Publikationsdatum:
2022-05-26
Beschreibung:
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 1987
Beschreibung:
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.
Beschreibung:
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.
Schlagwort(e):
Ocean bottom
;
Seismic tomography
;
Knorr (Ship : 1970-) Cruise
Repository-Name:
Woods Hole Open Access Server
Materialart:
Thesis
Format:
application/pdf
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