Geodetic Determination of Relative Plate Motion in Central California

VOL. 78, NO. 5
IOURNAL OF GEOPHYSICAL RESEARCH
FEBRUARY 10, 1973
Geodetic Determination of Relative Plate Motion in Central California
J. C. SAVAGEA•V R. O. Bvxwoxm
National Center ]or Earthquake Research
U.S. GeologicalSurvey, Menlo Park, Cali]ornia 94025
Geodetic data along the San Andreas fault between Parkfield and San Francisco,California
(latitudes 36øN and 38øN, respectively),have been re-examinedto estimate the current relative movement between the American and Pacific plates acrossthe San Andreas fault system.
The average relative right lateral motion is estimated to be 32 ñ 5 mm/yr for the period
1907-1971.Between 36øN and 37øN it appears that most, if not all, of the plate motion is
accommodatedby fault creep. Although strain is presumably accumulating north of 37øN
(San FranciscoBay area), the geodeticevidencefor accumulationis not conclusive.
We have re-examinedthe geodeticdata per-
slip (called fault creep) occursat the surface;
this behavior presumablyrepresentsan extenmotion betweenthe Pacificand Americanplates sion of the area of stable sliding on the fault
along the 280-kin-long segment of the San surface upward through the entire crust, so
Andreas fault that extends southeast from San
that the motion measured acrossthe segment
Francisco (Figure 1). The data consistof re- over a few years is similar to the motion in the
peated surveys of triangulation networks over rigid-block model shown in the lower part of
the period 1907-1962, repeated surveys over Figure 2. In other regions (e.g., the sectionof
the period 1960-1970of the geodimeternetwork the San Andreas fault from just north of Holshown
in Figure1, anddirectmeasurements
of lister to well north of San Francisco)the upper
fault creepmostly in the period 1968-1971.The portion of the fault surface appears to be
objective is to determinethe relative motion of locked, and strain should accumulate in the
geodetic stations (e.g., Santa Ana and Mr.
vicinity of the fault (upper sketchesin FigToro in Figure 1) on oppositesides of, and at
ure 2). In measuring relative motion of the
some distance from, the faults in the San plates acrossa sectionwhere the fault is locked,
Andreas system.
it is of coursenecessaryto be sure that the
Figure 2 showstwo simplemodelsof plate measurementsextend far enough beyond the
motion alonga transformfault. The platesslip fault to spanthe zoneof strain accumulation.
past one anotherin responseto shearstresses This distanceturns out to be surprisinglylarge.
tinent
to
the
determination
of
the
relative
that presumablyoriginate from the drag of
A roughestimateof it can be obtainedfrom a
mantle convectioncurrentsupon the bottom of dislocation model similar to that shown in the
the lithosphereplates.Our presentunderstand- upper part of Figure 2. For simplicitywe asing of the San Andreas fault system suggests sume that slip on the fault is zero down to
that below about 15-kin depth the platesslip depthD but is an amountb everywherebelow
past one another fairly uniformly alonga steep that depth. Then, in the coordinatesystem
contact; i.e., a conditionof continuousstable shownin Figure 2, the strike slip displacement
sliding may very well prevail at depth, as is v and the tensor shear strain e• on the free
evidenced by rather uniform long-term dis- surfaceare givenfrom the simplescrewdislocaplacementrates measuredat the surfaceand tion model[Weertmanand Weertman,1964] by
by an apparent absenceof earthquakefoci at
depthsgreaterthan about 15 km. Along certain
segments
of the fault (e.g.,Parkfieldto Hollister;
e• - (bD/27r)(x
•' -{- D2)-1
see Figure 1) continuousor quasi-continuous
v- (b/•r)
arctan
(x/D)
(1)
Both quantitiesare plottedas functionsof x/D
on the right-handsideof Figure2. Most of the
Copyright (•) 1973 by the American Geophysical Union.
832
833
SAVAGE AND BURFORD: ]•ELATIVE PLATE MOT•O•
15 km, as is commonlyaccepted,measurements
TAMALPAIS.• •-/•"-d "•/,
must
'•_ ('-)X•6.'•
MTDIABLO
SA. •
%X•
/
FRANCISC,
OI ""•\ '/
SIERRA.X}
"-'
MORENA•
I•,
o
I
"]•"•
'
/
"• LOMb'(
2 ¾0
"• -5_*-•4,•1o*.SANTA
•
ANA
e•w•.x•
.
3" 0
RO
$6
øN X•
KM
, ....
extend
to 100 km on either
side of the
fault. It is this requirement that creates the
principal obstacleto interpreting geodeticdata
on plate movement: any result can be challengedon the basisthat measurementsmay not
have extended sufficiently far from the fault.
On the other hand, where the rigid-block type
of motion obtains, there is no particular difficulty in measuringrelative block motion.
5•0
TRIANGULATION
DATA
Part of the primary schemeof triangulation
in California spans the San Andreas fault
system.Repeated surveysof this systemby the
U.S. Coast and Geodetic Survey have furnished
PARKFIELD
ß
Fig. 1. The San Andreas fault system between
Parkfield and San Francisco. The principal faults,
shown by heavy sinuous lines, are, from west to
east, the Safi Andreas, Hayward, and Calaveras.
The solid triangles represent stations in the primary triangulation scheme.The lines connecting
triangles show the California geodimeter network.
invaluable
data
on relative
movement
across
the fault system. Whirten [1948] comparedthe
geodeticpositionsof the stations shownin Figure 3 as determined from tke surveys of 1906,
1922, and 1946 to determine the relative move(The four lines farthest south are identified by
ment of the stations. The geodetic positions
numbers25, 28, 30, 32. The other lines are identiwere calculatedsubject to the assumptionthat
fied in Figure 7.) Observationsof fault creep (in
millimeters per year) are shownby numbers along the position and length of the line Mocho-Mt.
the faults. Fault creep data marked by * are
Diablo was the same for each survey. The
from Nason [1971]; other data. are from R. O.
results of Whitten's analysis are shown by
Burford (unpublished data, 1971).
arrows originating at each station in Figure 3.
It appearsthat Santa Ana and Tamalpais have
relative movement is concentrated close to the
undergone
no significant net displacement in
fault (e.g., 50% of the relative motion occurs
the
period
1906-1946, whereas Mr. Toro,
between x - --D and x -- +D, but, to inGavilan,
Loma
Prieta, and Sierra Morena have
clude 90% of the relative motion, measurements
must be made across a zone extending from all moved northwest approximately parallel to
x -- --6.3D to x -- +6.3D. Thus if D is ,•bout the fault at average rates of 49, 48, 32, and 36
exy
O
t•
-15
-3
0
+15
i•.•l•• X/O
•$
X/D-•
DISPLACEMENT
STRAIN
••
'ß
I•xy
tt,tt
tttJ
y
-$
0
+3
X / O ---P
Fig. 2. Simple models for fault motion showingthe distribution of shear strain and strike
slip displacementon the surface.The upper sketchesrefer to a locked-fault model, in which
slip on the fault occursonly at depths greater than D. The lower sketchesrefer to the rigidblock model, in which slip is approximately uniform with depth.
834
SAVAGE AND BURFORD: RELATIVE PLATE MOTION
mm/yr, respectively.Becausethese results are
contaminated by appreciable errors accumulated in carrying the triangulation network
forward from the base Mocho-Mt. Diablo, it is
probablethat the 17-mm/yr differencebetween
the lowest and highestrates is not significant.
The magnitudesof the accumulatederrors are
suggestedby the apparent movementat Santa
Ana (1 meter northwest in 1906-1922 and !
meter southeast in 1922-1946). Presumably
Santa Ana remained fixed relative to Mocho-
Mr. Diablo during the entire'period (all three
stations are on the same plate well back from
the fault), and the apparent motionsin 19061922 and 1922-1946 merely representaccumuFig. 3. Movement of stations in the primary
lated survey error. Thus correctionsof similar
scheme of triangulation from a comparisonof the
magnitudeand direction (i.e., 1 meter southeast adjusted surveys of 1906, 1922, and 1946 [after
for 1906-1922
and 1 meter northwest
for 1922-
1946) shouldbe appliedto the displacements
at
Mr.
Toro and Gavi!an.
Such corrections
would
clearly make the motionsat thesestationsmore
uniform for the tw(• periodsof time. The point
we wish to make here is that significanterrors
accumulate in triangulation adjustments,but
the relative motion of nearby stations is preserved.
The relative motion between nearby stations
can frequentlybe calculateddirectly from the
observedchangesin azimuth. For example,in
cases where the azimuth between two stations is
Whitten, 1948]. The arrows represent the apparent
station displacementin the periods 1906-1922and
1922-1946.
ment parallel to the strike of the San Andreas
fault; for the other lines in Figure 4 we have
assumedthat the relativedisplacement
is parallel to the fault. As can be seen, the data do
define reasonablylinear trends. The slopesof
the trends have been calculated
from a least-
squares analysis of all data except the 1882
survey (the 1882 survey was excludedfrom the
approximatelyperpendicularto a fault, the fit because the interval 1882-1906 would concomponentparallelto the fault of the displace- tain the sudden displacementsassociatedwith
ment betweenthose two stationsis simply the the San Francisco earthquake of 1906); the
productof the azimuthchangeand the distance slopes,along with the standard deviation indibetween stations. Such a calculation would
catedby the least-squares
analysis,are shownin
apply, for example,to the line Santa Ana-Mt. Figure 4.
Although it does not form part of the priToro in Figure 3. Alternatively, if one assumes
that the relative motion is parallel to the fault
strike, the magnitude of the displacementcan
readily be calculatedfrom changesin azimuth
even though that azimuth is not even approximately perpendicularto the fault strike. Whirten
[1948] and Meade [1963] have applied these
techniquesto the triangulation network in Figure 3. We have used their data for surveysin
1882, 1906, 1922, 1947, 1951, !957, and 1962 to
show in Figure 4 the calculated relative displacement as a function of time. For the lines
approximately perpendicularto the fault strike
(Mr. Diablo-SierraMorena, Mocho-LomaPrieta,
Santa Ana-Mt. Toro, and Santa Ana-Gavilan)
the method gives the componentof displace-
mary triangulation network, Farallon L.H., a
lighthouseon a small island about 36 km southwest of the San Andreas fault (Figure 5), has
been intersectedin surveysof 1855, 1882, 1906,
1922, 1948, and 1957. The accuracy of the
azimuth to Farallon L.H. is, of course,somewhat
lower than azimuths in the primary scheme.
Nevertheless,the positionof Farallon L.H. (so
far seaward from the fault) makes it desirable
to attempt to ascertain its motion from the
observedchangesin azimuth.Figure 5 showsthe
two lines to the Farallon L.H., one from Tamalpais and one from Sonoma Mountain, which
have been used to calculate the fight lateral
relative motion of Farallon
L.H. For the motion
SAVAGEAND BURFORD' RELATIVE PLATE MOTION
of FarallonL.H. relativeto Tamalpaiswe added
the azimuth of Mr. Diablo from Tamalpais in
the main-schemetriangulation to the observed
(i.e., unadjusted) angle at Tamalpais from Mr.
ROSS
'•RMTN.
•.
%
835
'
\
'
\
,
38'
30
N-
•.SO.OMA
•
"%
P
Diablo to Farallon L.H. to calculate the azimuth
MTN
,
AMALPAIS
-
•MT•DIABLO
f
E
1900
1950
.
.
! ! ! ! ! ! ! ! i
•
I MT.
DIABLO
- MT.TAMALPAIS
•
LrJ
E
ß
-2'16
•
mm/yr
•
•N
,
,
•RIETA
•
,•
'•
/
/
-I
• 2t
<[
28 * I0 mm/yr
•
MORENA
0 ,••BLO-SIERRA
•o
2
3,9'8
Fig. 5. Map showing the location of Farallon
L.H. relative to the main scheme of triangulation.
The faults (heavy lines) are, from left to right,
the San Andreas fault, the Hayward-Rogers
Creek-Healdsburg system, and the CalaverasGreen Valley system.
mm/yr
0
'•1
MOCHO
- SIERRA
MORENA
</: -2
of Farallon L.H. The results (Figure 6) suggest
a right lateral motion of 8 ñ i mm/yr for
Farallon L.H. relative to Tamalpais. That mo-
• 2:F
MOCHOLOMA
•-,"J"-'
,,••,•--•,,,
, PRIETA
30"$
mm/yr
k-
0
,
•
....
:
MOCHO- MT. TORO
<[
47"
17 mm/yr
tion can also be calculated by using the
angle from Farallon L.H. to Ross Mountain
(seeFigure 5) measuredat Tamalpais. Because
the line Tamalpais-RossMountain is parallel
to the San Andreas fault, it is reasonableto
assume that
.j 2
.••..•_•.
'(
26'6
• 2
mm/yr
ANA
- GAVII..AN
I
•
22 * 13mm/yr
bJ
ANA
- MT.TORO
>
- 2.[ SANTA
•..J
' TaGAVILA.
-' MT.
TORO
; ;';:
I I _1, • '
LrJ_11
I
I
I
1900
I
i
I
I
I
I
-4"6
mm/yr
ß
1950
Fig. 4. Relative right lateral displacement as
a function of time for stations in the primary
triangulation scheme (Figure 3) as calculated
from observed changesi• azimuth [from Whirten,
1948; Meade, 1963]. The average rate of motion
in millimeters per year as calculated by a leastsquares linear fit to the postearthquake (1906)
data is shown beside each plot. The uncertainty
quoted is one standard deviation, as obtained
from the least-squaresanalysis. The pre-1906 observations (open circles) were not included in the
least-squaresfit.
its azimuth
remains
fixed. Thus
the changein azimuth of Farallon L.H. from
Tamalpais shouldbe equal to (but of opposite
sign from) the change in the angle Farallon
L.H. to RossMountainmeasuredat Tamalpais.
That analysissuggestsa right lateral motion of
Farallon L.H. relative to Tamalpais of 25 ñ 11
mm/yr, an estimate that, becauseof its large
standarddeviation,doesnot differ significantly
(i.e., within the 95% confidencelevel) from the
earlier estimate of 8 ñ 1 mm/yr. The second
estimate (25 ñ 11 mm/yr) does suggestvery
strongly, however,that the standard deviation
(ñ1 mm/yr) of the earlier estimate is much
too low, a result that is not very surprising in
view of the fact that the least-squaresfit was
basedon only 3 data points.
The right lateral motion of Farallon L.H.
relative to SonomaMountain (Figure 5) can be
estimatedfrom the changein the anglebetween
Mr. Diablo and Farallon L.H. measured at
Sonoma Mountain. Because the line Mt. Diablo-
SonomaMountain is practically parallel to the
San Andreas fault, its azimuth is not likely
to changeappreciably.Thus the changein the
836
SAVAGEAND BURFORD:RELATIVEPLATE MOTION
and Mt. Diablo, as is assumedin Whitten's
adjustment.If it has not remainedfixed, its
movement would introduce a bias tending to
causethe right lateral movement of Farallon
E
i TAMALPAIS
TOFARALLON
L.H.
0
9•)6'' : : 19•)0
'
mm/yr
6d'l
L.H. to be overestimated. Thus there is reason
to believe that Whit•en's estimatemay be too
high.
A triangulationnetwork (SalinasRiver valley
net,) extendingnorthwestfrom Parkfield along
Z SONOMA
MT.TOFARALLON
L.H.
21"
_$L
ø
•-•'•
12
1950
the San Andreas fault to within about 30 km
of Hollister was established in 1944 and re-
Fig. 6. Relative right lateral displacement as a
function
of time
for
Farallon
L.I-I.
relative
to
Tamalpais and Sonoma Mountain (Figure 5) as
calculated from observedchangesin azimuth. The
average rate of motion in millimeters per year as
calculated by a least-squareslinear fit to the postearthquake (1906) data is shown beside each plot.
The uncertainty quoted is one standard deviation. The pre-earthquakedata (open circles) were
not included in the least-squaresfit.
surveyedin 1963. The network coversa zone
extendinglaterally from a few kilometersnortheast of the San Andreas fault to about 40 km
southwestof the fault. Meade and Small [1966]
compared
geodetic
positions
calculated
fromthe
two adjustedsurveysto detect relative movement across the San Andreas fault. That com-
parisonshowedclearlythat all the significant
relative movement occurred within a few kilom-
etersof the fault; there was no significantrelaangle Mt. Diablo to Farallon L.H. at Sonoma tive motion between stations on the same side
Mountain is equal to the change in azimuth of the fault (i.e., on the samefault block). The
of Farallon L.H. to Sonoma Mountain.
The
measurementsare quite consistentwith the
changesin the azimuth so calculatedsuggesta rigid-blockmodelshownin the lower half of
right lateral motion of Farallon L.H. relative Figure 2. Meade and Small [1966] also calcuto SonomaMountain of 21 ñ 12 mm/yr (Fig- lated the right lateral componentof motion
ure 6).
acrossthe fault from the changein azimuth of
Dr. C. A. Whitten (quotedby Bolt [1970, p: six lines that crossedthe fault approximately
34] ) has investigatedthe movementof Farallon perpendicular
to strike.They foundthe average
L.H. relative to Mr. Diablo, SonomaMountain, rate of right lateral slip acrossthe fault was
and RossMountain as indicatedby the surveys 32 ñ 1 mm/yr.
of 1882, 1906, 1922, 1948, and 1957. Whirten
A triangulationnetwork (the Monterey Bay
adjustedthe resultssubjectto the assumption arc) with a high density of stationswas exthat Mt. Diablo, Sonoma Mountain, and Ross tended in 1930 from the coastline north of
Mountain all remainedfixed, and he found that Mt. Toro to Santa Ana, crossingthe San AnFarallon
L.H.
moved N 14øW at the rate of
46 mm/yr. The indicatedmotionis quite closely
parallel to the strike of the San Andreasfault
(N 35øW) in this area. Whitten's value should
be comparedwith our own estimateof 21 ñ 12
mm/yr for the right lateral motion of Farallon
L.H:
relative
to Sonoma Mountain.
The stan-
dreasfault systemnear Hollister (seeFigure 1
for locations).The network was resurveyedin
1951 and 1962. This network has been discussed
by Whirten[1960]andby SavageandBur[ord
[1970]. Savageand Burfordcomparedthe observedanglesfor the three surveysand concludedthat within the accuracyof the surveys
dard deviation of our own estimate (ñ12
there was no evidence for strain accumulation
mm/yr) is sufiqcientlylarge that the estimate
itself is probablynot inconsistentwith Whitten's
value. However, we believethat Whitten's estimate is subjectto very appreciableuncertainty.
It is very doubtfulthat RossMountain, scarcely
in the blocks on either side of the fault system.
The relative motion between blocks appeared
to be accommodatedby slip on San Andreas
and Calaveras faults or strain accumulation in
that underwent 3.5 meters of slip in 1906, has
very narrow zones centered on those faults.
Savageand Burford estimatedthe combined
relativefight lateral slip acrossthe SanAndreas
remained
and Calaveras faults to be 25 ñ
6 km from the section of the San Andreas fault
fixed
relative
to
Sonoma
Mountain
I mm/yr.
SAVAGEAND BURFORD.' [RELATIVE [PLATE Mo•xoN
all the 1951 data in Figure 4 are too low; in
fact, the valuesare so low that they require left
rather than right lateral motion acrossthe San
Andreas fault system in the period 1947-1951.
122'00'
+ 38'00'
0 +M 2
BI.O
•• 1,,
,•
1957-
?
1951
-1957
.KM, 3,0
An error in the 1951 azimuth
of the line Mr.
Diablo-Mocho would also explain this anomaly.
Thus we feel that the large right lateral motions
inferred from the 1951-1962 San FranciscoBay
area triangulation data are probably the consequenceof horizontal refraction in the baseline
MOGHO
stations in the San Francisco Bay area from com-
observations
parison of the adjusted triangulation surveys of
1951, 1957,and 1962.In the calculationthe length
tive movement
and azimuth of the line Mt. Diablo-Mocho
assumed to remain invariant in the interval
837
were
19'51-
1962. The data are from Pope et al. [1966].
average right lateral strain accumulationacro•
the 70-km-widezone to be 0.04 ñ 0.08 •
strain/yr which is equivalentto 3 ñ 5 mm/yr
right lateral relative motionof the end points
alternative
to this
explanationis to accept a highly irregular relabetween the Pacific and Ameri-
can plates (left lateral slip in 1947-1951, very
rapid right lateral slip in 1951-1957, and no
slip in 1957-1962).We believethat an error in
the azimuth
Excludingslip on the faults,they estimatedthe
of 1951. The
observations in 1951 is much more
likely. An error of this magnitudecan be tolerated in a long-termseriesof observationssuch
as are shownin Figure 4; however,it will completely dominate the changesin a period as
short as 1951-1957.
of the arc. Thus, the analysis of Savage and
GEODIMETER DATA
Bur/ord[1970]leadsto an estimat•of 28 ñ 5
mm/yr right lateral motion for a point near
Mt. Toro relative to a point near Santa Ana.
Finally, a high-densitytriangulation network
Figure 1 shows the northern part of the
California geodimeternetwork, which extends
was established in 1951 in the area between
Mr. Diablo and Mocho on the east and Sierra
establishedin 1959 and was surveyedapproximately a•nually until 1968 by the California
alongthe SanAndreasfault. Th9 networkwas
Morena and Tamalpaison the west (Figure 7).
Division of Water
This network was resurveyedin 1957 and 1962.
Pope et al. [1966] have shownthat the surveys
suggesta large fight lateral movement (80 cm
for Sierra Morena relative to Mocho) distributed acrossthe network during the period
1951-1957 but negligible and rather random
motion in the period 1957-1962.These motions
are indicatedby arrowsin Figure 7. As Whirten
[1959] pointed out, a closeexaminationof the
displacementpattern for the period 1951-1957
suggeststhe possibilityof a clockwiserotation
been surveyed by the California Division of
Mines and Geology. Line length measurements
are available for a period of 9 to 12 years for
eachof the lines shownin Figure 1, and the data
of the whole network
The geodimeternetwork near Hollister contains a rigid configuration (Figure 8) that
permits a unique solution (exclusiveof displacementsgeneratedby rigid-bodymotionsof
the figure as a whole) for the relative motions
relative to the fixed base
line Mt. Diablo-Mocho. Such an apparent rotation would result if the observation from either
end of the base line to the other end were de-
flected by horizontal refraction. Dr. C. A.
Whitten (verbal communication,1971) has informed us that up to 4 secondsof horizontal
refraction has recently been observed from
Mocho, and it doesseempossiblethat the 1951
surveymight have been similarly affected.This
possibilityis supportedby the observationthat
Resources. Since 1968 it has
for most of these lines exhibit a trend in time.
We have taken this trend as definedby a leastsquareslinear fit to representthe average rate
of changeof line length (dL/dt), and that value,
along with its standard deviation, will be used
in this analysisto estimate fault slip and strain
accumulation.
of the five stations involved. Savage and Bur-
[ord [1971] have alreadydiscussed
this solution,
and it will sufficehere 'to review it briefly. We
chose to calculate motions relative to Browns,
and for the moment we neglect the effects of
possiblerigid rotationsof the figure by requir-
838
SAVAGEAND BURFORD.'RELATIVE PLATE MOTION
ing that the azimuth of line 21 remainfixed.
Then the observedrates of lengtheningof the
lines involved determine the relative motions of
the stationsas shownby the arrowsin Figure 8.
The restriction to a fixed azimuth
for line 21
canbe removedby addingany motiongenerated
by a rigid rotation of the trilateration figure
about Browns. Clearly such a rotation imposes
a componentof motion perpendicularto the
fault system at Fairview as well as at other
stations.Becausethe length of line 21 has not
changedsignificantly(dL/dt = 2 ñ I mm/yr)
and the line itself is practically parallel to the
San Andreas fault, we do not think it likely
that station Fairview has moved significantly
with respectto Browns.Thus we doubt that
any significantrotation should be imposedon
the trilateration figure, and consequentlywe
believe that
the annual rate vectors shown in
Figure 8 are approximately correct.
I•I•/YR
0
2O
DISPLACEMENT
0
I
Kid
ß
I
20
,
I
Fig. 8. Trilateration network near Hollister.
The arrows represent the average rate of displacement of the stations relative to Browns for
the period 1960-1970as inferred from the average
rate of lengthening of the observed lines.
tivity was basedon measurements
at the end
points and occasionally
on a balloonmeasurement at the midpoint [Ho[mann, 1968]; later
measurements(shownas solidpoints in the figshownin Figure 8 can be obtainedby making ure) weremadewith laserinstruments(geodolite
the plausibleassumptionthat all motionsare or Model 8 Geodimeter), and the correctionfor
parallel to the San Andreasfault system.Then atmosphericrefractivity was measuredsimulthe changesin the lengthsof the pairs of lines taneouslyfrom an aircraft flying along the line
that link Fairview to Fremont dictate the relaof sight. The later measurementsare of course
tive right lateral motion between those points. somewhatmore accurate, but a careful examiLines 17 and 23 require a relative motion of nation of the data indicatesno systematicerror
32 ñ 5 mm/yr, lines 18 and 22 require 27 _ 4 was introduced by changing the method of
mm/yr, and lines 20 and 21 require 27 ñ 3 measurement.The linear least-squaresfit to the
mm/yr. (The same assumption requires that data for each line is shown in Figure 10, and
Sargent and Gilroy both move at the rate of the slope of that line (dL/dt) along with its
standard deviation is shown beside the line in
15 ñ 4 mm/yr relative to Fairview.) Fault
creep has been measuredat several localities Figure 9.
alongthe Calaverasand San Andreasfaultsnear
The configuration(•f the geodimeternet near
Hollister (seeFigure 1). The averagevalue for Loma Prieta (Figure 11) is suchthat one can
fault creep on the Calaveras in this sector is estimate the relative right lateral motion across
seento be 15 ñ 2 mm/yr and on the San An- the San Andreas fault system,which at that
dreas 10 ñ 2 mm/yr. Thus it would appear latitude consistsof three strands (the Calaveras,
that all the relative movementdetectedby the the Hayward, and the San Andreasproper).
geodimeternetwork can be accountedfor by Lines 6 and 13 in Figure 11 are nearly colinear
and can be taken together to approximate a
observedfault creep.
Next let us considerthe geodimeternetwork singlelong line from Loma Prieta t(• Allison.
north of Hollister (Figures I and 9). The ge- Then the triangle Loma Prieta-Allison-Mt.
odimeter data for this area are generally less Hamilton forms a rigid configurationthat can
satisfactory than data found elsewhere,and we be analyzedfor relativemotions(againexcludhave chosen to show the individual length ing possiblemotio.nsgeneratedby rigid rotameasurementsas a function of time (Figure 10), tions of the figure as a whole). We shall calso that the reader can judge for himself the culate the motions relative to Mr. Hamilton
uncertaintiesin the analysis.Before 1969 mea- and, to eliminatethe contributionsfrom rigid
surements were made with a Model 2A Geodimrotation .of the figure, hold the azimuth (•f line
eter, and the correctionof atmosphericrefrac- 7 fixed. Then the rates of lengtheningof lines
A similar solution of the trilateration
network
SAVAGEAND BURFORD: RELATIVE PLATE MOTION
839
quire that the motions of the stations relative to
Mr. Hamilton be as they are shown by the
dashedarrows and numbers in Figure 11. The
indicated relative right lateral motion acrossthe
fault systemin this area appearsto be 38 ñ 7
mm/yr (Eagle Rock relative to Mt. Hamilton).
The configurationof the geodimeternetwork
southeastof Hollister (lines 25, 28, 30, and J32
in Figure 1) doesnot permit a uniquesolution
for the relative motion acrossthe fault system.
However, it appears reasonableto assumethat
the motionsare approximat.elyparallel to the
fault trend, in which caseit is easy to infer the
am.ount of relative right lateral motion required
between stations at the ends of each line. The
values for the individual lines are shown in
Fig. 9. Geodimeter network in the vicinity of
San Francisco Bay. The numbers beside the lines
represent the average rate of lengthening (with
standard deviation) of the line in millimeters per
year. The Hollister trilateration network (Figure
8) is shown in the lower right.
Table 1, and a meanvalue for the relativeright
lateral motion appearsto be 32 ñ 3 mm/yr for
the decade 1960-1970. Fault creep has been
measured at several localities along this section
of the fault (Figure 1). The measurementsgen-
erally refer to the right lateral motion observed
6, 7, 10, and 13 determinethe motionsof Allison
and Loma Prieta, as is shown by the arrows in
Figure 11. The restrictionto a fixed azimuth for
line 7 can be removed by adding any motions
generated by a rigid-body rotation of the triangle Loma Prieta-Allison-Mt. Hamilton about
Mt. Hamilton. Such rotations add a component
of motionat Allisonthat is perpendicularto the
trend of the San Andreas system. Inasmuch as
the predominantmort.on at all stations should
be at least approximatelyparallel to the San
Andreasfault, the additionalcomponentat Allison producedby the acceptablerotationsmust
be rather
small. The rotational
contribution
across a zone about 100 meters wide centered
on the active trace of the fault. The measure-
ments in this section range from 18 to 36
mm/yr, with an averagevalue of about 25 ñ 6
mm/yr. There is reason to believe that such
direct measurementsof fault creep may underestimate'the
over-all
relative
motion
within
the
fault zone [Savageand Bur/ord, 1971], and for
this reasonthe higher valuesmay be more reliable; nevertheless,the averagemeasuredvalue
for fault creep (25 ñ 6 mm/yr) is not significantly different from the average inferred from
geodimetermeasurements(32 ñ 3 mm/yr).
at
Loma Prieta must be of almost equal magnitude
to that at Allison (lines 7 and 10 are of similar
CONCLUSIONS
The
measurements
of relative
motion
across
length) but will be almost parallel to the San
Andreasfault (becauseline 10 is almostperpen-
the San Andreasfault systembetweenParkfield
dicular to the fault). Thus we have established
that Loma Prieta must moveparallelto the fault
trend at a rate of about 31 ---+6 mm/yr plus or
minusa few millimetersper year associated
with
possiblerigid-body rotations of the trilateration
figure. The latter contribution is small compared with the uncertainty in the former estimate and can be neglected.We can proceed
further by assumingthat the other stations in
Figure 11 also move parallel to the trend of the
For conveniencethat segmentof the fault has
been divided into three sections(northern, central, and southern), with the dividing points
San
Andreas
fault.
The
observed
values
of
dL/dt for the variouslines (Figure 9) then re-
and San Francisco
are summarized
in Table
2.
about 30 km northwest and 30 km southeast of
Hollister. In quoting triangulation resultsfrom
Figure 4 in Table 2, only lines approximately
normal to the San Andreas fault system have
been used, so as to obtain more reliable esti-
matesof relativeright'lateralmotion.
For the southernsectionthe evidenceis quite
goodthat the measuredvalue (32 mm/yr) does
representthe total relative right lateral motion
840
SAVAGE
ANDBURFORD'
RELATIVE
PLATEMOTION
LINE
I
DL/DT-+10* I mm/yr
/ LINE 2
0
o
0
-5
I LINE 5
I LINE 6
+20I
.DL/DT+14
"2•m/yr
IDL/DT-6"2
mm/yr
LINE 4
DL/DT+2"I mm/yr
o
.....
I
/ DL/DT'+Sa'2
mm/yr
+201
"
LINE
I LINE
9
I LINE
7
.
IO
DL/DT--5
•,$ mm/yr
0DL•o":•
o15
o mm/yr
-I0
o o o •)
_.1 0,•,ø,
,, ,o,,6',
, :: ,,
',/,,oS'
......
'
-I0
?,',;oS
LIN[
I$
DL/DT--I$
LINE II
DL/DT--7
I LINE 12
o
o
•4
mm/yr
•' 3 mm/yr
DL/DT=+5
"2 mm/yr
_,q,,
T,
+101 o o '
-I0
- 20
;'o'q
......
LINE
15
DL/OT ß -8"
4 mm/yr
/ LINE 16
I LINE 14
I DL/DT+7"$mm/yr
+11fDL/DT-+I
•'3
mm/yr
o
ß
o
o,
-IO
ß
•
.....
i i i
i
+Iø
t o•
Fig. 10. Change
of linelengthasa function
of timefor geodimeter
linesin theSanFran-
ciscoBay area.The opendata pointsrepresent
measurements
by the CaliforniaDepartment
of Water Resources,
and the solid pointsrepresentmore recentmeasurements,
in which
atmospheric
reftactivitywasdetermined
simultaneously
froman aircraft.The straightline
in eachplot represents
the least-squares
linearfit to the data.The slopeof that lineis quoted
with its standarddeviation as the averagerate of lengthenin• (dL/dt).
angulationdata for the SalinasRiver valleynetwork, 1944-1963 [Meade and Sin.all, 1966],
relativeplate motionhasbeenmeasured
in the
centralsectionis also quite good.The primary
schemeof triangulation (Figures 3 and 4)
show no evidence of relative motion within the
shows that Mr. Toro and Gavilan both move
block southwest of the fault (i.e., no measur-
within the limits of significanceat the same
rate relative to Santa Ana, as we shouldexpect
acrossthe San Andreas fault system.The tri-
ablestrain). Moreover,the relativeright lateral
motionmeasuredby the geodimeternetworkis
in reasonableagreementwith fault creep measured within 100 meters of the active fault trace.
Thus the evidence certainly suggestsrigidblock motion of the type shown in the lower
sketch of Figure 2. The evidencethat all the
for the rigid-blockmodel.Similarlyno significant movement between Mt. Toro and Gavilan
(bothon the samefault block)is demonstrated
in Figure 4 (in fact, the indicatedmotion,
thoughnot significantly
differentfrom zero,is
left lateral). However, becauseof the large
SAVAGE AND ]•URFORD' RELATIVE PLATE MOTION
841
observedfault creepon the Calaveras(about 14
mm/yr) and the San Andreas (about 12
mm/yr) faults is adequateto explain the observedplate motion,as wasdiscussed
previously
[Savage and Bur/ord, 1971]. Finally, the various estimates
of relative
block
movement
for
the centralsectionin Table 2 are quite consistent
despitethe fact that they refer to measurements
that span zonesof differentwidth acrossthe
fault system.In the analysisof the northern
section(Table 2), there is somewhatgreateruncertaintythat the geodeticcontroldoesextend
far enougheitherto the eastor to the westto be
sure that
all the relative movement between
plates has been included.The excellentagreeFig. 11. Station displacement rates relative to
Mt. Hamilton for stations in the Bay area geodimeter
network
as inferred
from
the rate
of
changeof line lengths. In most casesthe information was inadequate to determine the motion
without imposing the constraint that it was
parallel to the San Andreas fault (N 40øW); for
those cases displacement rate is indicated by a
dashed rather
than solid arrow. The rates of dis-
placement (with standard deviations) in millimeters per year are shown beside the arrows.
standard
deviations these data do not exclude
the possibilityof somerelative movementbetween stations on the same fault block (e.g.,
within the 95% confidence level Mt. Toro
could move northwest at the rate of 8 mm/yr
relative to Gavilan). Similarly the analysisof
Savage and Bur/ord [1970] demonstrated no
significantrelative motion in the fault blocks
on either side of the fault system, but they
could not exclude such movement
below the
level of significancein their analysis (about
7 mm/yr across 70 km at the 95% confidencelevel). The most convincingevidencethat
the rigid-blockmodel applies is the fact that
ment between the movement of Loma Prieta
relative to Mocho (30 -- 3 mm/yr from Figure
4) and relativeto Mt. Hamilton (31 _ 6 mm/yr
from Figure 11) suggeststhe absenceof significant relative motion between Mocho and Mt.
Hamilton. The fact that all the stations on the
block southwest of the San Andreas fault in
Figure 11 moveat the samevelocityrelativeto
Mt. Hamilton suggests
that none of the relative
motion extends appreciably west of the San
Andreasfault. However,the standarddeviations
associated with both observations are large
enoughthat we cannotexcluderelative motion
TABLE 2.
Summary of Relative
Motions of Blocks
on Opposite Sides of the San Andreas Fault
System between Parkfield
and San Francisco
Section
of
Fault
Triangulation
mm/yr
Geodimeter
Lines
South
of
Period
32
ñ
'1'
32
ñ 3
Central
26
22
ñ
6•
ñ 13õ
28
ñ 4
58
ñ 7
Line
Data Are
Available
Right
dL/dt,
mm/yr
Northern
Lateral
Motion,
mm/yr
28
30
32
1959
1959
1960
1959
to
to
to
to
1971
1971
1971
1971
28
25
28
35
ñ
ñ
ñ
ñ
1
2
3
3
35
28
29
37
ñ
ñ
ñ
ñ
2
2
3
3
sll
ñ 10ô
8**
59
ñ
21
ñ 12•
*Salinas River Valley network,
[Meade and Small, 1966].
1944 to 1965
$Santa Ana-Gavilan,
1906 to 1962, Figure 4.
õSanta Ana-Mt. Toro, 1906 to 1962, Figure 4.
IIMontereyBay arc, 1950 to 1962 [Savageand
Burford,
25
28
Hollister
Relative
for which
Geodimeter Data,
1960 to 1970
mm/yr
Southern
2s
TABLE 1. Rate of Change of Line Length
and Indicated Right Lateral Motion for
Data,
1970].
ôMt. Diablo-Sierra
Morena, 1906 to 1957,
Figure 4.
**Mocho-Sierra Morena, 1906 to 1957, Figure 4.
ttSonoma Mountain-Farallon
L.H.,
1906 to
1957, Figure 6.
842
within
SAVAGEAND BURFORD: RELATIVE PLATE MOTION
the block east ,of the Calaveras
fault
or west of the San Andreasfault. Perhapsmore
direct evidence on the lack of relative movement
cepting a rate near 32 mm/yr. Moreover, the
fault movement in the southern and central sec-
tions seemsto be predominantly,if not entirely,
within the western block is the absence of a
of the rigid-block type (lower sketch in Figure
significantchangein length for geodimeterline 2). Thus the problemof whether geodeticmea14 (Figure 9), which lies wholly within that surementsextend sufficientlyfar from the fault
block. Less definite evidence for the lack of
to include all significant movement is not inrelative movement within the eastern blocks is
volved. After examiningall the uncertaintieswe
affordedby the absenceof significantchangein have rather subjectively assigneda standard
length of line 4 (Figure 9). The primary tri- deviation of ñ5 mm/yr to the estimatedrelative
angulationdata for the northern sectionin Table plate motion, so that the final value we propose
2 extend only to Sierra Morena, which lies only is 32 ñ 5 mmfyr.
about 3 km .southwest of the San Andreas fault.
The motion of Tamalpais at the extremenorth
The stationapparently was displaced1.7 meters end of the network may be anomalous.The art,o the northwest. relative to Mocho-Mt. Diablo
rows in Figure 3 certainly suggestno net moveat the time of the 1906 San Franciscoearthquake ment of Tamalpais with respectto Mr. Diablo
[Hay)•ord a•d Baldwin, 1908]. This of course and Mocho.However,whenthe repeatedobserindicatesthat the sameamount of relative right vations are plotted as a function of time (Figlateral motion must have accumulated to the
ure 4) the uncertainty of the relative motion
west of the station in an unknown interval of
(--2 -- 16 mmfyr) is clearly demonstrated;
time preceding the earthquake of 1906. Thus indeed, the data in that figure indicate that
the primary triangulation data for the northern within the 95% confidencelimits the motion
sectionin Table 2 may underestimatethe rela- couldbe identicalwith that at Loma Prieta (30
tive plate motion by an amount of the order of mmfyr right lateral), a station that occupiesa
17 mm/yr (based on the 100-year recurrence comparablepositionrelativeto the fault system.
interval between great earthquakes). On the
There doesappear to be a pattern of motion
other hand, absenceof any significantshortening across both the San Andreas fault and the Calain geodimeterline 14 indicatesthat relative right veras-Hayward fault system in Figure 11. A
lateral motion is not accumulatingwest of Sierra right lateral motion of about 19 _ 2 mm/yr
Morena at present,and the supplementaltri- acrossthe Calaveras-Hayward system is sugangulationdata (last entry in Table 2), which gestedby the indicatedmovementsof Red Hill,
extend far seawardto Farallon L.H., suggestan American,and Morgan relative to Mr. Hamileven lower rate of relative motion across the
ton. Similarly a right lateral motion of about
San Andreas system than do the primary tri7 ñ 2 mm/yr acrossthe San Andreas fault is
angulation data.
suggestedby the indicatedmovementsof Nike,
Examination of Table 2 shows that, within Mindego, and Eagle Rock relative to Loma
the limits of error in measurement,all the esti- Prieta and Shelford. The motion of the stations
mates are consistentwith a relative right lateral onthe•southwest
sideof the Calaveras-Hayward
motion of about 32 mm/yr acrossthe San An- system(i.e., the line of stationsRed Hill, Amerdreas fault system.This agreement obtains de- ican, and Morgan) relative to those on the
spite the fact that in the southern section all n,ortheast side of the San Andreas fault (i..e.,
the accommodationappearsto be concentrated stationsLoma Prieta and Shelford) can be estiwithin I km of the fault trace, in the central mated only from the data for the single consection it appears to be concentrated on two necting geodimeterline, line 13. The indicated
faults (San Andreas and Calaveras) about 20 relative right lateral motion is 16 _ 5 mm/yr, a
km apart, and in the northernsectionit appears rate that is surprisingly large in view of the
t(• be spread over more than 40 km. It is true fact that line 13 doesnot crossone of the recogthat the agreementwouldbe more impressiveif nized throughgoingfaults. It is unfortunatethat
the standard deviations of the individual meano other suitably oriented geodimeter line
surementwere somewhatsmaller; nevertheless, crossesthe gap from the southwestside of the
the remarkablecoincidenceof so many different Calaveras-Hayward fault system to the northmeasurementsis compelling evidence for ac- east side of the San Andreas fault; it would be
SAVAGEAND BURFORD.'RELATIVE PLATE MOTION
of great interestto have an independentcheck
on the resultsindicatedby line 13.
The data in Figure 6 on the motion of Farallon L.H. are roughlyconsistent
with the picture
.outlinedabove.The relative right lateral movement of 8 ----+1 mm/yr from 'Tamalpais to
843
The data for lines 1, 2, and 3 (Figure 9) do
not appear to be consistentwith any simple
interpretation. The change in line i certainly
suggestsright lateral motion across the Calaveras fault, but the changein line 3 is inconsistentwith suchmotion. Line 2 appearsto be
Farallon L.H. is quite consistentwith the ob- shorteningat the rate of 5 mm/yr, whereas
served 7-mm/yr movement acrossthe San Ansimple right lateral motion modelswould sugdreasfault. The right lateral movement(21 _--+ gest that it shouldbe extendingat a very low
12 mm/yr) of Farallon L.H. relative to Sonoma rate. Perhaps an argument c.ould be advanced
Mountainis reasonably
well within the expected for omitting line 3 because of the scatter in
range For a relative movement across the San line length measurementsshown in Figure 8.
Andreas fault system of 32 ----+5 mm/yr. A
(However, we find it hard to seehow the trend
value somewhatlower than 32 mm/yr might could be reversed to a negative slope even by
be expectedfor the line Sonoma Mountain to rejecting data.) But even if line 3 is omitted, it
Farallon L.H. because it may not span the is difficult to explain the seeminglyinconsistent
completezone of the San Andreas fault system behavior of lines I and 2.
(Figure 5). Thus the data for the northern
The geodimeterdata in Figure 9 do not really
tier of stations (Mt. Diablo, Tamalpais,Sonoma fit either of the modelsof Figure 2. The rigidMountain, and Farallon L.H.) are consistent block model (lower sketchin Figure 2) doesnot
with our over-all interpretation.
apply becausethe measuredfault creep appears
The sites of fault creep observationsin the inadequateto explain the observedmotion and
also.becauseline 13 has lengthenedsignificantly
fault system in central California are shown in
Figure 1. Within the area shownin Figure 11, even though it does not cross a recognized
fault creep has been observedon the Calaveras throughgoingfault. The locked-faultmodel (upand Hayward faults but not on the San Andreas per sketchin Figure 2) apparentlydoesnot apfault. (An offsetof about 6 mm in an alignment ply becauselines 3, 4, and 14 would be expected
array establishedin 1968 on the San Andreas to shortenappreciably,whereasthey actually
fault near the crossingwith geodimeterline 15, lengthen,althoughby an insignificantamount.It
shownin Figure 9, has been observed,but it is is not completely clear that the discrepancies
not clear that this motion is a real measure of
are wholly due to the inadequacies of the
fault creep.It is sh.own in Figure I as question- m.odels; it is possiblethat the difficulty is due
able.) The observationsillustrated in Figure 1 to extraneoustrends in the data (e.g., incomsuggestthat about 10 mm/yr of fault creepoc- plete observationof fault creep,stationinstabilcurs alongthe Hayward-Calaverasfault system ity leading to erroneousvalues of dL/dT, or
in the area wherelines6, 7, 9, and 10 crossthe apparent trends in Figure 10 introduced by
system.The pattern of fault creepsuggests
that random error). We think someof the difficulty
slip is transferred from the Calaveras to the in interpreting the data may be associatedwith
Hayward near the point where the latter the branching of the Hayward fault from the
branchesoff from the former (i.e., just southof Calaveras fault in the vicinity of lines 9 and
the crossingwith line 9). Becausethe transfer 10. It is likely that strain may be distributed in
.of slip is not a simpleprocess,we are not at all a somewhatunusualmanner at such a Y juncsure that. the measurementsof fault creep re- tion.
ported here representall the deformationwithin
It is perhapsworth showingthat the geodimthe fault zone. We suspectthere may be con- eter data, in the San Francisco Bay area do
centrated deformation within the thin wedge not fit a pattern of uniform strain accumulathat separatesthe Calaverasand Hayward faults tion.Figure
12 shows
the• average
strainrate
in this area. However,the measuredvalues(10 (L -• dL/dt) as a function of azimuth for each
mm/yr) for fault creep in this area are some- of the geodimeterlines.'The continuouscurve
what less than the displacement across the representsthe uniform strain rate field, deterHayward-Calaveras system inferred from geo- mined by least squares,that best fits the obdimeter measurements(19 _ 2 mm/yr).
served data. The principal strain rates for the
844
SAVAC,
E AND BURFORD: RELATIVE PLATE MOTION
•
FIT
FOR
UNIFORM
STRAIN
5
sured near Ross Mountain, of the surface de-
NICEI• T
earthquakeof 1906 to calculate'the depth to
whichsuddenslip extendedat the time of the
earthquake.He useda screwdislocati.
on model
LEASTSQUARE
}
/
formation
•36 N
14
'
•Js•
•o'
,•s' %
qeo.
associated with
the San Francisco
that is just the complementof the one used in
(1): constantslip to a depth D and no slip
beneaththat depth.From the observed
deforma13
9
Fig. 12. Average strain rates for lines in the
Bay area geodimeternetwork as a function of
azimuth (measured clockwise from north). The
pointsare identifiedby line number,and the error
bars representone standard deviation on either
side of the point. If the strain field in the Bay
area were uniform, a sinusoidalcurve would fit
the points. The best uniform field fit as determinedby leastsquaresis shownby the heavyline,
and the principal axes of the best-fit strain field
are shown as an inset in the figure.
tion Chinnew estimatedD to be not more than
6 km and perhapsas smallas 2 km. Again,because of the oversimplifiedmodel, this depth
estimate must be underst.oodas representing
some intermediate depth within the zone of
transition from completeslip to no slip. Presumablythe regionthat slipsduring the earthquake is the regionthat did not slip continuously before the earthquake. Thus we would
expectthat Chinnery's
estimate
of D (2-6 km)
shbuldbe approximately
the sameas our own
estimate (9 km); this of course assumesthat
the regionof fault creepdoesnot progressively
approach closerto the surface until failure oc-
field are alsoshownin the figure. It is clear that
the field is not a reasonable fit to the observations.
Meade [1971] has discussedmeasurements
from 1906 (postearthquake)
to 1969on a small
triangulationfigure (8-km aperture) that spans
the San Andreas fault
near Ross Mountain
(Figure 5). The data indicate approximately
uniform right lateral shearstrain accumulation
at the rate of 0.55 --+ 0.05 • strain/yr. (The
value quoted is for the tensor shear, not the
engineeringshear,asusedby Meade; the former
is half of the latter.) There wasno indicationof
fault c•reepalongthe trace of the 1906 rupture,
which passesthrough the triangulation figure.
The locked-fault model (upper sketch Figure 2)
is presumedto apply to the San Andreasfault
in this region,and we can use equation I to obtain an estimate of the depth D below which
slip occurs.Using our estimateof relative plate
motion (b - 32 mm/yr) and Meade's measurement (0.55 • strain/yr) for the peak strain
(i.e., x = 0 in equation 1), we find D -- 9 km.
Becausethe simplescrewdislocationmodel (no
slip above depth D and constantslip below) is
grossly oversimplified,the value of D quoted
must be understoodas representingsomeintermediate depth within the zone •f transition
from completeslip to no slip.
Chinnery [1961] has usedthe values,mea-
curs.Chinnery'scomparativelylow value of D
wou|d indicatea proportionatelylower value of
b (e.g., 16 mm/yr); such a low value would
suggestthat part of the relative plate motion is
being carried by the northern continuationsof
the Hayward and Calaveras faults (Rodgers
Creek-Healdsburg
faults and GreenValley fault,
respectively).However,in view of the tremendous oversimplificationsinv.olved in the screw
dislocationmodels,it is possiblethat the differencebetweenthe two estimatesof D is not significant.
Acknowledgments. The 1968 geodimeter data
in Figure 10 were furnished through the courtesy
of the California Division of Mines and Geology;
the data for 1969-1971 were furnished through a
cooperative effort of the California Division of
Mines and Geology, the U.S. Geological Survey,
and the University of California, Berkeley
(through NSF grant GA 12205 to B. A. Bolt and
F. H. Mofiqtt). All the triangulation measurements were made by the U•S. Coast and Geodetic
Survey (now National Geodetic Survey).
Publication has been authorized by the Director, U.S. Geological Survey.
REFERENCES
Bolt, B. A., Cause of earthquakes, in Earthquake
Engineering, edited by R. L. Wiegel, pp. 21-45,
Prentice Hall, Englewood Cliffs, N.J., 1970.
Chinnery, M. A., Deformation of the ground
SAVAGEAND BURFORD: RELATIVE PLATE MOTION
around surface faults, Bull. Seismol. Soc. Amer.,
51, 355-372, 1961.
Hofmann, R. B., Geodimeter fault movement investigations in California, Calif. Dep. Water
Resour. Bull., no. 116, part 6, 183 pp., 1968.
Hayford, J. F., and A. L. Baldwin, Geodetic measurementsof earth movements, in The California
Earthquake o[ April 18, 1906, vol. 1, pp. 114-145,
Carnegie Institution of Washington, D.C., 1908.
Meade, B. K., Horizontal crustal movements in
the United States, 25 pp., U.S. Coast Geod.
Surv., Washington, D.C., 1963.
Meade, B. K., Horizontal movement along the
San Andreas fault system,Roy. $oc. N. Z. Bull.,
9, 175-179, 1971.
Meade, B. K., and J. B. Small, Current and recent
movement on the San Andreas fault, Calif.
Div. Mines Geol. Bull., 190, 385-391, 1966.
Nason, R. D., Investigations of fault creep slippage in Northern and Central California, Ph.D.
thesis, 231 pp., Univ. of Calif., San Diego, 1971.
Pope, A. J., J. L. Stearns, and C. A. Whitten,
Surveys for crustal movement along the Hayward fault, Bull. Seismol. $oc. Amer. 56, 317323, 1966.
845
Savage, J. C., and R. O. Burford, Strain accumulation in California, Bull. Seismol. $oc. Amer.,
60, 1877-1896,1970.
Savage, J. C., and R. O. Burford, Discussion of
paper by C. H. Scholz and T. J. Fitch, 'Strain
accumulation along the San Andreas fault,' J.
Geophys. Res., 76, 6469-6479, 1971.
Weertman, J., and J. R. Weertman, Elementary
Dislocation Theory, 213 pp., Macmillan, New
York, 1964.
Whitten, C. A., Horizontal earth movement, vicinity of San Francisco, California, Eos Trans.
AGU, 29, 318-323, 1948.
Whitten, C. A., Notes on remeasurement of triangulation net in the vicinity of San Francisco,
Calif., Calq. Div. Mines Geol. Spec. Rep. 57,
56-57, 1959.
Whitten, C. A., Horizontal movement in the
earth's crust, J. Geophys. Res., 65, 2839-2844,
1960.
(Received May 23, 1972;
revised October 4, 1972.)