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NASA’s InSight mission on Mars—first glimpses of the planet’s interior from seismology
No full textInternational audienceNASA’s InSight mission is thefirst lander to deploy a seismometer on a pla-netary body since more than 40 years. With a year of seismic data from Mars, new discoveries on Mars’tectonics and interior structure are just emergin
Crustal thickness from seismic noise correlations in preparation for the InSight mission to Mars
Get PDFDie Entstehung terrestrischer Planeten wirft noch viele Fragen auf. Die Untersuchung des
Inneren des Planeten Mars mit Hilfe seismischer Wellen kann zur Beantwortung dieser
Fragen beitragen. Große Teile der Marskruste sind aus der Frühzeit der Entwicklung
des Planeten erhalten, da Mars keine Plattentektonik aufweist. Informationen über die
Mächtigkeit und den Aufbau der Kruste können somit zu unserem Verständnis der Entstehung und Entwicklung terrestrischer Planeten beitragen.
Die InSight Mission zum Mars wird ein Seismometer auf der Oberfläche des Planeten
absetzen. Um die seismischen Daten bestmöglich auszunutzen, wird in dieser Arbeit eine
Methode entwickelt und vorgestellt, die aus seismischem Rauschen die Reflektivität des
Untergrundes abbildet und die Dicke der Kruste bestimmt. Die hier entwickelte Methode
macht von ‘passiver seismischer Interferometrie’ Gebrauch. Durch die Berechnung von
Auto- und Kreuzkorrelationen erhält man die Reflektionsantwort des Untergrundes. Anschließend wird die Methode mit Hilfe von Daten getestet, die an seismischen Stationen
in Zentral- und Osteuropa aufgenommen wurden. Diese Stationen befinden sich in Gebieten, wo die Erdkurste unterschiedliche Mächtigkeiten aufweist, welche vergleichbar
sind mit Abschätzungen für die InSight Landestelle.
Die Analyse der Daten besteht aus Glätten des Spektrums, Auto- oder Kreuzkorrela-
tion der Phase, im Zeit-Frequenz-Bereich nach Phase gewichtetes Stapeln und Bandpassfiltern (hauptsächlich im Bereich 1-2 Hz). Diese Schritte lassen sich erfolgreich auf Daten
von Horizontal- und Vertikalkomponenten anwenden. Die Daten der Vertikalkomponenten resultieren in jeweils ein Autokorrelationsergebnis für die einzelnen Messstationen.
Die Daten der Horizontalkomponenten ergeben zwei Autokorrelationen, in Nord-Süd-
und Ost-West-Richtung, sowie zwei Kreuzkorrelationen von gemischter Richtung. Die
berechneten Korrelationen sind zeitlich und räumlich stabil, aber es lässt sich für alle
Komponenten ein klarer Zusammenhang mit anthropogen erzeugtem Rauschen erkennen.
Die Korrelationen, die für die Vertikalkomponenten berechnet werden, konvergieren nach
∼6 Monaten zu einem stabilen Ergebnis. Die Korrelationen der horizontalen Komponenten benötigen dafür mehr Daten. Dies gilt insbesondere für die Kreuzkorrelationen. Aus
Informationen über seismische Geschwindigkeiten und geschätzte Moho-Tiefen wird ein
Unsicherheits-Ziel-Fenster berechnet. In diesem Fenster wird eine Reflektivitätsänderung
erwartet. Im Fall der Vertikalkomponenten wird die Reflektivitätsänderung, welche im
Zusammenhang mit der Moho steht, automatisch innerhalb des Ziel-Fensters bestimmt.
Für die Horizontalkomponenten muss dieser Vorgang manuell durchgeführt werden, um
alle vier Korrelationsergebnisse zu berücksichtigen. Die bestimmten Laufzeiten werden
mit Hilfe der a priori seismischen Geschwindigkeiten in Tiefe umgerechnet.
Die berechneten Moho-Tiefen stimmen mit Ergebnissen anderer Studien überein und
ergeben konsistente Resultate für die Daten von Horizontal- und Vertikalkomponenten. Außerdem können weitere Reflektoren bei größeren und niedrigeren Tiefen identifiziert
werden. Durch Kombination der bestimmten Laufzeiten der Korrelationen von Vertikal-
und Horizontalkomponenten lassen sich Abschätzungen des vp/vs-Verhältnisses erhalten. Jedoch sind diese Verhältnisse sehr variabel und ergeben teilweise unrealistische
Werte. Diese Variabilität lässt sich zum Beispiel durch nicht-vertikale Einfallswinkel
und daraus resultierende abweichende Laufwege der seismischen Wellen erklären. Aus
diesem Grund müssen sie mit Vorsicht gehandhabt werden. Trotzdem können die Autokorrelationen der Horizontalkomponenten zusätzlich genutzt werden um mögliche krustale
Anisotropie unterhalb der Messstationen zu detektieren.
Zusammenfassend lässt sich sagen, dass die Krustenmächtigkeit aus seismischem
Rauschen für eine einzelne Station extrahiert werden kann, für verschiedene Seismometerarten, wenn nur geringe Mengen an Daten vorhanden sind, ohne starke seismische
Quellen und unabhängig vom vorliegenden Untergrund. Dies zeigt das Potential der
vorgestellten Methode im Hinblick auf die InSight Mission.The formation of terrestrial planets is still poorly understood. Studying the interior of Mars by analysing seismic waves can help to answer questions about that process. In the absence of plate tectonics, Mars has retained much of its crust from early stages of the planet’s evolution. Knowledge of its crustal thickness and structure can therefore
contribute to the understanding of terrestrial planet formation and evolution.
In preparation for the InSight seismometer installation on Mars, a method is developed
to image the reflectivity of the subsurface and extract the crustal thickness from ambient
seismic noise. The developed method makes use of passive seismic interferometry by
calculating auto- and single-station cross-correlations to obtain the reflection response of
the subsurface. It is then tested on seismic stations across Central and Eastern Europe
with varying crustal thicknesses, comparable to estimates for the InSight landing site.
A processing scheme of spectral smoothing, phase auto- or cross-correlation, time-frequency domain-phase weighted stacking and bandpass filtering, predominantly between 1-2 Hz, is introduced. This processing scheme works well for the vertical and
horizontal component data. One autocorrelation result is obtained for each station from
the vertical component data. The horizontal component data gives two autocorrelation
results, in the North-South and East-West directions, and two cross-correlation results of
mixed directions. The obtained correlations are spatially and temporarily stable, but for
all components a correlation with the presence of cultural noise can be observed. The vertical component correlations converge to a stable result after ∼6 months of data, whereas
the horizontal component correlations require more data, especially the cross-correlations.
Prior information, in the form of seismic velocity and the predicted Moho depth, are included as an uncertainty target window of expected reflectivity change. The reflectivity
change associated with the Moho is automatically determined inside the uncertainty window in case of the vertical component autocorrelations. For the horizontal component
correlations, this needs to be done manually to include all four correlation results. The
determined lag times are converted to depths using the prior information about the seismic
velocities.
The obtained Moho depths compare well with results obtained from other studies using different methods and are consistent across vertical and horizontal component correlations. Furthermore, it is possible to identify additional deeper and shallower reflectors.
The combination of the determined lag times of Moho related reflectivity change from the
horizontal and vertical component correlations gives the possibility to obtain estimates of
the vp/vs-ratios. However, these ratios show large variability and sometimes unreasonable
values and therefore need to be treated with caution. This variability can be explained by
e.g., non-vertical incidence, and the resulting different paths of the waves. The horizontal
component autocorrelations can nevertheless be used to detect possible crustal anisotropy below the station.
In general, crustal thickness can be extracted from seismic noise correlations for a
single station, for different seismometer types, when only moderate amounts of data are
available, without the need for strong seismic sources, and independent of the structure
of the subsurface. All this makes the introduced method a promising tool for the InSight
mission
Moho depth across the Trans-European Suture Zone from P- and S-receiver functions
No full textThe Mohorovicic discontinuity, Moho for short, which marks the boundary between crust and mantle, is the main first-order structure within the lithosphere. Geodynamics and tectonic evolution determine its depth level and properties. Here, we present a map of the Moho in central Europe across the Teisseyre-Tornquist Zone, a region for which a number of previous studies are available. Our results are based on homogeneous and consistent processing of P-and S-receiver functions for the largest passive seismological data set in this region yet, consisting of more than 40 000 receiver functions from almost 500 station. Besides, we also provide new results for the crustal vP/vS ratio for the whole area.
Our results are in good agreement with previous, more localized receiver function studies, as well as with the interpretation of seismic profiles, while at the same time resolving a higher level of detail than previous maps covering the area, for example regarding the Eifel Plume region, Rhine Graben and northern Alps. The close correspondence with the seismic data regarding crustal structure also increases confidence in use of the data in crustal corrections and the imaging of deeper structure, for which no independent seismic information is available.
In addition to the pronounced, stepwise transition from crustal thicknesses of 30 km in Phanerozoic Europe to more than 45 beneath the East European Craton, we can distinguish other terrane boundaries based on Moho depth as well as average crustal v(P)/v(S) ratio and Moho phase amplitudes. The terranes with distinct crustal properties span a wide range of ages, from Palaeoproterozoic in Lithuania to Cenozoic in the Alps, reflecting the complex tectonic history of Europe. Crustal thickness and properties in the study area are also markedly influenced by tectonic overprinting, for example the formation of the Central European Basin System, and the European Cenozoic Rift System. In the areas affected by Cenozoic rifting and volcanism, thinning of the crust corresponds to lithospheric updoming reported in recent surface wave and S-receiver function studies, as expected for thermally induced deformation. The same correlation applies for crustal thickening, not only across the Trans-European Suture Zone, but also within the southern part of the Bohemian Massif
Upper mantle structure across the Trans-European Suture Zone imaged by S-receiver functions
No full textWe present a high-resolution study of the upper mantle structure of Central Europe, including the western part of the East European Platform, based on S-receiver functions of 345 stations. A distinct contrast is found between Phanerozoic Europe and the East European Craton across the Trans-European Suture Zone. To the west, a pronounced velocity reduction with depth interpreted as lithosphere-asthenosphere boundary (LAB) is found at an average depth of 90 km. Beneath the craton, no strong and continuous LAB conversion is observed. Instead we find a distinct velocity reduction within the lithosphere, at 80-120 km depth. This mid-lithospheric discontinuity (MLD) is attributed to a compositional boundary between depleted and more fertile lithosphere created by late Proterozoic metasomatism. A potential LAB phase beneath the craton is very weak and varies in depth between 180 and 250 km, consistent with a reduced velocity contrast between the lower lithosphere and the asthenosphere. Within the Trans-European Suture Zone, lithospheric structure is characterized by strong heterogeneity. A dipping or step-wise increase to LAB depth of 150 km is imaged from Phanerozoic Europe to 20-22 degrees E, whereas no direct connection to the cratonic LAB or MLD to the east is apparent. At larger depths, a positive conversion associated with the lower boundary of the asthenosphere is imaged at 210-250 km depth beneath Phanerozoic Europe, continuing down to 300 km depth beneath the craton. Conversions from both 410 km and 660 km discontinuities are found at their nominal depth beneath Phanerozoic Europe, and the discontinuity at 410 km depth can also be traced into the craton. A potential negative conversion on top of the 410 km discontinuity found in migrated images is analyzed by modeling and attributed to interference with other converted phases
Crustal S-Wave Velocity from Apparent Incidence Angles: A Case Study in Preparation for InSight
No full textISSN:1572-9672ISSN:0038-6308ISSN:0038-630
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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