential faults and ring fractures is commonly observed. Finite element modelling is used here to investigate the strain field that may be expected from such intrusive events. Two simple vertical scenarios are explored, one for a caldera with a central block of thickness to diameter ratio of ~1:1 (similar to Rabaul) and one with a ratio much less than 1:1 (similar to the Valles type). Surface deformation in both cases is similar with central uplift, the development of a moat (or trough) like feature just outside of the intersection of the azimuth of the intruded ring fault and the free surface, and broader scale tumescence at a scale several times larger than the calderas radius. The response of the block and sub-caldera magma chamber for the two scenarios, however, is different. The blocks are in effect squeezed; the high aspect ratio one deforms upwards at the surface and downwards at its base, whereas the low aspect ration one experiences up arching (or bending) of the central part of the caldera block. Central uplift still occurs when only a short arc of a ring fracture system or a circumferential fault is intruded. In both models, tumescence in the centre of the caldera from single ring dyke intrusion can only account for decimetres to metres of surface uplift. Repeated intrusions over tens to hundreds of thousands of years, however, may cause incremental up doming of the caldera block leading to larger scale resurgent features. The amount of uplift possible due to squeezing of a high aspect ratio block is limited. It is proposed, however, that where bending of plate-like blocks occur above a decompressible and/or malleable magma body, ring fault intrusion may be a significant contributor to resurgence. In the simple conceptual models shown here, the amount of ring dyke-induced central uplift will be >40–50% of the width of the ring complex. In the geological record the accumulation of intrusions into some ring fractures has led to annular or arcuate plutons of hundreds of meters to several kilometres in thickness. At certain calderas such intrusions may be a control on the marked concentration of uplift within the restricted area defined by the caldera faults. The complex nature of the horizontal displacements associated with the intrusion of ring and arcuate dykes is also explored. Intrusion into ring fracture zones will tend to take place into those sectors of the annular zone which are perpendicular to the least compressive stress vector. This may be a factor in the observed difference for caldera evolution in extensional and compressional areas. The unrest at several modern calderas is tentatively related to circumferential fault intrusion.Editorial responsibility: J. Stix  相似文献   

8.
Summary The number and size of salt particles produced by the bursting of air bubbles in sea water has been measured. Bubbles of diameters varying between 1/2 and 2 mm each produced about 300 nuclei the sizes of which, under the electron microscope, were mainly between 0.1 and 0.5 diameter. They appeared to consist mainly of sodium chloride, the smallest ones containing only 10–15 g of salt.These results, together with measurements of the size distribution of salt nuclei collected over the oceans in areas of spray formation, indicate that the total concentrations of salt nuclei over the oceans in winds of up to 15 m sec–1 probably do not exceed 100 cm–3. The corresponding rate of production of salt nuclei at the sea surface is estimated to be 1000 cm–2 sec–1. It is therefore inferred that sea spray contributes perhaps only one-fifth of the nuclei involved in cloud formation, the majority being the products of combustion, either natural or man-made.  相似文献   

9.
The Boundary Element Method (BEM), a numerical technique for solving boundary integral equations, is introduced to determine the earth's gravity field. After a short survey on its main principles, we apply this method to the fixed gravimetric boundary value problem (BVP), i.e. the determination of the earth's gravitational potential from measurements of the intensity of the gravity field in points on the earth's surface. We show how to linearize this nonlinear BVP using an implicit function theorem and how to transform the linearized BVP into a boundary integral equation using the single layer representation. A Galerkin method is used to transform the boundary integral equation using the single layer representation. A Galerkin method is used to transform the boundary integral equation into a linear system of equations. We discuss the major problems of this approach for setting up and solving the linear system. The BVP is numerically solved for a bounded part of the earth's surface using a high resolution reference gravity model, measured gravity values of high density, and a 50 50 m2 digital terrain model to describe the earth's surface. We obtain a gravity field resolution of 1 1 km2 with an accuracy of the order 10–3 to 10–4 in about 1 CPU-hour on a Siemens/Fujitsu SIMD vector pipeline machine using highly sophisticated numerical integration techniques and fast equation solvers. We conclude that BEM is a powerful numerical tool for solving boundary value problems and may be an alternative to classical geodetic techniques.  相似文献   

10.
The EISCAT VHF radar (69.4°N, 19.1°E) has been used to record vertical winds at mesopause heights on a total of 31 days between June 1990 and January 1993. The data reveal a motion field dominated by quasi-monochromatic gravity waves with representative apparent periods of 30–40 min, amplitudes of up to 2.5 m s–1 and large vertical wavelength. In some instances waves appear to be ducted. Vertical profiles of the vertical-velocity variance display a variety of forms, with little indication of systematic wave growth with height. Daily mean variance profiles evaluated for consecutive days of recording show that the general shape of the variance profiles persists over several days. The mean variance evaluated over a 10 km height range has values from 1.2 m2s–2 to 6.5 m2s–2 and suggests a semi-annual seasonal cycle with equinoctial minima and solsticial maxima. The mean vertical wavenumber spectrum evaluated at heights up to 86 km has a slope (spectral index) of -1.36 ± 0.2, consistent with observations at lower heights but disagreeing with the predictions of a number of saturation theories advanced to explain gravity-wave spectra. The spectral slopes evaluated for individual days have a range of values, and steeper slopes are observed in summer than in winter. The spectra also appear to be generally steeper on days with lower mean vertical-velocity variance.  相似文献   

11.
Microgravity changes with time, not always consistent with the Bouguer-corrected free air gradient, have been recorded and associated with cyles of eruptive activity at Krafla, Kilauea, Pacaya and Etna volcanoes. In contrast, over the non-erupting yet active fumarolic vents at Poás (Costa Rica), real-time gravity observations over three periods during the years 1983–1985 have identified ca. 140µGal amplitude, cyclic gravity variations. Their decrease in amplitude with distance from the active crater, coupled with a static sub-surface structural model, have allowed the effects of a variety of possible causative dynamic phenomena to be evaluated. It is concluded that cyclic changes in the average density (by ca. 0.03 Mg m–3) in the magma pipe at depths below 500 m, rather than variation in magma chamber or water table geometry, are responsible for the observed gravity variations. Specifically, anaverage of 1 % fluctuation in the volume of gas in crystal-free magma, a process driven by thermal convection cycles, probably accounts for the density/gravity change.  相似文献   

12.
The authors conducted a Rn222 survey in wells of the Larderello geothermal field (Italy) and observed considerable variations in concentrations. Simple models show that flow-rate plays an important part in the Rn222 content of each well, as it directly affects the fluid transit time in the reservoirs. Rn222 has been sampled from two wells of the Serrazzano area during flow-rate drawdown tests. The apparent volume of the steam reservoir of each of these two wells has been estimated from the Rn222 concentration versus flow-rate curves.List of symbols Q Flow-rate (kg h–1) - Decay constant of Rn222 (=7.553×10–3 h–1) - Porosity of the reservoir (volume of fluid/volume of rock) - 1 Density of the fluid in the reservoir (kg m–3) - 2 Density of the rock in the reservoir (kg m–3) - M Stationary mass of fluid filling the reservoir (kg). - E Emanating power of the rock in the reservoir (nCi kg rock –1 h–1). - P Production rate of Rn222 in the reservoir: number of atoms of Rn222 (divided by 1.764×107) transferred by the rock to the mass unit of fluid per unit time (nCi kg fluid –1 h–1). - N Specific concentration of Rn222 in the fluid (nCi kg–1) - Characteristic time of the steam reservoir at maximum flow-rate (=M/Q)  相似文献   

13.
Summary The geothermal gradient in the Carpathian Basin lies between 40–70 C/km. According to careful measurements in shafts the value of terrestrial heat flow in the southern part of Hungary is (2.055–3.066)·10–6 cal/cm2 sec. These measurements are believed the first ever attempted in continental Europe. Systematic heat flow measurement are being extended to other part of this country.  相似文献   

14.
Summary In adjusting measured values in sets A(r*), v(r*) and f(r*) by means of a power function in the form of P=Kr* a region of discontinuity of the approximating curves was found at the distance r*11.5 m kg –1/3. It is assumed that this discontinuity was caused by the varying character of the source of seismic waves. For scaled distances r*>11.5 m kg –1/3 the explosion was considered to be a spherical source from the point of view of the charge geometry and of the distance of the pick-up from the centre of the charge, whereas if r*<11.5 m kg –1/3 the explosion in the borehole had the character of a cylindrical source. The difference of the two types of sources was reflected in the exponent with both the functions A(r*) and v(r*), so that for r*>11.5 m kg –1/3 –4.0 and–2.4, and for r*<11.5 m kg –1/3 –2.5 and–1.5. For the same intervals of scaled distance in the set f(r*)1.4 and1.2.  相似文献   

15.
Simple models, like the well-known point source of dilation (Mogis source) in an elastic, homogeneous and isotropic half-space, are widely used to interpret geodetic and gravity data in active volcanic areas. This approach appears at odds with the real geology of volcanic regions, since the crust is not a homogeneous medium and magma chambers are not spheres. In this paper, we evaluate several more realistic source models that take into account the influence of self-gravitation effects, vertical discontinuities in the Earths density and elastic parameters, and non-spherical source geometries. Our results indicate that self-gravitation effects are second order over the distance and time scales normally associated with volcano monitoring. For an elastic model appropriate to Long Valley caldera, we find only minor differences between modeling the 1982–1999 caldera unrest using a point source in elastic, homogeneous half-spaces, or in elasto-gravitational, layered half-spaces. A simple experiment of matching deformation and gravity data from an ellipsoidal source using a spherical source shows that the standard approach of fitting a center of dilation to gravity and uplift data only, excluding the horizontal displacements, may yield estimates of the source parameters that are not reliable. The spherical source successfully fits the uplift and gravity changes, overestimating the depth and density of the intrusion, but is not able to fit the radial displacements.  相似文献   

16.
The 1991–1993 eruption was probably the largest on Mt. Etna for 300 years. Since then the volcano has entered an unusually quiescent period. A comprehensive record of gravity and ground deformation changes presented here bracket this eruption and give valuable insight into magma movements before, during and after the eruption. The gravity and deformation changes observed before the eruption (1990–1991) record the intrusion of magma into the summit feeder and the SSE-trending fracture system which had recently been active in 1978, 1979, 1983 and 1989, creating the feeder dyke for the 1991–1993 eruption. In the summit region gravity changes between 1992 and 1993 (spanning the end of the eruption) reflect the withdrawal of magma from the conduit followed more recently (1993–1994) by the re-filling of magma in the conduit up to pre-eruption levels. In contrast, in the vicinity of the fracture zone, gravity has remained at the 1991–1992 level, indicating that no withdrawal has occurred here. Rather, magma has solidified in the fracture system and sealed it such that the 1993–1994 increase in magma level in the conduit was not accompanied by further intrusion into the flanks. Mass calculations suggest that a volume of at least 107 m3 of magma has solidified within the southeastern flank of the volcano.  相似文献   

17.
Summary Mean equatorial gravity has been computed from geopotential models GEM-10C, GEM-7, GEM-T1, GEM-T2, GEM-T3, JGM-1, JGM-2, JGM-3 and OSU91A and compared to the normal equatorial gravity, e=978 032·699 × 10–5 m s–2, computed from four given parameters defining the Earth's level ellipsoid. In all models ge>e.  相似文献   

18.
A sediment budget is constructed for the slope and narrow continental shelf off the Sepik River in order to estimate the relative importance of turbid plumes versus bottom gravity transport through a near-shore submarine canyon in the dispersal of sediment across this collision margin. 210Pb geochronology and inventories of Kasten cores are consistent with the northwestward dispersal of sediment from the river mouth via hypopycnal and possible isopycnal plumes. Sediment accumulation rates are 5 cm yr−1 on the upper slope just off of the Sepik mouth, decreasing gradually to 1 cm yr−1 toward the northwest, and decreasing abruptly offshore (<0.2 cm yr−1 at 1200 m water depth). A sediment budget indicates that only about 7–15% of the Sepik River sediment discharge accumulates on the adjacent open shelf and slope. The remainder presumably escapes offshore via gravity flows through a submarine canyon, the head of which extends into the river mouth. The divergent sediment pathways observed off the Sepik River (i.e., surface and subsurface plumes versus sediment gravity flows through a canyon) may be common along high-yield collision margins of the Indo–Pacific archipelago, and perhaps are analogous to most margins during Late Quaternary low sea-level conditions.  相似文献   

19.
In the mid-fifteenth century, one of the largest eruptions of the last 10 000 years occurred in the Central New Hebrides arc, forming the Kuwae caldera (12x6 km). This eruption followed a late maar phase in the pre-caldera edifice, responsible for a series of alternating hydromagmatic deposits and airfall lapilli layers. Tuffs related to caldera formation ( 120 m of deposits on a composite section from the caldera wall) were emitted during two main ignimbritic phases associated with two additional hydromagmatic episodes. The lower hydromagmatic tuffs from the precaldera maar phase are mainly basaltic andesite in composition, but clasts show compositions ranging from 48 to 60% SiO2. The unwelded and welded ashflow deposits from the ignimbritic phases and the associated intermediate and upper hydromagmatic deposits also show a wide compositional range (60–73% SiO2), but are dominantly dacitic. This broad compositional range is thought to be due to crystal fractionation. The striking evolution from one eruptive style (hydromagmatic) to the other (magmatic with emission of a large volume of ignimbrites) which occurred either over the tuff series as a whole, or at the beginning of each ignimbritic phase, is the most impressive characteristic of the caldera-forming event. This strongly suggests triggering of the main eruptive phases by magma-water interaction. A three-step model of caldera formation is presented: (1) moderate hydromagmatic (sequences HD 1–4) and magmatic (fallout deposits) activity from a central vent, probably over a period of months or years, affected an area slightly wider than the present caldera. At the end of this stage, intense seismic activity and extrusion of differentiated magma outside the caldera area occurred; (2) unhomogenized dacite was released during a hydromagmatic episode (HD 5). This was immediately followed by two major pyroclastic flows (PFD 1 and 2). The vents spread and intense magma-water interaction at the beginning of this stage decreased rapidly as magma discharge increased. Subsequent collapse of the caldera probably commenced in the southeastern sector of the caldera; (3) dacitic welded tuffs were emplaced during a second main phase (WFD 1–5). At the beginning of this phase, magma-water interaction continued, producing typical hydromagmatic deposits (HD 6). Caldera collapse extended to the northern part of the caldera. Previous C14 dates and records of explosive volcanism in ice from the south Pole show that the climactic phase of this event occurred in 1452 A.D.  相似文献   

20.
Summary In Northern Bohemia 33 research flights were made during which concentration of giant condensation chloride nuclei was measured up to a height of 3,500 m above the earth's surface. Chloride particles were determined by traces left by them in a sheet of gelatine with silver nitrate (Liesegang circles). The actual size of the particles was obtained by comparison of particles of a known size, falling in a sedimentation tube, with images in the sensitive sheet, and the result was adapted to the analysis of samples during the flight. For each level at which a sample was exposed the spectrum of the sizes of nuclei was determined. During the year the average concentration of the giant chloride nuclei at a height of 100 m above the earth was 7.3×10–3 cm–3. At greater heights concentration of the nuclei decreased successively, so that at 2,000 m it was only 0.3×10–3 cm–3 and above the level of 3,000 m the nuclei occurred sporadically, on the whole. Under the influence of atmospheric exchange the concentration of the nuclei in the boundary layer up to 1,000 m above the earth's surface changes considerably both during the day and during the year. The highest concentration occurs at a height of several hundred metres above the earth in the winter month, at the morning and evening hours, i.e. during a more stable temperature lapse rate. From the established concentrations of the nuclei relation between the turbulent diffusion coefficient and the height above the earth was determined. Its average value during the year increases up to a height of 300 to 400 m above the earth. Above this level it decreases roughly according to the relation ofKz –6/7. The exponent changes not only with temperature stratification (with the time of day and year), but also with the average wind speed. For sodium-chloride particles of 2.5 in diameter the maximum value of turbulent diffusion coefficient (at level of 400 m), reaches about 18×103 cm2 sec–1 while the minimum value is about 5×103 cm2 sec–1 (above 2,000 m).  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号