Seismic signals in geothermal areas of active volcanism: a case study from ‘La Fossa’, Vulcano (Italy)

Since the last eruption (1888–1890) volcanism at Vulcano, Aeolian Archipelago, southern Tyrrhenian Sea, has taken the form of persistent fumarolic activity. The gas-vapour phases of the geothermal systems are mainly discharged within two restricted areas about 1 km apart from each other, in the northern part of the island. These areas are La Fossa crater, and the beach fumaroles of the Baia di Levante. Fluids released at the two main fumarolic fields display quite different chemical and temperature characteristics, implying different origins. The local seismicity essentially takes the form of discrete shocks of shallow origin (depth1 km) at La Fossa, usually with energy < 10¹³ ergs. They are thought to be related to the uprise of pressurized hot gases and vapours discharged at the crater fumaroles. The present investigation points to the existence of two principal categories of seismic events (called M-shocks and N-shocks). These are short events (normally < 10 s). M-type shocks are thought to be due to resonance vibrations within the interior of the volcano, probably driven by the excitation of shock-waves within cavities deeply affected by deposition and alteration of self-sealant hydrothermal minerals. N-type events display features that resemble those of volcano-tectonic earthquakes, but have no recognizable S-phases. Here they are tentatively attributed to microfracturing of rocks which have been extensively hydrothermally altered. Results of the present study permit a preliminary conceptual model of the local shallow seismic processes in the framework of geochemical modelling of fumarolic activity and geological inferences from geothermal drilling.     

Potential cauchy-poisson waves generated by submarine eruptions of kick 'em Jenny volcano

Kick 'em Jenny is the only known currently active submarine volcano in the Lesser Antilles. The volcano has erupted at least 10 times since first being discovered in 1939 and the summit has shoaled from a depth of 232 m in 1962 to its present-day depth of 150 m. Kick 'em Jenny is located in a province of explosive volcanism, has a known history of explosive eruptions and erupts magma of an explosive type. Future eruptions are likely to become increasingly more violent as the effect of the overlying water pressure becomes less. A preliminary study (Smith and Shepherd, 1993) suggests that Kick 'em Jenny is a prime candidate for tsunamigenic eruptions on a potentially hazardous scale, possibly affecting the whole of the eastern Caribbean region.The classic approach to problems of water waves generated by sudden disturbances of the free surface makes use of the Cauchy-Poisson-Lamb theory. A large number of theoretical developments to this theory have been made for specific forms of surface disturbance. A development by Unoki and Nakano (1953a, b) considers both two- and three-dimensional Cauchy-Poisson waves generated by finite initial elevations and impulses applied to a free surface of infinitely deep water. Unoki and Nakano's results compared well to the wave systems recorded following submarine eruptions of the Myojinsho Reef volcano in 1952–53.Given the similarity of the two situations, Unoki and Nakano's theory is applied to Kick 'em Jenny to provide estimates of potential Cauchy-Poisson wave heights throughout the eastern Caribbean for a range of eruption magnitudes. The results show that, although the waves generated are unlikely to pose much of a threat to the eastern Caribbean as a whole, they should be considered a hazard to the islands immediately adjacent to the volcano including Grenada, the Grenadines, and St Vincent.     

An investigation of upper mantle heterogeneity beneath the Archaean and Proterozoic crust of western Canada from Lithoprobe controlled-source seismic experiments

Observations of upper mantle reflectivity at numerous locations around the world have been linked to the presence of a heterogeneous distribution of rock types within a broad layer of the upper mantle. This phenomenon is observed in wide-angle reflection data from Lithoprobe's Alberta Basement Transect [the SAREX and Deep Probe experiments of 1995] and Trans-Hudson Orogen Transect [the THoRE experiment of 1993]. SAREX and Deep Probe image the Archaean lithosphere of the Hearne and Wyoming Provinces, whereas THoRE images the Archaean and Proterozoic lithosphere of the Trans-Hudson Orogen and neighbouring areas.Finite-difference synthetic seismograms are used to constrain the position and physical properties of the reflective layer. SAREX/Deep Probe modelling uses a 2-D visco-elastic finite-difference routine; THoRE modelling uses a pseudospectral algorithm. In both cases, the upper mantle is parameterized in terms of two media. One medium is the background matrix; the other is statistically distributed within the first as a series of elliptical bodies. Such a scheme is suitable for modelling: (1) variations in lithology (e.g., a peridotite matrix with eclogite lenses) or (2) variations in rheology (e.g., lenses of increased strain within a less strained background).The synthetic seismograms show that the properties of heterogeneities in the upper mantle do not change significantly between the two Lithoprobe transects. Beneath the Trans-Hudson Orogen in Saskatchewan, the layer is best modelled to lie at depths between 80 and 150 km. Based on observations from perpendicular profiles, anisotropy of the heterogeneities is inferred. Beneath the Precambrian domains of Alberta, 400 km to the west, upper mantle heterogeneities are modelled to occur between depths of 90 and 140 km. In both cases the heterogeneous bodies within the model have cross-sectional lengths of tens of kilometers, vertical thicknesses less than 1 km, and velocity contrasts from the background of − 0.3 to − 0.4 km/s. Based on consistency with complementary data and other results, the heterogeneous layer is inferred to be part of the continental lithosphere and may have formed through lateral flow or deformation within the upper mantle.     

The growth, collapse and quiescence of Teno volcano, Tenerife: new constraints from paleomagnetic data

Tenerife basically consists of three Miocene shield volcanoes, the Anaga, the Teno and Central shield, as well as the Pliocene Cañadas volcano. The temporal evolution and structural significance of each volcano with respect to the history of Tenerife is still a matter of debate. We present paleomagnetic results in order to enhance the view of the volcanic history of the Teno volcano by means of magnetostratigraphy. It is found that the initial subaerial phase shows reverse magnetizations throughout. After two major sector collapses, dominantly normally magnetized lavas extruded. Comparisons of observed magnetic polarities with the geomagnetic polarity timescale show that these volcanic activities occurred within 0.4 Myr between 6.3 and 5.9 Ma. Significantly younger flows, ～ 5.3 Myr old according to their radiometric age, revealed again normal polarity throughout. The absence of inversely magnetized lavas in-between the two normal periods indicates a volcanic hiatus or erosional phase. The evolutionary sequence and the estimated high production rates for the initial building phase are similar as would be expected for a hotspot volcano. The average geomagnetic field for 6.0 ± 0.2 Ma is close to an axial dipole field showing a slight far-sided/right-handed effect. The field strength, determined by Thellier-type intensity determinations, corresponds to a virtual axial dipole moment of 4.9 × 10²² A m². This value is approximately half of the present day field strength, but similar to values obtained for the mid-Miocene. It also corresponds to the proposed tertiary low-field level of the geomagnetic dipole moment.     

Crustal structure across the Grand Banks–Newfoundland Basin Continental Margin – II. Results from a seismic reflection profile

New multichannel seismic reflection data were collected over a 565 km transect covering the non-volcanic rifted margin of the central eastern Grand Banks and the Newfoundland Basin in the northwestern Atlantic. Three major crustal zones are interpreted from west to east over the seaward 350 km of the profile: (1) continental crust; (2) transitional basement and (3) oceanic crust. Continental crust thins over a wide zone (∼160 km) by forming a large rift basin (Carson Basin) and seaward fault block, together with a series of smaller fault blocks eastwards beneath the Salar and Newfoundland basins. Analysis of selected previous reflection profiles (Lithoprobe 85-4, 85-2 and Conrad NB-1) indicates that prominent landward-dipping reflections observed under the continental slope are a regional phenomenon. They define the landward edge of a deep serpentinized mantle layer, which underlies both extended continental crust and transitional basement. The 80-km-wide transitional basement is defined landwards by a basement high that may consist of serpentinized peridotite and seawards by a pair of basement highs of unknown crustal origin. Flat and unreflective transitional basement most likely is exhumed, serpentinized mantle, although our results do not exclude the possibility of anomalously thinned oceanic crust. A Moho reflection below interpreted oceanic crust is first observed landwards of magnetic anomaly M4, 230 km from the shelf break. Extrapolation of ages from chron M0 to the edge of interpreted oceanic crust suggests that the onset of seafloor spreading was ∼138 Ma (Valanginian) in the south (southern Newfoundland Basin) to ∼125 Ma (Barremian–Aptian boundary) in the north (Flemish Cap), comparable to those proposed for the conjugate margins.     

东准噶尔纸房地区晚石炭世巴塔玛依内山组陆相火山-沉积体系特征

东准噶尔巴塔玛依内山组是典型陆相火山．沉积体系(盆地)，主体由基性及中酸性火山熔岩组成，火山碎屑岩及火山碎屑沉积岩较少，通过对纸房地区晚石炭世巴塔玛依内山组陆相火山岩的岩石化学、地球化学特征进行系统研究，认为它属于造山期后固结初期，新陆壳裂谷向高原火山岩演化的同岩浆源、同沉积盆地、同火山作用的钙碱系列双峰式火山岩．     

利用ERS-2 SAR图像纹理分析方法揭示长白山天池火山近代喷发物空间分布特征

简单介绍了SAR图像的纹理特征以及正交小波变换纹理提取方法。论述了SAR图像的纹理特征参与分类的重要性。以长白山天池火山为例,通过对ERS2SAR图像进行纹理分析,提取了SAR图像两个层次的尺度变化、时频局部化和方向性纹理特征。并将SAR纹理特征与TM图像及DEM进行复合,利用多源信息各自的优势,进行了BP神经元网络分类,从较大范围对长白山天池火山735±15aB.P.大喷发的喷发物空间分布进行评价。获取了长白山天池火山近代喷发物的空间分布及规模。这对长白山天池火山未来喷发危险性初步评价、火山地质制图及火山灾害预测有重要意义。