1,721,010 research outputs found
Applicability and bias of VP/VS estimates by P and S differential arrival times of spatially clustered earthquakes - why the method of Lin and Shearer (2007) will yield biased results in nearly all realistic configurations
No full textEstimating small-scale VP/VS variations at depth can be a powerful tool to infer lithology and hydration of a rock, with possible implications for frictional behavior. In principle, from the differential arrival times of P and S phases from a set of spatially clustered earthquakes, an estimate of the local VP/VS can be extracted, because the VP/VS is the scaling factor between the P and S differential times for each pair of earthquakes. We critically review the technique proposed by Lin and Shearer (2007), in which the mean value over all stations is subtracted from the differential arrival times of each pair of events in order to make the method independent of a priori information on origin times. The demeaned differential P and S arrival times are plotted on a plane, and the VP/VS ratio is estimated by fitting the points on this plane.
We tested the method by both theoretical analysis and numerical simulations of P and S travel times in several velocity models. We found that the method returns exact values of VP/VS only in the case of a medium with homogeneous VP/VS , whereas, when a VP/VS gradient is present, the estimates are biased as an effect of systematic differences between P and S takeoff angles. We demonstrated that this bias arises from the demeaning of the arrival times over the stations. In layered models with VP/VS decreasing with depth, we found that VP/VS is overestimated or underestimated, respectively, for takeoff angles larger or smaller than 90°. In mosst realistic local earthquake monitoring settings, the take-off angles are not equally distributed but there will be a dominance of downward going rays, resulting in an overall bias. We calculated analytically the dependence of this bias on the takeoff angles. Additional simulations showed that the difference between the calculated and the expected VP/VS is reduced for simple horizontally layered velocity structures (<0.06), whereas it is 0.27 in a more realistic velocity model mimicking a subduction zone
Local Vp/Vs ratio in the vicinity of the Tocopilla (Chile) earthquake (Mw 7.7, 14/11/2007) inferred by differential P- and S- travel times
No full textWe have inferred the Vp/Vs ratio along the segment of the Peru-Chile subduction
margin corresponding to the rupture area of the Tocopilla earthquake (Mw 7.7, 14/11/2007).
This event nucleated in Northern Chile and broke the southern ~100 km of the ~500 km
Northern Chile Southern Peru seismic gap, which at the time had not seen an earthquake of
this magnitude since the M~9 event of 1877. Tocopilla event activated two main co-seismic
slip patches: one around the epicenter and another north-east of the Mejillones Peninsula.
We have applied the Lin and Shearer (2007) approach to the aftershock sequence of
the Tocopilla event. In this approach, the relative time shift between the S phases of a pair of
nearby events at one station are plotted as function of the time shifts between the P phases of
the same pair. The process is repeated for a cluster of events. If the events are close enough
to assume a locally uniform Vp/Vs ratio and the P-reciprocal wavefront can be approximated
as planar, the points lay on a line, whose slope is an estimation of the local Vp/Vs. The
technique is extended to a set of stations demeaning the time shifts from each pair of events.
The time shifts are inferred maximizing the cross-correlation function between the event pairs.
The technique has been applied to clusters of events sharing similar waveforms and
spatially clustered hypocentres. We have adopted a robust linear regression and have
assigned a statistical error to the best fit.
Clusters have been identified along the whole profile of the subducting slab, although
most clusters falls within a sub-vertical branch of the subduction interface hosting a major
aftershock (Michilla earthquake, 16/12/2007, Ml 6.8) and its aftershocks. This branch falls
inside the subducted Nazca Plate at depths of 40-50 km, north-east of the Mejillones
Peninsula, and shows Vp/Vs mostly in the range 1.7-1.8. Clusters of the plate interface
shallower than about 30 km show Vp/Vs around 1.9, while at intermediate depths (30-40 km)
Vp/Vs is ~1.8. We speculate on the existence of hydrated crust producing the highest Vp/Vs
(~1.9 or larger) observed at shallow depths
Applicability and bias of the Vp/Vs estimates by differential arrival times of cluster of earthquakes
No full textEstimating small-scale V-P/V-S variations at depth can be a powerful tool to infer lithology and hydration of a rock, with possible implications for frictional behavior. In principle, from the differential arrival times of P and S phases from a set of spatially clustered earthquakes, an estimate of the local V-P/V-S can be extracted, because the V-P/V-S is the scaling factor between the P and S differential times for each pair of earthquakes. We critically review the technique proposed by Lin and Shearer (2007), in which the mean value over all stations is subtracted from the differential arrival times of each pair of events in order to make the method independent of a priori information on origin times. The demeaned differential P and S arrival times are plotted on a plane, and the V-P/V-S ratio is estimated by fitting the points on this plane.
We tested the method by both theoretical analysis and numerical simulations of P and S travel times in several velocity models. We found that the method returns exact values of V-P/V-S only in the case of a medium with homogeneous V-P/V-S, whereas, when a V-P/V-S gradient is present, the estimates are biased as an effect of systematic differences between P and S takeoff angles. We demonstrated that this bias arises from the demeaning of the arrival times over the stations. In layered models with V-P/V-S decreasing with depth, we found that V-P/V-S is overestimated or underestimated, respectively, for takeoff angles larger or smaller than 90 degrees. Moreover, we calculated analytically the dependence of this bias on the takeoff angles.
Our simulations also showed that the difference between the calculated and the expected V-P/V-S is reduced for simple horizontally layered velocity structures (< 0.06), whereas it is 0.27 in a more realistic velocity model mimicking a subduction zone
Inference of small-scale Vp/Vs ratio along the rupture area of the Tocopilla earthquake, Northern Chile (Mw 7.7, 14/11/2007)
No full textWe have inferred the Vp/Vs ratio along the segment of the Peru-Chile subduction margin corresponding to the rupture area of the Tocopilla earthquake (TE, Mw 7.7, 14/11/2007). This event nucleated in Northern Chile and broke the southern ~100 km of the ~500 km Northern Chile Southern Peru seismic gap, which had not seen an earthquake of this magnitude since the M~9 event of 1877. TE activated two main co-seismic slip patches: one around the epicenter and another north-east of the Mejillones Peninsula. We have applied the Lin and Shearer approach [1] to the aftershock sequence of TE. In this approach, the relative time shift between the S phases of a pair of nearby events at one station are plotted as function of the time shift between the P phases of the same pair. The process is repeated for a set of events. If the events are close enough to assume a uniform local Vp/Vs and the P-reciprocal wavefront can be approximated as planar, the points lay on a line, whose slope is an estimation of the local Vp/Vs. The technique is extended to a set of stations demeaning the time shifts from each pair of events. The time shifts are inferred maximizing the cross-correlation function between the event pairs. The technique has been applied to clusters of events sharing similar waveforms and spatially clustered hypocentres. We have adopted a robust linear L2 regression and have assigned a statistical error to the best fit. Most clusters are identified within a sub-vertical branch of the subduction interface hosting a major aftershock (Michilla earthquake, 16/12/2007, Ml 6.8) and its aftershocks. This branch falls inside the subducted Nazca Plate at depths of 40-50 km, north-east of the Mejillones Peninsula, and shows Vp/Vs mostly in the range 1.8-1.9. Clusters close to the Mejillones Peninsula and to the epicenter displays Vp/Vs around 1.7 and 1.8, respectively. References [1] - Lin, G., & Shearer, P. (2007). Estimating local Vp/Vs ratios within similar earthquake clusters. Bulletin of the Seismological Society of America, 97 (2), 379-388
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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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