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Geodesy in the Year 2000: An Historical Perspective
John B. Rundle
Division 6231
Sandia National Laboratories
Albuquerque, NM 87185
INTRODUCTION: GEODESY IN THE YEAR 1900
At 0512 hours Pacific Standard Time on the morning of- April 18th,
1906, the city of San Francisco was destroyed by a major earthquake.
Subsequent study determined the approximate magnitude to have been in
excess of 8, the event having ruptured more than 400 kilometers of the
nearby San Andreas fault. In the words of the Carnegie Commission,
which was empaneled to investigate the earthquake and its causes (Lawson
et al., 1908~:
"The shock was violent in the region about the Bay of San Francisco,
and with few exceptions inspired all who felt it with alarm and
consternation. In the cities many people were injured or killed,
and in some cases persons became mentally deranged, as a result of
the disasters which immediately ensued from the commotion of the
earth. The manifestations of the earthquake were numerous and
varied. It resulted in the general awakening of all people asleep,
and many were thrown from their beds. In the zone of maximum
disturbance persons who were awake and attending to their affairs
were in many cases thrown to the ground. Many persons heard
rumbling sounds immediately before feeling the shock. Some who were
in the fields report having seen the violent swaying of trees so
that their top branches seemed to touch the ground, and others saw
the passage of undulations of the soil. Several cases are reported
in which persons suffered from nausea as a result of the swaying of
the ground. Many cattle were thrown to the ground, and in some
instances horses with riders in the saddle were similarly thrown.
Animals in general seem to have been affected with terror.''
It was well known at the time that earthquakes are caused by the
relief of elastic strain in the earth's crust. The Pittsburgh Post of
April 19, 1906, page 6, recounts:
... "it is probable that earthquakes are caused by the same stresses
in the earth's crust, partly due to contractions, that tilt and fold
rock strata into mountains. The strain to which the rocks are thus
subjected, when suddenly relieved by the rocks giving way, produces
many earthquakes."
9
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Furthermore, according to the Post:
"Earthquakes ... have usually, when carefully studied, been traced
to some line of rock weakness as a fault. Such earthquakes are
merely phenomena accompanying rock movements that may in time
greatly modify the earth's surface.''
While the San Francisco earthquake was not the first to be
scientifically investigated, its report had the greatest impact, both
upon scientific thought in general, and upon geodesy in particular.
Other reports by Robert Mallet on the 1857 Naples earthquake, by R. D.
Oldham on the 1897 Assam earthquake, and by John Milne on the 1880
Yokohama earthquake were instrumental in establishing seismology as a
scientific discipline. But the 1908-1910 Carnegie Commission reports on
the San Francisco earthquake stand alone, because it was there that the
elastic rebound theory of earthquakes was first introduced by Harry
Fielding Reid (Reid, 1910~. The primary data supporting the hypothesis
that earthquakes represent a rebound from a state of previously stored
elastic strain energy were geodetic survey measurements conducted
between 1851 and 1906 (Figure 1, taken from Hayford and Baldwin, 1908~.
To Reid, these data indicated that the earth's crust had, over the
preceding decades, undergone a systematic deformation whose effect was
to place the San Andreas fault into a state of disequilibrium. The
earthquake was thus a result of forces in the earth's crust returning
the system to -a state nearer to equilibrium. As Reid showed, -the survey
data indicated that the sense of sudden motion of monuments near the
fault at the time of the earthquake was the reverse of steady motion
that occurred prior to the event, indicating the release of stored
elastic energy.
The geodetic technology in common use by the Coast Survey in the
latter part of the nineteenth, and early part of the twentieth
centuries, relied primarily upon triangulation for measurements of
horizontal position (Hosmer, 1919~. In this method, permanent marks
were fixed to the ground in networks of regular triangular patterns,
over which were conducted measurements to determine the angles subtended
by lines-of-sight between the marks. For the most part, the earlier
triangulations were conducted during daylight hours, by sighting on a
sun-reflecting heliotrope with a telescope precisely calibrated in
angular direction. The most commonly used telescope was of the type
known as a "Direction Instrument", first designed in England by Ramsden
in 1787. Other telescopes of lesser precision were the "Repeating
Instruments" designed in France in the year 1790. Beginning in 1902,
triangulations were primarily conducted at night, it being realized that
thermal instabilities in the atmosphere produce unacceptable lateral
refraction which can be remedied by observing through thermally stable
nighttime air. Acetylene lamps were used initially, later supplanted by
incandescent electric lights. Generally speaking, observations were
obtained by mounting the instruments and the lights at the tops of
wooden, and later metal towers, whose heights ranged up to more than one
hundred feet.
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It was using these instruments and technologies that the data
in the Elastic Rebound hypothesis were obtained. At the time,
combination of standard observing and network adjustment methods were
expected to yield first order accuracies in line lengths of about 1 part
in 25,000 (Hosmer, 1919~. To obtain these accuracies, instructions such
as those issued to Clem Garner, chief of a survey party operating near
San Francisco in 1922, were typical (Bowie, 1924~:
used
the
"You will, therefore, take special precautions against conditions
which would cause horizontal refraction and will adopt such an
observing program as will secure triangle closing errors with 2.5"
as a maximum, and with not more than 1" as a mean. It is
recommended that each direction at a station be measured on at least
two nights with not less than 12 acceptable directions on each
night. One direction at each station may have only 16 acceptable
positions observed and these may be on a single night if by so doing
a day may be saved and provided further that the closures are within
the above limits."
It was realized by the geodetic community at least as early as 1924
(e.g., Bowie, 1924), that the decade-long changes in triangulation
networks observed throughout California were the result of earth
movements, and that these earth movements were related to the San
Andreas fault. In these early papers, one can see a clear emphasis on
analyzing changes in triangulation angles between successive surveys as
a means of understanding the physical role of fault movements. In
modern terminology, we refer to this as relative positioning. But the
most important rationale for the establishment of regular survey
campaigns, from a political and economic point of view, has always been
for cadastral, or boundary determination purposes. The 1850-1900 coast
surveys were primarily made for coastal navigation of ships, not for
boundary determinations, as shown by the fact that the published maps of
the lines of sight also listed sailing instructions in the lower left
corner. To improve positional accuracy, astronomic observations of
monuments (Laplace Stations) in the network were obtained to control
orientation, and to obtain geoid slopes for scientific studies. With
proper care, positional accuracy of one part in 100,000 was achieved.
In the early twentieth century astronomical longitudes of points in
the field were determined by using a portable transit to measure the
time of passage of stars past the local meridian. The principle sources
of error were the accuracy with which the exact transit time could be
determined by the observer, and undetermined errors in polar motion and
universal time (UTAH. Moreover, since local transit time is supposed to
be taken with reference to the geometrical reference ellipsoid,
deflection of the vertical induced by anomalous masses implies a
pointing error of the transit telescope, and thus errors in the inferred
longitude. Upon comparison of the observed transit time to the
precalculated Ephemerides, the longitude could be obtained.
Alternately, a comparison could be made to a transmitted time
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calibration signal. Prior to 1922, these signals were received in the
field over telegraph lines, but subsequently were transmitted via radio.
Astronomical latitudes were determined by observing the elevation of
stars above the horizon using a zenith telescope. Again, a major error
source arose from deflection of local vertical. Using these techniques,
it was found possible to astronomically measure to about 0.10" in
latitude, and 0.003'' in longitude, albeit at considerable effort and
expense.
With the passage of time, instrumentation for land-based horizontal
positioning evolved. Triangulation was still the primary method for
obtaining horizontal positions until about 1960, when electro-optical
distance measuring instruments were developed, such as the Geodimeter
and the Geodolite. When temperature and humidity are measured at the
time of ranging, and appropriate corrections for atmospheric refraction
are applied, line-lengths can be obtained over networks of monuments
with typical spacings of kilometers to -tens of kilometers, with
accuracies in the range of parts per million or better. Other land-
based instruments have followed, including multiwavelength electro-
optical distance measuring apparatus, which measure and apply
atmospheric refraction corrections automatically.
Although not as important as horizontal triangulation for analyzing
motions related to strike-slip faulting, leveling measurements had, by
the late nineteenth century, reached a high state of technical
accomplishment (Hosmer, 1919~. In fact, the techniques and technology
used then are, in all essential aspects, basically the same as those in
use today. As in triangulation, leveling measurements are made over
networks of marks (benchmarks) fixed on the earth's surface, generally
along roads, railroad beds, and other gently sloping paths with good
access. The major advance in leveling technology occurred with the
discovery of an alloy of 35% nickel and 65% steel called "invar".
Discovered originally by C. E. Guillaume, Director of the International
Bureau of Weights and Measures near Paris, France, invar is distinct in
having an extremely low coefficient of thermal expansion (~.1 ppm/°C),
due to a special heat treatment used in its preparation. By 1906, the
Coast Survey had begun using invar for measurement tapes and leveling
rods, and has continued this practice to the present. The performance
of invar has been found to be generally satisfactory, except possibly in
leveling measurements of extreme accuracy, when instabilities in
material structure may cause unpredictable changes in length of the
tapes at the level of parts per million. More important sources of
error in precise leveling measurements arise from unequal atmospheric
refraction effects over forward and backward sightings, and systematic
errors in rod calibration. With more modern self-leveling telescopes,
an additional source of error has been found to arise from deflections
of the compensator pendulum induced by nearby electromagnetic sources
such as power lines. Still, rigorous field tests demonstrate that
accuracies achieved in leveling are about 10 mm over 100 kilometer
distances, and about 5 mm over 1000 meters of elevation change.
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In contrast to positioning, which is the primary geodetic observable
of interest in the study of active faulting, gravity is of principal
importance in studying the geophysical structure of the earth. Prior to
about 1800, it was thought that the matter comprising the earth and its
surface features was of roughly uniform density, and that any
deformation of the earth was of an essentially elastic nature (Jeffreys,
1976~. However, following the 1855 survey of India, J. H. Pratt found
that the gravitational attraction of the Himalayas observed by Everest,
the Surveyor General of India, was only about one-third as large as it
should be, if the mountains were treated as uncompensated masses.
Shortly thereafter, G. B. Airy, then the Astronomer Royal, proposed in
1855 that mountains floated on a substratum capable of deforming
inelastically in response to the excess gravitational load. Pratt, in
1859, proposed an alternate hypothesis, in which mountains of lower
density than the substratum ride passively on the underlying rigid
material. Both of these mechanisms are still invoked today, in studying
the isostatic compensation of surface features of the earth. The
applicability of both mechanisms continues to be a subject of research,
in addition to other problems related to the structure and dynamics of
the earth.
Early gravity meters were based upon measuring the period of an
accurately calibrated pendulum. The first of these, the half-seconds
invariable pendulum apparatus, was designed and perfected in 1882 by
Sterneck in Austria (Hosmer, 19191. In 1890, T. C. Mendenhall,
Superintendent of the Coast and Geodetic Survey modified the design,
producing an instrument which was used successfully for many years. The
basic design involves the comparison of the locally measured period of
the pendulum to a chronometer of known calibration. Due to the nature
of the measurement, the amount of time needed for a single observation
was typically on the order of 8 to 12 hours. The accuracy of the
observations was typically a few ppm, that is to say, a few milligal.
Another instrument also in use at the time, and one whose importance
has undergone a resurgence due to studies of the fundamental nature of
gravity, was the Eotvos torsion balance. In this instrument, two masses
are fixed to the ends of a long, slender rod, which is suspended at the
end of a long fiber. While the nature of these masses was not
considered important for routine applications, it has since assumed
considerable importance, for reasons to be discussed in this volume.
Under the action of a spatially varying gravity field, the rod tends to
turn into the plane of a great circle oriented perpendicular to the
local meridian, called the prime vertical. By measuring the torsion in
the fiber, the intensity of the local gravitational field can be
deduced. Other designs were also in use at the time, but all were
eventually supplanted by the far more durable and portable gravity
meters. Accuracies of the torsion balance were similar to those of
modern portable gravity meters, a few microgals.
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Unlike pendulum meters and torsion balances, gravity meters, which
first began to appear in the 1930's, are in essence a mass suspended on
the end of a sensitive spring (e.g., Telford et al., 19769. The early
gravity meters were stable, that is to say, they had a linear dependence
of deflection on deviation of the length of the spring from a relatively
large, nonzero value. The major problem with an arrangement of this
type is that the signal of interest is the deviation about the mean
deflection, thus producing a relatively poor sensitivity. The Gulf and
Boliden gravity meters are of this type. Generally speaking, the stable
meters have fallen into disuse.
By contrast, the unstable gravity meters have inherently a far
superior sensitivity, as-small deviations are magnified substantially.
The most important of these instruments is the LaCoste-Romberg
gravimeter, which utilizes a zero length spring. In 1934, L. J. B.
LaCoste found a method of producing a spring-balance system in which the
restoring force depends inversely on the length of the spring. The
result is a spring-balance system of great sensitivity. In fact, the
sensitivity becomes unbounded as the actual length of the spring
approaches zero. In practice, such meters are used in a null mode,
where rotations of an adjustment screw are used to restore the mass to
its original position. The deviation of local gravity from a reference
value is then proportional to the number of turns of the screw. Major
sources of error are~atmospheri~c pressure changes and temperature
variations. For these reasons, the mass and spring assembly are
enclosed in a pressure and temperature controlled environment.
Most recently, transportable absolute gravity meters have been
developed which are based upon timing the free fall of a mass. In this
case, the mass is a corner cube reflector, and its velocity is measured
with a laser interferometer system. Accuracies are typically in the
range of a few microgals, if several hundred drops of the cube are
measured.
CURRENT PROBLEMS: APPROACHING THE YEAR 2000
With the advent of the space age, following the launch of Sputnik in
1957, the science of geodesy entered a new era. As the chapters in this
volume describe, a variety of new space-based observational techniques
are under development which will allow significantly new approaches to
old problems, and provide means for addressing scientific questions
which were previously insoluble. As is implied by the foregoing
historical discussion, current problems in geodesy revolve principally
around positioning and gravity field determination. To this list can be
added a new and rapidly evolving topic related to fundamental tests of
natural laws, principally General Relativity, and the inverse-square law
of classical Newtonian gravitation (see the chapter by Paik).
Precise positioning measurements play a critical role in many
geodynamical problems, some of which are summarized in Walter (1984~.
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Following the approach of Jordan and Minster (1988), it is possible to
classify the most important of these problems into two categories,
secular and transient, and to classify the data types needed to address
these problems according to whether motions are predominantly vertical
or horizontal (see Chapter 21. Of the four distinct combinations, each
typically demands its own approach, and to obtain a solution, each
typically has its own accuracy standards.
The division of crustal motions into transient and secular reflects
the increasing recognition that even now, space-based geodetic
measurements are succeeding in defining the time averaged, long-term
motion of the plates to subcentimeter accuracies. That is, relative
plate motion rates, when measured from the stable interior of plates,
and averaged over tens of years, are in most cases essentially identical
to rates averaged over millions of years. Thus, the predictions of the
global plate motion models (e.g., Minster and Jordan, 1978) are being
confirmed with ever-increasing confidence. Where deviations from these
rates occur, it seems clear that additional physical processes are at
work on a more local scale, which are unrepresented by the global
models. As a result, it is becoming possible to address problems
related to fluctuations about the long term rates, that is to say,
processes which are responsible for producing transient crustal
movements. As the long-term motions of arbitrary points becomes known
to accuracies of several millimeters per year or better, it will become
possible to subtract these motions from the instantaneous rates with
confidence that what remains will have physical significance. The
observations needed to carry out this program will have the
characteristic properties of being frequent, spatially dense
observations over geodetic networks of large scale, with positional
accuracies of several millimeters.
In terms of dynamical problems related to plate motions, it is thus
possible to think in terms of a variety of scales, both spatial and
temporal. The average rates given by the global rigid plate motion
models are defined over the preceding 2-3 million years, and the typical
spatial scales represented are on the order of plate dimensions, that is
to say, thousands of kilometers. Physical understanding of the
processes responsible for these observations will come from increasingly
sophisticated global convection models, which even now are yielding
considerable insight into the long term evolution of the earth. As
motions on these space-time scales become better defined, many of the
frontier-level problems will increasingly be focused on dynamical
processes spanning decade and less time scales, over spatial dimensions
of tens to hundreds of kilometers. Motions on these time scales have
considerable importance in a variety of phenomena, including
earthquakes, polar motion and length of day, and post-glacial rebound
studies. To the extent that transient dynamical processes of the solid
earth are reflected in observed polar motion, the understanding of such
diverse phenomena as climatic changes and ocean circulation will be
greatly enhanced. Thus, in effect, knowledge of rigid plate motions
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provides a space-time reference frame, as well as kinematic boundary
conditions, for understanding much shorter-lived motions on time scales
of hours to hundreds of years.
Transient tectonic motions on spatial scales of hundreds to a
thousand or so kilometers are often termed "regional'' deformations. It
is typically upon these spatial scales that one sees significant
departures from the predictions of the rigid plate motion models. These
departures are in the form of space-time variations in average motion,
due, for example, to earthquakes, transient motions of the
asthenosphere, and aseismic slip, as well as in the existence of complex
boundary zones of deformation. Examples of the former can be seen at
well-studied plate boundaries such as southern California, Japan, and
Alaska. Examples of the latter can be seen in the western United
States, the Alpide belt, Tibet, and the East African Rift Zone. Of the
transient motions, perhaps the most interesting is the apparent
migration of stress and strain along plate boundary zones. One of the
best examples is the sequence of earthquakes which occurred along the
North Anatolian fault zone in Turkey, between 1939-1967. These
remarkable events, all of about magnitude 7, appeared to originate with
the earthquake of December 26, 1939, which occurred in the northeastern
part of the country. Subsequently, in 1942, 1943, 1944, 1953, 1957, and
1967, a sequence of events occurred progressively farther to the west,
at an "average" rate of migration of something like 100 meters/day. In
addition to these, an event occurred in 1966 to the east of the
epicenter of the 1939 event. With the occurrence of these earthquakes,
most of the north Anatolian fault, together with the northern segment of
the east Anatolian fault, had ruptured. The most fascinating question
is whether such a migration effect is a ubiquitous feature of fault
zones. In fact, there is an accumulating, but still small, body of
evidence which suggests causal relations in space and time between major
earthquakes on a variety of plate boundaries. These effects have been
observed in the Nankai region of southwest Japan, the Imperial Valley of
California the central Nevada seismic zone, and in recent paleoseismic
studies of the San Andreas fault zone of southern California.
In addition to regional-scale events, an important class of local-
scale episodic displacements are associated with volcanism. In a number
of large caldera structures around the world, such as Long Valley in
California, Rabaul in Papua New Guinea, and the Campi Flegrei west of
Naples, Italy, displacements of perhaps a meter have occurred over
intervals of a few years, without as yet an accompanying eruption.
Typically, these caldera structures have areas of perhaps 500-1000 km .
In another case, that of Mount Saint Helens, vertical motions of
hundreds of meters occurred over about two months, culminating in the
devastating eruption of May 18, 1980. These crustal motions occur at
such a rapid pace that conventional land-based surveying cannot provide
a detailed space-time picture of the course of the inflation, with the
consequence that valuable information on the source process is lost.
Space geodetic systems, by contrast, have the flexibility to observe
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large networks of points relatively cheaply, or to operate unmanned in a
continuous mode of operation.
The most important and interesting problems in understanding
dynamical processes associated with earthquakes and volcanoes are at
regional scales, over time intervals of hours to hundreds of years.
Slip along fault zones, for example, is accommodated by a combination of
seismic and aseismic deformation which can be detected by geodetic
methods. Given the fact that both slip in fault zones and volcanic
phenomena occur over finite, nonzero regions in space and time, the
fundamental dynamical question is how the system becomes organized in a
physical sense into the space-time structures observed. Thus, the
emerging point of view in the earth sciences is to seek a generic
understanding of the dynamics of complex systems. This is a
fundamentally process-oriented approach, rather than the more
traditional observation-driven approach, and mirrors the same
fundamental change occurring in other fields of science. Implementation
of this approach emphasizes the need for models to aid in understanding,
the need for observations to test the models then following as a logical
consequence. These observations will involve spatially dense,
temporally frequent positioning and surface gravity change observations.
The evolution in the frontier for current research involving- the
earth's gravity field has been no less dramatic than for the dynamics of
the earth's crust. At the turn of the century, discrimination between
differing compensation mechanisms, and study of the the tides and the
dynamical figures of the earth and moon received the greatest attention.
By contrast, gravity observations now play a critical role in almost all
aspects of earth science research. These areas include understanding
the structure of the earth's lithosphere and its interaction with the
underlying asthenosphere, thermal structure of the continental
lithosphere and the nature of the driving forces of plate tectonics, and
the composition and rheology of the mantle (NASA, Report of a Gravity
Workshop, 1987; see also the chapter by McNutt). To address these
questions, accuracy requirements are typically in the microgal range
over horizontal distances of tens to hundreds of kilometers. In
addition to these structural questions, repeat measurements of gravity
at the microgal level are a ready means of measuring, on a temporally
continuous basis, small vertical motions of the crust. Typically, these
vertical motions are most important in post-glacial rebound studies,
where one expects to observe changes of a few microgals per year, and in
association with volcanoes or thrust faults, where changes of tens to
hundreds of microgals may be observed over time intervals of hours to
years.
In studies of the oceanic lithosphere, important questions involve
the degree to which structural features formed by dynamical processes,
such as mid-oceanic ridges, fracture zones, and seamounts, are
isostatically compensated, and upon what time scale this process occurs.
Of late, data from several satellite altimeter missions, including
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SEASAT, GEOSAT, and GEOS-3, have played an important role in mapping the
oceanic geoid. With these missions, uniform mapping of the ocean's
gravity field and geoid have been achieved for the first time. However,
due to military constraints, these data are usually obtained only in
modes where the orbits have been made to repeat every few weeks. Hence,
spatial coverage in the along-track direction is far better than in the
cross-track direction. By contrast, bathymetry data obtained by
shipborne instruments such as Seabeam or SeaMARC are able to provide far
more detailed topographic coverage on the local scale of features such
as fracture zones and mid-oceanic ridges. Nevertheless, there are
problems that can only be addressed by the use of satellite geodetic
data, such as the origin of the large gravity anomalies observed at
oceanic trenches, and the mechanisms by which mid-plate swells and
plateaus are supported.
For the continental lithosphere, gravity data allow models of
rifting and continental extension to be systematically tested. For
example, locations of continental rift zones are often associated with
prominent gravity anomalies. Gravity data can also provide information
on the depth and lateral extent of sedimentary basins, as well as upon
the nature and extent of the roots of mountain belts. Moreover, gravity
data offer important constraints on the deep structure of the
continental lithosphere, whose thickness and physical properties are not
at present well understood.
But perhaps the most critical role which gravity observations play
is in determining the long term dynamics of the interior of the earth.
It is clear from a variety of observations that the motions in the
mantle are driven by thermal convection, implying the existence of
laterally heterogeneous density variations. In concert with recent
advances in seismic data analysis techniques, the gravity field of the
earth, together with the shape of the geoid, provide the most important
constraints on the density contrasts in the earth's deep interior. To
the extent that these density variations are physically related to the
convective processes which drive the plates over hundred-million year
time scales, gravity data play a key role in unraveling the mechanism
for long term mantle dynamics. And of course, since it is widely
recognized that the source of the geomagnetic dynamo is undoubtedly
convective motions within the fluid outer core, geoid and gravitational
observations place critical constraints on the generation of the earth's
magnetic field.
The most important problem areas in understanding the long term
dynamics of the earth involve understanding the depth of penetration of
subducted slabs, that is to say, the vertical scale of mantle
convection; determining the viscosity structure of the mantle;
understanding the possible role of small scale (100 kilometer
wavelength) convection in the upper mantle; and investigation of the
still-unresolved source of the long-term stability of mantle plumes. In
general, these problems require global gravity field coverage, with
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accuracies on the order of milligals, and resolution of roughly 100
kilometers. In addition, there exists considerable interest in
obtaining an improved marine geoid, at the level of 0.1 meter accuracy
over wavelengths of 100-200 kilometers. The rationale lies in the
search for an improved understanding of general circulation and dynamic
topography in the oceans, which is in turn motivated by questions
related to atmosphere-ocean interaction, and problems related to global
climate change.
As a final note, precise determinations of the earth's gravity field
have taken on new importance in light of recent suggestions that
classical Newtonian gravitation should be revised to admit shorter range
interactions (the popularly termed fifth- and sixth-forces). These
suggestions stem from analysis of anomalous keon decays seen in a few
accelerator experiments, new geophysical determinations of G from
experiments in mine shafts, from gravity observations in boreholes in
icecaps and on towers, and from reanalysis of the classical Eotvos
experiments. As yet this controversy is unresolved, but the several-
hundred meter wavelengths suggested for the interaction range should be
visible in some precise satellite tracking experiments, through precise
gravity "radiometry, and by more conventional terrestrial means. In
addition, there is considerable interest in using satellite gravity
studies to test predictions of General Relativity, such as the rate of
precession of a space-borne spinning gyroscope by the Lense-Thirring
effect, otherwise known as the tt dragging of inertial frames. The rate
of precession depends on the intensity of the local gravitational field.
ACKNOWLEDGEMENT S
I am indebted to my colleagues on the committee on geodesy, namely,
C. Goad, T. Dixon, E. Metzger, J. B. Minster, R. Sailor, R. Stein, and
H. Orlin, for reviews. The work contained in this paper was supported
under contract DE-AC04-76DP00789.
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REFERENCES
Bowie, W., Earth Movements in California Spec. Publ. 106 273 U.S.
, , ,
Govt. Printing Office, Washington, DC, 1924.
Douglas, N. B., Satellite Laser Ranging and Geologic Constraints on
Plate-Tectonic Motion, M. S. Thesis, University of Miami, 1988.
Hayford, J. F. and A. L. Baldwin, The Earth Movements in the California
Earthquake of 1906, in Lawson, A. C. and others, The California
Earthquake of April 18. 1906, Report of the State Earthquake
Investigation Commission, published by the Carnegie Institution of
Washington, DC; Volume I, 1908.
Hosmer, G. L., Geodesy Including Astronomical Observations Gravity
Measurements and Method of Least Squares, John Wiley & Sons, New
York, 1919.
Jeffreys, H., The Earth its Origin. History and Physical Constitution,
Cambridge University Press, Cambridge, 1976.
Jordan, T. H. and J. B. Minster, Beyond Plate Tectonics: Looking at
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Astrophysics and Geophysics, Proc. IAU Symp. 129, ed. M. J. Reid and
J. M. Moran, Reidel, Dordrecht, 1988.
Lawson, A. C. and Others, The California Earthquake of April 18 1906,
Report of the State Earthquake Investigation Commission, published
by the Carnegie Institution of W~hinat~n Within fir. Troll T
1908
~ ~rat ~ ~ ~A ~ L L~ = ~ 1 ~ ~~ ~ ? V ~ 1 ~111= 1 ~
Minster, J. B. and T. H. Jordan, Present Day Plate Motions, J.
Geophys.Res., 83, 5331 - 5354, 1978.
NASA, Geophysical and Geodetic Requirements for Global Gravity Field
Measurements, 1987-2000, Report of a Gravity Workshop, Colorado
Springs, 1987, published by GeodYnamics Branch nix Perth ~r;=-rm
and Applications, NASA, 1987
Reid, H. F., The California Earthquake of April 18. 1906, Report of the
State Earthquake Investigation Commission, published by the Carnegie
Institution of Washington, Washington, DC; Volume II, The Mechanics
of the Earthquake, 1910.
Telford, W. M., L. P. Geldart, R. E. Sheriff, and D. A. Keys, Applied
Geophysics, Cambridge University Press, Cambridge, 1976.
Walter, L. S., Geodynamics, NASA Conference Publication 2325, published
by Earth Science and Applications Division, NASA, 1984.
OCR for page 21
21
~-
l ~
s - ~g
Tomol~
MAP OF THE
COAST RANGE REGION
or
MIDDL E CAL I FOR N I
LEGEND
37°oo Foutl o' ,906
~Uovemont ot I.C)C
Suceelsi,. mov~ment. ot 1868 ond 1906
~Combined movement. of 1868 ond 1906
_, , _ .
123°100
__ . ... _
122° 00
+
\~
i, /, ~,th4~ Dioblo
Fo re l l on L:+ ~/ ~
I ~ I / / ~` ~ ~ ~ ,,'` - ~unc' ~
~E ~ece' ~.locho
Pi" H~ ~ \\
S-~o jreno~__ `; - ~ \
~ g~roo i
~n~o~u] Alt~
SCOI. of orrowS I0tO o. showr hero. Pt. P'nol L.~\Pi~
122~104' ~
O S 10 me~r'
, 12 3°1 OC' _ -
\
~_`Sonic
,,° ~no
~ /"'
, ~n
I ~e
~t. roro
Figure 1. Crustal motions, obtained by first order triangulation near
San Francisco, California, during the years 1851-1868, and 1868-1906.
Vector motions are due to the earthquakes of 1868 and 1906, as well as
to long term interseismic crustal motion.
OCR for page 22
22
5+ YR IN:TE~LATE BSLNS
8
Go
_ l
8
.
Do
a:
1
L:3
~ $
-
U)
(sigma < 15 mm/yr)
HIPSTER ~ JORDAN (1978) MODEL VALUES
1~
//
//
' W}
. ~
. ~
//
SLR v.s. MODEL:
BEST FIT LITHE (+---~)
SLOPE = 0.9416
CORR = 0 9739
- 1 ~0 so -80.~
~o.c~ o.oo
MODEL R.~= - S~I~
1
~.00 80. ~LOG:
Figure 2. Rates of Baseline length change obtained by Satellite Laser
Ranging, compared with rates predicted by the global, rigid plate motion
model of Minster and Jordan (1978~. Departures of the observed rates
from the predicted rates are due to violation of the assumptions
inherent in the rigid plate models. Figure taken from Douglas (1988)
Representative terms from entire chapter:
gravity meters
