58 research outputs found
The tectonics and three-dimensional structure of spreading centers : microearthquake studies and tomographic inversions
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1987Two-thirds of the Earth's surface has been formed along a global system of
spreading centers that are presently manifested in several different structural forms,
including the classic rift valley of the Mid-Atlantic Ridge, the more morphologically
subdued East Pacific Rise, and the pronounced en echelon structure of the
Reykjanes Peninsula within southwestern Iceland. In this thesis, each of these
different spreading centers is investigated with microearthquake studies or
tomographic inversion of travel times. Results of these studies are used to
constrain the spatial variability of physical properties and processes beneath the
axis of spreading and, together with other observations, the temporal
characteristics of crustal accretion and rifting.
In Chapter 2 the theoretical basis of seismic body-wave travel-time tomography
and techniques for the simultaneous inversion for hypocentral parameters and
velocity structure are reviewed. A functional analysis approach assures that the
theoretical results are independent of model parameterization. An important
aspect of this review is the demonstration that travel time anomalies due to path
and source effects are nearly independent. The discussion of the simultaneous
inverse technique examines theoretically the dependence of tomographic images
on the parameterization of the velocity model. In particular, the effects of
parameterization on model resolution are examined, and it is shown that an
optimum set of parameters averages velocity over localized volumes. Chapter 2
ends with the presentation of the results of tomographic inversions of synthetic data
generated for a model of the axial magma chamber postulated to exist beneath the
East Pacific Rise. These inversions demonstrate the power of the tomographic
method for imaging three-dimensional structure on a scale appropriate to
heterogeneity along a spreading ridge axis.
Chapter 3 is the first of two chapters that present the results of a
microearthquake experiment carried out within the median valley of the
Mid-Atlantic Ridge near 23° N during a three week period in early 1982. In this
chapter, the experiment site, the seismic network, the relocation of instruments by
acoustic ranging, the hypocenter location method, and the treatment of arrival time
data are described. Moreover, hypocentral parameters of the 26 largest
microearthquakes are reported; 18 of these events have epicenters and focal
depths which are resolvable to within ±1 km formal error at the 95% confidence
level. Microearthquakes occur beneath the inner floor of the median valley and
have focal depths generally between 5 and 8 km beneath the seafloor. Composite
fault plane solutions for two spatially related groups of microearthquakes beneath
the inner floor indicate normal faulting along fault planes that dip at angles of 30°
or more. Microearthquakes also occur beneath the steep eastern inner rift
mountains. The rift mountain earthquakes have nominal focal depths of 5-7 km
and epicenters as distant as 10-15 km from the center of the median valley. The
depth distribution and source mechanisms of these microearthquakes are
interpreted to indicate that this segment of ridge axis is undergoing brittle failure
under extension to a depth of at least 7-8 km.
In Chapter 4, the population of earthquakes considered in Chapter 3 is doubled
and is used to define seismicity trends, to improve source mechanisms, and to
estimate seismic moment and source dimensions of selected events. From a total
of 53 microearthquakes, 23 are located beneath the inner floor and the epicenters
of 20 of these occur within approximately 1 km of a line which strikes N25° E; this
seismicity trend is over 17 km in length. For 12 events located along the seismicity
trend, the composite fault plane solutions clearly indicate normal faulting along
planes that dip near 45°. The seismic moments of inner floor microearthquakes
are in the range 1017_1020 dyn cm, and a B value of 0.8±0.2 is determined for
events with moments greater than 1018dyn cm. Epicenters of rift mountain
earthquakes do not appear to define linear trends; however, over a 24 hour period
a high concentration of activity within a small area was observed. The seismic
moments of events beneath the inner rift mountains vary between 1018 and 1020
dyn cm and define a B value of 0.5±0.1.
Also in Chapter 4, a tomographic inversion of travel times from earthquakes
and local shots indicates a region of relatively lower velocities at 1-5 km depth
beneath the central portion of the median valley inner floor, presumably the site of
most recent crustal accretion. Results of microearthquake analysis and
tomographic inversion are synthesized with local bathymetry and the record of
larger earthquakes in the region to suggest that this section of the median valley
has been undergoing continued horizontal extension and modest block rotation
without crustal-level magma injection for at least the last 104 yr.
In Chapter 5, the simultaneous inverse technique is applied to a
microearthquake data set collected at the Hengill central volcano and geothermal
comp,lex in southwestern Iceland. Arrival time data from 153 well-located
microearthquakes and 2 shots, as recorded by 20 vertical component
seismometers, are used to image velocity heterogeneity within a 14 x 15 x 6 km3
volume that underlies the high-temperature Hengill geothermal field. The dense
distribution of sources and receivers within the volume to be imaged permits
structure to be resolved to within ±1 and ±2 km in the vertical and horizontal
directions, respectively. The final model of stuctural heterogeneity is characterized
by distinct bodies of anomously high velocities: two of these bodies are continuous
from the surface to a depth of about 3 km, and each is associated with a site of past
volcanic eruption; the third body of high velocity lies beneath the center of the
active geothermal field at depths of 3-4 km.
The results of this thesis demonstrate that microearthquake surveying and
seismic tomography are powerful tools for investigating the spatial variability of the
dynamic processes that accompany the generation and early evolution of oceanic
lithosphere.This research was supported by the National Science Foundation, under grants
EAR-8416192 and EAR-8617967, and by the Office of Naval Research, under
contract N00014-86-K-0325
Three-dimensional seismic structure of the Mid-Atlantic Ridge (35°N) : evidence for focused melt supply and lower crustal dike injection
Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 110 (2005): B09101, doi:10.1029/2004JB003473.We gathered seismic refraction and wide-angle reflection data from several active source experiments that occurred along the Mid-Atlantic Ridge near 35°N and constructed three-dimensional anisotropic tomographic images of the crust and upper mantle velocity structure and crustal thickness. The tomographic images reveal anomalously thick crust (8–9 km) and a low-velocity “bull's-eye”, from 4 to 10 km depth, beneath the center of the ridge segment. The velocity anomaly is indicative of high temperatures and a small amount of melt (up to 5%) and likely represents the current magma plumbing system for melts ascending from the mantle. In addition, at the segment center, seismic anisotropy in the lower crust indicates that the crust is composed of partially molten dikes that are surrounded by regions of hot rock with little or no melt fraction. Our results indicate that mantle melts are focused at mantle depths to the segment center and that melt is delivered to the crust via dikes in the lower crust. Our results also indicate that the segment ends are colder, receive a reduced magma supply, and undergo significantly greater tectonic stretching than the segment center.This research was
supported by U.S. National Science Foundation grants OCE-0203228 and
OCE-0136793; support for V. Lekic was provided by the IRIS undergraduate
internship program
Observing Strategies for the NICI Campaign to Directly Image Extrasolar Planets
We discuss observing strategy for the Near Infrared Coronagraphic Imager
(NICI) on the 8-m Gemini South telescope. NICI combines a number of techniques
to attenuate starlight and suppress superspeckles: 1) coronagraphic imaging, 2)
dual channel imaging for Spectral Differential Imaging (SDI) and 3) operation
in a fixed Cassegrain rotator mode for Angular Differential Imaging (ADI). NICI
will be used both in service mode and for a dedicated 50 night planet search
campaign. While all of these techniques have been used individually in large
planet-finding surveys, this is the first time ADI and SDI will be used with a
coronagraph in a large survey. Thus, novel observing strategies are necessary
to conduct a viable planet search campaign.Comment: 12 pages, 10 figures, submitted to Proceedings of the SPI
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Seismic anisotropy beneath the Juan de Fuca plate system: Evidence for heterogeneous mantle flow
Here we use SKS shear wave splitting observations from ocean-bottom seismometer data to infer patterns of mantle deformation beneath the Juan de Fuca plate and its adjoining boundaries. Our results indicate that the asthenosphere beneath the Juan de Fuca plate responds largely to absolute plate motion with an anisotropic layer developing rapidly near the ridge and persisting into the subduction zone. Geographically restricted deviations from this pattern indicate the presence of secondary processes. At discrete plate boundaries, such as the Blanco transform fault, seismic anisotropy is attributed to relative plate motion within a narrow zone (<50 km). Beneath the deforming southern Gorda plate region—a diffuse plate boundary—splitting observations similarly suggest deformation dominated by relative motion between the rigid Juan de Fuca and Pacific plates but distributed over a broad zone (∼200 km). Our results are inconsistent with toroidal flow around the southern edge of the subducting slab due to rollback, as suggested by onshore studies. Instead, reorganization of upper mantle flow associated with plate fragmentation seems to dominate the anisotropic signature of southern Cascadia
The Gemini NICI Planet-Finding Campaign: The Frequency of Giant Planets Around Debris Disk Stars
We have completed a high-contrast direct imaging survey for giant planets
around 57 debris disk stars as part of the Gemini NICI Planet-Finding Campaign.
We achieved median H-band contrasts of 12.4 mag at 0.5" and 14.1 mag at 1"
separation. Follow-up observations of the 66 candidates with projected
separation < 500 AU show that all of them are background objects. To establish
statistical constraints on the underlying giant planet population based on our
imaging data, we have developed a new Bayesian formalism that incorporates (1)
non-detections, (2) single-epoch candidates, (3) astrometric and (4)
photometric information, and (5) the possibility of multiple planets per star
to constrain the planet population. Our formalism allows us to include in our
analysis the previously known Beta Pictoris and the HR 8799 planets. Our
results show at 95% confidence that 5MJup
planet beyond 80 AU, and 3MJup planet outside
of 40 AU, based on hot-start evolutionary models. We model the population of
directly-imaged planets as d^2N/dMda ~ m^alpha a^beta, where m is planet mass
and a is orbital semi-major axis (with a maximum value of amax). We find that
beta 1.7. Likewise, we find that beta < -0.8 and/or amax
< 200 AU. If we ignore the Beta Pic and HR 8799 planets (should they belong to
a rare and distinct group), we find that
3MJup planet beyond 10 AU, and beta < -0.8 and/or alpha < -1.5. Our Bayesian
constraints are not strong enough to reveal any dependence of the planet
frequency on stellar host mass. Studies of transition disks have suggested that
about 20% of stars are undergoing planet formation; our non-detections at large
separations show that planets with orbital separation > 40 AU and planet masses
> 3 MJup do not carve the central holes in these disks.Comment: Accepted to ApJ on June 24, 2013. 67 pages, 17 figures, 12 table
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An upper mantle seismic discontinuity beneath the Galápagos Archipelago and its implications for studies of the lithosphere‐asthenosphere boundary
An upper mantle seismic discontinuity (the Gutenberg or G discontinuity), at which shear wave velocity decreases with depth, has been mapped from S‐to‐p conversions in radial receiver functions recorded across the Galápagos Archipelago. The mean depth of the discontinuity is 91 ± 8 km beneath the southeastern archipelago and 72 ± 5 km beneath surrounding regions. The discontinuity appears deeper beneath the portion of the Nazca plate that we infer passed over the Galápagos mantle plume than elsewhere in the region. We equate the depth of the G discontinuity to the maximum depth extent of anhydrous melting, which forms an overlying layer of dehydrated and depleted mantle. We attribute areas of shallow discontinuity depth to the formation of the dehydrated layer near the Galápagos Spreading Center and areas of greater discontinuity depth to its modification over a mantle plume with an excess temperature of 115 ± 30°C. The G discontinuity lies within a high‐seismic‐velocity anomaly that we conclude forms by partial dehydration and a gradual but steady increase in seismic velocity with decreasing depth after upwelling mantle first encounters the solidus for volatile‐bearing mantle material. At the depth of the solidus for anhydrous mantle material, removal of remaining water creates a sharp decrease in velocity with depth; this discontinuity may also mark a site of melt accumulation. Results from seismic imaging, the compositions of Galápagos lavas, and rare‐earth‐element concentrations across the archipelago require that mantle upwelling and partial melting occur over a broad region within the dehydrated and depleted layer. We conclude that the G discontinuity beneath the archipelago does not mark the boundary between rigid lithosphere and convecting asthenosphere
The Cascadia Initiative : a sea change In seismological studies of subduction zones
Author Posting. © The Oceanography Society, 2014. This article is posted here by permission of The Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 27, no. 2 (2014): 138-150, doi:10.5670/oceanog.2014.49.Increasing public awareness that the Cascadia subduction zone in the Pacific Northwest is capable of great earthquakes (magnitude 9 and greater) motivates the Cascadia Initiative, an ambitious onshore/offshore seismic and geodetic experiment that takes advantage of an amphibious array to study questions ranging from megathrust earthquakes, to volcanic arc structure, to the formation, deformation and hydration of the Juan De Fuca and Gorda Plates. Here, we provide an overview of the Cascadia Initiative, including its primary science objectives, its experimental design and implementation, and a preview of how the resulting data are being used by a diverse and growing scientific community. The Cascadia Initiative also exemplifies how new technology and community-based experiments are opening up frontiers for marine science. The new technology—shielded ocean bottom seismometers—is allowing more routine investigation of the source zone of megathrust earthquakes, which almost exclusively lies offshore and in shallow water. The Cascadia Initiative offers opportunities and accompanying challenges to a rapidly expanding community of those who use ocean bottom seismic data.The Cascadia Initiative is supported by
the National Science Foundation; the
CIET is supported under grants OCE-
1139701, OCE-1238023, OCE‐1342503,
OCE-1407821, and OCE-1427663
to the University of Oregon
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