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Episodic block motion and convergence along the Calaveras fault in Central California
Tectonophysics, 71 (1981) 87--94 Elsevier Scientific Publishing Company, Amsterdam -- Printed in the Netherlands
87
EPISODIC BLOCK MOTION AND CONVERGENCE ALONG THE CALAVERAS FAULT IN CENTRAL CALIFORNIA
L.E. SLATER
Cooperative Institute for Research in Environmental Sciences (CIRES), University o f Colorado at Boulder, Boulder, CO 80309 (U. S. A.) (Received July 1, 1980)
ABSTRACT Slater, L.E., 1981. Episodic block motion and convergence along the Calaveras fault in central California. In: P. Vysko~il, R. Green and H. M~'lzer (Editors), Recent Crustal Movements, 1979. Tectonophysics, 71: 87--94. A geodetic array covering approximately 100 k m : near Hollister, California has been monitored daily whenever possible since September 1975 using a multiwavelength distance-measuring (MWDM) instrument with a demonstrated precision of 1 part in 107. The MWDM array straddles the Calaveras fault, a portion of the transform fault system between the North American and Pacific tectonic plates. The deformation of the region typically occurs during episodes of significant movement, lasting several weeks, interspersed by periods of relative quiescence. An analysis of the MWDM data suggests that most of the observed crustal movements can be explained by rigid-block motion parallel to the strike of the Calaveras fault. However, there is a significant component of the motion that appears to be due to a convergence of the crustal blocks towards the Calaveras fault. This convergent trend, if it represents long-term crustal movement behavior, may be responsible for the small, elongated hills situated along the fault trace.
INTRODUCTION
Crustal deformation near HoUister, California has been monitored since September 1 9 7 5 using a multiwavelength distance-measuring (MWDM) instrument (Siater, 1975). Line length changes have been measured daily, whenever possible, within a geodetic array covering approximately 100 km 2 (Fig. 1). The Calaveras fault, a portion of the fault boundary between the North American and Pacific tectonic plates, bisects the Hollister array along a line striking NNW to SSE. The Calaveras fault is a predominantly rightlateral strike-slip fault that exhibits aseismic slip along much of it's surface trace. Three other active faults near the MWDM array are also shown in Fig. 1. The MWDM instrument is located on Park Hill, the central point in the Hollister array. The instrument site is approximately 150 m east of the active surface trace of the Calaveras fault. The array is composed of 9 pri-
0040-1951/81/0000--0000/$ 02.50 © 1981 Elsevier Scientific Publishing Company
88
q•4'•G,
~
r.
AFAIRv IEW
/
~IRA FOOTHL IL I 5km I
Fig. 1. Map of the MWDM array near Hollister, California showing the locations of the Calaveras, Busch, San Andreas, and Sargent faults. The MWDM instrument is located at the central station in the t o w n of Hollister.
mary lines 3--9 km in length that terminate at permanently installed retroreflectors located at varying distances from the Calaveras fault. The MWDM instrument is capable of making line length measurements to a precision of 1 part in 107 (precision is defined here as the ratio of the standard deviation of many repeated length measurements to their mean value) (Slater and Huggett, 1976). Figure 2 shows an example of the data collected during the first 1432 days of the MWDM survey. Each data point represents a daily mean value calculated from approximately 30 consecutive 10-second determinations of that specific line. These data clearly illustrate the significant variations in strain rate that occur within the array as first reported by Huggett et al. (1977). A detailed inspection of all the data from the entire
89 40 KNOB, 4.5 km A
50
Q~
3
,o
i ..J
I
I 1976
I 1977
I 1978
1979
Fig. 2. MWDM data collected on the line to Knob during the first 1432 days. The abrupt increase in line length identified by the letter C was associated with a large creep event that occurred near the MWDM instrument site, thereby causing the instrument to displace in a manner lengthening the line to Knob. The abrupt increase in line length identified by the letter E appears to be a coseisrnic response associated with a magnitude 5.9 earthquake approximately 25 km NNW o£ Hol]ister, California, on the Calaveras fault (Coyote Lake).
array shows that much of the deformation of the region occurs during episodes of significant movement, lasting several weeks, interspersed by periods of relative quiescence. Slater and Burford (1979) suggested that this episodic behavior corresponded to large-scale, essentially rigid-block motion associated with deep-seated aseismic movement on the fault extending througho u t much of the active seismic zone. THE
RIGID-BLOCK
HYPOTHESIS
The first tests of the rigid-block hypothesis utilized only the data collected during the first 817 days of the MWDM survey. During this initial 2 year period considerably less than average rainfall was recorded near Hollister; only one line (Hollair-Easy) exhibited significant, rainfall related, benchmark instability. The first rigid-block model assumed that the MWDM instrument site was located on the eastern block. All line length changes were considered to be caused by block motion parallel to the strike of the Calaveras fault (approximately N20°W locally). Such a simple approach worked reasonably well for all the fault-crossing lines extending o n t o the western block. Most of the observed minus calculated ( O - C) residuals were within measurement errors. The O -- C residuals indicated significant strain accumulation in the eastern block, however, where the model predicted no line-length changes. A second, slightly more complicated, rigid-block model was considered
90
A ()
A+C, i 31
A 0,2
& A+I5 0,0
/
144./
/4
L- j
MWDM SITE ~,,~3 5 A+0.3
3.(? [
,:,-0.3 Fig. 3. The observed m i n u s calculated MWDM survey. The finite t h i c k n e s s o f lines. The m o d e l m o v e m e n t (relative to by the heavy arrow and o f the MWDM mm/a.
_
5 km
AO.O
(O -- C) residuals for the first 8 1 7 days o f the the fault z o n e is indicated by the pair o f parallel the eastern b l o c k ) o f the western block is s h o w n instrument site by the light arrow. All units are
next. This model allowed the fault to be considered as a zone of finite thickness and located the MWDM instrument site within that zone. Both blocks were allowed to move in arbitrary directions relative to the MWDM site but no rotations were considered. The O -- C residuals of this model are shown in Fig. 3. With the exception of the +1.3 mm/a residual on the line to Hollair-Easy all other line residuals are within the measurement errors. Because the retro-reflector at Pereira appears to be located within a broad shear zone south of Hollister its O - C residual was not calculated. Figure 3 shows the relative movement of the MWDM instrument site and the western block relative to the eastern block. The best-fit model predicts 14.4 mm/a of right-lateral movement parallel to the Calaveras fault for the western block and 3.5 mm/a for the MWDM site relative to the eastern block. There also appears to be a significant c o m p o n e n t of movement normal to the trace of the Calaveras fault (3.1 mm/a and 3.0 mm/a for the western block and the MWDM site relative to the eastern block, respectively). The m o t i o n of Park Hill (the MWDM site} relative to the eastern block is similar in both direction and amplitude to results reported by Savage et al. (1979).
Fig. 4. T o p o g r a p h i c m a p o f Hollister area s h o w i n g e l o n g a t e d hills along the Calaveras and Sargent faults. Part o f the MWDM array is also s h o w n .
91 T
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MAJOR FAULTS IN REGION (DASHED WHERE NFERRED] CREEP METER SITES MWDM
BASE
Nm J
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~. . . . . . . r,~®
{,}'2
The convergent trend of the western and eastern blocks, if true and representing long-term crustal movement behavior, may offer a possible explanation for the occurrence of the small, elongated hills situated along the fault trace (see Fig. 4). R. Nason (pers. commun., 1976) suggested that these hills are constructional features rather than erosional remnants. If the MWDM instrument site is, in fact, located within the fault zone and free to exhibit arbitrary motion, the analysis of the MWDM data set is considerably more complicated and does not provide a unique interpretation. There is, however, a simple approach that removes any movement of the MWDM site if we assume rigid block behavior outside of the finite thickness of the fault zone. Inspection of Fig. 1 reveals that the lines to Knob and Foothill are nearly parallel and that the lines to Fairview and Poison are nearly parallel. If the line lengths of Knob and Foothill are added together and the line lengths of Fairview and Poison are added together we can effectively eliminate any movement of the MWDM site. The remaining 2 compos-
45
2 5 5 ~ 285
345~375 405 435 qf/a495~ { 4651r "~52! 555
58~!
\
\ \
645
~}45 825
855
1035~
°6,tio9;\
1005
\
1185'
1335 5mm
L___~
1305
1395
Fig. 5. Relative m o t i o n o f the western block calculated from the c o m p o s i t e lines, K n o b + F o o t h i l l and Fairview + Poison. The median day n u m b e r appropriate to the 30 day average is listed b y each data point. An o p e n circle indicates a period o f missing data w h e r e it has been assumed that adjacent data points define a linear trend during that time period. The straight dashed line indicates the assumed strike o f the Calaveras fault, N 2 0 ° W .
93
ire lines allow the determination of an arbitrary amount of slip and its direction between t w o rigid blocks. This approach was applied to the entire 1432 day data set. Because of a scarcity of data on some of the longer lines a 30 day average was formed on each of the 4 lines. The appropriate averages were then summed and the amount and direction of motion were calculated for that 30 day period. The results of that calculation are shown in Fig. 5. The straight dashed line represents the path that the western block would take relative to the eastern block if pure right-lateral strike-slip movement occurred along the Calaveras fault (assumed to strike N20°W). The segmented solid line represents the calculated path of the western block relative to the eastern block, the median day number appropriate to each 30 day average is listed by each data point. An open circle indicates a period of missing data where it has been assumed that adjacent data points define a linear trend during that time period. During the first 825 days the trend is quite similar to that presented in Fig. 3, right-lateral slip parallel to the Calaveras fault amounted to 14.1 mm/a and convergence normal to the fault amounted to 4.1 mm/a. The cause of the rapid divergent trend between day 825 and day 915 is uncertain b u t rainfall related effects can not be ruled out. Beginning on day 915 the convergent trend establishes itself again and continues for approximately one year. Movement during the first 1305 days suggests an average right-lateral slip parallel to the Calaveras fault amounting to 16.8 mm/a and convergence normal to the fault amounting to 3.1 mm/a. From day 1305 until day 1425 a significant divergent trend is again obvious in Fig. 5, unlike the earlier divergent trend, however, no substantial rainfall effects are likely to be responsible for this large change. A divergent trend could also be interpreted as a reduction in the normal stress across the fault and it is interesting to observe that 2 of the largest earthquakes to occur in the region during the MWDM project t o o k place during this interval; a magnitude 4.8 near San Jose on day 1325 and a magnitude 5.9 near C o y o t e L a k e on day 1416. Both earthquakes were located on the Calaveras fault north of Hollister. CONCLUSIONS
The preliminary analysis of the first 1432 days of data from the Hollister MWDM array suggests that a relatively simple rigid-block model adequately predicts much of the deformation observed in the region. The model requires that the fault zone be considered as having a finite thickness and that the relative motion of the two rigid blocks not be confined to only the direction of the Calaveras fault trace. During most of the first 1432 days of the MWDM survey the model indicated significant convergence of the blocks toward the Calaveras fault. If this convergent trend is real and represents longterm crustal movement behavior it offers a possible explanation for the small, elongated hills situated along the fault trace. The final 120 days of data considered in this paper suggest a substantial divergent trend which
94 c o i n c i d e d w i t h 2 of the largest e a r t h q u a k e s t o o c c u r in t h e region during the entire p e r i o d o f the MWDM survey. This d i v e r g e n t t r e n d m a y , p e r h a p s , indicate a p e r i o d o f t h e r e d u c t i o n o f t h e n o r m a l stress across t h e fault. It m u s t be a c k n o w l e d g e d , h o w e v e r , t h a t this p a r t i c u l a r m o d e l and interp r e t a t i o n is n o t u n i q u e ; it is m e r e l y t h e simplest. ACKNOWLEDGEMENTS I wish t o t h a n k t h e city o f Hollister, California a n d t h e m a n y p e o p l e t h e r e w h o kindly allowed c o n t i n u e d access to t h e i r p r o p e r t y during t h e entire p e r i o d o f the MWDM survey. O p e r a t i o n o f the MWDM s y s t e m was s u p p o r t e d b y t h e U.S. D e p a r t m e n t of t h e I n t e r i o r , G e o l o g i c a l Survey, u n d e r c o n t r a c t s 1 4 - 0 8 - 0 0 0 1 - 1 5 2 6 3 and 1 4 - 0 8 - 0 0 0 1 - 1 5 8 7 7 . REFERENCES Huggett, G.R., Slater, L.E., and Langbein, J., 1977. Fault slip episodes near Hollister, California: Initial results using a multiwavelength distance-measuring instrument. J. Geophys. Res., 82: 3361--3368. Savage, J.C., Prescott, W.H., Lisowski, M., and King, N., 1979. Geodolite measurements of deformation near Hollister, California, 1971--1978. J. Geophys. Res., 84: 7599-7615. Slater, L.E., 1975. A Multiwavelength Distance-Measuring Instrument for Geophysical Experiments. Ph.D. Thesis, Univ. of Washington, Seattle, Washington. Slater, L.E. and Burford, R.O., 1979. A comparison of long-baseline strain data and fault creep records obtained near Hollister, California. Tectonophysics, 52: 481--496. Slater, L.E. and Huggett, G.R., 1976. A multiwavelength distance-measuring instrument for geophysical experiments. J. Geophys. Res., 81 : 6299--6306.
87
EPISODIC BLOCK MOTION AND CONVERGENCE ALONG THE CALAVERAS FAULT IN CENTRAL CALIFORNIA
L.E. SLATER
Cooperative Institute for Research in Environmental Sciences (CIRES), University o f Colorado at Boulder, Boulder, CO 80309 (U. S. A.) (Received July 1, 1980)
ABSTRACT Slater, L.E., 1981. Episodic block motion and convergence along the Calaveras fault in central California. In: P. Vysko~il, R. Green and H. M~'lzer (Editors), Recent Crustal Movements, 1979. Tectonophysics, 71: 87--94. A geodetic array covering approximately 100 k m : near Hollister, California has been monitored daily whenever possible since September 1975 using a multiwavelength distance-measuring (MWDM) instrument with a demonstrated precision of 1 part in 107. The MWDM array straddles the Calaveras fault, a portion of the transform fault system between the North American and Pacific tectonic plates. The deformation of the region typically occurs during episodes of significant movement, lasting several weeks, interspersed by periods of relative quiescence. An analysis of the MWDM data suggests that most of the observed crustal movements can be explained by rigid-block motion parallel to the strike of the Calaveras fault. However, there is a significant component of the motion that appears to be due to a convergence of the crustal blocks towards the Calaveras fault. This convergent trend, if it represents long-term crustal movement behavior, may be responsible for the small, elongated hills situated along the fault trace.
INTRODUCTION
Crustal deformation near HoUister, California has been monitored since September 1 9 7 5 using a multiwavelength distance-measuring (MWDM) instrument (Siater, 1975). Line length changes have been measured daily, whenever possible, within a geodetic array covering approximately 100 km 2 (Fig. 1). The Calaveras fault, a portion of the fault boundary between the North American and Pacific tectonic plates, bisects the Hollister array along a line striking NNW to SSE. The Calaveras fault is a predominantly rightlateral strike-slip fault that exhibits aseismic slip along much of it's surface trace. Three other active faults near the MWDM array are also shown in Fig. 1. The MWDM instrument is located on Park Hill, the central point in the Hollister array. The instrument site is approximately 150 m east of the active surface trace of the Calaveras fault. The array is composed of 9 pri-
0040-1951/81/0000--0000/$ 02.50 © 1981 Elsevier Scientific Publishing Company
88
q•4'•G,
~
r.
AFAIRv IEW
/
~IRA FOOTHL IL I 5km I
Fig. 1. Map of the MWDM array near Hollister, California showing the locations of the Calaveras, Busch, San Andreas, and Sargent faults. The MWDM instrument is located at the central station in the t o w n of Hollister.
mary lines 3--9 km in length that terminate at permanently installed retroreflectors located at varying distances from the Calaveras fault. The MWDM instrument is capable of making line length measurements to a precision of 1 part in 107 (precision is defined here as the ratio of the standard deviation of many repeated length measurements to their mean value) (Slater and Huggett, 1976). Figure 2 shows an example of the data collected during the first 1432 days of the MWDM survey. Each data point represents a daily mean value calculated from approximately 30 consecutive 10-second determinations of that specific line. These data clearly illustrate the significant variations in strain rate that occur within the array as first reported by Huggett et al. (1977). A detailed inspection of all the data from the entire
89 40 KNOB, 4.5 km A
50
Q~
3
,o
i ..J
I
I 1976
I 1977
I 1978
1979
Fig. 2. MWDM data collected on the line to Knob during the first 1432 days. The abrupt increase in line length identified by the letter C was associated with a large creep event that occurred near the MWDM instrument site, thereby causing the instrument to displace in a manner lengthening the line to Knob. The abrupt increase in line length identified by the letter E appears to be a coseisrnic response associated with a magnitude 5.9 earthquake approximately 25 km NNW o£ Hol]ister, California, on the Calaveras fault (Coyote Lake).
array shows that much of the deformation of the region occurs during episodes of significant movement, lasting several weeks, interspersed by periods of relative quiescence. Slater and Burford (1979) suggested that this episodic behavior corresponded to large-scale, essentially rigid-block motion associated with deep-seated aseismic movement on the fault extending througho u t much of the active seismic zone. THE
RIGID-BLOCK
HYPOTHESIS
The first tests of the rigid-block hypothesis utilized only the data collected during the first 817 days of the MWDM survey. During this initial 2 year period considerably less than average rainfall was recorded near Hollister; only one line (Hollair-Easy) exhibited significant, rainfall related, benchmark instability. The first rigid-block model assumed that the MWDM instrument site was located on the eastern block. All line length changes were considered to be caused by block motion parallel to the strike of the Calaveras fault (approximately N20°W locally). Such a simple approach worked reasonably well for all the fault-crossing lines extending o n t o the western block. Most of the observed minus calculated ( O - C) residuals were within measurement errors. The O -- C residuals indicated significant strain accumulation in the eastern block, however, where the model predicted no line-length changes. A second, slightly more complicated, rigid-block model was considered
90
A ()
A+C, i 31
A 0,2
& A+I5 0,0
/
144./
/4
L- j
MWDM SITE ~,,~3 5 A+0.3
3.(? [
,:,-0.3 Fig. 3. The observed m i n u s calculated MWDM survey. The finite t h i c k n e s s o f lines. The m o d e l m o v e m e n t (relative to by the heavy arrow and o f the MWDM mm/a.
_
5 km
AO.O
(O -- C) residuals for the first 8 1 7 days o f the the fault z o n e is indicated by the pair o f parallel the eastern b l o c k ) o f the western block is s h o w n instrument site by the light arrow. All units are
next. This model allowed the fault to be considered as a zone of finite thickness and located the MWDM instrument site within that zone. Both blocks were allowed to move in arbitrary directions relative to the MWDM site but no rotations were considered. The O -- C residuals of this model are shown in Fig. 3. With the exception of the +1.3 mm/a residual on the line to Hollair-Easy all other line residuals are within the measurement errors. Because the retro-reflector at Pereira appears to be located within a broad shear zone south of Hollister its O - C residual was not calculated. Figure 3 shows the relative movement of the MWDM instrument site and the western block relative to the eastern block. The best-fit model predicts 14.4 mm/a of right-lateral movement parallel to the Calaveras fault for the western block and 3.5 mm/a for the MWDM site relative to the eastern block. There also appears to be a significant c o m p o n e n t of movement normal to the trace of the Calaveras fault (3.1 mm/a and 3.0 mm/a for the western block and the MWDM site relative to the eastern block, respectively). The m o t i o n of Park Hill (the MWDM site} relative to the eastern block is similar in both direction and amplitude to results reported by Savage et al. (1979).
Fig. 4. T o p o g r a p h i c m a p o f Hollister area s h o w i n g e l o n g a t e d hills along the Calaveras and Sargent faults. Part o f the MWDM array is also s h o w n .
91 T
•
I
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~
]' ] / a.
,~\ \
r
\.,
<
4 A;
% /
s i
L
A L
N
'
5"" ,
T
O
E
I
S
"~
'¼~..I
\\
S
~
"0
T
i,, _.
I • ,o
//
--
,~.
\"
,,-
,
/ LEGEND
~, \
MAJOR FAULTS IN REGION (DASHED WHERE NFERRED] CREEP METER SITES MWDM
BASE
Nm J
LINE
~. . . . . . . r,~®
{,}'2
The convergent trend of the western and eastern blocks, if true and representing long-term crustal movement behavior, may offer a possible explanation for the occurrence of the small, elongated hills situated along the fault trace (see Fig. 4). R. Nason (pers. commun., 1976) suggested that these hills are constructional features rather than erosional remnants. If the MWDM instrument site is, in fact, located within the fault zone and free to exhibit arbitrary motion, the analysis of the MWDM data set is considerably more complicated and does not provide a unique interpretation. There is, however, a simple approach that removes any movement of the MWDM site if we assume rigid block behavior outside of the finite thickness of the fault zone. Inspection of Fig. 1 reveals that the lines to Knob and Foothill are nearly parallel and that the lines to Fairview and Poison are nearly parallel. If the line lengths of Knob and Foothill are added together and the line lengths of Fairview and Poison are added together we can effectively eliminate any movement of the MWDM site. The remaining 2 compos-
45
2 5 5 ~ 285
345~375 405 435 qf/a495~ { 4651r "~52! 555
58~!
\
\ \
645
~}45 825
855
1035~
°6,tio9;\
1005
\
1185'
1335 5mm
L___~
1305
1395
Fig. 5. Relative m o t i o n o f the western block calculated from the c o m p o s i t e lines, K n o b + F o o t h i l l and Fairview + Poison. The median day n u m b e r appropriate to the 30 day average is listed b y each data point. An o p e n circle indicates a period o f missing data w h e r e it has been assumed that adjacent data points define a linear trend during that time period. The straight dashed line indicates the assumed strike o f the Calaveras fault, N 2 0 ° W .
93
ire lines allow the determination of an arbitrary amount of slip and its direction between t w o rigid blocks. This approach was applied to the entire 1432 day data set. Because of a scarcity of data on some of the longer lines a 30 day average was formed on each of the 4 lines. The appropriate averages were then summed and the amount and direction of motion were calculated for that 30 day period. The results of that calculation are shown in Fig. 5. The straight dashed line represents the path that the western block would take relative to the eastern block if pure right-lateral strike-slip movement occurred along the Calaveras fault (assumed to strike N20°W). The segmented solid line represents the calculated path of the western block relative to the eastern block, the median day number appropriate to each 30 day average is listed by each data point. An open circle indicates a period of missing data where it has been assumed that adjacent data points define a linear trend during that time period. During the first 825 days the trend is quite similar to that presented in Fig. 3, right-lateral slip parallel to the Calaveras fault amounted to 14.1 mm/a and convergence normal to the fault amounted to 4.1 mm/a. The cause of the rapid divergent trend between day 825 and day 915 is uncertain b u t rainfall related effects can not be ruled out. Beginning on day 915 the convergent trend establishes itself again and continues for approximately one year. Movement during the first 1305 days suggests an average right-lateral slip parallel to the Calaveras fault amounting to 16.8 mm/a and convergence normal to the fault amounting to 3.1 mm/a. From day 1305 until day 1425 a significant divergent trend is again obvious in Fig. 5, unlike the earlier divergent trend, however, no substantial rainfall effects are likely to be responsible for this large change. A divergent trend could also be interpreted as a reduction in the normal stress across the fault and it is interesting to observe that 2 of the largest earthquakes to occur in the region during the MWDM project t o o k place during this interval; a magnitude 4.8 near San Jose on day 1325 and a magnitude 5.9 near C o y o t e L a k e on day 1416. Both earthquakes were located on the Calaveras fault north of Hollister. CONCLUSIONS
The preliminary analysis of the first 1432 days of data from the Hollister MWDM array suggests that a relatively simple rigid-block model adequately predicts much of the deformation observed in the region. The model requires that the fault zone be considered as having a finite thickness and that the relative motion of the two rigid blocks not be confined to only the direction of the Calaveras fault trace. During most of the first 1432 days of the MWDM survey the model indicated significant convergence of the blocks toward the Calaveras fault. If this convergent trend is real and represents longterm crustal movement behavior it offers a possible explanation for the small, elongated hills situated along the fault trace. The final 120 days of data considered in this paper suggest a substantial divergent trend which
94 c o i n c i d e d w i t h 2 of the largest e a r t h q u a k e s t o o c c u r in t h e region during the entire p e r i o d o f the MWDM survey. This d i v e r g e n t t r e n d m a y , p e r h a p s , indicate a p e r i o d o f t h e r e d u c t i o n o f t h e n o r m a l stress across t h e fault. It m u s t be a c k n o w l e d g e d , h o w e v e r , t h a t this p a r t i c u l a r m o d e l and interp r e t a t i o n is n o t u n i q u e ; it is m e r e l y t h e simplest. ACKNOWLEDGEMENTS I wish t o t h a n k t h e city o f Hollister, California a n d t h e m a n y p e o p l e t h e r e w h o kindly allowed c o n t i n u e d access to t h e i r p r o p e r t y during t h e entire p e r i o d o f the MWDM survey. O p e r a t i o n o f the MWDM s y s t e m was s u p p o r t e d b y t h e U.S. D e p a r t m e n t of t h e I n t e r i o r , G e o l o g i c a l Survey, u n d e r c o n t r a c t s 1 4 - 0 8 - 0 0 0 1 - 1 5 2 6 3 and 1 4 - 0 8 - 0 0 0 1 - 1 5 8 7 7 . REFERENCES Huggett, G.R., Slater, L.E., and Langbein, J., 1977. Fault slip episodes near Hollister, California: Initial results using a multiwavelength distance-measuring instrument. J. Geophys. Res., 82: 3361--3368. Savage, J.C., Prescott, W.H., Lisowski, M., and King, N., 1979. Geodolite measurements of deformation near Hollister, California, 1971--1978. J. Geophys. Res., 84: 7599-7615. Slater, L.E., 1975. A Multiwavelength Distance-Measuring Instrument for Geophysical Experiments. Ph.D. Thesis, Univ. of Washington, Seattle, Washington. Slater, L.E. and Burford, R.O., 1979. A comparison of long-baseline strain data and fault creep records obtained near Hollister, California. Tectonophysics, 52: 481--496. Slater, L.E. and Huggett, G.R., 1976. A multiwavelength distance-measuring instrument for geophysical experiments. J. Geophys. Res., 81 : 6299--6306.