Volcanic eruptions can emit large amounts of ash into the atmosphere, which can have significant impacts on infrastructure, human health, agriculture and air traffic. Remote sensing instruments can efficiently detect airborne ash plumes, and the measured spectra can be exploited to obtain information on the physical characteristics of ash (grain size distribution, number concentration of particles, optical depth). The key parameter on which all such satellite retrievals depend is the complex refractive index (CRI) which remains one of the largest sources of uncertainty in the retrieval process. Here we present a complementary dataset of refractive indices of volcanic ash, according to Deguine et al. (2020), to cover the major explosive eruptions occurred during the past 50 years. These CRIs were obtained using an innovative experimental methodology which consists in measuring simultaneously the extinction spectra in the IR and UV/visible domain and the size distribution of ash in suspension in a carrier gas. This methodology has been successfully applied on samples collected from various eruptions and deposits in Indonesia (Kelud), Chile (Chaitén), Italy (Stromboli), Russia (Karymsky), Tanzania (Rungwe, Mount Meru), Ethiopia (Corbetti), Philippines (Taal, Pinatubo) and USA (Mount St. Helens). Moreover, the sensitivity of the CRI to chemical composition and mineralogical structure (amorphous/crystalline fraction) has been investigated and shows a strong dependence of the CRI on these parameters.
<p>A water quality problem exists in populated areas along the flanks of Mt. Meru in northern Tanzania, with excessively high fluoride (F-) concentrations exceeding the WHO drinking water standards (1.5 mg/L). Little is known about the potential sources of F<sup>-</sup>among the various rocks types forming the Meru aquifers. Nineteen samples (Debris avalanche deposits (DAD n=4), lava flows n=6, brecciated lava n=4, pumice n=2, scoria n=1, ash n=1, carbonitic n=1) representative of the materials covering the slopes of Mt. Meru were characterized for their mineralogical, chemical, and total and water-soluble F<sup>-</sup> compositions. Mt. Meru is mainly composed of alkaline volcanic rocks of basaltic to phonolitic composition. The total F<sup>-</sup> analysis indicated that F<sup>-</sup>occurs in all rock types with a mean value varying per rock type from 0.6 to 3.2 g/kg. The DAD in the east and northwest of Mt.Meru contained the highest amount of F<sup>- </sup>(mean 3.1&#177;0.17 g/kg), whilst the lava flow samples had the lowest mean value (0.6&#177;0.25 g/kg). Water rock-interaction experiments further revealed the highest release of F<sup>- </sup>in the analyzed DAD samples, possibly associated with their weathering status that progressively converted the primary minerals into secondary clay-bearing minerals assemblage, and favoring F<sup>- </sup>release into the interacting water. Unlike DAD, pumice and ash have a moderately high level of total F<sup>-</sup> (1.76&#177;0.04 g/kg) yet; release a minimal amount of it through leaching. Petrographic observations showed that the analyzed volcanic rocks consist of volcanic glass and rare F<sup>-</sup>-bearing accessory minerals (amphibole, titanite, biotite, and apatite), among others. Using electron microprobe analysis, the F<sup>-</sup> concentrations were found to be as high as 3- 6.5 g/kg in the glassy groundmass and up to 4 g/kg, 5 g/kg, and 45 g/kg in accessory phases of titanite, amphibole, and fluorapatite, respectively. Comparing the abundance and the composition of the glassy groundmass with the mineral phases, the former harbors most of the total F<sup>-</sup>content. The findings of leaching experiments are congruent with past water quality which show that, low F<sup>-</sup> is found in water from lava and tephra-dominated areas at higher altitudes and Mt. Meru west, respectively. This new information could guide future explorations for safer locations to place wells for water consumption. Itcould also be of interest for other East African Rift sectors and similar volcanic settings.</p><p>Keywords<strong>: </strong>Northern Tanzania, East African Rift, Meru volcano, fluoride contamination, volcanic rocks, leaching</p><p>&#160;</p>
Abstract Understanding the long‐term earthquake recurrence pattern at subduction zones requires continuous paleoseismic records with excellent temporal and spatial resolution and stable threshold conditions. South central Chilean lakes are typically characterized by laminated sediments providing a quasi‐annual resolution. Our sedimentary data show that lacustrine turbidite sequences accurately reflect the historical record of large interplate earthquakes (among others the 2010 and 1960 events). Furthermore, we found that a turbidite's spatial extent and thickness are a function of the local seismic intensity and can be used for reconstructing paleo‐intensities. Consequently, our multilake turbidite record aids in pinpointing magnitudes, rupture locations, and extent of past subduction earthquakes in south central Chile. Comparison of the lacustrine turbidite records with historical reports, a paleotsunami/subsidence record, and a marine megaturbidite record demonstrates that the Valdivia Segment is characterized by a variable rupture mode over the last 900 years including (i) full ruptures ( M w ~9.5: 1960, 1575, 1319 ± 9, 1127 ± 44), (ii) ruptures covering half of the Valdivia Segment ( M w ~9: 1837), and (iii) partial ruptures of much smaller coseismic slip and extent ( M w ~7.5–8: 1737, 1466 ± 4). Also, distant or smaller local earthquakes can leave a specific sedimentary imprint which may resolve subtle differences in seismic intensity values. For instance, the 2010 event at the Maule Segment produced higher seismic intensities toward southeastern localities compared to previous megathrust ruptures of similar size and extent near Concepciόn.
Abstract Standard geodetic models simplify magma sheet injection to the opening of geometrically simple dislocations in a linearly elastic, homogeneous medium. Intrusion geometries are often complex, however, and non‐elastic deformation mechanisms can dominate the response of heterogeneous rocks to magma‐induced stresses. We used three‐dimensional near‐surface displacements of a scaled laboratory experiment in which a steeply inclined analog magma sheet was injected into granular material. We ran forward models and inverted for eight parameters of an “Okada‐type” tensile rectangular dislocation in a homogeneous, isotropic, and linearly elastic half‐space. Displacements generated by a forward model largely mismatch the experimental displacements, but full or restricted non‐linear inversions of geometrical parameters reduce the residual displacements. The intrusion opening, dip, depth, and to a lesser degree length and width mismatch the most between the experiment and inversion results, whereas location and strike mismatch the least. Our results challenge assumptions made by many analytical and geodetic models.
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