EM Monitoring of Crustal Processes Including the Use of the Network-MT Observations

There are several kinds of coupling mechanisms which can convert mechanical, chemical or thermal energies due to seismic or volcanic activities into electromagnetic energies. As a result of concentrated efforts in laboratory and theoretical research, the basic relationship between the intensity of electromagnetic sources and changes in mechanical, chemical and thermal state is becoming established. Also with the progress of the electromagnetic simulation techniques, it has been possible to evaluate in situ sensitivity. Based on this progress and also due to extensive improvement in measuring techniques, many field experiments have been performed to elucidate subsurface geophysical processes underlying the preparation stage, onset, and subsequent healing stage of earthquakes and volcanic eruptions. In volcanic studies, many studies have reported the measurement of electromagnetic signals which were successfully interpreted in terms of various driving mechanisms. Although there have been numerous reports about the existence of precursory electromagnetic signals in seismic studies, only a few of them could be successfully explained by the proposed mechanisms, whereas coseismic phenomena are often consistent with those mechanisms including the absence of detectable signals. In many cases, one or two orders of higher sensitivity were required, especially for precursory signals. Generally, electromagnetic methods are more sensitive to near-surface phenomena. It will be necessary to discriminate electromagnetic signals due to these near-surface sources, which often possess no relationship with the crustal activities. Further efforts to enhance in situ sensitivity through improvements in observation techniques and in data processing techniques are recommended. At the same time, multi-disciplinary confirmation against the validity of electromagnetic phenomena will inevitably be necessary. A Network-MT observation technique has been developed to determine large-scale deep electrical conductivity structure. In the method, a telephone line network or purpose-built long baseline cables are utilized to measure voltage differences with long electrode separations. Because of the averaging effect of the electric fields, static shift problems due to small-scale, near-surface lateral heterogeneities can be alleviated. Several field experiments revealed regional scale deep electrical conductivity structures related to slab subduction or its stagnation, which enable us to elucidate underlying physical processes caused by the slab motion. The technique can also be applied to monitor the electric potential field related to crustal activities. The annual variation of the potential field and electrical conductivity in the French Alps were interpreted to be caused by the annual variation of lake water level. The method was also used to monitor the regional scale spatio-temporal variation of the SP field and electrical conductivity before and at the onset of earthquakes and volcanic eruptions.     

The geomorphology of planetary calderas

Satellite-derived observations of the geomorphology of calderas on Earth, Mars and Venus can be used to learn more about shield volcanoes. Examples of terrestrial basaltic volcanoes from the Galapagos Islands, Hawaii, and the Comoro Islands show how these volcanoes contrast with examples found on Mars and Venus. Caldera structure, degree of infilling, and the location of vents on the flanks are used to interpret each volcano's recent history. The geometry of the caldera floor can be used to infer some of the characteristics of the magma storage system, and the orientation of the deep magma conduits. The formation of benches within the caldera and the effects of the caldera on the distribution of flank eruptions are considered, and it is evident that most calderas on the planets are/were dynamic features. Presently, deep calderas, with evidence of overflowing lavas and ponded lavas high in the caldera wall, show that these calderas were once shallow. Similarly, shallow calderas filled with ponded lavas are evidence that they were once deeper. It is probably a mistake, therefore, to place great significance on caldera depth with regard to the position, shape, or size of subsurface plumbing.     

Formation and failure of volcanic debris dams in the Chakachatna River valley associated with eruptions of the Spurr volcanic complex, Alaska

The formation of lahars and a debris avalanche during Holocene eruptions of the Spurr volcanic complex in south-central Alaska have led to the development of volcanic debris dams in the Chakachatna River valley. Debris dams composed of lahar and debris-avalanche deposits formed at least five times in the last 8000–10,000 years and most recently during eruptions of Crater Peak vent in 1953 and 1992. Water impounded by a large debris avalanche of early Holocene (?) age may have destabilized an upstream glacier-dammed lake causing a catastrophic flood on the Chakachatna River. A large alluvial fan just downstream of the debris-avalanche deposit is strewn with boulders and blocks and is probably the deposit generated by this flood. Application of a physically based dam-break model yields estimates of peak discharge (Q_p) attained during failure of the debris-avalanche dam in the range 10⁴<Q_p<10⁶ m³ s⁻¹ for plausible breach erosion rates of 10–100 m h⁻¹. Smaller, short-lived, lahar dams that formed during historical eruptions in 1953, and 1992, impounded smaller lakes in the upper Chakachatna River valley and peak flows attained during failure of these volcanic debris dams were in the range 10³<Q_p<10⁴ m³ s⁻¹ for plausible breach erosion rates.Volcanic debris dams have formed at other volcanoes in the Cook Inlet region, Aleutian arc, and Wrangell Mountains but apparently did not fail rapidly or result in large or catastrophic outflows. Steep valley topography and frequent eruptions at volcanoes in this region make for significant hazards associated with the formation and failure of volcanic debris dams.     

长白山天池火山密码破译势在必行

长白山天池火山是中新世以来多次喷发并造成严重灾害的火山。通过十几年的火山地质及火山地震监测研究认为长白山天池火山近期不会马上喷发，但它是一座具有潜在喷发危险性的活动火山，并提出建立长白山火山观测站的重要意义，讨论了天地火山密码的破译需要调动各方面有利因素和有利条件。     

Mascali,Mount Etna Region Sicily: An Example of Fascist Planning During the 1928 Eruption and Its Continuing Legacy

The 1928 eruption of Etna, Sicily, although the largest such event this century, has not been studied in detail. In this paper the nature of the eruption, the destruction it caused – including the complete devastation of the town of Mascali (pre-eruption population 2,000) – and emergency responses of the authorities to it are reviewed in the context of fascist politics and planning priorities. It is contended that, although at one level the response to the 1928 eruption was successful, at another fascism merely continued and enhanced a reactive, propitiatory approach to hazard mitigation. We argue that this legacy was not successfully overcome until the middle of the nineteen eighties. Finally contemporary Italian moves towards a more proactive approach to disaster planning, both generally and in the context of Etna, are discussed.     

Was a dacite dome implicated in the 9,500 <Emphasis Type="SmallCaps">b.p.</Emphasis> collapse of Mt Ruapehu? A palaeomagnetic investigation

Constraining the process by which volcanoes become unstable is difficult. Several models have been proposed to explain the driving forces which cause volcanic edifices to catastrophically collapse. These include models for destabilisation of volcanic flanks by wedging due to dyke intrusion and the weakening of mechanical properties by pressurisation of pore fluids. It is not known which, if any, of the models are relevant to particular sector collapse events. Recent developments in the palaeomagnetic estimation of emplacement temperatures of volcaniclastic rocks have shown that even relatively low emplacement temperatures can be recorded by volcaniclastics with high fidelity. We have carried out a palaeomagnetic study of emplacement temperatures to investigate the role of igneous activity in the initiation of the 9,500 b.p. Murimotu sector collapse of Mt Ruapehu, New Zealand. This debris avalanche deposit has three fades which are stratigraphically superimposed, and the lowermost fades contains three lithological assemblages representing different segments of the edifice which were transported with little internal mixing within the flow. We have determined that some of the dacite-bearing assemblage 1, fades 1 was hot (∼350 °C) during transport and emplacement, whereas none of the other lithological assemblages of fades contained hot material. Our interpretation is that a dacite dome was active on the ancient Ruapehu edifice immediately prior to the Murimotu sector collapse. The partially cooled carapace of the dome and material shed from this part was incorporated into the avalanche deposit, along with cold lavas and volcaniclastics. We have not found evidence for incorporation of material at or close to magmatic temperatures, at least in the sampled locations. Our palaeomagnetic work allows us to develop a comprehensive, new palaeomagnetic classification of volcaniclastics. Published online: 25 January 2003 Editorial responsibility: D. Dingwell     

Geochemistry of the thermal springs and fumaroles of Basse-Terre Island,Guadeloupe, Lesser Antilles

 The purpose of this work was to study jointly the volcanic-hydrothermal system of the high-risk volcano La Soufrière, in the southern part of Basse-Terre, and the geothermal area of Bouillante, on its western coast, to derive an all-embracing and coherent conceptual geochemical model that provides the necessary basis for adequate volcanic surveillance and further geothermal exploration. The active andesitic dome of La Soufrière has erupted eight times since 1660, most recently in 1976–1977. All these historic eruptions have been phreatic. High-salinity, Na–Cl geothermal liquids circulate in the Bouillante geothermal reservoir, at temperatures close to 250  °C. These Na–Cl solutions rise toward the surface, undergo boiling and mixing with groundwater and/or seawater, and feed most Na–Cl thermal springs in the central Bouillante area. The Na–Cl thermal springs are surrounded by Na–HCO₃ thermal springs and by the Na–Cl thermal spring of Anse à la Barque (a groundwater slightly mixed with seawater), which are all heated through conductive transfer. The two main fumarolic fields of La Soufrière area discharge vapors formed through boiling of hydrothermal aqueous solutions at temperatures of 190–215  °C below the "Ty" fault area and close to 260  °C below the dome summit. The boiling liquid producing the vapors of the Ty fault area has δD and δ¹⁸O values relatively similar to those of the Na–Cl liquids of the Bouillante geothermal reservoir, whereas the liquid originating the vapors of the summit fumaroles is strongly enriched in ¹⁸O, due to input of magmatic fluids from below. This process is also responsible for the paucity of CH₄ in the fumaroles. The thermal features around La Soufrière dome include: (a) Ca–SO₄ springs, produced through absorption of hydrothermal vapors in shallow groundwaters; (b) conductively heated, Ca–Na–HCO₃ springs; and (c) two Ca–Na–Cl springs produced through mixing of shallow Ca–SO₄ waters and deep Na–Cl hydrothermal liquids. The geographical distribution of the different thermal features of La Soufrière area indicates the presence of: (a) a central zone dominated by the ascent of steam, which either discharges at the surface in the fumarolic fields or is absorbed in shallow groundwaters; and (b) an outer zone, where the shallow groundwaters are heated through conduction or addition of Na–Cl liquids coming from hydrothermal aquifer(s). Received: 9 November 1998 / Accepted: 15 July 1999     

吉林长白山天池火山岩年代-40Ar-39Ar

本文综合分析了长白山天池火山岩的类型及年代学研究结果,在此基础上对白头山期两期粗面岩进行了Ar-Ar年代学研究,确定了这两期火山活动的时代分别为123±7和123±10千年.文章还讨论了全新世天池火山喷发的环境意义及对东亚气候变化的影响.     

Seismic quantification of the explosions that destroyed the dome of Volcán de Colima, Mexico, in July–August 2003

In July–August 2003, the andesitic lava dome at Volcán de Colima, México, was destroyed by a sequence of explosions that replaced the 2×10⁶ m³ dome with a crater 200 m across and 30 m deep. The two strongest explosions occurred on July 17 and August 28. The initial low-frequency impulses that they produced, which were recorded on broadband seismic records, allowed an estimation of the counter forces of the initiating process as being equal to 0.3×10¹¹ N and 1×10¹¹ N for the July and August events, respectively. The seismic characteristics follow the Nishimura-Hamaguchi scaling law for volcanic explosions, reflecting self-similarity in the processes initiating explosive events. The results also show that counter forces can discriminate between the sizes of explosive eruptions that are assigned the same magnitude by conventional methods of classification such as the Volcanic Explosivity Index. The increasing use of broadband seismometers may therefore provide the basis for using counter forces to determine the magnitude of explosive eruptions.     

1983–2003 decaying rate of deflation at Askja caldera: Pressure decrease in an extensive magma plumbing system at a spreading plate boundary

New deformation data from the Askja volcano, Iceland, show that the volcano's caldera has been deflating continuously for over 20 years, and confirm that the rate of subsidence is slowing down. The decay in subsidence rate can be fitted with a function of the form e ^{− t / τ} , where τ is 39 years. Reanalysis of GPS data from 1993–1998 show that these data can be fitted with a model calling for two Mogi point sources, one shallow, and another one much deeper (16.2 km depth). Pressure decrease occurs in both sources. The deeper source is responsible for observed horizontal contraction towards Askja at distances that cannot be explained by the shallower source. Plate spreading of 19 mm/year distributed evenly over about 100-km-wide zone is also favoured by the data.