1,720,986 research outputs found
Building Icelandic Igneous Crust by Repeated Melt Injections
Full text linkObservations of microseismicity provide a powerful tool for mapping the movement of melt in the crust. Here we record remarkable sequences of earthquakes 20 km below the surface in the normally ductile crust in the vicinity of Askja Volcano, in northeast Iceland. The earthquakes occur in swarms consisting of identical waveforms repeating as frequently as every 8 s for up to 3 h. We use template waveforms from each swarm to detect and locate earthquakes with an automated cross-correlation technique. Events are located in the lower crust and are inferred to be the result of melt being injected into the crust. During melt intrusion high strain rates are produced in conjunction with high pore fluid pressures from the melt or exsolved carbon dioxide. These cause brittle failure on high-angle fault planes located at the tips of sills. Moment tensor solutions show that most of the earthquakes are opening cracks accompanied by volumetric increases. This is consistent with the failure causing the earthquakes by melt injection opening new tensile cracks. Analysis of the magnitude distribution of earthquakes within a swarm reveals a complicated relationship between the imposed strain rates and the fluids that cause brittle failure. The magnitude of the earthquakes is controlled by the distance fluids can migrate along a fault, whereas the frequency of the events is controlled by the strain rate. Faults at the tips of sills act to focus melt transport between sills and so must be an important method of transporting melt through the lower crust
Seismic anisotropy reveals a dynamic link between adjacent magmatic segments prior to dyke Intrusion
Full text linkSeismic anisotropy has increasingly been proposed as a tool in the monitoring of magmatic systems and potential forecasting of volcanic eruptions. We present a detailed study of how seismic anisotropy evolves in an active magmatic rift segment before, during, and after a dyke intrusion in the Afar depression, Ethiopia. Results show that seismic anisotropy prior to the dyke intrusion is controlled by a complex and deforming magma plumbing system beneath the adjacent Dabbahu and Manda‐Hararo magmatic segments. Approximately eight days prior to the dyke intrusion in the Dabbahu segment, the pattern of anisotropy, coupled with lower crustal seismicity, is best explained by the inflation of a lower crustal magma reservoir in the Manda‐Hararo segment. This is the only clearly observed precursory change in seismic anisotropy. During the dyke intrusion, the magnitude of seismic anisotropy increases twofold, before rapidly returning to predyke values once the intrusion has ended. Combining our observations with models of magmatically induced crustal stress, we propose that when the deep magma reservoir beneath the Dabbahu segment becomes overpressured, inflation is triggered in the magma reservoir of the neighboring Manda‐Hararo segment. This provides strong evidence for a hydraulic link between the deep magma systems of the neighboring rift segments and that magma reservoirs beneath the Dabbahu segment can be fed by the lateral flow of magma from an adjacent segment. Our results demonstrate that seismic anisotropy has the potential to be a powerful tool for monitoring deformation in the magma plumbing systems of active volcanoes
Triggered earthquakes suppressed by an evolving stress shadow from a propagating dyke
Full text linkLarge earthquakes can generate small changes in static stress: increases that trigger aftershock swarms, or reductions that create a region of reduced seismicity—a stress shadow. However, seismic waves from large earthquakes also cause transient dynamic stresses that may trigger seismicity. This makes it difficult to separate the relative influence of static and dynamic stress changes on aftershocks. Dyke intrusions do not generate dynamic stresses, so provide an unambiguous test of the stress shadow hypothesis. Here we use GPS and seismic data to reconstruct the intrusion of an igneous dyke that is 46 km long and 5 m wide beneath Bárðarbunga Volcano, central Iceland, in August 2014. We find that during dyke emplacement, bursts of seismicity at a distance of 5 to 15 km were first triggered and then abruptly switched off as the dyke tip propagated away from the volcano. We calculate the evolving static stress changes during dyke propagation and show that the stressing rate controls both the triggering and then suppression of earthquake rates in three separate areas adjacent to the dyke. Our results imply that static stress changes help control earthquake clustering. Similar small static stress changes may be important for triggering seismicity near geothermal areas, regions being hydrofractured and deflating oil and gas fields
Motion in the north Iceland volcanic rift zone accommodated by bookshelf faulting
Full text linkAt mid-ocean ridges, the movements between rift segments are usually accommodated by transform faults that are oriented perpendicular to the rift axis. Analysis of seismic data from rift segments exposed in Iceland shows that such movements can also occur through the rotation of several small faults and crustal blocks that slip like books tilting on a shelf
Seismicity of the Bora-Tullu Moye Volcanic Field, 2016–2017
Full text linkThe Bora ‐ Tullu‐Moye volcanic field is a geothermal energy prospect in the central Main Ethiopian Rift, but little is known about the seismicity of the region. Here we document seismic activity between Feb 2016 and Oct 2017, locating more than a 1000 seismic events of local magnitude 0 to 2.7. This provides new insights into fluid movement and deformation beneath what we only now realize is a complicated volcanic system. A discrete cluster of events lies beneath Tullu‐Moye (TM), but, surprisingly, most of the seismicity lies in two clusters that are beneath neither the Bora or TM edifices. In these regions, we use earthquake cluster orientations, fault‐plane‐solutions and fast seismic shear‐wave orientations to show that seismicity is triggered by hydrothermal circulation of fluids along pre‐existing fractures. The fractures trend in multiple directions and are, in general, not parallel to rifting related structures. Instead, the fractures are parallel to structures created during previous caldera forming eruptions at both Bora and TM. Highly fractured regions such as this could be attractive targets for geothermal power generation. We estimate a minimum depth for a magmatic body beneath TM to be 6.5 km using the mapped brittle‐ductile transition. Frequency analysis of the earthquake waveforms reveal the bulk of the events to be volcano‐tectonic but some low‐frequency (LF) seismicity is present at a depth of 5 km beneath the TM edifice triggered by high pore fluid pressures
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Focused seismicity triggered by flank instability on Kīlauea’s Southwest Rift Zone.
No full textSwarms of earthquakes at the head of the Southwest Rift Zone on Kīlauea Volcano, Hawaiʻi, reveal an interaction of normal and strike-slip faulting associated with movement of Kīlauea's south flank. A relocated subset of earthquakes between January 2012 and August 2014 are highly focused in space and time at depths that are coincident with the south caldera magma reservoir beneath the southern margin of Kīlauea Caldera. Newly calculated focal mechanisms are dominantly dextral shear with a north-south preferred fault orientation. Two earthquakes within this focused area of seismicity have normal faulting mechanisms, indicating two mechanisms of failure in very close proximity (10's of meters to 100 m). We suggest a model where opening along the Southwest Rift Zone caused by seaward motion of the south flank permits injection of magma and subsequent freezing of a plug, which then fails in a right-lateral strike-slip sense, consistent with the direction of movement of the south flank. The seismicity is concentrated in an area where a constriction occurs between a normal fault and the deeper magma transport system into the Southwest Rift Zone. Although in many ways the Southwest Rift Zone appears analogous to the more active East Rift Zone, the localization of the largest seismicity (>M2.5) within the swarms to a small volume necessitates a different model than has been proposed to explain the lineament outlined by earthquakes along the East Rift Zone
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