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OCR for page 23
Implications of Precise Positioning
Jean-Bernard H. Minster, Thomas H. Jordan, Bradford H. Hager,
Duncan C. Agnew, Leigh H. Royden
INTRODUCTION
One of the most exciting developments in crustal kinematics over the
last two decades has been the birth of space-geodetic positioning
techniques capable of achieving accuracies of one cm or better. The
principal motivation for using these techniques for precise point
positioning is to take advantage of the significant increase in
technological capabilities that they represent in order to address
directly important tectonic problems that cannot be tackled economically
at present by ground-based geodetic techniques and classical field
geology. The main techniques which have matured over the past decade or
so include Very Long Basel ine Intefferometry (VLBI) and Satel l i te Laser
Ranging ( SIR) .
More recently, the less burdensome--from the point of view of field
operations--Global Positioning System (GPS) has gained considerable
popularity for the study of regional deformation problems. Over the
technological and scientific horizon, we may count as future candidates
the Geodynamic Laser Ranging System (GLRS) which is being considered as
a facility instrument on the EOS platform, as well as alternatives
developed in Europe, such as the French DORIS and the German PRARE
systems. Convenience considerations aside, the main advantage of these
new techniques, from the geologist's point of view, is that they should
permit (in principle) frequent resurveys of dense networks. This,
together with improved control of vertical displacements, represents a
completely new, enhanced capability, which will allow geologists to
address problems that have SO far eluded them.
It may seem difficult to believe that the ability to measure the
relative positions of two points on the earth separated by 100 or even
10,000 km has a measurable socio-economic impact. The applications to
earthquake prediction and volcanic surveillance are, of course, often
invoked, but somehow seem too remote to justify an immediate development
effort. Nevertheless, in the past decade, space geodesy has begun to
provide useful constraints on the solution of difficult geological
problems of immediate importance, such as the distribution of crustal
deformation both east and west of the San Andreas Fault in central
California (e.g., Jordan and Minster, 1988b). This issue is an
interesting one from a scientific point of view, but it also has great
practical importance, in view of the high population density and
numerous critical facilities found along the Pacific coast of the U.S.
In the next two decades, we must extend our experience to other
tectonically active areas which have significant human as well as
scientific importance.
23
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24
CRUSTAL MOTIONS AND DEFORMATIONS
Deformation of the earth's lithosphere covers a broad spectrum of
temporal and spatial scales, from seconds to aeons and from mineral
grains to planetary dimensions. We rely below on the discussion of
Jordan and Minster (1988a).
Table 1 categorizes a subset of lithospheric motions that cause
geologically and geophysically significant deformations. It is
convenient to discriminate secular motions persisting on geological time
scales of thousands to millions of years from transients associated
with, for example, seismic and volcanic events. Practical research is
more concerned with the transients, because they tend to disturb human
activities. Secular motions also warrant vigorous study, however, since
they provide the kinematical framework for describing transients and
understanding their driving mechanisms.
The most significant long-term deformations are those related to
plate tectonics. Although local tectonic movements near plate
boundaries display large vertical components and time-dependent
behavior, the net motions between the stable interiors of large blocks
are forced by viscous damping and gravity to be nearly steady and
horizontal. The characteristic tangential velocity of the plate system
is about 50 mm/yr, which gives rise to displacements easily measured by
geodetic methods. Horizontal secular motions have been observed both by
ground-based networks (e.g., Savage, 1983) and by space-geodetic systems
(e.g., Christodoulidis et al., 1985; Herring et al., 1986~. Though
their application to geodesy is relatively new, space-based techniques
have already revolutionized the science of terrestrial distance
measurement. They are contributing new information about active
tectonics, particularly on the planetary scales previously inaccessible
to ground-based surveys (Figure 1~.
Rigid-plate motions. In the ocean basins, most of the deformation
related to horizontal secular motion occurs in well-defined, narrow
zones that are the boundaries of a dozen or so large lithospheric
plates. The current plate velocities are constrained by three basic
types of data collected along these submerged boundaries: (1) spreading
rates on mid-ocean ridges from magnetic anomalies, and directions of
relative motion from (2) transform-fault azimuths and (3) earthquake
slip vectors. The first self-consistent global models were synthesized
soon after the formulation of plate tectonics (LePichon, 1968), and
significant refinements were made throughout the next decade (Chase,
1972; Minster et al., 1974~. Third generation models were published by
1978 (Chase, 1978; Minster and Jordan, 1978) and are still in use. Work
has been recently completed at Northwestern University on an improved
fourth-generation plate-motion model name NOVEL-1 (DeMetz et al., 1989),
which remedies most of the problems identified with earlier models (see
also Gordon et al., 1988~.
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25
Since the reference frame is arbitrary, the angular velocity vectors
describing the instantaneous relative motions among M rigid-plates are
specified by 3 (N - 1) ~ independent components, which are derived by
least- squares inversion of a carefully selected, globally distributed
data set. As shown in Table 2, the increasing sizes of the data sets
upon which successive generations of models have been based reflect
continued vigorous research activity in geology and geophysics since the
advent of plate tectonics.
Table 3 lists the instantaneous rotation vectors ~ "Euler vectors " ~
that describe the relative motions of the plates in NOVEL-1. It is
reproduced from the presentation by R. Gordon and his co-workers at the
October 17 - 21, 1988 NASA Crustal Dynamics Principal Investigators
Meeting, held in Munich, Germany (Gordon et al., 1988). The convention
adopted in this table is that the first plate moves counterclockwise
relative to the second plate. The nomenclature is as follows: at -
Africa, an - Antartica, ar - Arabia, au - Australia, ca - Caribbean, co
- Cocos, eu - Euras ia, in - India, na - North American, nz - Nazca, pa
Pacific, sa - South America. Table 3 also lists the one- sigma error
ellipses (marginal distributions) attached to the Euler vector
estimates. The two-dimensional marginal distribution of the pole
pos ition is specified in each case by the angular lengths of the
principal axes and the azimuth (maX of the maj or axis, and the one-
dimens tonal marginal distribution of the angular rotation rate is
specified by its standard deviation Am.
The magnetic anomalies employed in the various rigid-plate motion
models mentioned above average the rates over the last 2 - 3 million
years, about the shortest time span for which good spreading rates can
be ob tained on a global teas is . Although this interval is hardly
" instantaneous " from a geodetic point of view, it is geologically brief,
and the small plate displacements that take place during it are well
described by infinitesimal (as opposed to finite) rotations. It will
probably be some time before the global plate-tectonic models can be
significantly improved by space geodesy. Because the geological data
sets are large and the inverse problem is strongly overdeter~nined, the
formal uncertainties in the angular velocity components are already
quite small, and correspond to formal uncertainties of 1 or 2 mm/yr in
the predicted rates of relative motions. More importantly, the fourth
generation NOVEL- 1 model listed in Table 3 is consistent, at the 1- 2
m~7/yr level, with the hypothesis that maj or plates behave rigidly over a
million-year time scale. Moreover, there is growing evidence that the
rates - of - change of geodetic baselines spanning plate boundaries are
consistent with the geological estimates (e.g., Herring et al., 1986),
provided that the endpoints are located within stable plate interiors.
(This is comforting, both as a check on the techniques and as
corroboration of the geophysical expectation that the instantaneous
velocities between points in stable plate interiors are dominated by
secular plate motions. ) This means that, for those plates whose motions
are well constrained by geological observations, direct geodetic
measurements will not add significant constraints to the estimate of
secular velocities. In any case, given the level of
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26
internal consistency of the geological models, to contribute to the
improvement of existing models of present-day motion among the major
plates, the tangential components of relative velocities on interplate
baselines with endpoints located within stable plate interiors must be
resolved to an accuracy on the order of 1 mm/yr.
It is therefore clear that geologically-based global plate motion
models provide in fact a kinematic ref erence frame in which to analyze
short-term geodetic observations, as well as the kinematic boundary
condi tions which must be satisfied by models of plate boundary
deformation zones. In other words, the most important and interesting
geodetic signals with characteristic time scales ranging from 1 hour to
100 years are detected as departures from predictions of million-year
average rates based on geological rigid-plate models.
However, there exist examples for which the reliability of currently
available global models is difficult to assess, and some improvement
could be made from space-geodetic data. For example, Southeast Asia is
assumed to be part of the large Eurasian plate, but the active tectonics
of China imply it should be moving as a separate entity. Because it is
completely surrounded by complex zones of deformation, its motion
relative to Eurasia will be difficult to quantify without space-geodetic
observations. Similarly, we note that geological rates are lacking
across convergent boundaries, such as trenches. This is one reason why
the motion of the Philippine plate relative to its neighbors is not well
known. Again, space geodesy should provide useful constraints. Closer
to home is the case of the Pacific-North America plate pair, whose
relative rate of motion can be directly measured only on a tiny ridge
segment in the mouth of the Gulf of California. The recent modeling
work of DeMetz et al. (1987) (reflected in NOVEL-1) yields a relative
plate velocity along that boundary that is 15-20% slower than earlier
estimates. Since this rate is critical to models of deformation in the
western United States, a geodetic check on the Pacific-North America
angular velocity will be very valuable.
With these exceptions and a few others, however, the global networks
of VLBI and SLR stations are just too sparse to control plate motions as
tightly as the geological observations. The impact of geodetic
observations on the development of these plate-motion standards will be
relatively minor, at least for the next few years. Of course, this does
not say that interplate observations made by space-geodetic methods will
not reveal new and interesting phenomena associated with other
categories of motion listed in Table 1; particular attention should be
focused on time-dependent signals, including the possibility that plate
speeds and directions have changed significantly during the 2-My
averaging period of the geological data. Our point is to emphasize that
the exciting issues for space geodesy lie beyond the now-classical
descriptions of mayor plate motions.
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27
Departures from rigid-plate motions. The various types of departures
from the predictions of the rigid-plate models fall into two major
categories: (1) large scale and regional scale non-rigid behavior
(plate deformation and plate boundary zones of deformation), and (2)
non-steady motions, including in particular post-seismic strains and
aseismic deformations.
Plate deformation. There are conspicuous instances where the ideal
rigid-plate model fails to describe adequately the complexities of
present-day tectonic interactions, especially within the continents and
along their margins. Examples can be found in regions undergoing
compression due to plate collision, such as the Alpide Belt, and more
particularly, Tibet, or the northern margin of the South American plate,
as well as regions dominated by extensional tectonics, such as the
African Rift Zone and the western U.S. Such regions are often
characterized by spectacular landscapes and have long attracted the
attention of geologists.
Perhaps the most outstanding example of large-scale continental
deformation is Tibet. Collision of the Indian subcontinent with the
Asian continent about 50 million years ago has been followed by roughly
2000-4000 km of convergence across Tibet and the Himalayas, resulting in
the most impressive region of young and active deformation on earth
(Molnar and Tapponnier, 1975, 1978~. Although it is generally accepted
that deformation within Tibet has resulted in movement of crustal
fragments, with dimensions of tens to hundreds of kilometers, in
directions oblique or even orthogonal to the overall direction of
convergence between India and Asia, the details of present rates and
directions of motion are still sketchy at best (e.g., Lyon-Casn and
Molnar, 1985; Molnar et al., 1987). The use of space-geodetic
techniques to unravel the complex kinematical picture of this region,
even at the reconnaissance level of detail, should certainly be
recognized as an important target for the next two decades.
Another young example of active tectonics which space geodesy will
help understand better is the Mediterranean region. There we have "back
arc" type basins adjacent to zones of coeval subduction and convergence,
such as the Aegean-Hellenic systems, the Tyrrhenian-Appennine/Calabrian
system, and the Pannonian-Carpathian system (see, for example,
Malinverno and Ryan, 1986; McKenzie, 1972, 1978; Mercier, 1977; Royden
et al., 1983; Scandone, 1979~. These continental systems present an
excellent opportunity to study the interaction of active extensional and
convergent processes because, unlike most oceanic systems, a reasonably
large amount of the region is exposed above sea-level, so that geodetic
and geologic field studies are practical. Again the importance of
reaching a more precise understanding of the current tectonic evolution
of the area is enhanced by the high population density. Accordingly, a
substantial long-term multi-national effort, the Wegener-Medlas Project,
has been undertaken in 1984 to refine the kinematic picture of the
Mediterranean region, and has begun to yield SLR data capable of placing
OCR for page 28
28
useful constraints on 'the geological models (e.g., Wilson, 1987). The
continuation of this project on a regional scale, and the densification
of the network in critical areas (using, for example, GPS campaigns)
will remain an important~component of tectonic studies of the region in
the coming years.
A particularly interesting third example is the western United
States, where the interaction between the North American plate and the
northwestward moving Pacific plate is spread out over broad zones of
deformation, and wher'e the available geodetic data sets are
substantially larger. Although California's San Andreas Fault can be
identified as a major locus of movement on the Pacific-North America
plate boundary, the likelihood that significant crustal deformation is
occurring both east and west of the San Andreas has long been recognized
by geologists. Geological and geodetic observations of the present rate
of slip along the fault in central California (about 34 mm/yr) is
significantly lower than the rate predicted from successive generations
of rigid-plate motion models, including NOVEL-1 (about 50 mm/yr). This
so-called "San Andreas discrepancy" has been analyzed by Minster and
Jordan (1984, 1987), using both geological 'information and geodetic
data. Their conclusion was that even the short 4 year record of VLBI
observations available to them for the relevant baselines (see Figure 2)
was sufficient to place useful constraints on the integrated deformation
east of the San Andreas across the Basin and Range, and by vector
addition, on the integrated deformation west of the San Andreas across
the California margin (see also Weldon and Humphreys, 1985~.
Jordan and Minster (1988a) reviewed the use of space-geodetic
observations to solve geological problems, with a focus on the various
types of secular horizontal motions listed in Table 1. They concluded
that many geological and geophysical problems related to such motions
are indeed currently being addressed by space-geodetic experiments,
provided that critical measurements are made at accuracies not feasible
by conventional techniques. In particular, they state that measuring
the velocities between crustal blocks to +5 mm/yr can place geologically
useful constraints on the integrated deformation rates across
continental plate-boundary zones, such as the western U.S., the
Mediterranean and Tibet.
However, it must be emphasized that baseline measurements in
geologically complicated zones of deformation are useful only to the
extent that the relationship of the endpoints to geologically
significant crustal blocks is understood. Some antennas have a long
history of participation in VLBI experiments, so that their motions in
the VLBI reference frame are becoming well known; but they lie within
complex zones of faulting, and their motions in kinematical frames fixed
to local geology are not at all known. For example, the baseline rates
for the Owens Valley Radio Observatory (OVRO) in California relative to
the Westford', Massachusetts, and Ft. Davis, Texas, antennas have been
measured to a precision of about 2 mm/yr (Figure 2~. In their analysis
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29
of Basin and Range extension, Minster and Jordan (1987) have assumed
OVRO moves with the Sierra Nevada-Great Valley block, but unfortunately,
it is separated from the Sierra Nevada by a major system of faults, one
of which broke in the great 1872 Owens Valley earthquake. Until the
position of OVRO is regularly resurveyed in a local geodetic network
which includes stations planted firmly on the Sierra block, the
geological implications of the VLBI data will remain in doubt.
Consequently, we can recommend that the establishment of frequently (or
even continuously) surveyed local geodetic networks of sufficient
density, around major geodetic sites in active areas should receive high
priority.
The discussion has been focused so far on rather localized
deformation, that is, on the occurrence of deformation zones with
horizontal scales significantly less than overall plate dimensions.
Whether the major tectonic plates, which are found to behave rigidly to
an excellent approximation over million-year time scales, are actually
undergoing nonlocal steady or episodic deformation on shorter time
scales, is another important scientific question which bears directly on
our understanding of the mechanical behavior of the lithosphere on a
large scale, as well as our models of the force systems that drive plate
tectonics. In order to resolve this issue, we will require geodetic
coverage on a global scale, using techniques capable of delivering mm/yr
accuracy for baselines 10,000 km long. Only space geodesy can meet such
requirements, and, in the absence of any kind of ''ground truth", we must
rely on intercomparisons between independent techniques to evaluate the
actual performance of the systems.
Nonsteady motions. The problems of time dependence of the motions and
non-rigid behavior of the plates, two issues which lie beyond the now-
classical descriptions of major plate motions, are exciting applications
of current and future space geodetic systems. The fundamental
underlying scientific problem is the transmission of strain (or stress)
in the lithosphere. This question is intimately related to the problem
of coupling between earthquakes, evolution of volcanic eruption, and
ultimately to the problem of predicting catastrophic events.
The time scales involved range from a fraction of an hour to
centuries or longer, and span a range in which the physical phenomena
are very poorly understood, primarily because the measurements are
sparse, infrequent, and mostly very recent. Thus the evidence for
episodic (as opposed to steady) motions along plate boundaries and
within plate boundary deformation zones is insufficient at the present
time to map the time and spatial scales involved.
A space-geodetic system capable of high sampling rate and dense
spatial coverage over large areas will make possible a nearly
unprecedented exploration of how crustal deformation varies with time.
Much too little is known about this, the only data so far available
comes from geodetic measurements that are too infrequent, too
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30
insensitive, or too localized to provide conclusive evidence. Space
geodesy will provide information that is crucial to understanding the
physics of the earthquake process. The motivation for these
measurements can be understood in the context of a simple model of the
mechanical properties of the crust and upper mantle (Figure 31.
Rocks respond to applied stress in a way very similar to the
children's toy, Silly Putty, i.e., they are viscoelastic. They respond
elastically to rapid variations in stress, but undergo irreversible
deformation by creeping flow under low but sustained applied stresses.
If the stresses are high enough, rocks fail brittlely; the result of
this sudden failure is an earthquake. Low temperature and low confining
pressures favor brittle failure, while high temperatures promote creep
by decreasing the effective viscosity of rocks. The theological
behavior also depends upon rock type, crustal rocks being more prone to
creep than mantle rocks at the same ambient conditions. Pore fluids and
volatiles also have effects. Although the details are poorly
understood--indeed, an important goal of geodetic monitoring is to
obtain the observations needed to understand the details-- the general
variation in effective strength of the crust and upper mantle is
illustrated schematically in Figure 3.
Large earthquakes generally nucleate near one of the maxima in the
strength versus depth curve and propagate rapidly through the adjacent
brittle region. However, the deformation associated with an earthquake
is not limited to this seismic rupture. Aftershocks typically relieve
stress on slip-deficient areas of the fault plane and extend the rupture
to adjoining regions. Aseismic creep on the fault plane, perhaps also
extending into the ductile regime, may increase the total amount of
displacement associated with an earthquake. Viscoelastic adjustments of
the Earth's crust and mantle cause redistribution of strain after a
major earthquake. Stress is relieved by flow in the ductile regions,
allowing elastic strain to accumulate in the stronger, more elastic
regions.
A general feeling for the interaction of the viscous and elastic
properties of rocks in the crust and mantle may be obtained by
considering the simple model originally developed by Elsasser (1969) and
extended by others (e.g., Melosh, 1976; 1977, 1983; Cohen, 1984; Rundle
and Jackson, 1977; Rundle, 1988a,b). This model consists of an elastic
layer of thickness he overlying a viscous layer of thickness he and a
rigid base. An initial sudden displacement of a fault diffuses outward
with a diffusivity ~ = he/, where ~ is the Maxwell time of the
system. Repeated jerky offsets on plate boundary faults result in very
smooth motion some distance away, corresponding to the steady velocities
of plate interiors.
For regional scale problems, such as southern California (Figure4),
the elastic layer can be taken to be the upper crust, the viscous layer
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31
the lower crust, and the rigid substratum is the upper mantle. The
displacement from an earthquake on a given fault will diffuse through
the crust, causing regional strain and perhaps influencing other faults,
on a time scale that depends upon the effective viscosity of the lower
crust (e.g., Turcotte et al., 1984~. Crustal rheology is poorly
constrained, but reasonable estimates indicate that strains may
propagate 30 km in 50 years. As can be seen from Figure 4, there have
been many earthquakes in southern California in the past 50 years with
significant source dimensions. Based on Elsasser's model, these events
would be expected to have viscoelastic strain migration associated with
them. Observing this strain migration could constrain crustal rheology.
Over a scale of tens of kilometers around the fault, Thatcher (1983)
showed geodetic evidence for time-dependent strain rates after great
earthquakes on the San Andreas. He showed that his rather sparse data
could be fit both by a model in which a viscoelastic asthenosphere
underlays an elastic lithosphere, and by a purely elastic model with
exponentially-decaying afterslip on the fault. Thus, the change in
strain rates could be due to the eons titutive properties either of the
fault zone or of the asthenosphere, but we do not know which. It is
crucial to separate these effects in order to understand better the
physics of earthquakes and transmission of stress through the crust.
The distribution of postseismic strain in time and space would provide
important constraints on the physics of faulting and on the material
properties of the fault zone and surrounding Earth. Redistribution of
stress and strain to adjacent faults would provide important information
related to earthquake prediction.
We discuss below possible time-varying strains that might be
observed following earthquakes in the context of specific recent
experience in southern California. We then examine another possible
source of time-dependent strain not associated with individual
earthquakes.
Post-seismic strains. A variety of observations have made it clear that
the extremely rapid strain of a fault rupture is followed by strain
rates that are high, compared to the long-term average measured for the
fault zone, but the data are lacking to determine the physics
responsible. To begin closest to the fault, ruptures that break to the
surface commonly show substantial slip in the hours and days after the
actual earthquake. Does this reflect equal amounts of slip at greater
depth, or is the deeper part of the fault stable, this afterslip merely
being caused by the gradual propagation of deep slip through the near-
surface layers?
A particularly clear example of this kind of ambiguity has recently
been provided in southern California by the Superstition Hills
earthquake sequence of November 1987. The first large event was the
Elmore Ranch earthquake (Ms~6.2), followed 12 hours later by the main
Superstition Hills event (Ms~6.6), and of course many aftershocks.
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Seismicity patterns and surface rupture showed two conjugate faults, the
Elmore Ranch earthquake being on a fault conjugate to the San Jacinto
fault zone and the mainshock on a fault (Superstition Hills) parallel to
the zone. Figure 4 shows the location of the Pinon Flat Observatory
(PFO) relative to these conjugate events (labeled "1987"~.
We are fortunate to have available data from the long-base strain
instruments at Pinon Flat Observatory (e.g., Wyatt et al., 19821; since
these have been properly "anchored" to depth, they give results (out to
periods of a year) that are better than any existing geodetic
measurement. They thus provide a window, if only at one place, for what
we might expect from future space-geodetic systems, and promise to be a
source of "ground truth" for these measurements. Strains from these two
events were recorded by two of the laser strainmeters at PFO; they are
dominated by the coseismic offsets (e.g., Figure 59. These offsets and
those recorded by other instruments at PFO are in reasonable agreement
with results for a dislocation in a half-space, if seismically-derived
parameters are used to define the dislocation. The frequent sampling of
these data allows us to look at preseismic and postseismic deformations
in some detail, with the best data coming from the fully-anchored NW
laser strainmeter. During the interval between the two earthquakes, the
largest signal on this record consists of microseisms plus some small
residual drift in the instrument. We can certainly rule out any
anomalous strains during this time above the level of about 3% of the
eventual coseismic offset, and for the final 1000 seconds above about
the 0.5% level.
It is hard to believe that the Superstition Hills earthquake was not
triggered by the Elmore Ranch earthquake; but it is clear that no simple
model of elastic strain and brittle failure can explain the 12-hour
delay between events: if the Superstition Hills fault were close to
failure, it should have ruptured during the dynamic strains generated by
the Elmore Ranch earthquake, or at least soon after it, when the elastic
stress changes had been fully imposed. We can think of several models,
all speculative, to account for this.
1. All the surrounding material is elastic, and the
Superstition Hills fault failed by brittle rupture at a critical stress
level. The 12-hour delay was caused by afterslip on the Elmore Ranch
fault; only after this had gone far enough was the applied stress
sufficient for failure. This model would appear to be ruled out by the
PFO data, which show no obvious afterslip (this would appear in
proportion to the coseismic strain); any stress changes from this cause
could be at most 10% of the stress changes at the time of the first
earthquake. It could be that a small additional stress was enough, but
this seems unduly ad hoc.
2. All parts of the system could have responded elastically,
but the fault actually failed by some kind of stress corrosion. The
model results of Tse and Rice (1986) show something like this; for a
realistic friction law, earthquakes in their fault model begin with slow
slip over a very small depth range, rapidly accelerating to seismic
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slip, which would probably be consistent with the strainmeter data.
However, this assumes a steady increase of applied stress; whether it
still would result in inter-earthquake slip undetectable by the
strainmeters would require additional modeling.
3. The Superstition Hills fault could have failed brittlely
(as in the first hypothesis) with the delay between earthquakes being
due to stress diffusion (Elsasser, 1969; Melosh, 1976, 1977, 1983)
outward from around the Elmore Ranch fault. The physical model proposed
recently by Rundle (1988a,b) may be invoked to account for the effects
of interactions between faults (e.g., Rundle and Kanamori, 19879. We
may assume that the first earthquake occurred in the brittle upper crust
(Figure 4), underlain by a ductile lower crustal "asthenosphere." Just
after the earthquake, the stresses in both regions are described by
elasticity (for otherwise the coseismic strain step at PFO would not
match the halfspace solution), but the ductile zone then begins to flow,
interacting with the overlying elastic layer to cause a diffusion of
stress outward from the fault, and hence increasing the stress on the
Superstition Hills fault. For the usual estimates of crustal viscosity,
we would expect little change in stress over 12 hours. If, however, the
rheology is time-dependent or obeys power-law flow, the effective
viscosity close to the fault (where the largest stress changes occur)
could be quite low, allowing rapid diffusion of stress, which would tend
to slow down as stress levels smooth out. Again, more detailed modeling
will be needed to see if such stress diffusion could occur without
causing strains in the overlying lithosphere large enough to be detected
by the distant strainmeters at PFO.
Space Geodesy could have helped distinguish between these models, if
measurements had been collected in the time interval between events.
Multiple surveys of an array of monuments around these faults would have
provided the best evidence possible on changes of strain between them,;
had none been seen, we would have good evidence for some kind of delayed
failure on the fault itself, rather than a delay from stress
propagation. It is crucial that planning for space-geodetic systems
allow a fast enough response time to make critical observations like
these possible.
Aseismic Deformations. Aseismic deformations, in the form of strain
episodes not associated with seismic events, have been hypothesized.
One of the classic examples is the now infamous Palmdale Bulge. Repeat
leveling of the region near Palmdale on the "Big Bend" segment of the
San Andreas fault (see Figure 5) showed an apparent uplift of up to 30
cm, followed by subsidence (Castle et al., 1976~. This feature has been
interpreted to result from episodic slip on a horizontal slip zone in
the lower crust (Thatcher, 1979; Rundle and Thatcher, 19809. It was
later discovered that a least part of the inferred bulge was the result
of atmospheric refraction errors (Holdahl, 1982), with an additional
smaller error due to miscalibration of survey rods (Jackson et al.,
19831. The revised amplitude of the uplift is close enough to
measurement error to be ambiguous (Stein, 1987), leading some
geophysicists to dismiss the entire phenomenon. The controversy still
OCR for page 42
42
2. The most important and interesting geodetic signals
averaged over 1 hour to 100 years take the form of d epartures from
predictions of million-year average rates based on geological rigid-
plate models.
3-. The most significant departures from rigid-plate motions
occur in zones of 100 to 1000 km width, within which differential
motions are accommodated by a combination of seismic slip and aseismic
deformation.
4. Measuring velocities between crustal blocks to +5 mm/yr
will provide geologically useful constraints on the integrated
deformation rates across continental plate-boundary zones, such as the
western U.S. and Tibet.
5. It is only through the integration of geodetic and geologic
field studies that active deformation can be related to earlier activity
within an evolving tectonic system. Coordination of geodetic and
geologic studies is therefore necessary to establish the temporal
dependence of crustal deformation on a geologic time scale (e.g.,
Royden, 1988~.
6. The establishment of frequently (or even continuously)
surveyed local geodetic networks of sufficient density, around major
geodetic sites in active areas, should receive high priority.
7.: The development and systematic deployment of affordable and
easily deployable space-geodetic systems with cm to mm precision and
high sampling rates--that is, "occupation" frequency ranging from hourly
to weekly--will permit investigation of geophysical phenomena,
particularly the earthquake cycle, in a range of spatial and temporal
scales never explored before.
8. Space-geodetic observations yield constraints on crustal
kinematics; to achieve an improved understanding of the dynamics and
thus, a better grasp of the underlying physical phenomena, we must rely
on a broad combination of geophysical and geological observations as a
way to extend the geodetic signals to longer time scales and to
extrapolate surface information to crustal and mantle depths.
Based on the discussion held at the Erice workshop, and in view of
the conclusions listed above, the panel on the "Long-Term Dynamics of
the Solid Earth" formulated the following recommendations concerning the
continued development and application of precise point positioning
techniques:
Over the next 20 years, major efforts in applying precise
post tioning techniques shout d be aimed primarily at:
a . Continued large seal e reconnaissance surveys wi th station
spacing on the order of 102 km, to improve our understanding of the
OCR for page 43
43
kinematic evolution of
continental deformation.
b . Sus twined
cen time ter - 1 evel accuracy,
ex t ens i ve,
l argely unexpl ored zones of
, repeated measurements of dense networks at
to determine the time dependence and spatial
distribution of deformation within and across zones of intense tectonic
activity. Measurement frequencies should range from daily to annually
over a decade or more, with station spacing from 3 to 30 ken, and network
dimensions from 10 to 1000 km. In regions of complex deformation,
geodetic measurements should be complemented by comprehensive tectonic
and structural studies and careful estimates of displacements and
displacement rates on geologic time seal es .
c.
Con tinned improvemen t of capabi 1 i ti es,
to achi eve:
- -ail l imeter- l evel accuracy in both horizontal and vertical
components for detail ed subaerial studies, system cal iteration, and
al timately, l ow- cos t rou tine depl oyment .
- - centimeter- l evel accuracy in both horizontal and vertical
componen ts f or sea - bo t tom sys t ems .
. .
OCR for page 44
44
REFERENCES
Bird and Rosenstock, Geol. Soc. Amer. Bull , 95, 946-957, 1984.
Castle et al., Science, 192, 251-253, 1976.
Chase, Geophys. J. R. Astr. Soc., 29, 117-122, 1972.
Chase, Earth Planet. Sci. Lett., 37, 353-368, 1978
Christodoulidis et al., J. Geophys. Res., 90, 9249-9264, 1985.
Cohen, J. Geophys. Res., 89, 4538-4544, 1984.
DeMets et al., Geophys. Res. Lett., 14, 911-914, 1987.
DeMets et al., 1989, (in preparation).
Elsasser, in The Application of Modern Physics to the Earth and
Planetary Interiors, S. K. Runcorn, ed. 223-246, 1969.
England et al., J. Geophys. Res., 90, 3551-3557, 1985.
Gordon et al., presented at NASA CDP P.I. Meeting, Munich? Germany,
Oct. 17-21, 1988.
Herring et al., J. Geophys. Res., 91, 8341- 8347, 1986.
Hodahl, J. Geophys. Res., 87, 9374-9388, 1982.
Humphreys et al., Geophys. Res. Lett., 11, 625-627, 1984.
Jachens et al., Science, 219, 1215 - 1217, 1983.
Jackson et al., Tectonophysics, 97, 73-83, 1983.
Jordan and Minster, in The Impact; of VLBI on Astrophysics and
Geophysics, (M. J. Reid and J. M. Moran, eds.) Proc. IAU Symposium
129, 341-350, 1988a.
Jordan and Minster, Scientific American, August 48-58, 1988b.
LePichon, J. Geophys. Res., 73, 3661-3697, 1968.
Lyon-Caen and Molnar, Tectonics, 4, 513-538, 1985.
Malinverno and Ryan, Tectonics, 5, 227-245, 1986.
McKenzie, Geophys. J. Roy. Astr. Soc., 30, 109-185, 1972.
McKenzie, Geophys. J. Roy. Astr. Soc., 55, 217-254, 1978.
Melosh, J. Geophys. Res., 81, 5621-5632, 1976.
Melosh, Pure Appl. Geophys., 15, 429-439, 1977.
Melosh, Geophys. Res. Lett., 10, 47-50, 1983.
Mercier, Bull. Geol. Soc. Pr., 7, 663-672, 1977.
Merifield et al., Tech. Rep. 83-3, Lamar-Merifield Geol. Inc. Santa
Monica, CA, 1983.
Minster and Jordan, J. Geophys. Res., 83, 5331-5354, 1978.
Minster and Jordan, Pac . Sec . Soc . Econ. Paleontol . Mineral, 38 , 1- 16 ,
1984.
Minster and Jordan, Geophys. Res., 92, 4798-4804, 1987.
Minster et al., Geophys. J. Roy. Astr. Soc. , 36, 541-576, 1974.
Molnar and Tapponnier, Science' 189, 419-426, 1975.
Molnar and Tapponnier, Geophys. Res., 83, 5361-5375, 1978.
Molnar et al., Geology, 15, 249-253, 1987.
Royden et al., Tectonics, 2, 63-90, 1983.
Royden, in Geodetic Studies and Crustal Dynamics, U.,S. Geodynamics
Committee Progress Report, 3.1-3.10, 1988.
Rundle, J. Geophys. Res., 93, 6237-6254. 1988a.
Rundle, J. Geophys. Res., 93, 6255-6274, 1988b.
Rundle and Jackson, Geophys. J. Roy Astr. Soc., 49, 575-592, 1977.
Rundle and Kanamori, J. Geophys. Res., 92, 2606 - 2616, 1987.
OCR for page 45
45
Rundle and Thatcher, Seismol. Soc. Amer. Bull., 70, 1869-1886, 1980.
Sauber et al., J. Geophys. Res., 88, 2213-2219, 1983.
Savage, Ann. Rev. Earth Planet. Sci., 11, 11-43, 1983.
Savage and Gu, J. Geophys. Res., 90, 10301-10309, 1985.
Savage et al., J. Geophys. Res., 92, 4785-4797, 1987.
Scandone, Bull. Soc. Geol. Ital., 98, 27-34, 1979.
Snay et al., Royal Soc. New Zealand Bull, 24, 131-140, 1986.
Spiess, IEEE Trans. on Geoscience and Remote Sensing GE23~4), 502-510,
1985.
Stein, Rev. Geophys., 25, 855-863, 1987.
Thatcher, J. Geophys. Res, 84, 2351-2370
Thatcher, Nature, 299, 12, 1983.
Tse and Rice. J. Geo~hYs. Res
, 1979.
~ ~ _ , 91, 9452-9472, 1986.
Turcotte et al., J. Geophys. Res., 89, 5801-5816, 1984.
Weldon and Humphreys, Tectonics, 5, 33-48, 1985.
Wilson, Geojournal, 14.2, 143-161, 1987.
Wyatt et al., Bull.
Zhang,
Seismol. Soc. Amer., 72, 1701-1715, 1982.
Rate Amount and StYle of late Cenozoic Deformation of Southern
Ningxia Northeastern Margin of Tibetan Plateau
Inst. of Tech., Cambridge, MA, 1987
.
Ph.D. Thesis, Mass.
OCR for page 46
46
TABLE 1. Tstoes on_
. . . .
HORIZONTAL
SECULAR HANSEN r
Plate motions (Pre,co,pos~yscismic
Boundary-zonc ~lonics Fault coop
Inmplatc dctoTmanon Stress redistribution
Tectonic motions (Prc,co,postyscismic
ll~nnal subsidence Magma inflation
vERncAL Diapinsm Tidal loading
Cn~s1.al landing
Past-glacial rebound
Cmtonic cpeuog~y
.
1 aDlc I. Data sets used In ~bf~'' Plate monon models
2nd generation, e.g. Rhi1(~), 1974
3rd generation, e.g. RA12(~), 1978
Magnetic
rates
68
110
4th pcncranon. NOVEL-1 (3) 1988 277
fransform
faults
62
78
121
~ ,.
blip
vectors
06
142
724 _
Minster at al., (1974~. (2) Min. tcr and Jordan (1978~. (3) Go cordon ct al., (15 ~881.
Groun~l-based metilods
1, ,1 1, _1 1 1
1~3 10-2 10-1 10° 101 102
Basclinc Length (km)
Total
data
236
330
1122
-- SLR
VLBI -----
-- GPS ----- - - - ? ?
103 104
Figure 1 Spatial scales sampled by various geodetic methods.
OCR for page 47
~:::::::::~:~::::~::::::~:~::::::::::::~:::::~::::::~:::::::~:::::::::~::~:::~::::::::::::::
: ~I:
i: ~ ~ ~ ~ : : ~::
:~'i:. :i, a: ~ .:::
~ ~ Mix :~: :::
::::::::::::::::::::::::::::::::-:::::::::::::::::: ~ ~ ~ i' x ~ : :~ ~
- ~ * . i i::~ i x i ~ i
- *O * Ibex - ~ x
:.:. ~ it: i i ~:i i : ~ .~ i: ~ :~: :~: x
At,, ,: :: ~ I ~ .:.~ ~ ~ x
i . x i: ii
i ~ ~ : i.. hi.i~,~...,..~...,.~,.~.........
i ~ ~ .~...: ~ me:
'~ ~ .~ A.
~ ......... :.: i' . ' ~* ... :i a /~X,.,:,, ~ .,,, .~,~,~
i . ~ = ~ ~ ~
~/ i.~*~xi~
. i,.~ i ::: i i ~ ~
:::: i ~ ~ ~. ~i ::: ~ ..~ ~:
~ ~ ~ ~ :
~ .: i., ~ ~ ~ ~ ::::::::::::::::::::::::::::::::::::::::::::::
: . :
: : :::::: : i:.\ At: :~::::
:: :.~: ~
.:: ::~ ~
: by* :~ ~ ~ ~ ~ it: ~ ~ ~ ~ ~ :.' :~:: :
- I* ~ ~ ~ ~ ~ ~ ~ ~ ~ :.) :::::
:.::...:..:.: ~ i :: ~.:~ .: i. ~ . :. . ~ :: :: At: at: ~ ~ :: At: ~ ~
if"' '' ' 'I 2 2 ~
","''W'.."':'""""""""',"'~".'.'."'"."""'""'.'""''~""'"''""'~""'.''''' _ "'".'"'"'~'""'.:',.:,:':.:.''"""'''\"'~""'~' ''it ''"*< " ''''
."~"'~.,'~',"'.~"::::~,~' '':""'~"",,'':""""':':""':':':'~ "''::'~' '.'.'''.... .' 'at . .''. ~'..~"~"'~.~.~'~,~,~.'.~.".'..~'~'.,"~ .~i. .:..,, "'. ,. i.
:..:. i: :~..::::~ .:: ::::: ~ ~ : :: :::::: :'': a. ~.~ .~ .. i. ~ at: . ~.~ :, ... . .~: '. ~ .. ~ . ?.
:.::: ...,,~.,.~....~,.,~..,. ~,,.~ ,.'.''.,:', ~ ~,.~ ~ .,' ~ ~ ~ ~.~ ~'~ ~ All.. '~'~'~'~ ~.~ ~.~.~'~.~'~
~'.'~'~'~'~'~""'"""''~'"'~"'4'~ "*"'~"''2'~"
:::::::.::::::: ,.;?'::.:::::::
.' ~,~,"~.''.".2",",.',.
~'~ I'm '' ~'~'~.~'~,~ ~.~ '', ~ .~, ,~ . .
:~:::~':.~'~:~:.:~:~::::~::~:~:~:::~:: ::::::::::::.:::::'~:~:::::~:~:~:: ~''::::~:~:~:~:~:::::~:::::::~::~.::~:::::~:~:~:~:~:~:~
:: :~: :.: ::.: :.:.:X~:? :~: :.?,: X.~. *My: *::: t:: :: C: :x~;:~:~:: it :'*: my: ::: :::
~.~.~ ~.~ .:: ~ Aims: :~ . ? :>i.~ * :?:x~.C*.Si.., ~ ?~*:'.ii~:? i We':. If.
~.~,'~.''X~i, ::'. ',:,
~'~"""'"'~'~'~'~'~"?''?'.
: x ?x
::::::::: :,: :,:~:::::::~:::::::::::::::::.::: ::::: :::::::::: :::::::~:~::::::::::: ::: :::::~: ::::::::::::::::: :::::::: ::: :.:.:..:.:.:.: :.:::.:.:.:.:.:.:.:::.: :.:.::
. i : ': ~ :: ~ ~ ~ ~.~.~ ~ ~ ~ ~it: ~? ~ ~ ~ ~ ~ ~ ~ ~' ?) ?'
. ~9?1 . ..
. ..:'~':::':::::''-:~-:,:::::::,::.::::.:: ::..~.:~::::::::::~'::::::::.-~?::~:::::::::~ -??: ?5X.~ ~ ~ ~ *
. ~ ~ ~ . ~ . ~ i ail. ? i ~ ~ ~ ~ ~ ~ ~ . ~ ~ . ~ . . ? ? ~ ~ i
i ~ ~.~ :::' i. :~?? ::: ~ :.:.:.: :: :~ . ~ .. ~. ~.~ ~ ~ ~ ~ ~ .
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:^gi ~ ~ ':::. ~.~.~.~.~ ~..~.~.~..~. :: :.~ a.?: i?, i-:..*.. .?i: if,~.?,X .?i x *. i· .~?.,` :* hi *a? i:
,~,~,.~,: it,, ~. ~.......... : ?~:.,~o. ~ ~ c. ~.~:~ ~ c ~.~.~.~.~c~
~ct NCs;~'~ix;$~>,j,,(~>i~cc~ ~
::::::::::::::::::::::::::::::::::::::::::: lo:: ,::: ,:::,:.: ::: :::::: :::::::::::::::::::
~'~'~ ~ ~ ~ ~ ~,~,~ ~,~ ~ ~ ~ ,~,:s,, ~s~,.: ,, ~ ~ ~ ~,~,~,~,~ ~ ~,~ ~ ~ ~
Figure 2. The San Andreas fault in central California is one element of the
western U.S. plate boundary zone. In addition, we must account for
contributions from crustal deformation both east and west of the fault. The
integrated deformation west of the fault, which consists of NW-SE extension
across the Great Basin, can be measured directly from the rates-of-change
of VLBI baselines monitored by NASA's Crustal Dynamics Project. Combining this
estimate with the geologically and geodetically observed rate of slip on the
San Andreas, and the rigid-plate estimate of total motion between the Pacific
and North America plates, we can derive geologically useful constraints on
the integrated rate of deformation across the California margin, west of
the fault. (From Jordan and Minster, 1988b. Reproduced with permission and
by courtesy of Scientific American. j
.
STRENGTH
Zone of coseismic ~ A
brittle failure ~
,... ., .. , . ~
Y::::.:::::::,::s
Zone of post- /$ ~
seismic creep / ~t
/ \....
HOT OR HIGH STRESS
REGIME
,
rye
ELASTIC
.` , _
....` , _
: A\} ~-DUCTILE FLOW
. . : ,~c
, :
i,,' \ CRUST
. -/ \
:l VISCO- BRITTLE FAILURE
[ ELASTIC \
VISCOELASTIC
COLD OR LOW STRESS
REGIME
Figure 3. Schematic diagram of the strength and mode of deformation of
the crust and upper mantle. Postseismic deformation occurs as the
result of creep on the extension of the fault plane or stress relaxation
in the viscoelastic regions.
OCR for page 48
48
=~t~l~llL,.,~,,I]-llllilllllll--l-lllllll-1lIll-lllilllllllilllIll~lllllllllll ,,l,,,It~ IlI
l ·# e
·1~,"""''
~ Ni.~
~ 7ht= '.
~\/;: ~ ~7
e
20 KH ~ `~. · , ----~ · ~ ~- h
rm r~ l- l I l l l l ~ T l l ~ I l I ~ I I l l l l l r ~ l I l l r I l l ~ l ~ l ~ I r I I l l ~ I 1 1 '} I ! ~ t ~ ~ ~1 1 i
121 ° 120. Il9e 118. 117- 116. 115
37.
- 36
-35
.` _
-,I' _
,' -34.
_
t
- 3 3 ~
_
Figure 4. Maj or fault lines and earthquake epicenters in southern
California. All events of magnitude >4e 5 in the past 55 years are
shown, w' th rupture zones of the largest events shaded. Fault
abbreviations SAF, San Andreas; SJF, San Jacinto; EF, Elsinore; NIF,
Newport-Inglewood; GF, Garlock; and IF, Imperial. PFO is the Pinon Flat
Observatory.
PfO Residual Stra~n - Superstition Hills Sequence
.... Al ~
All corrections applied,
dynomic stroin skipped, . . NW-SE Stro~n
predicted tides subtrocted,
and 5,, S2, S s fit from
longer Series
._
C
-
._
o
-
S.~/. Eq.
E.R. Eq.
327.6 328.0 328.4 328.8 329.2 329.6
Time (day number - 1987)
Figure 5. Superstition Hills events recorded at The Pinon Flat
Observatory (See Figure 4~.
OCR for page 49
49
1 ol4)
-
12
10 ~
10
10 ~
~n _
'
10
~n
._
. ~
~ _a l_
~ Plate Tectonics
_ _
Boundary
Z 0 n
Te~ton ic:
-10 yr
Tectonic /
Magmatic
Cycles
Stral n
E v e n t
. Volcanic
E v e n t ~
. Post-Glaclal
t. Rebound
~......
Post-sel~m Ic
Rebound
~' &
E a r t h q u a k e
C 0 u p I i n 9
1o2
_1 yr
1
1 `' f 10 103
Spatial scale X, km
_1 d
104
Figure6. Map of geodetic signals in terms of spatial and temporal scales.
OCR for page 50
50
Displacement of 10 ~
10
10 ~
-
. 10 -
-
-
-
In
6
~ 10
F
10
10 -
EDM, monthly
~ ~ 3 o c c u p a ~ i o n s / y r
Observatory ~/~
2-Color
EDM
d a I I y
1 mm system
A/ we. k I y
-stem
rly
10° 10 1o2 103 104
Spatial scale X, km
2
-10 yr
_1 yr
_1 d
_1 hr
Figure 7. Detection capability of various geodetic techniques at the
10 ~ 7 s train level .
OCR for page 51
51
Displacement of 10 ~
10
10 -
. 1o8
In
-
In 6
10
-
_ .
10
10
2
2 - C o 1 o r
EDIVI
dally
E D M , m 0 n t h I y
OCR for page 52
52
Table 3. _ _
Plate
Pair
na-pa
cope
china
c~nz
nz-pa
nz-an
nz-sa
an-pa
pa-au
eu-pa
co ca
nz ca
cu-na
af-na
aft
na-sa
af-sa
an-se
na ca
ca-sa
au-an
af-an
au-af
au-in
in-af
ar-af
ins
ar-cu
Tutu
in-ar
~ 1
ON
I^on~de Longitude
°E
1
48.7 -78.2 0.78
36.8 -108.6 2.09
27.9 -120.7 1.42
4.8 -124.3 0.95
55.6 -90. 1 1.42
40.5 -95.9 0.54
56.0 -94.0 0.76
64~3 -84.0 0.91
-60.1 -178.3 1.19
61.1 -85.8 0.90
24.1 -1 19.4 1.37
56.2 -104.6 0.58
(dcg/My)
Pacific Region
Atlantic Region
. .
69.4 135.8 0.92
78.8 38.3 0.25
21.0 -20.6 0.13
16.3 -58.1 0.15
62.5 -39.4 0.39
86.4 -40.6 0.28
-74.3 -26. 1 0.1 1
50.0 -65.3 0.19
Indian Ocean and African Regions
Error Ellipse
t5max t5min (man too
(deg/M,,)
1.3 1.3 -61 0.01
1.0 0.6 -33 0.05
1.8 0.7 -67 0.05
2.9 1.5 -88 0.05
1.8 0.9 -1 0.02
4.5 1.9 -9 0.02
3.6 1.5 -10 0.02
1.2 1.0 81 0.01
1.0 0.9 -58 0.02
1.3 1.1 90 0.02
2.5 1.2 -60 0.06
6.5 3.2 -3 1 0.04
4.1 1.3 -11 0.01
3.7 1.0 77 0.01
6.0 0.7 -4 0.09
5.9 3.7 -9 0.01
2.6 0.8 -11 0.01
3.0 1.2 -24 0.01
25.5 2.6 -52 0.03
15.1 4.3 -2 0.03
13.2 38.2 0.68
5.6 -39.2 0.13
12.4 49.8 0.66
-5.5 77.1 0.31
23.6 28.5 0.43
24.1 24.0 0.42
24.4 17.7 0.53
24.6 1 3.7 0.52
15.1 40.5 0.72
3.0 9 1.5 0.03
1.3 1.0 -63 0.00
4.4 1.3 -42 0.01
1.2 0.9 -39 0.0 1
7.4 3.1 -47 0.07
8.8 1.5 -74 0.06
4.9 1.3 -65 0.05
8.8 1.8 -79 0.06
5.2 1.7 -72 0.05
2.1 1.1 -4S 0.01
26.1 2.4 -58 0.04
- 1
Representative terms from entire chapter:
plate boundary
