1,721,086 research outputs found
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
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Rupture Characteristics of Large Earthquakes
Full text linkThe occurrence of many large and/or destructive global earthquakes over the past ten years has provided unprecedented seismic, geodetic, and tsunami recordings that reveal complex rupture processes advancing our understanding of earthquake physics. This thesis research has focused on seismological analysis of recent large earthquakes to extract observational insights that address two fundamental questions, “how do great earthquake rupture?”, and “what controls large earthquakes?”. We approach these two questions by providing an improved seismological understanding of large earthquake rupture processes, exploring the variation of kinematic source parameters, and placing the ruptures into the context of tectonic plate motions that drive the deformation.Given the great diversity of earthquakes, various seismic tools have been explored to give a better robust characterization of large earthquake ruptures. It includes W-phase point source inversion, back projection of seismic array data to map the space-time distribution of high-frequency coherent seismic radiation, determination of broadband source spectra and radiated energy, waveform inversion for co-seismic finite-source slip distribution, and forward modeling of and joint inversion with tsunami and GPS data. By applying these methods, I have studied large events located in different areas, including 1) megathrusts (subduction zone plate boundaries) along the Japan trench, Middle American Trench, and globally; 2) the large transform fault boundary near Scotia-Sea-Antarctic plate boundary, and 3) intraplate events in subducted slabs near the Philippine trench, at intermediate depth (70-300 km) beneath Rat Island earthquake and in the mantle transition zone (300-700 km) beneath Sea of Okhotsk and Ogasawara Islands. The controlling parameters for earthquake-related hazards (e.g. tsunami and strong ground shaking) and earthquake physical mechanisms (e.g. brittle failure, thermal weakening process, stress transfer) have been investigated with an emphasis on the frequency-dependence seismic radiation
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Evaluating Shallow Slip Extent in Large Megathrust Earthquakes by Analyzing P and Pdiff Coda Arrivals for Water Reverberations
Full text linkObservations of shallow focus megathrust earthquakes are difficult to obtain due to limited instrumentation in the near-source underwater region, and teleseismic observations have limited resolution that is strongly influenced by poorly resolved shallow seismic velocity structures near the toe of the accretionary prism (Lay et al., 2012). Slip under deep water can establish strong water reverberations that persist into seismic coda that is not usually inverted for the source. In this study, P wave coda is examined for many recent major and great earthquakes with independently determined slip distributions and known tsunami excitation to evaluate the prospect for rapidly constraining the up-dip rupture extent of large megathrust earthquakes. For narrowband filtered (7-15 s) data at an 80°-120° distance range, final event average rms Pcoda/rms P (RMS C/P) ratios above 0.610 are found exclusively for events that are believed to have ruptured to shallow depths, while ratios < 0.610 are found for all but one of ruptures that did not extend to the trench. Average RMS C/P ratios computed at all azimuths for each event are larger at azimuths in the direction of the trench for all cases, and events with deep slip beneath continental margins have pronounced reduction of reverberative coda amplitudes in the landward directions. This procedure is successful in separating ruptures that have shallow slip from those that do not for events larger than MW 7.5. It is likely to be very effective for smaller events as well, as their entire rupture area will span a smaller depth range than for many of these events. Given the overall success of the classification using all azimuths, which is the simplest for fast implementation, an optimal azimuthal windowing measure is not necessary. Our specific procedure is of somewhat limited value for assessing likelihood of enhanced tsunami excitation relative to the seismic moment magnitude of the event for very nearby coastlines; however, it could still be valuable for tsunamis with coastal arrival times that are longer than the average 15-18-minute lag time required for measuring numerous relative coda levels in the 80°-120° distance range
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Effects Of Near-Source Heterogeneity On Wave Fields Emanating From Crustal Sources Observed At Regional And Teleseismic Distances
Full text linkNear-source path effects imprint the wave field emanating from a seismic source and, if not well resolved, can obscure the details of source characteristics determined from observations of the seismic waves at regional and teleseismic distances (>200 km). These effects are particularly strong for crustal sources such as shallow earthquakes and underground nuclear explosions. First, I explore 2D effects of random seismic P-wave velocity heterogeneity resulting from volumetric heterogeneity in the upper mantle and variability of the Moho on the amplitude decay of the regional phase Pn. Results indicate that the pattern of amplitude decay due to geometric spreading for a simple Earth model is more complex than that for an Earth model containing strong heterogeneity in the mantle lid. Next, I implement the representation theorem in a method which collects displacement and strain components output from a 3D finite difference program capable of including realistic surface topography and geologic structure in a 3D velocity model, and calculates teleseismic 3D Green functions (3DGFs) to specified receiver locations. Green functions produced from a 3D source model match Green functions produced from a 1D source model for theoretical source-receiver geometries. This new method is then applied to the problem of constraining the source depth and location of the three nuclear tests conducted by North Korea, by using a realistic topography model for the mountainous test region to calculate 3DGFs for several possible locations of each event. Amplitude ratios of P and pP from 3DGFs are correlated to those in observed stacked traces. Results show a sensitivity of this method to source depth and location across the test site region with source depths slightly greater than published estimates, but relative locations consistent with other studies. Finally, I determine a rupture model of the 2008 Wenchuan earthquake using 3DGFs calculated in a velocity model containing the dramatic topographic contrast in the fault area, and compare results with a rupture model produced using 1DGFs. Rupture models derived from 1D and 3D synthetic source regions are very similar to each other, and both are consistent with other studies
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The 3 May 2006 (Mw 8.0) and 19 March 2009 (Mw 7.6) Tonga Earthquakes: Intraslab Compressional Faulting Below the Megathrust
Full text linkThe Tonga subduction zone is among the most seismically active regions and has the highest plate convergence rate in the world. However, recorded thrust events confidently located on the plate boundary have not exceeded Mw 8.0, and the historic record suggests low seismic coupling along the arc. We analyze two major thrust fault earthquakes that occurred in central Tonga in 2006 and 2009. The 3 May 2006 Mw 8.0 event has a focal mechanism consistent with interplate thrusting, was located west of the trench, and caused a moderate regional tsunami. However, long-period seismic wave inversions and finite-fault modeling by joint inversion of teleseismic body waves and local GPS static offsets indicate a slip distribution centered ~65 km deep, about 30 km deeper than the plate boundary revealed by locations of aftershocks, demonstrating that this was an intraslab event. The aftershock locations were obtained using data from 7 temporary seismic stations deployed shortly after the mainshock, and most lie on the plate boundary, not on either nodal plane of the deeper mainshock. The fault plane is ambiguous and investigation of compound rupture involving co-seismic slip along the megathrust does not provide a better fit, although activation of megathrust faulting is responsible for the aftershocks. The 19 March 2009 Mw 7.6 compressional faulting event occurred below the trench; finite-fault and W-phase inversions indicate an intraslab, ~50 km deep centroid, with ambiguous fault plane. This event also triggered megathrust faulting. There continues to be a paucity of large megathrust earthquakes in Tonga
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Toward resolving stable high-resolution kinematic rupture models of large earthquakes by joint inversion of seismic, geodetic and tsunami observations
Full text linkIn this thesis, I summarize the research that I have done at UC Santa Cruz involving my development of joint inversion approaches using hr-GPS, teleseismic body and surface waves, regional seismic, campaign GPS, InSAR and tsunami datasets, to investigate the kinematic rupture patterns of large earthquakes. In eight different studies of rupture models of the 2011 Tohoku earthquake, 2012 Indo-Australia earthquake, 2012 Costa Rica earthquake, 2013 Craig earthquake, 2010 Mentawai earthquake, 2013 Pakistan earthquake, 2010 Chile earthquake and 2014 Iquique earthquake, I adopted each available dataset progressively in my joint inversion algorithm, so that in my current approach I can model all of the types of datasets simultaneously. As noted in this thesis, the teleseismic datasets provide good temporal resolution of the rupture process, while geodetic datasets have good spatial resolution. Tsunami datasets have good spatial resolution of slip near the trench. The joint inversion combines the advantage of each dataset, yielding stable and high- resolution rupture models with detailed spatial and temporal information. Resolving a robust and detailed rupture model helps us to understand co-seismic rupture properties, such as depth dependent energy release patterns, super-shear rupture, and tsunami excitation. Comparing the inter-seismic locking pattern and post-seismic stress release pattern with the co-seismic rupture model helps to investigate the locking and releasing behavior of the fault plane through the earthquake cycle, the stress release level of large earthquakes and the relationship between the main shock ruptures, aftershocks and non-seismogenic deformation
Excursions to the base of the mantle.
No full textThe core mantle boundary (CMB) region, consisting of the core mantle interface itself and the few hundred kms above and below it, is investigated in two different types of large-scale seismological studies. In the first study, a large set of long-period S phases is analyzed to determine large scale shear velocity structure in the CMB region beneath Alaska. Subdivision of the data by depth, source region, and source to receiver azimuth establishes that the observations exhibit stable behavior, which is consistent with extensive local stratification of D\sp{\prime\prime}. The preferred model, SYLO, has a 2.75% velocity increase 243 km above the core mantle boundary and a negative velocity gradient within the D\sp{\prime\prime} layer. Analysis of core-penetrating phases indicates that the outermost 100 km of the core below the fast D\sp{\prime\prime} region underlying Alaska must be slower than the PREM structure. In the second study, a set of short-period P-wave amplitude profiles near the core shadow zone is utilized to explore lateral variations in P velocity structure at the base of the mantle. Various radially symmetric models are tested by comparison of the data with amplitudes measured from synthetics. The observed amplitude versus distance profiles exhibit significant regional variations of the apparent shadow zone boundary, with as much as a 5\sp\circ shift in onset distance, but it is possible to model the overall behavior using simple, regionally varying positive P velocity gradients in the lowermost mantle. The strong lateral variations required by the data support the presence of compositional heterogeneity in D\sp{\prime\prime}. The data also indicate that if there is a P velocity discontinuity corresponding to the S discontinuity beneath Alaska, it must be less than 1% in magnitude, indicating a change in Poisson's ratio across the discontinuity. The results from these two studies, as well as those from previous studies of the CMB region (which are briefly reviewed), are used to evaluate several simple schemes for the role of D\sp{\prime\prime} in mantle convection.PhDGeologyUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/104449/1/9034551.pdfDescription of 9034551.pdf : Restricted to UM users only
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
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