Fault Zone Structure Determined through the Analysis of Earthquake Arrival Times

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Fault Zone Structure Determined through the Analysis of Earthquake Arrival Times
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Author
Michelini, A.
Publication Date
1991-10-01
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LBL-31534
Lawrence Berkeley Laboratory
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.UNIVERSITY OF CALIFORNIA
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EARTH SCIENCES DIVISION
Fault Zone Structure Determined through the Analysis
of Earthquake Arrival Times
A. Michelini
(ph.D. Thesis)
October 1991
u. C. Law~ence Berkeley Lab~ratory
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r Library, Berkeley
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Prepared for the U.S. Department of Energy under _Contract Number DE-AC03-76SF00098
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information, apparatus, product, or process disclosed, or represents that its use would not
infringe privately owned rights. Reference herein to any specific commercial product,
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necessarily constitute or imply its endorsement, recommendation, or favoring by the
United States Government or any agency thereof, or the Regents of the University of
California. The views and opinions of authors expressed herein do not necessarily state or
reflect those of the United States Government or any agency thereof or the Regents of the
University of California.
LBL-31534
Fault Zone Structure Determined through the
Analysis of Earthquake Arrival Times
Alberto Michelini
(ph.D. Thesis)
Department of Geology and Geophysics
University of California
and
Eanh Sciences Division
Lawrence Berkeley Laboratory
University of California
Berkeley, California 94720
October 1991
This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences,
Engineering and Geosciences Division, of the U.S. Deparunent of Energy under Contract No. DE-AC03-
76SFO0098.
,.
Copyright © 1991 by Alberto Michelini
The United States Department of Energy has the right to use this thesis
for any purpose whatsoever, including the right to reproduce all or any part thereof.
Fault Zone Structure Determined Through
the Analysis of Earthquake Arrival Times
by
Alberto Michelini
Abstract
This thesis develops and applies a technique for the simultaneous determination of P
and S wave velocity models and hypocenters from a set of arrival times. The velocity
models are parameterized in terms of cubic B-splines basis functions which permit
the retrieval of smooth models that can be used directly for generation of synthetic
seismograms using the ray method. In addition, this type of smoothing limits the
rise of instabilities related to the poor resolving power of the data. Vp /Vs ratios
calculated from P and S models display generally instabilities related to the different
ray-coverages of compressional and shear waves. However, Vp/Vs ratios are important
for correct identification of rock types and this study introduces a new methodology
based on adding some coupling (i.e., proportionality) between P and S models which
stabilizes the Vp /Vs models around some average preset value determined from the
data. Tests of the technique with synthetic data show that this additional coupling
regularizes effectively the resulting models.
The method has been applied to two data sets recorded at the northern and
southern ends of the creeping zone in California-Lorna Prieta and Parkfield. The
results of both analyses indicate that relatively high-velocity bodies appear to control
the mode of deformation, the seismicity pattern and the extent of rupture in larger
events.
At Parkfield the anomalous body (Vp ~ 6.5 km/sec) extends on the southwest
side of the San Andreas fault, deeper than 5 km. Its velocity appears to be too high
for the granitic composition of the Salinian block. A Vp/Vs anomaly (Vp/Vs ~ 1.9)
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characterizes the hypocentral zone of the 1966 Parkfield mainshock and it is suggested
to be produced by high fluid-pore pressures.
The anomalous high-velocity body at Lorna Prieta (Vp ~ 7.0 km/sec) extends
deeper than 8 km on the southwest side of the San Andreas fault. The October 18,
1989, mainshock hypocenter occurred at the northern termination of this anomalous
body and ruptured through it.
az:...7.~~
Thomas V. McEvilly
Dissertation Committee Chair
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Ai miei genitori
(To my parents)
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Acknowledgements
Berkeley represents an important stage of my life-I received a scientific education,
I met wonderful friends and I met my wife Luisa.
I came to Berkeley for the first time in September 1984 to spend a period of 8
months as a visiting scholar at the Seismographic Station. When I arrived they used
to call me doctor until I decided to apply to graduate schooP.
Soon I met my contemporaneous graduate students with whom I have spent and
•
appreciated these years in school. Some of them have become very close friends of
mine and I will spend a few words on some of the peculiarities that have made them
unique. The first person I became close after my arrival was Anthony with whom I
have shared the "joy of fractals" as applied to the rims of the Grand Canyon. Al
though I have always tried avoiding other Italians at Berkeley, I could not elude the
"Renaissance Group" of Walter Alvarez and Sandro Montanari. Caffe was the ingre
dient and often the pretext for widely different conversations. I became some kind
of outside member (membership was only opened to geologists !). Sandro is a very
atypical Italian and this is probably the reason we have become close friends through
the years. His house was (and is) always an "attractor" for people, conversations
and discussions blended with "great" music and food. It was at their house that I
was first introduced to Luisa. During the early years of this "Cal adventure", Vicki
and I shared good times and we supported each other through the difficult ones. In
part, I owe to her if I am now writing this. Jay the "easterner" ? Jay started a year
ahead of me and has been like my "constant" in this tenure. Although on different
scales, our thesis topics were similar and we both benefited from that. Since he grad
uated, I felt compelled to do the same! What about Michael, Joel and Don ? They
are very different but they all share a very precious quality-generosity. I met Bill
when we were both taking courses, the so called "informal" curriculum that, whether
you like it or not, you were advised to take (I am glad I took it !). Bill and I have
spent these years sharing the same data and working on complementary projects. Of
course, the approach was different-the British versus the Italian! In a nice way
he has often ridiculed my Mediterranean heritage while I mocked his strictly British
stubbornness. Bill and Mary (his wife) have become through the years some of my
closest friends and the time we have spent together is filled with little unforgettable
anecdotes. They used to live on a sailing boat and one day we nearly went aground
close to the Golden Gate because the boat would not tack easily-it had not been
designed for Bay but for "proper" ocean sailing, Bill remarked later ! I am grateful
to Tom McEvilly for picking me up after my "orals" in a time when I was drifting
aimlessly. After some testing we tuned onto the same frequency and his enthusiasm,
understanding, and advice have contributed to these years. I feel that Tom, Bill and
1 The title "dottore" is awarded in Italian universities after a curriculum that falls between a
bachelor and a master, rarely a Ph.D ..
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