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Crustal structure from teleseismic bodywave databy Submitted to the Department of Earth, Atmospheric, and Planetary Sciences on May 25, 1990 in partial fulfillment of the requirements for the degree of Doctor of Science ABSTRACT
In this thesis we have developed and implemented a series of techniques
to determine information about the crustal velocity structure of the earth
beneath a network of seismic stations from the analysis of teleseismic P-waveforms.
We examined the usefulness of methods which utilize vertical component teleseismic
P-seismograms recorded on 2 seismic monitoring networks. The first is located
in the northeastern United States and is utilized as a test area for the new
methods, and the second is in Larderello, Italy, the site of one of the world's
largest geothermal energy production facilities which is currently being explored
with a variety of geological and geophysical methods.
The main general conclusion of this study is that the analysis of vertical
component teleseismic P-waveforms can provide very useful information about
the crustal velocity structure of the earth. It has long been recognized that
the delays in travel time of direct P-waves can image the broad lateral extent
of the low velocity zone in Larderello. To improve this model of the earth
structure we examined the waveforms for primary reflections from deep velocity
discontinuities which either have regional extent or are isolated to the vicinity
of individual receivers. The measure of the travel times from these phases
(although much more difficult to make than the direct arrival) hold valuable
information about the crust. We developed two methods to extract this information
from the vertical component teleseismic P-waveforms. The first is the application
of a simulated annealing technique to the problem of relative travel time
determination and works on the premise that within a window in each waveform
a wavelet is common to all stations recording the same event. We use this
optimization method to locate the Moho in New England and to determine accurate
measures of direct arrivals in the Larderello data. The second method relies
upon an important data transformation which simplifies and regularizes the
waveforms. This transformation is a two-step process, where we first determine
the source wavelet common to all receivers for each recorded event and then
convert each source into a simple and repeatable zero-phase wavelet. Once
transformed, we take advantage of the wide variety of event incidence angles
present in the New England and Larderello data sets. Each primary reflection
two-way travel time is dependent on the event incidence angle (or ray parameter),
and we exploit this dependency to determine the relative travel times and
average velocities to major discontinuities in the crust by using a ray parameter
trajectory stacking scheme (called the rpt method).
To extract all of the available information about the crustal velocity structure
out of teleseismic waveforms, one must incorporate the entire waveform into
the analysis. To this end, we have developed and applied a waveform inversion
method to refine the details of the velocity model sketched by the previous
techniques. This method is based on the calculation of sensitivity functions,
or partial derivatives, of the predicted seismogram to changes in each of
the parameters which are used in the calculation of the synthetic waveform.
This waveform matching scheme uses the misfit to the data and the Frechet
kernel to update the model, and with this process we can resolve important
velocity features in the crust.
In addition to these general conclusions we have determined a number of specific
important and interesting details about the velocity structure of the Larderello
geothermal area. The travel time residual inversion yielded information about
the size and extent of the low velocity feature in the crust. This intrusive
body is about 20 km by 20 km in lateral extent and exists from depths of about
6 km to below 40 km. The strong travel time residual in the area (about 1
second over about 30 km) indicated a region of intense reduced velocity to
by at least 20% (melts of igneous rocks are reduced in velocity by 30
to 40%). The rpt method was applied to the Larderello data to
help clarify this picture of the crust, and we found that beneath most stations
in the region, strong velocity discontinuities exist at depths of 20 to 25
km. This regional feature is interrupted in the central portion of the area
where a negative gravity anomaly is strongest and where temperatures are most
elevated. This area has a number of more isolated velocity contrasts.
Our waveform inversion technique confirms many of the findings of the previous
applications to teleseismic data and supplements them with detailed information
about the crustal velocity structure (particularly in the upper 3 to 10 km).
This part of the crust is difficult to image with conventional reflection
techniques utilizing vertical component teleseismic waveform data (direct
arrivals, primary reflections and full P-waveforms) and two data enhancement
techniques (simulated annealing and source equalization) can reveal some of
the fine details of the velocity structure of the crust.
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