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

Skeletal and Dental Health of Precontact Marquesans: The Bioarchaeology of the Human Skeletons from Ha‘atuatua, Nuku Hiva, Marquesas Islands

Skeletal and dental indicators (e.g., stature, linear enamel hypoplasia, cribra orbitalia, trauma, dental pathologies, and other evidence of disease) recorded in 45 subadult and 36 adult skeletons from the NHaa 1 site at Ha‘atuatua, Nuku Hiva, northern Marquesas, are used to examine the health, diet, and lifestyle of precontact Marquesans during the Expansion Period (ca. A.D. 1300–1600). Limited comparisons with skeletons from Hane on Ua Huka and other Pacific Island series augment this study. In addition to elevated numbers of subadult deaths, many during the first year of life, significant palaeopathology suggestive of infection, anemia, or metabolic disease was noted for seven subadults. In contrast, very little palaeopathology was noted in the adults and no significant sex differences for most indicators of health. With few exceptions, the skeletal and dental indicators of health in the Ha‘atuatua and Hane series were very similar. Compared to other precontact Pacific series, the Ha‘atuatua males were tall and similar to other East Polynesians. Higher frequencies of stress fracture in the lower back at Ha‘atuatua may be linked to activities associated with landscape changes and the construction of stone megalithic structures. The skeletal and dental indicators of health observed in the Ha‘atuatua burials are most like those reported for other East Polynesian series. The precontact inhabitants of the Marquesas were generally healthy, contrary to expectations of increased disease frequencies and evidence of warfare during the Expansion Period at Ha‘atuatua. These new bioarchaeological data broaden our understanding of the health and lifestyle of precontact Polynesians

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

The Gemini NICI Planet-Finding Campaign: The Offset Ring of HR 4796 A

We present J, H, CH_4 short (1.578 micron), CH_4 long (1.652 micron) and K_s-band images of the dust ring around the 10 Myr old star HR 4796 A obtained using the Near Infrared Coronagraphic Imager (NICI) on the Gemini-South 8.1 meter Telescope. Our images clearly show for the first time the position of the star relative to its circumstellar ring thanks to NICI's translucent focal plane occulting mask. We employ a Bayesian Markov Chain Monte Carlo method to constrain the offset vector between the two. The resulting probability distribution shows that the ring center is offset from the star by 16.7+/-1.3 milliarcseconds along a position angle of 26+/-3 degrees, along the PA of the ring, 26.47+/-0.04 degrees. We find that the size of this offset is not large enough to explain the brightness asymmetry of the ring. The ring is measured to have mostly red reflectivity across the JHK_s filters, which seems to indicate micron-sized grains. Just like Neptune's 3:2 and 2:1 mean-motion resonances delineate the inner and outer edges of the classical Kuiper Belt, we find that the radial extent of the HR 4796 A and Fomalhaut rings could correspond to the 3:2 and 2:1 mean-motion resonances of hypothetical planets at 54.7 AU and 97.7 AU in the two systems, respectively. A planet orbiting HR 4796 A at 54.7 AU would have to be less massive than 1.6 Mjup so as not to widen the ring too much by stirring.Comment: Accepted to A&A for publication on April 23, 2014 (15 pages, 9 figures, 4 tables