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1.
Discrete explosive bursts are known from many volcanic eruptions. In maar–diatreme eruptions, they have occurred in debris-filled volcanic vents when magma interacted with groundwater, implying that material mobilized by such explosions passed through the overlying and enclosing debris to reach the surface. Although other studies have addressed the form and characteristics of craters formed by discrete explosions in unconsolidated material, no details are available regarding the structure of the disturbed debris between the explosion site and the surface. Field studies of diatreme deposits reveal cross-cutting, steep-sided zones of non-bedded volcaniclastic material that have been inferred to result from sedimentation of material transported by “debris jets” driven by explosions. In order to determine the general processes and deposit geometry resulting from discrete, explosive injections of entrained particles through a particulate host, we ran a series of analogue experiments. Specific volumes of compressed (0.5–2.5 MPa) air were released in bursts that drove gas-particle dispersions through a granular host. The air expanded into and entrained coloured particles in a small crucible before moving upward into the host (white particles). Each burst drove into the host an expanding cavity containing air and coloured particles. Total duration of each run, recorded with high-speed video, was approximately 0.5–1 s. The coloured beads sedimented into the transient cavity. This same behaviour was observed even in runs where there was no breaching of the surface, and no coloured beads ejected. A steep-sided body of coloured beads was left that is similar to the cross-cutting pipes observed in deposits filling real volcanic vents, in which cavity collapse can result not only from gas escape through a granular host as in the experiments, but also through condensation of water vapour. A key conclusion from these experiments is that the geometry of cross-cutting volcaniclastic deposits in volcanic vents is not directly informative of the geometry of the “intrusions” that formed them. An additional conclusion is that complex structures can form quickly from discrete events.  相似文献   

2.
The Ferrar large igneous province of Antarctica contains significant mafic volcaniclastic deposits, some of which are interpreted to fill large vent complexes. Such a complex was re-examined at Coombs Hills to map individual steep-sided cross-cutting bodies in detail, and we found several contrasting types, two of which are interpreted to have filled subterranean passageways forcefully opened from below into existing, non-consolidated debris. These transient conduits were opened because of the propagation of debris jets – upward-moving streams of volcaniclastic debris, steam, magmatic gases +/− liquid water droplets – following explosive magma–aquifer interaction. Some debris jets probably remained wholly subterranean, whereas others made it to the surface, but the studied outcrops do not allow us to differentiate between these cases. The pipes filled with country rock-rich lapilli-tuff or tuff-breccia are interpreted to have formed following phreatomagmatic explosions occurring near the walls or floor of the vent complex, causing fragmentation of both magma and abundant country rock material. In contrast, some of the cross-cutting zones filled with basalt-rich tuff-breccia or lapilli-tuff could have been generated following explosions taking place within pre-existing basalt-bearing debris, well away from the complex walls or floor. We infer that once focused jets were formed, they did not incorporate significant amounts of existing debris while travelling through them; instead, incorporation of fragments from the granular host took place near explosion sites. Other basalt-rich tuff-breccia zones, accompanied by domains of in situ peperite and coherent basalt pods, are inferred to have originated by less violent processes.  相似文献   

3.
Physicochemical and microbiological characteristics of the bathing waters in Varna’s Black Sea coastal area were investigated during year 2007 at 23 monitoring stations. Most of the determined physicochemical parameters—pH, mineral oils, surface active substances, phenols, dissolved oxygen, nutrients and microbiological parameters—“Total coliforms”, “Faecal coliforms”, “Faecal streptococci” were in compliance with the guidline’ limits and exhibited good water quality. Ammonium and phosphate’s pollution above the limits was determined at the South beach, Officers beach and Central beach situated in Varna’s central bathing zone. For the period of 13.08 to 24.10.2007—70% of the South beach samples analyzed for NH4+ exceeded the limits 60 times and the concentrations of PO43− exceeded the limits 17.5 times. Some deviations from the guidline’ limits regarding the microbiology were exhibited at the same beaches. A conclusion is made that the area of study is not yet seriously threatened, in spite of the rapid recreation during the last years.  相似文献   

4.
 The Middle Jurassic Tuttle Lake Formation in the northern Sierra Nevada, California, comprises a thick volcaniclastic sequence deposited in a submarine island-arc setting and penetrated by numerous related hypabyssal intrusions. A composite andesite-diorite intrusive complex ≥4.5 km long and ≥1.5 km thick was emplaced while the host Tuttle Lake sediments were still wet and unconsolidated. Large parts of the intrusive complex consist of peperite formed where andesitic magma intruded and intermixed with tuff, lapilli-tuff and tuff-breccia. The southern half of the complex consists of augite-phyric andesite containing peperite in numerous small, isolated pockets and in more extensive, laterally continuous zones. The peperites comprise three main types recognized previously in other peperite studies. Fluidal peperite consists of small (≤30 cm), closely spaced, at least partly interconnected, globular to amoeboid andesite bodies enclosed by tuff. This peperite type developed during intrusion of magma into fine-grained wet sediment along unstable interfaces, and fluidization of the sediment facilitated development of complex intrusive geometries. Blocky peperite and mixed blocky and fluidal peperite formed where magma intruded coarser sediment and underwent variable degrees of brittle fragmentation by quenching and dynamic stressing of rigid margins, possibly aided by small steam explosions. The northern half of the intrusive complex consists predominantly of a different type of peperite, in which decimetre-scale plagioclase-phyric andesite clasts with ellipsoidal, elongate, or angular, polyhedral shapes are closely packed to widely dispersed within disrupted host sediment. Textural features suggest the andesite clasts were derived from conduits through which magma was flowing, and preserved remnants of the conduits are represented by elongate, sinuous bodies up to 30 m or more in length. Disruption and dispersal of the andesite clasts are inferred to have occurred at least partly by steam explosions that ripped apart a network of interconnected feeder conduits penetrating the host sediments. Closely packed peperite is present adjacent to mappable intrusions of coherent andesite, and along the margin of a large mass of coarse-grained diorite. These coherent intrusions are considered to be major feeders for this part of the complex. Examples of magma/wet sediment interaction similar in scale to the extensive peperites described here occur elsewhere in ancient island-arc strata in the northern Sierra Nevada. Based on these and other published examples, large-scale peperites probably are more common than generally realized and are likely to be important in settings where thick sediment sequences accumulate during active volcanism. Careful mapping in well-exposed terrains may be required to recognize large-scale peperite complexes of this type. Received: 8 June 1998 / Accepted: 4 December 1998  相似文献   

5.
 Investigation of well-exposed volcaniclastic deposits of Shiveluch volcano indicates that large-scale failures have occurred at least eight times in its history: approximately 10,000, 5700, 3700, 2600, 1600, 1000, 600 14C BP and 1964 AD. The volcano was stable during the Late Pleistocene, when a large cone was formed (Old Shiveluch), and became unstable in the Holocene when repetitive collapses of a portion of the edifice (Young Shiveluch) generated debris avalanches. The transition in stability was connected with a change in composition of the erupting magma (increased SiO2 from ca. 55–56% to 60–62%) that resulted in an abrupt increase of viscosity and the production of lava domes. Each failure was triggered by a disturbance of the volcanic edifice related to the ascent of a new batch of viscous magma. The failures occurred before magma intruded into the upper part of the edifice, suggesting that the trigger mechanism was indirectly associated with magma and involved shaking by a moderate to large volcanic earthquake and/or enhancement of edifice pore pressure due to pressurised juvenile gas. The failures typically included: (a) a retrogressive landslide involving backward rotation of slide blocks; (b) fragmentation of the leading blocks and their transformation into a debris avalanche, while the trailing slide blocks decelerate and soon come to rest; and (c) long-distance runout of the avalanche as a transient wave of debris with yield strength that glides on a thin weak layer of mixed facies developed at the avalanche base. All the failures of Young Shiveluch were immediately followed by explosive eruptions that developed along a similar pattern. The slope failure was the first event, followed by a plinian eruption accompanied by partial fountain collapse and the emplacement of pumice flows. In several cases the slope failure depressurised the hydrothermal system to cause phreatic explosions that preceded the magmatic eruption. The collapse-induced plinian eruptions were moderate-sized and ordinary events in the history of the volcano. No evidence for directed blasts was found associated with any of the slope failures. Received: 28 June 1998 / Accepted: 28 March 1999  相似文献   

6.
Regional architecture of geochronology and differential cooling pattern show that the Dabie orogen underwent a thermal doming extension during 140–85 Ma. This extension resulted in widespread re-melting of the Dabie basement, intense volcanic activities in North Huaiyang and the formation of fault-controlled depressions in the Hefei basin. This thermal doming extension can be further divided into two consecutive evolving stages, i.e. the intensifying stage (140–105 Ma) and the declining stage (105–85 Ma). In the first stage (140–105 Ma), the thermal doming mainly was concentrated in the Dabie block, and to a less degree, in the Hongan block. The thermal doming structure of the Dabie block is configured with Macheng-Yuexi thermal axis, Yuexi/Luotian thermal cores and their downslide flanks. The orientation of thermal axis is dominantly parallel to the strike of orogen, and UHP/HP units together with metamorphic rocks of North Huaiyang constitute the downslide flanks. The Yuexi core differs from the Luotian core in both the intensity and the shaping time. To some extent, the Hongan block can be regarded as part of downslide systems of the Dabie doming structure. The doming process is characterized by thermal-center’s migration along the Macheng-Yuexi thermal axis; consequently, it is speculated to be attributed to the convective removal of thickened orogenic root, which is a process characterized by intermittance, mi gration, large-scale and differentiation. During the declining stage (105–85 Ma), the dome-shaped figure still structurally existed in the Dabie orogen, but orogenic units cooled remarkably slow and magmatic activities stagnated gradually. Study on the thermal doming of Dabieshan Mountains can thus provide detailed constraints on the major tectonic problems such as the UHP/HP exhumation model, the boundary between North Dabie and South Dabie, and the orogenesis mechanism.  相似文献   

7.
The Method of Dynamic Calibration (MDC) of stations of the International Monitoring System (IMS) was developed for calibrating regions where no underground nuclear explosions were carried out, with the purpose of providing conditions for implementation of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) in nontrivial cases. Initially, the MDC had been presented in [Kedrov, 2001; Kedrov et al., 2001; Kedrov O.K. and Kedrov E.O., 2003] and then considered in detail in [Kedrov et al., 2008]. The core of MDC relates to adapting diagnostic parameters for the identification of underground nuclear explosions (UNE) and earthquakes elaborated for the region of Eurasia, taken as a basic region (BR), for other researched regions that differ from BR in the character of the attenuation of seismic waves. The unique characteristic of this method lies in the fact that calibration of diagnostic parameters with the help of attenuation coefficients b Δ at varied source-station traces is implemented using only natural seismicity data within the limits of an explored region and does not require special underground chemical explosions. The MDC algorithm is implemented in the research program ”Kalibr”, which was tested by using the experimental data from Eurasia region. It is shown in this work that MDC can be used for calibration of regions where a very low level of natural seismicity is observed. According to the results of the calibration of diagnostic parameters at IMS stations in several regions of North America, Africa, and Asia, the approximate classification of propagation conditions for seismic signals at source-station traces in platform and tectonically active regions is made. The results for the development of two research programs, “Spektr” and “Signal”, are presented; this software is intended for automation of calculation procedures for spectral diagnostic parameters of UNEs’ and earthquakes’ identification by amplitude spectra of P waves and by the maximal amplitudes of P, S, and LR signals. The application of these programs allowed us to accelerate the whole calibration procedure for a particular source-station trace using the ”Kalibr” program.  相似文献   

8.
Saucer-shaped dolerite and sandstone intrusions are common in sedimentary basins world-wide. We have conducted a series of scaled experiments simulating the process of magma emplacement in sedimentary basins, with particular attention on the formation of saucer-shaped sills. The model materials were (1) cohesive fine-grained silica flour, representing brittle crust; and (2) molten low-viscosity oil, representing magma. The experiments were performed in both homogeneous and layered models. In all the experiments, oil injection resulted in doming of the surface. In the homogeneous models, the injected oil formed cone sheets and sub-vertical dykes. Cone sheets formed for shallow injection (1–3 cm), and vertical dykes formed for deeper injection (4–5 cm). In layered models, the injected oil always formed saucer-shaped intrusions. Our experimental results show that (1) sill intrusion results in the formation of a dome, with melt erupting at the rim; (2) layering controls the formation of sills and saucer-shaped sills; (3) saucer-shaped sills are fed from the bottom and the fluid flows upward and outward; and (4) the diameter of saucer-shaped sills increase with increasing emplacement depth. The systematic relation between domes and sills and the depth-dependence of sill diameters show that saucer-shaped intrusions result from the interaction between a growing flat-lying shallow sill and doming of the free surface. We conclude that saucer-shaped intrusions represent fundamental geometries formed by shallow magma intrusion in stratified basins.  相似文献   

9.
Deep-sea limu o Pele are shards of basaltic glass commonly described as “bubble walls.” When first identified they were inferred to form in submarine fire fountains, but were then reinterpreted as the products of hydrovolcanic volcanism, formed when submarine lava flows entrapped and vaporised seawater. Limu discovered below the c 3 km critical depth of seawater, where superheated water exists as a supercritical fluid instead of a vapour, led to the hydrovolcanic model of limu o Pele formation being discarded in favour of a magmatic CO2-driven, “strombolian-like” model. This revised magmatic mechanism has been widely accepted by the scientific community. We describe a newly discovered limu o Pele-rich deposit at ~1,052 mbsl on the northeast summit plateau region of Lō`ihi Seamount, Hawai`i. The limu at this site is concentrated in a chemically monomict ash lens interbedded with thin lava sheets that are separated from overlying volcaniclastic material by a discontinuity. The geometry and geochemistry of the deposit provide compelling evidence for a hydrovolcanic, sheet flow-related origin. The exceptional abundance and preservation of limu at this site allows 4 morphologic subtypes of limu- thin film, plateau-border, convex film, and Pele’s hair- to be identified and linked to portions of the isolated rupturing bubbles from which they are derived. We extend our discussion to beyond this new Lō`ihi deposit, by including a review of limu o Pele occurrences and thermodynamic considerations that demonstrate the hydrovolcanic model of limu formation to be more tenable than the magmatic model at all depths, including below the critical depth of seawater.  相似文献   

10.
The Milos volcanic field includes a well-exposed volcaniclastic succession which records a long history of submarine explosive volcanism. The Bombarda volcano, a rhyolitic monogenetic center, erupted ∼1.7 Ma at a depth <200 m below sea level. The aphyric products are represented by a volcaniclastic apron (up to 50 m thick) and a lava dome. The apron is composed of pale gray juvenile fragments and accessory lithic clasts ranging from ash to blocks. The juvenile clasts are highly vesicular to non-vesicular; the vesicles are dominantly tube vesicles. The volcaniclastic apron is made up of three fades: massive to normally graded pumice-lithic breccia, stratified pumice-lithic breccia, and laminated ash with pumice blocks. We interpret the apron beds to be the result of water-supported, volcaniclastic mass-How emplacement, derived directly from the collapse of a small-volume, subaqueous eruption column and from syn-eruptive, down-slope resedimentation of volcaniclastic debris. During this eruptive phase, the activity could have involved a complex combination of phreatomagmatic explosions and minor submarine effusion. The lava dome, emplaced later in the source area, is made up of flow-banded lava and separated from the apron by an obsidian carapace a few meters thick. The near-vertical orientation of the carapace suggests that the dome was intruded within the apron. Remobilization of pyroclastic debris could have been triggered by seismic activity and the lava dome emplacement. Published online: 30 January 2003 Editorial responsibility: J. McPhie  相似文献   

11.
After 33 years of repose, one of the most active volcanoes of the Kurile island arc—Sarychev Peak on Matua Island in the Central Kuriles—erupted violently on June 11, 2009. The eruption lasted 9 days and stands among the largest of recent historical eruptions in the Kurile Island chain. Satellite monitoring of the eruption, using Moderate Resolution Imaging Spectroradiometer, Meteorological Agency Multifunctional Transport Satellite, and Advanced Very High Resolution Radiometer data, indicated at least 23 separate explosions between 11 and 16 June 2009. Eruptive clouds reached altitudes of generally 8–16 km above sea level (ASL) and in some cases up to 21 km asl. Clouds of volcanic ash and gas stretched to the north and northwest up to 1,500 km and to the southeast for more than 3,000 km. For the first time in recorded history, ash fall occurred on Sakhalin Island and in the northeast sector of the Khabarovsky Region, Russia. Based on satellite image analysis and reconnaissance field studies in the summer of 2009, the eruption produced explosive tephra deposits with an estimated bulk volume of 0.4 km3. The eruption is considered to have a Volcanic Explosivity Index of 4. Because the volcano is remote, there was minimal risk to people or infrastructure on the ground. Aviation transport, however, was significantly disrupted because of the proximity of air routes to the volcano.  相似文献   

12.
Peperites formed by mixing of magma and wet sediment are well exposed along Punta China, Baja California, Mexico, where two sills intrude a section of lava flows, limestones, and volcaniclastic rocks. Irregular lobes and dikes extend from the sills several meters into host sediments, including highly comminuted flow top breccias (lithic lapilli tuff breccias) and shelly micrites, whereas intrusive contacts with lava flows are sharp and planar. Where one sill intruded both coarse-grained volcaniclastic rock and fine-grained limestone, textural differences between the hosts produced strikingly different styles of peperite. Blocky masses of the basaltic intrusions up to 1 m in size were dispersed for distances up to 3 m into host lithic lapilli tuff breccias; the blocks consequently underwent in situ fragmentation as they were rapidly quenched. The high degree of dispersion resulted from steam explosions as the magma enveloped pockets of water in the coarse-grained permeable host. Elutriation of fine-grained material from vertical pipes in tuff breccia above the lower sill provides evidence for meter-scale fluidization of the host. The contact zone between the basaltic magma and the shelly micrite host resembles a mixture of two viscous, immiscible fluids (fluidal peperite). Intrusion occurred behind a stable vapor film which entrained lime mud particles and carried them off grain by grain as magma advanced into the host. Thin-section-scale elutriation pipes formed. Microglobular peperite represents a frozen example of a fuel-coolant interaction (FCI) between basaltic magma and fluidized micrite host. The intimate intermixing of magma and host at the submillimeter level is attributed to fluid instabilities developed along the magma-vapor-host interface. Such intimate intermixing of magma and water-bearing fragmental debris is commonly a precursory step toward explosive hydrovolcanism.  相似文献   

13.
In this paper, one of the distribution-free tests — randomization test, is briefly described. It doesn’t need any distribution assumption and its related parameter estimation and is applicable to random and nonrandom sample. Then it is used to the test of migration of strong earthquakes on the Xianshuihe Fault Belt and “immunity” of large earthquakes in the large northern reigon of China. The test results show that there is 98.7% confidence degree for the migration of strong earthqueks on the Xianshuihe Fault Belt and “immunity” of earthqueks withM S⩾8 toM S⩾7 is significant in the large northern region of China. The obtained test results and the test method itself have certain application in the practice. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 484–489, 1993.  相似文献   

14.
Forensic seismology revisited   总被引：1，自引：0，他引：1  
The first technical discussions, held in 1958, on methods of verifying compliance with a treaty banning nuclear explosions, concluded that a monitoring system could be set up to detect and identify such explosions anywhere except underground: the difficulty with underground explosions was that there would be some earthquakes that could not be distinguished from an explosion. The development of adequate ways of discriminating between earthquakes and underground explosions proved to be difficult so that only in 1996 was a Comprehensive Nuclear Test Ban Treaty (CTBT) finally negotiated. Some of the important improvements in the detection and identification of underground tests—that is in forensic seismology—have been made by the UK through a research group at the Atomic Weapons Establishment (AWE). The paper describes some of the advances made in identification since 1958, particularly by the AWE Group, and the main features of the International Monitoring System (IMS), being set up to verify the Test Ban. Once the Treaty enters into force, then should a suspicious disturbance be detected the State under suspicion of testing will have to demonstrate that the disturbance was not a test. If this cannot be done satisfactorily the Treaty has provisions for on-site inspections (OSIs): for a suspicious seismic disturbance for example, an international team of inspectors will search the area around the estimated epicentre of the disturbance for evidence that a nuclear test really took place. Early observations made at epicentral distances out to 2,000 km from the Nevada Test Site showed that there is little to distinguish explosion seismograms from those of nearby earthquakes: for both source types the short-period (SP: ∼1 Hz) seismograms are complex showing multiple arrivals. At long range, say 3,000–10,000 km, loosely called teleseismic distances, the AWE Group noted that SP P waves—the most widely and well-recorded waves from underground explosions—were in contrast simple, comprising one or two cycles of large amplitude followed by a low-amplitude coda. Earthquake signals on the other hand were often complex with numerous arrivals of similar amplitude spread over 35 s or more. It therefore appeared that earthquakes could be recognised on complexity. Later however, complex explosion signals were observed which reduced the apparent effectiveness of complexity as a criterion for identifying earthquakes. Nevertheless, the AWE Group concluded that for many paths to teleseismic distances, Earth is transparent for P signals and this provides a window through which source differences will be most clearly seen. Much of the research by the Group has focused on understanding the influence of source type on P seismograms recorded at teleseismic distances. Consequently the paper concentrates on teleseismic methods of distinguishing between explosions and earthquakes. One of the most robust criteria for discriminating between earthquakes and explosions is the m b : M s criterion which compares the amplitudes of the SP P waves as measured by the body-wave magnitude m b, and the long-period (LP: ∼0.05 Hz) Rayleigh-wave amplitude as measured by the surface-wave magnitude M s; the P and Rayleigh waves being the main wave types used in forensic seismology. For a given M s, the m b for explosions is larger than for most earthquakes. The criterion is difficult to apply however, at low magnitude (say m b < 4.5) and there are exceptions—earthquakes that look like explosions. A difficulty with identification criteria developed in the early days of forensic seismology was that they were in the main empirical—it was not known why they appeared to work and if there were test sites or earthquakes where they would fail. Consequently the AWE Group in cooperation with the University of Cambridge used seismogram modelling to try and understand what controls complexity of SP P seismograms, and to put the m b : M s criterion on a theoretical basis. The results of this work show that the m b : M s criterion is robust because several factors contribute to the separation of earthquakes and explosions. The principal reason for the separation however, is that for many orientations of the earthquake source there is at least one P nodal plane in the teleseismic window and this biases m b low. Only for earthquakes with near 45° dip-slip mechanisms where the antinode of P is in the source window is the m b:M s criterion predicted to fail. The results from modelling are consistent with observation—in particular there are earthquakes, “anomalous events”, which look explosion-like on the m b:M s criterion, that turn out to have mechanisms close to 45° dip-slip. Fortunately the P seismograms from such earthquakes usually show pP and sP, the reflections from the free surface of P and S waves radiated upwards. From the pP–P and sP–P times the focal depth can be estimated. So far the estimated depth of the anomalous events have turned out to be ∼20 km, too deep to be explosions. Studies show that the observation that P seismograms are more complex than predicted by simple models can be explained on the weak-signal hypothesis: the standard phases, direct P and the surface reflections, are weak because of amongst other things, the effects of the radiation pattern or obstacles on the source-to-receiver path; other non-standard arrivals then appear relatively large on the seismograms. What has come out of the modelling of P seismograms is a criterion for recognising suspicious disturbances based on simplicity rather than complexity. Simple P seismograms for earthquakes at depths of more than a few kilometres are likely to be radiated only to stations that lie in a confined range of azimuths and distances. If then, simple seismograms are recorded over a wide range of distances and particularly azimuths, it is unlikely the source is an earthquake at depth. It is possible to test this using the relative amplitudes of direct P and later arrivals that might be surface reflections. The procedure is to use only the simple P seismograms on the assumption that whereas the propagation through Earth may make a signal more complex it is unlikely to make it simpler. From the amplitude of the coda of these seismograms, bounds can be placed on the size of possible pP and sP. The relative-amplitude method is then used to search for orientations of the earthquake source that are compatible with the observations. If no such orientations are found the source must be shallow so that any surface reflections merge with direct P, and hence could be an explosion. The IMS when completed will be a global network of 321 monitoring stations, including 170 seismological stations principally to detect the seismic waves from earthquakes and underground explosions. The IMS will also have stations with hydrophones, microbarographs and radionuclide detectors to detect explosions in the oceans and the atmosphere and any isotopes in the air characteristic of a nuclear test. The Global Communications Infrastructure provides communications between the IMS stations and the International Data Centre (IDC), Vienna, where the recordings from the monitoring stations is collected, collated, and analysed. The IDC issues bulletins listing geophysical disturbances, to States Signatories to the CTBT. The assessment of the disturbances to decide whether any are possible explosions, is a task for State Signatories. For each Signatory to do a detailed analysis of all disturbances would be expensive and time consuming. Fortunately many disturbances can be readily identified as earthquakes and removed from consideration—a process referred to as “event screening”. For example, many earthquakes with epicentres over the oceans can be distinguished from underwater explosions, because an explosion signal is of much higher frequency than that of earthquakes that occur below the ocean bed. Further, many earthquakes could clearly be identified at the IDC on the m b : M s criterion, but there is a difficulty—how to set the decision line. The possibility has to be very small that an explosion will be classed by mistake, as an earthquake. The decision line has therefore to be set conservatively, consequently with routine application of current screening criteria, only about 50% of earthquakes can be positively identified as such. Various methods have been proposed whereby a “determined violator” could avoid the provisions of a CTBT and carry out a test that would be either undetected or detected but not identified as an explosion. The increase in complexity and cost of such a test should discourage any State from attempting it. In addition, there is always the possibility of some stations detecting the test, the test being identified as suspicious, and so subject to an OSI. With time as the IMS becomes more efficient and effective it will act increasingly to deter anyone contemplating a clandestine test, from going ahead. What has emerged is several robust criteria. The criteria include: location, which when combined with hydro-acoustic data can identify earthquakes under the sea; m b : M s; and depth of focus. More detailed study is required of any remaining seismic disturbance that is regarded as suspicious: for example, is close to a site where nuclear tests have been carried out in the past. Any disturbance that is shown to be explosion-like, may be the subject of an OSI. One surprise is how little plate tectonics has contributed to resolving problems in forensic seismology. Much of the evidence for plate tectonics comes from seismological studies so it would be expected that the implications for Earth structure arising from forensic seismology would be consistent with plate-tectonic models. So far the AWE Group have found little synergy between plate tectonics and forensic seismology. It is to be hoped that the large volume of seismological data of high quality now being collected by the IMS and the increasing number of digital stations, will result in a revised Earth model that is consistent with the findings of forensic seismology, so that a future review of progress will show that the forensic seismologist can draw on this model in attempting to interpret apparently anomalous seismograms.
A. DouglasEmail:
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15.
AMS radiocarbon age dating of planktonic foraminifera in volcaniclastic deposits on Loihi Seamount yields ages ranging from 590 years before present (y BP) at 10 cm depth to 5,880 y BP at 1,007 cm depth in an 11-m-thick section exposed along inward facing, caldera-bounding faults on the eastern side of Loihi’s summit. The accumulation rate of the deposit was about 0.37 cm/y from 5,880 to 3,300 y BP and it consisted of subequal amounts of alkalic and tholeiitic fragments. The rate slowed dramatically at about 3,300 y BP to an average 0.04 cm/y and the particles that have accumulated since consist mostly of alkalic glass fragments. The decrease in accumulation rate could indicate a decrease in volcanic activity at Loihi beginning about 3,300 y BP. This lower level of activity appears to be continuing today.  相似文献   

16.
Three stages of collapse and doming of the inner subsided block are recognized in the Miocene Kakeya cauldron. The mechanism of the first collapse is not clear, but the second and third are volcanic in origin. The second collapse was triggered by eruptions of silicic andesite lava flows and pyroclastic ejecta. The boundary fault between the subsided block and its surroundings is nearly vertical. The subsided block formed a distinct basin structure, and its marginal part was intensely deformed by faulting. The third collapse took place cylindrically, accompanied by voluminous eruptions of dacitic pyroclastic materials. The collapsed block formed a basin structure with a gently dipping marginal part. The doming of the inner subsided block was due to increase of pressure in a magma chamber.The structure formed by the second collapse is not consistent with the concept of the subterranean structure of either the so-called «Krakatau»- (funnel-shaped) type or Valles-type calderas. The second collapse is transitional between «Krakatau»- and Valles-type calderas or a new type of volcanic depression. The features of the third collapse and the resurgent doming are similar to those of Valles-type calderas, except for the size of cauldrons and composition of magmas related to collapse. The similarities indicate that the Kakeya cauldron was formed in an extensional tectonic setting similar to that for Valles-type calderas.  相似文献   

17.
A method of correcting the seismometers’ responses discrepancy for differential measurements of strains and rotations in the seismic far field is proposed. The method concerns differential calibration of the whole seismometers’ array by electric current. A model of corrective filtering of the obtained differential signals in the Z domain is given. Two methods of the filter parameters’ estimation are introduced; they are based on the seismometers response to the calibration. A practical test of the methods — an analysis of a recorded seismic event — is added. Significant reduction of differences between simultaneous seismic recordings was obtained, which is interpreted as cleaning of differential signal from spurious elements.  相似文献   

18.
Microtextural characteristics of fresh ejecta from Stromboli volcano were examined from three periods of differing eruption style and intensity in 2002. Activity shifted from relatively weak and infrequent ash-charged explosions during January through May into two broad cycles of waxing activity in June through late September, and late September through December, followed by the onset on 28 December of the 2002/2003 effusive eruption. Analyzed sets of lapilli from May, September/October, and 28 December show contrasts in the physical properties of magma resident in the shallow conduit during this range of activity. Three distinct textures are observed among the analyzed pyroclasts: low density (LD) with an abundance of subspherical bubbles, the presence of large, irregularly shaped bubbles, and a light-to-transparent glass matrix; transitional texture (TT) with an intermediate number of subspherical bubbles, a high frequency of large, irregularly-shaped bubbles, and a honey colored glass matrix; and high density (HD) with sparse relatively small bubbles, conspicuous large irregular bubbles, and a dark glass matrix. Observational and quantitative data (density, vesicle size) indicate that these textures are linked through variable residence time in Stromboli’s shallow conduit, with an ongoing evolution from LD to HD magma. Calculations suggest that residual LD magma will evolve to HD texture in a period of hours to days. Contrasting amounts of the LD, TT, and HD magmas are present in each sample, with the most TT in May, the most LD in September/October, and the most HD in December. This implies that the shallow magma had a different rheology at each collection period. The viscosity of LD and HD magmas are calculated to be in the range of 2,000 to 2,600 and 3,000 to 5,000 Pa s, respectively, which, with their changing proportions, must have implications for rates of bubble slug ascent and processes of fragmentation. This study suggests that an increasing maturity of magma in Stromboli’s shallow conduit (with resultant increase in viscosity) feeds back to reduce the intensity of explosions, whereas a steady flux of LD magma favors more powerful explosions.  相似文献   

19.
    
According to the requirement of the project “Establishment of the Physical Model of Earthquake Precursor Fields”, this paper elucidates the train of thinking for research on the project and some scientific problems which must be studied; the elucidation emphasizes that the core of this project is to study the conditions and processes of the generation of strong earthquakes. The paper first outlines the origin and development of the “strong-body earthquake-generating model” proposed by the author in the 1980’s; and then proves the reasonableness of the model from three aspects, namely: deep structures, mechanical analysis and rock fracture experiments. By studying the tomographic image for the northern part of North China, it can be seen that the sources of strong earthquakes are all distributed in high-velocity bodies, or in the contact zone between high-velocity and low-velocity bodies but nearer to the high-velocity body. It has been affirmed through studies of the mechanical models of hard and soft inclusions that the existence of a hard inclusion is an important condition for the high concentration of large amounts of strain energy. A lot of theoretical and experimental studies have been made to investigate the conditions for rock instability; the results have consistently indicated that rock instability, sudden fracture and stress drop would be possible only if the stiffness of the source body is greater than the environmental stiffness. This subject is one of the “8th Five-Year Plan” of the SSB, China.  相似文献   

20.
White Island is an active andesitic-dacitic composite volcano surrounded by sea, yet isolated from sea water by chemically sealed zones that confine a long-lived acidic hydrothermal system, within a thick sequence of fine-grained volcaniclastic sediment and ash. The rise of at least 106 m3 of basic andesite magma to shallow levels and its interaction with the hydrothermal system resulted in the longest historical eruption sequence at White Island in 1976–1982. About 107 m3 of mixed lithic and juvenile ejecta was erupted, accompanied by collapse to form two coalescing maar-like craters. Vent position within the craters changed 5 times during the eruption, but the vents were repeatedly re-established along a line linking pre-1976 vents. The eruption sequence consisted of seven alternating phases of phreatomagmatic and Strombolian volcanism. Strombolian eruptions were preceded and followed by mildly explosive degassing and production of incandescent, blocky juvenile ash from the margins of the magma body. Phreatomagmatic phases contained two styles of activity: (a) near-continuous emission of gas and ash and (b) discrete explosions followed by prolonged quiescence. The near-continuous activity reculted from streaming of magmatic volatiles and phreatic steam through open conduits, frittering juvennile shards from the margins of the magma and eroding loose lithic particles from the unconsolidated wall rock. The larger discrete explosions produced ballistic block aprons, downwind lobes of fall tephra, and cohesive wet surge deposits confined to the main crater. The key features of the larger explosions were their shallow focus, random occurrence and lack of precursors, and the thermal heterogeneity of the ejecta. This White Island eruption was unusual because of the low discharge rate of magma over an extended time period and because of the influence of a unique physical and hydrological setting. The low rate of magma rise led to very effective separation of magmatic volatiles and high fluxes of magmatic gas even during phreatic phases of the eruption. While true Strombolian phases did occur, more frequently the decoupled magmatic gas rose to interact with the conduit walls and hydrothermal system, producing phreatomagmatic eruptions. The form of these wet explosions was governed by a delicate balance between erosion and collapse of the weak conduit walls. If the walls were relatively stable, fine ash was slowly eroded and erupted in weak, near-continous phreatomagmatic events. When the walls were unstable, wall collapse triggered larger discrete phreatomagmatic explosions.  相似文献   

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