Toomey Douglas R.
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ArticleThe Cascadia Initiative : a sea change In seismological studies of subduction zones(The Oceanography Society, 2014-06) Toomey, Douglas R. ; Allen, Richard M. ; Barclay, Andrew H. ; Bell, Samuel W. ; Bromirski, Peter D. ; Carlson, Richard L. ; Chen, Xiaowei ; Collins, John A. ; Dziak, Robert P. ; Evers, Brent ; Forsyth, Donald W. ; Gerstoft, Peter ; Hooft, Emilie E. E. ; Livelybrooks, Dean ; Lodewyk, Jessica A. ; Luther, Douglas S. ; McGuire, Jeffrey J. ; Schwartz, Susan Y. ; Tolstoy, Maya ; Trehu, Anne M. ; Weirathmueller, Michelle ; Wilcock, William S. D.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.
ArticleThree-dimensional seismic structure of the Mid-Atlantic Ridge (35°N) : evidence for focused melt supply and lower crustal dike injection(American Geophysical Union, 2005-09-09) Dunn, Robert A. ; Lekic, Vedran ; Detrick, Robert S. ; Toomey, Douglas R.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.
ThesisThe tectonics and three-dimensional structure of spreading centers : microearthquake studies and tomographic inversions(Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1987-09) Toomey, Douglas R.Two-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.