The tectonics and three-dimensional structure of spreading centers--microearthquake studies and tomographic inversions

Thesis (Ph. D.)--Joint Program in Oceanography (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 1987.

Bibliographic Details
Main Author:	Toomey, Douglas R
Other Authors:	Mike Purdy, Sean C. Solomon.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 N250E; 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 450. The seismic moments of inner floor microearthquakes are in the range 101 7-1020 dyn cm, and a B value of 0.8±0.2 is determined for events with moments greater than 101 8 dyn 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 complex 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 structural heterogeneity is characterized by distinct bodies of enormously 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.
Format:	Thesis
Language:	eng
Published:	Massachusetts Institute of Technology 2010
Subjects:	Joint Program in Oceanography. Earth, Atmospheric, and Planetary Sciences. Woods Hole Oceanographic Institution. GC7.1
Online Access:	http://hdl.handle.net/1721.1/58497