Fifteen strong-motion accelerographs, each with the capability of writing the WWVB absolute time code on the recorded accelerogram, have been deployed in an elliptical array, at a station spacing of several kilometers, along the San Andreas Fault in the Bear Valley region of central California. Ten accelerograms were obtained for the June 22, 1973, earthquake (M = 3.9), located near the center of the array. Preliminary analyses of these accelerograms support previous suggestions that the crystalline rocks of the Gabilan Range possess higher material velocities and lower intrinsic absorption than do the Cretaceous and Cenozoic sedimentary rocks northeast of the fault zone. These accelerograms clearly indicate that a strong-motion accelerograph array of this sort can provide the basic data for source mechanism, wave propagation, and local ground-motion studies for earthquakes with magnitudes as small as 3.5-4.0

Dielman, Richard J.

Hanks, Thomas C.

Trifunac, Mihailo D.

Caltech Authors - Main

Bulletin of the 
Seismological Society of America 
Vol. 65 February 1975 No. 1 
AN ARRAY OF STRONG-MOTION ACCELEROGRAPHS IN BEAR VALLEY, 
CALIFORNIA 
BY RICHARD J. DIELMAN, THOMAS C. HANKS, AND MIHAILO D. TRIFUNAC 
ABSTRACT 
Fifteen strong-motion aceelerographs, each with the capability of writing the 
WWVB absolute time code on the recorded accelerogram, have been deployed in 
an elliptical array, at a station spacing of several kilometers, along the San Andreas 
Fault in the Bear Valley region of central California. Ten accelerograms were 
obtained for the June 22, 1973, earthquake (M = 3.9), located near the center of 
the array. Preliminary analyses of these accelerograms support previous sugges- 
tions that the crystalline rocks of the Gabilan Range possess higher material 
velocities ana lower intrinsic absorption than do the Cretaceous and Cenozoic 
sedimentary rocks northeast of the fault zone. These accelerograms clearly 
indicate that a strong-motion accelerograph array of this sort can provide the basic 
data for source mechanism, wave propagation, and local ground-motion studies 
for earthquakes with magnitudes as small as 3.5-4.0. 
INTRODUCTION 
This paper describes a new type of strong-motion accelerograph array currently 
operating in the Bear Valley region of central California and presents, in preliminary 
form, some data obtained from it for the first year's operation. This array, operated by 
the California Institute of Technology, is one element of a coordinated effort, involving 
also the University of California, Berkeley, the University of Nevada, and the University 
of Washington, to determine precise amplitude-frequency characteristics of the elastic 
radiation generated by intermediate magnitude earthquakes in the frequency range 
0.025 to 25 Hz. 
The San Andreas Fault in central California, between Parkfield and San Juan Bautista, 
is currently a zone of high seismicity in the intermediate magnitude range (3 ~< M < 5) 
and appreciable fault creep localized in a narrow zone identified as the present fault trace 
on geomorphological grounds. Within this section of the San Andreas Fault, a 40-km 
segment, approximately between San 13enito and the Cienega Road Winery (Almaden) 
has produced 5 M > 5 earthquakes, 13 M > 4.5 earthquakes, and 45 M > 4.0 earth- 
quakes in the past 35 years (T. V. McEvilly and L. R. Johnson, unpublished data). The 
Bear Valley region is loosely defined as the southeastern half of this 40-km segment 
(Figure 1). 
1 
2 RICHARD J. DIELMAN, THOMAS C. HANKS, AND MIHAILO D. TRIFUNAC 
The Bear Valley strong-motion array consists of 15 SMA-1 strong-motion accelero- 
graphs, individually equipped with CS-60-P WWVB time code radio receivers. Instru- 
ment locations are shown in Figure 1 and station data are presented in Table 1. The 
array has been deployed symmetrically across the presently active fault trace which 
separates low material velocity, highly attenuating Cretaceous and Cenozoic sedimentary 
rocks northeast Of and including the fault zone from the higher material velocity, less 
attenuating metamorphic and volcanic rocks of the Gabilan Range southwest of the fault 
zone (Boore and Hill, 1973; Kurita, 1973). 
I 36 ° 4 5 '  
% 
o P o i c i n e s  
, \  % 
% 
% 
B E A R  V A L L E Y  
S T R O N G - M O T I O N  A R R A Y  
O 5 IO 
~ ~ I 
KILOMETERS 
AGCR 
AWKR 
•UBR 
SMR • ~'///. x x " ~ %  o 
WBB 
• ,~ %% 
JSR, . " ',,2,, 
San 
FIG. 1. Bear Valley s trong-motion accelerograph array. Triangles indicate accelerograph sites identified 
with three-letter code (Table 1). Earthquake locations: 1.011573 0943 M = 4.2, (a) 011573 1023 M = 
3.7, (b)011573 1024 M = 3.3, (c) 011573 1441 M = 3.7; 2. 062273 0129 M = 3.9 (Table 2). Stipled area 
marks exposed crystalline rocks at the Gabilan Range. 
STRONG-MOTION ACCELEROGRAPHS IN BEAR VALLEY, CALIFORNIA 3 
o 
o 
o 
.o 
o 
z 
o 
z 
el 
e , , l e - I  ~ ' ~ l ~ , l t ' , t c , , l ¢ - . l e , I  
~ ~ ~  
0 
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~:~ ~ . ~  
~-~ 
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0 
~ ' ~  
~ o ~  
0 
~z 
-u- 6 
4 RICHARD J. DIELMAN~ THOMAS C. HANKS~ AND MIHAILO D. TRIFUNAC 
INSTRUMENTATION 
The SMA-1 strong-motion accelerograph installed in the Bear Valley Array is entirely 
battery operated and utilizes a triaxial arrangement of accelerometers together with 
photo-optical recording. The accelerograph remains in a standby condition until a 
vertical starter senses a sufficient degree of ground motion (0.01 g, adjustable); it then 
activates a light source and film transport motor. The instrument reaches full operation 
50 to 100 msec after being activated by the starter and continues to record as long as a 
motion of 0.01 g or greater is detected by the starter plus an additional 10 sec (adjust- 
able). The accelerographs installed in Bear Valley have been modified to operate for a 
period greater than 60 sec in order for the WWVB time code to be received in its entirety. 
The 70-mm perforated photographic film is transported through a recording camera at a 
speed of 1 cm/sec. The 50-ft film supply enables the instrument to record approximately 
25 consecutive events or record continuously for approximately 25 rain. 
The radio receiver used in Bear Valley is a solid-state, battery operated, portable unit 
(Figure 2) which receives a 60-KHz carrier signal transmitted by the National Bureau of 
Standards station, WWVB, in Fort Collins, Colorado. The 60-KHz carrier frequency is 
heterodyned with a beat frequency oscillator to produce an intermediate frequency of 
2 KHz which is then applied to a code stripper, automatic gain control, and audio 
amplifier circuits. The sensitivity of the receiver is sufficient to receive the 60-KHz carrier 
FIG. 2. Typical accelerograph installation with WWVB radio receiver mounted on top of strong-motion 
accelerograph. 
signal at any point within the continental United States. The antenna used in con- 
junction with the radio receiver is a circular, wire-wound, loop antenna (Figure 3) tuned 
at 60 KHz. 
STRONG-MOTION ACCELEROGRAPHS IN BEAR VALLEY, CALIFORNIA 5 
All but three accelerographs of this array are located in government, state, or privately 
owned single-story structures, in conjunction with battery-float chargers when ac power is 
available. Three sites (HSR, BCN, and WMV) are fully external (Figure 3); the accelero- 
FIO. 3. External station of the array. The accelerograph and radio receiver are housed in the steel 
seismometer vault, with supporting pole for loop antenna and solar panels. 
graph is housed in a steel seismometer vault, and batteries are charged with solar energy. 
The instrumental cost, at the time of installation, of a typical station of this array was 
approximately $1,800 (accelerograph, radio receiver, and antenna). When external siting 
6 R I C H A R D  J .  D I E L M A N ,  T H O M A S  C .  H A N K S ,  A N D  M I H A I L O  D .  T R I F U N A C  
8 
O 
~V "V V V 
V (N ' ~ I ' . ~  ~I  ~ V , - ~ . ~  ~I  V 
,..~ O ~r~ ~ O O .,...~ O T M  
~r ,'~, .4 eq e,i ~: -i N ei --~ .~ N V 
~D 
o o 
¢¢3 ¢e~ f¢~ o'3 ¢r~ 
q3 ,q- ¢,4 
e )  
o 
r,,O 
O 
r ~  
,it 
STRONG-MOTION ACCELEROGRAPHS IN BEAR VALLEY~ CALIFORNIA 7 
was necessary, an additional $350 was required for the seismometer vault and solar 
panels. Recent modifications of the radio receiver unit have allowed it to be housed 
within the accelerograph, at a slight reduction of cost for the instrumental package. 
Accelerograph stations in Bear Valley have experienced severe climatic variations 
which to date have not affected their performance or reliability. During the winter months, 
temperatures are often as low as 0°F, and the higher elevations in the area usually experi- 
ence light snowfall between November and March; conversely, summer temperatures are 
consistently in the 90 ° to 105°F range. Due to the reliability experienced during the 1st 
year's operation of the array, it is apparent that servicing intervals of 6 months will not 
significantly affect the performance of the instrumentation and have consequently been 
adopted. 
EARTHQUAKE ACCELEROGRAMS AND PRELIMINARY RESULTS 
As of October, 1973, 26 individual accelerograms have been obtained from the Bear 
Valley strong-motion array, 24 of which were recorded with the identifying WWVB radio 
signal. Of these, 12 record the earthquake 011573 0943 M = 4.2 and six identifiable after- 
shocks at as many as four stations, and ten record the earthquake 062273 0129 M = 3.9. 
For those earthquakes for which two or more accelerograms were taken, Table 2 sum- 
marizes earthquake source data and trigger (T) time, shear-wave (S) arrival time, and 
maximum amplitudes in millimeters as read on the original 70-mm film. Maximum 
accelerations may be estimated by dividing these data by the operating sensitivity 
(approximately 1.9 cm/g) and a factor of 2 to obtain zero-to-peak amplitudes. In addition, 
Table 2 summarizes these data for the earthquake 011573 1024 M = 3.3 recorded only at 
HSR. 
The earthquake 011573 0943 M = 4.2 was recorded at HSR, with maximum accelera- 
tions of approximately 10 per cent g, and at BCN. Two aftershocks, 1023 M = 3.7 and 
1441 M = 3.7, were recorded at two or more stations of the array. The aftershock 1024 
M = 3.3 was recorded only at HSR, but this earthquake is significant because it was 
recorded within the run time of the accelerogram for 1023 M = 3.7. The epicenters of 
these earthquakes are shown in Figure 1. The accelerograms of 0943 M = 4.2 are dis- 
played in Figure 4a and the accelerograms for 1024 M = 3.3 and 1441 M = 3.7 are 
displayed in Figure 4b. An aftershock with magnitude as small as 3.0 independently 
triggered the accelerograph at HSR. 
Because 1024 M = 3.3 did not trigger the accelerograph at HSR independently, a true 
S - P  time (shear-wave arrival time minus P-wave arrival time) can be obtained for this 
earthquake. A comparison of this value with the S -  T times at HSR of the other earth- 
quakes of this sequence indicates that the starting mechanism of the accelerograph is 
successfully triggering the instrument within 1/10 sec or so of the actual first motion for 
this particular source-station pair, even for earthquakes with magnitudes as small as 3.3. 
Ten accelerograms were obtained for the earthquake 062273 0129 M = 3.9 (Figure l, 
Table 2). The ten accelerograms are reproduced in Figure 5a and b. Maximum accelera- 
tions recorded for this earthquake were also in the range of 10 per cent g. 
The five accelerograms of Figure 5a all triggered well in advance of the S-wave arrival, 
which is generally clear in these five accelerograms. The S-wave is sharpest at BCN and 
PNM, both of which are located in the crystalline rocks of the Gabilan Range, and is 
enriched in high-frequency (,~ 10Hz) amplitudes at these stations relative to W K R and 
UBR northeast of the fault zone. The frequency content at JSR, which is located in the 
fault zone, is intermediate to these two extremes. The accelerogram obtained at GCR 
(Figure 5b) almost certainly triggered in advance of the S-wave arrival time (compare 
8 RICHARD J. DIELAMN, THOMAS C. HANKS, AND MIHAILO D. TRIFUNAC 
51.7 5 2 . 9  
HSR , I 
I I 
0 1 1 5 7 3  I I 
I t 
I i 
I 0 9 4 3 ' 2 9 . 9  ' i 
N 4 0 E  
DOWN 
S 5 0 E  
33 .2  35.1 
B C N  , I N 4 0 E  
i ? ' ~  
I t 
0 1 1 5 7 3  I , DOWN 
- -  [, i , ,  
I I 
0943:29.91 M S 5 0 E  
- -  i '  i:" 
FIG. 4a. Accelerograms of 011573 0 9 4 3  M = 4.2. T and S times are estimated with the vertical dashed 
lines; seconds and tenths of seconds in the minute of the earthquake are indicated. 
. . . .  7 
4 9 . 8  51.1 2 4 . 2  2 5 . 5  
H S R  I , N 4 0 E  
i~ ~ . ' ,~ .~ , ,~  . . . . . . . . .  
' I 011575 I D O W N  I r . I i............ 
I I 
I 
1 0 2 4 : 4 8 . 0  1 4 4 1 : 2 2 . 5  , ' S 4 0 E  
2 5 . 4  2 7 . 4  
B C N  N 4 0 E  
O I 1 5 7 5  D O W N  
1 4 4 1 : 2 2 . 5  S 5 0 E  
Fro. 4b. Accelerograms of 011573 1024 M = 3.3 with P and S times and 1441 M ---- 3.7 with T and S 
times. 
STRONG-MOTION ACCELEROGRAPHS IN BEAR VALLEY, CALIFORNIA 9 
trigger time and source-station geometry of GCR to those of the five accelerograms and 
stations of Figure 5b), but no clear identification of the S wave is possible at GCR. All of 
these observations are in accord with previous suggestions that transmission paths 
through the rocks northeast of the fault zone are more highly attenuating than paths 
through the crystalline rocks to the southwest of it (Kurita, 1973). Spectral analysis of 
these wave forms, however, will be necessary to verify this qualitative agreement, 
inasmuch as the longer duration of the shear-wave coda at JSR, UBR, and WKR may in 
part mitigate the clear difference in high-frequency excitation in the first ~ see of the S 
wave between these stations and BCN and PNM. 
The distribution of trigger and S-wave arrival times at these five stations are consistent 
with the results of Boore and Hill (1973) that material velocities are significantly greater 
southwest of the fault zone than northeast of it. Table 3 summarizes travel-time residuals 
of computed P minus observed T and computed S minus observed S. As an hypothesis to 
be checked, we assume that the trigger time of the instrument will grossly correlate with 
the arrival of the P wave for these accelerograms, as the favorable comparison of the 
S - T  and S - P  times for the several events of the January 15 sequence at HSR has 
indicated. Residuals were computed with respect to the same velocity model used to 
obtain the USGS location (Table 2), and, of course, the trigger and S-wave arrival times 
reported here are independent of this location. 
In Table 3, negative residuals indicate that the observed trigger or S-wave arrival time 
is early with respect to the computed P-wave and S-wave arrival, respectively, and 
positive residuals indicate the contrary. At BCN and PNM, the instrument triggered on or 
before the computed P wave, and the observed S waves arrive more quickly than the 
computed S waves. Both sets of residuals indicate that the actual compressional and 
shear-wave velocities are greater than those of the assumed velocity structure. At WKR 
and UBR, the opposite is true indicating actual compressional- and shear-wave velocities 
lower than those of the assumed velocity structure. Residuals at JSR, located within the 
TABLE 3 
TRAVEL TIME RESIDUALS OF 062273 0129 M = 3.9 AT FIVE STATIONS 
Southwest Fault Zone Northeast 
BCN W K R  
T =  0.00, S = - 0 . 3 0  T = 1.76, S = 0.94 
P N M  JSR UBR 
T =  - 0 . 8 ,  S =  - 0 . 5 6  T =  0.21, S =  0.35 T =  0.73, S =  1.26 
fault zone, are intermediate to these two extremes, suggesting material velocities only 
slightly lower than those of the assumed velocity structure. For the four stations BCN, 
PNM, UBR, and JSR, the pattern of trigger time and S-wave residuals nominally 
supports the hypothesis that the trigger time is generally correlatable with the P-wave 
arrival time, but in the case of WKR, the trigger time is clearly late with respect to first 
motion. 
CONCLUSIONS 
The broadest significance of the earthquake accelerograms and preliminary results 
discussed in this paper lies simply in the fact that a large number of accelerograms of 
10 RICHARD J. DIELMAN, THOMAS C. HANKS, AND MIHAILO D. TRIFUNAC 
14.0 15,1 N40E BCN 1 I 
I I 
06227:5 JJ I DOWN 
I,t | ~ , 
0129:12.2 
I I 
L, 
I I 
S40E 
14.6 16.0 
P NM ! ~,:., ... N50W 
i I' 
I I 
06227:5 ~ J DOWN 
I 
, I 
0129:12.2' 'p N40E 
15.0 17.1 
JaR ,, ,,~ S40W 
I I 
06227:5 i . . . .  i DOWN 
I E 
I I 
I N50W 0 1 2 9 : 1 2 . 2  1 
I 
16.2 IZI 
WKR , 
,, ,, 
06227:5 ~ ~, 
I I 
0129'. 12.2 ~ = ~ v ~  
I I 
S40W 
DOWN 
N50W 
15.8 18.5 
UBR .~>m~--~ N40E  
062275 
0t29:12.2 
I 
I I DOWN 
I 
I 
S50E 
m 
FIG. 5a. Accelerograms of  062273 0129 M = 3.9 with T and S times. 
STRONG-MOTION ACCELEROGRAPHS IN BEAR VALLEY, CALIFORNIA 11 
15.8 
GCR N40E 
06227:5 
0129:12.2 
DOWN 
S50E 
15.5 8VF N40E  
I 
06227:5 i~ DOWN 
I 
0 1 2 9 : 1 2 . 2 1  S 5 0 E  
I 
16.1 
SMR I - ~  N40E 
I 
I 
062275 1 DOWN 
- - r  
[ 
O129:12.2 '~ S50E 
MLR 16.6 ~ . ~  s 4ow 
062275 I , DOWN 
't 
% . -  
I 
I 
I 
0129:12.2 
I 
N50W 
16.6 
HSR N40E 
062275 DOWN 
0129:12.211 ' S50E 
FIG. 5b. More accelcrograms of 062273 0129 M = 3.9 with only T times given. 
12 RICHARD J. DIELMAN, THOMAS C. HANKS, AND MIHAILO D. TRIFUNAC 
small magnitude earthquakes (3 ~< M < 4) have been obtained by a rugged, reliable, and 
very inexpensive network of strong-motion accelerographs. The strong-motion accelero- 
graph, of course, was not designed for this purpose, but the instrumental capability of  
writing WWVB time code directly on the accelerogram together with a dense spacing 
( ~  5 km) of instruments will evidently allow for precise amplitude-frequency studies of  
small magnitude earthquakes in the frequency range of approximately 1 to 20 Hz at a 
number of  recording sites. With appropriate modifications to accommodate local radio 
time codes, this strong-motion instrumentation system could be used throughout the 
world and would be particularly valuable in those regions where seismological and earth- 
quake engineering investigations are yet in an embryonic stage. The accelerograms 
obtained for the 062273 0129 M = 3.9 earthquake bear ample witness to the capability 
of  such a system to provide the basic data for source mechanism, wave propagation, and 
local ground-motion studies from earthquakes with magnitude as small as 3.5--4.0 over a 
relatively broad frequency range. Because of the rapid increase of  the frequency of earth- 
quake occurrence with decreasing magnitude, the potential scope o f  "strong-motion 
seismology" investigations has been considerably broadened. 
ACKNOWLEDGMENTS 
We have appreciated the cooperation and patience of the property owners and local residents who have 
generously provided'space for instrument installations: AGH, the Almaden Winery and Mrs. L. Dewitt; 
BCN, WMV, and MLR, Charlotte Berberick; BVF, Bear Valley Fire Station and Richard T. Gilbert; 
GCR, George H. Callens; HSR, Hannes Schroll; JSR, Mr. P. James; PNM, Pinnacles National Monu- 
ment and Pat Patterson; SCO, Stone Canyon Observatory and Mr. R. Thompson; SMR, Jeff H. Schmidt; 
UBR, Charley Strohn; WBR, Mrs. L. E. Webb; WKR, Mr. F. Wilkinson; WMR, Thomas H. Williams. 
W. E. Ellsworth of NCER, U.S. Geological Survey, kindly provided the earthquake locations and 
determined the station residuals. We also thank the Seismic Engineering Branch of the U.S. Geological 
Survey for allowing us to incorporate three of their existing accelerograph stations into the Bear Valley 
strong-motion array. 
This research was supported in part by the Advanced Research Project Agency of the Department of 
Defense and was monitored by the Air Force Office of Scientific Research under Contract F44620--72-C- 
0097 and by the Earthquake Research Affiliates Program of the California Institute of Technology. 
REFERENCES 
Boore, D. M. and D. P. Hill (1973). Wave propagation characteristics in the vicinity of the San Andreas 
Fault, in Proc. Conf. Tectonic Probl. San Andreas Fault System, R. L. Kovach and A. Nur, Editors, 
pp. 215-223, Stanford Univ. Publ., Univ. Ser., Geol. Sci., 13. 
Kurita, T. (1973). Attenuation of shear waves along the San Andreas Fault in central California, Tech- 
nical Report No. 2, AFOSR-72-2392, 2392A, U.C. Berkeley. 
EARTHQUAKE ENGINEERING RESEARCH LABORATORY 
CALIFORNIA INSTITUTE OF TECHNOLOGY 
PASADENA, CALIFORNIA 91109 
Manuscript received April 10, 1974. 


English

An array of strong-motion accelerographs in Bear Valley, California

Bulletin of the 
Seismological Society of America 
Vol. 65 February 1975 No. 1 
AN ARRAY OF STRONG-MOTION ACCELEROGRAPHS IN BEAR VALLEY, 
CALIFORNIA 
BY RICHARD J. DIELMAN, THOMAS C. HANKS, AND MIHAILO D. TRIFUNAC 
ABSTRACT 
Fifteen strong-motion aceelerographs, each with the capability of writing the 
WWVB absolute time code on the recorded accelerogram, have been deployed in 
an elliptical array, at a station spacing of several kilometers, along the San Andreas 
Fault in the Bear Valley region of central California. Ten accelerograms were 
obtained for the June 22, 1973, earthquake (M = 3.9), located near the center of 
the array. Preliminary analyses of these accelerograms support previous sugges- 
tions that the crystalline rocks of the Gabilan Range possess higher material 
velocities ana lower intrinsic absorption than do the Cretaceous and Cenozoic 
sedimentary rocks northeast of the fault zone. These accelerograms clearly 
indicate that a strong-motion accelerograph array of this sort can provide the basic 
data for source mechanism, wave propagation, and local ground-motion studies 
for earthquakes with magnitudes as small as 3.5-4.0. 
INTRODUCTION 
This paper describes a new type of strong-motion accelerograph array currently 
operating in the Bear Valley region of central California and presents, in preliminary 
form, some data obtained from it for the first year's operation. This array, operated by 
the California Institute of Technology, is one element of a coordinated effort, involving 
also the University of California, Berkeley, the University of Nevada, and the University 
of Washington, to determine precise amplitude-frequency characteristics of the elastic 
radiation generated by intermediate magnitude arthquakes in the frequency range 
0.025 to 25 Hz. 
The San Andreas Fault in central California, between Parkfield and San Juan Bautista, 
is currently a zone of high seismicity in the intermediate magnitude range (3 ~< M < 5) 
and appreciable fault creep localized in a narrow zone identified as the present fault trace 
on geomorphological grounds. Within this section of the San Andreas Fault, a 40-km 
segment, approximately between San 13enito and the Cienega Road Winery (Almaden) 
has produced 5 M > 5 earthquakes, 13 M > 4.5 earthquakes, and 45 M > 4.0 earth- 
quakes in the past 35 years (T. V. McEvilly and L. R. Johnson, unpublished data). The 
Bear Valley region is loosely defined as the southeastern half of this 40-km segment 
(Figure 1). 
1 
2 RICHARD J. DIELMAN, THOMAS C. HANKS, AND MIHAILO D. TRIFUNAC 
The Bear Valley strong-motion array consists of 15 SMA-1 strong-motion accelero- 
graphs, individually equipped with CS-60-P WWVB time code radio receivers. Instru- 
ment locations are shown in Figure 1 and station data are presented in Table 1. The 
array has been deployed symmetrically across the presently active fault trace which 
separates low material velocity, highly attenuating Cretaceous and Cenozoic sedimentary 
rocks northeast Of and including the fault zone from the higher material velocity, less 
attenuating metamorphic and volcanic rocks of the Gabilan Range southwest of the fault 
zone (Boore and Hill, 1973; Kurita, 1973). 
I 36 ° 45 '  
% 
o Po ic ines  
, \  % 
% 
% 
BEAR VALLEY  
STRONG-MOTION ARRAY 
O 5 IO 
~ ~ I 
KILOMETERS 
AGCR 
AWKR 
•UBR 
SMR • ~'///. xx"~% o 
WBB 
• ,~ %% 
JSR, . " ',,2,, 
San 
FIG. 1. Bear Valley strong-motion accelerograph array. Triangles indicate accelerograph sites identified 
with three-letter code (Table 1). Earthquake locations: 1.011573 0943 M = 4.2, (a) 011573 1023 M = 
3.7, (b)011573 1024 M = 3.3, (c) 011573 1441 M = 3.7; 2. 062273 0129 M = 3.9 (Table 2). Stipled area 
marks exposed crystalline rocks at the Gabilan Range. 
STRONG-MOTION ACCELEROGRAPHS IN BEAR VALLEY, CALIFORNIA 3 
o 
o 
o 
.o 
o 
z 
o 
z 
el 
e , , le - I  ~ '~ l~, l t ' , t c , , l¢ - . le , I  
~ ~ ~  
0 
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~:~ .~ 
~-~ 
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0 
~'~ 
~o~ 
0 
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-u-  6 
4 RICHARD J. DIELMAN~ THOMAS C. HANKS~ AND MIHAILO D. TRIFUNAC 
INSTRUMENTATION 
The SMA-1 strong-motion accelerograph installed in the Bear Valley Array is entirely 
battery operated and utilizes a triaxial arrangement of accelerometers together with 
photo-optical recording. The accelerograph remains in a standby condition until a 
vertical starter senses a sufficient degree of ground motion (0.01 g, adjustable); it then 
activates a light source and film transport motor. The instrument reaches full operation 
50 to 100 msec after being activated by the starter and continues to record as long as a 
motion of 0.01 g or greater is detected by the starter plus an additional 10 sec (adjust- 
able). The accelerographs installed in Bear Valley have been modified to operate for a 
period greater than 60 sec in order for the WWVB time code to be received in its entirety. 
The 70-mm perforated photographic film is transported through a recording camera t a 
speed of 1 cm/sec. The 50-ft film supply enables the instrument to record approximately 
25 consecutive events or record continuously for approximately 25rain. 
The radio receiver used in Bear Valley is a solid-state, battery operated, portable unit 
(Figure 2) which receives a 60-KHz carrier signal transmitted by the National Bureau of 
Standards station, WWVB, in Fort Collins, Colorado. The 60-KHz carrier frequency is
heterodyned with a beat frequency oscillator to produce an intermediate frequency of 
2 KHz which is then applied to a code stripper, automatic gain control, and audio 
amplifier circuits. The sensitivity of the receiver is sufficient to receive the 60-KHz carrier 
FIG. 2. Typical accelerograph installation with WWVB radio receiver mounted on top of strong-motion 
accelerograph. 
signal at any point within the continental United States. The antenna used in con- 
junction with the radio receiver is a circular, wi e-wound, loop antenna (Figure 3) tuned 
at 60 KHz. 
STRONG-MOTION ACCELEROGRAPHS IN BEAR VALLEY, CALIFORNIA 5 
All but three accelerographs of this array are located in government, state, or privately 
owned single-story structures, inconjunction with battery-float chargers when ac power is 
available. Three sites (HSR, BCN, and WMV) are fully external (Figure 3); the accelero- 
FIO. 3. External station of the array. The accelerograph and radio receiver are housed in the steel 
seismometer vault, with supporting pole for loop antenna nd solar panels. 
graph is housed in a steel seismometer vault, and batteries are charged with solar energy. 
The instrumental cost, at the time of installation, of a typical station of this array was 
approximately $1,800 (accelerograph, radio receiver, and antenna). When external siting 
6 RICHARD J .  D IELMAN,  THOMAS C .  HANKS,  AND MIHAILO D.  TR IFUNAC 
8 
O 
~V "V  V V 
V (N '~ I ' .~ ~I ~ V , -~ .~ ~I V 
,..~ O ~r~ ~ O O .,...~ O TM 
~r ,'~, .4 eq e,i ~: -i N ei --~ .~ N V 
~D 
o o 
¢¢3 ¢e~ f¢~ o'3 ¢r~ 
q3 ,q- ¢,4 
e) 
o 
r,,O 
O 
r~ 
,it 
STRONG-MOTION ACCELEROGRAPHS IN BEAR VALLEY~ CALIFORNIA 7 
was necessary, an additional $350 was required for the seismometer vault and solar 
panels. Recent modifications of the radio receiver unit have allowed it to be housed 
within the accelerograph, at a slight reduction of cost for the instrumental package. 
Accelerograph stations in Bear Valley have experienced severe climatic variations 
which to date have not affected their performance or reliability. During the winter months, 
temperatures are often as low as 0°F, and the higher elevations in the area usually experi- 
ence light snowfall between November and March; conversely, summer temperatures are 
consistently in the 90 ° to 105°F range. Due to the reliability experienced during the 1st 
year's operation of the array, it is apparent that servicing intervals of 6 months will not 
significantly affect the performance of the instrumentation a d have consequently been 
adopted. 
EARTHQUAKE ACCELEROGRAMS AND PRELIMINARY RESULTS 
As of October, 1973, 26 individual accelerograms have been obtained from the Bear 
Valley strong-motion array, 24 of which were recorded with the identifying WWVB radio 
signal. Of these, 12 record the earthquake 011573 0943 M = 4.2 and six identifiable after- 
shocks at as many as four stations, and ten record the earthquake 062273 0129 M = 3.9. 
For those earthquakes for which two or more accelerograms were taken, Table 2 sum- 
marizes earthquake source data and trigger (T) time, shear-wave (S) arrival time, and 
maximum amplitudes in millimeters as read on the original 70-mm film. Maximum 
accelerations may be estimated by dividing these data by the operating sensitivity 
(approximately 1.9 cm/g) and a factor of 2 to obtain zero-to-peak amplitudes. In addition, 
Table 2 summarizes these data for the earthquake 011573 1024 M = 3.3 recorded only at 
HSR. 
The earthquake 011573 0943 M = 4.2 was recorded at HSR, with maximum accelera- 
tions of approximately 10 per cent g, and at BCN. Two aftershocks, 1023 M = 3.7 and 
1441 M = 3.7, were recorded at two or more stations of the array. The aftershock 1024 
M = 3.3 was recorded only at HSR, but this earthquake is significant because it was 
recorded within the run time of the accelerogram for 1023 M = 3.7. The epicenters of 
these earthquakes are shown in Figure 1. The accelerograms of 0943 M = 4.2 are dis- 
played in Figure 4a and the accelerograms for 1024 M = 3.3 and 1441 M = 3.7 are 
displayed in Figure 4b. An aftershock with magnitude as small as 3.0 independently 
triggered the accelerograph at HSR. 
Because 1024 M = 3.3 did not trigger the accelerograph at HSR independently, a true 
S-P  time (shear-wave arrival time minus P-wave arrival time) can be obtained for this 
earthquake. A comparison of this value with the S -  T times at HSR of the other earth- 
quakes of this sequence indicates that the starting mechanism of the accelerograph is 
successfully triggering the instrument within 1/10 sec or so of the actual first motion for 
this particular source-station pair, even for earthquakes with magnitudes as small as 3.3. 
Ten accelerograms were obtained for the earthquake 062273 0129 M = 3.9 (Figure l, 
Table 2). The ten accelerograms are reproduced in Figure 5a and b. Maximum accelera- 
tions recorded for this earthquake were also in the range of 10 per cent g. 
The five accelerograms of Figure 5a all triggered well in advance of the S-wave arrival, 
which is generally clear in these five accelerograms. The S-wave is sharpest at BCN and 
PNM, both of which are located in the crystalline rocks of the Gabilan Range, and is 
enriched in high-frequency (,~ 10Hz) amplitudes at these stations relative to WKR and 
UBR northeast of the fault zone. The frequency content at JSR, which is located in the 
fault zone, is intermediate to these two extremes. The accelerogram obtained at GCR 
(Figure 5b) almost certainly triggered in advance of the S-wave arrival time (compare 
8 RICHARD J. DIELAMN, THOMAS C. HANKS, AND MIHAILO D. TRIFUNAC 
51.7 52 .9  
HSR , I 
I I 
011573 I I 
I t 
I i 
I 0943 '29 .9  ' i 
N40E 
DOWN 
S50E 
33.2  35.1 
BCN , I N40E 
i ? '~  
I t 
011573 I , DOWN 
- -  [, i , ,  
I I 
0943:29.91 M S50E 
- -  i '  i:" 
FIG. 4a. Accelerograms of 011573 0943 M = 4.2. T and S times are estimated with the vertical dashed 
lines; seconds and tenths of seconds in the minute of the earthquake are indicated. 
. . . .  7 
49.8  51.1 24 .2  25 .5  
HSR I , N40E 
i~ ~. ' ,~.~,,~ . . . . . . . . .  
' I 011575 I DOWN I r . I i............ 
I I 
I 
1024:48 .0  1441:22 .5  , ' S40E  
25 .4  27 .4  
BCN N40E 
O I 1575  DOWN 
1441:22 .5  S50E 
Fro. 4b. Accelerograms of 011573 1024 M = 3.3 with P and S times and 1441 M ---- 3.7 with T and S 
times. 
STRONG-MOTION ACCELEROGRAPHS IN BEAR VALLEY, CALIFORNIA 9 
trigger time and source-station geometry of GCR to those of the five accelerograms and 
stations of Figure 5b), but no clear identification ofthe S wave is possible at GCR. All of 
these observations are in accord with previous suggestions that transmission paths 
through the rocks northeast of the fault zone are more highly attenuating than paths 
through the crystalline rocks to the southwest of it (Kurita, 1973). Spectral analysis of 
these wave forms, however, will be necessary to verify this qualitative agreement, 
inasmuch as the longer duration of the shear-wave coda at JSR, UBR, and WKR may in 
part mitigate the clear difference in high-frequency excitation in the first ~ see of the S 
wave between these stations and BCN and PNM. 
The distribution of trigger and S-wave arrival times at these five stations are consistent 
with the results of Boore and Hill (1973) that material velocities are significantly greater 
southwest of the fault zone than northeast of it. Table 3 summarizes travel-time r siduals 
of computed P minus observed T and computed S minus observed S. As an hypothesis to 
be checked, we assume that the trigger time of the instrument will grossly correlate with 
the arrival of the P wave for these accelerograms, as the favorable comparison of the 
S-T  and S-P  times for the several events of the January 15 sequence at HSR has 
indicated. Residuals were computed with respect o the same velocity model used to 
obtain the USGS location (Table 2), and, of course, the trigger and S-wave arrival times 
reported here are independent of this location. 
In Table 3, negative residuals indicate that the observed trigger or S-wave arrival time 
is early with respect o the computed P-wave and S-wave arrival, respectively, and 
positive residuals indicate the contrary. At BCN and PNM, the instrument triggered on or 
before the computed P wave, and the observed S waves arrive more quickly than the 
computed S waves. Both sets of residuals indicate that the actual compressional nd 
shear-wave velocities are greater than those of the assumed velocity structure. At WKR 
and UBR, the opposite is true indicating actual compressional- and shear-wave velocities 
lower than those of the assumed velocity structure. Residuals at JSR, located within the 
TABLE 3 
TRAVEL TIME RESIDUALS OF 062273 0129 M = 3.9 AT FIVE STATIONS 
Southwest Fault Zone Northeast 
BCN WKR 
T= 0.00, S = -0 .30  T = 1.76, S = 0.94 
PNM JSR UBR 
T= -0 .8 ,  S= -0 .56  T= 0.21, S= 0.35 T= 0.73, S= 1.26 
fault zone, are intermediate o these two extremes, suggesting material velocities only 
slightly lower than those of the assumed velocity structure. For the four stations BCN, 
PNM, UBR, and JSR, the pattern of trigger time and S-wave residuals nominally 
supports the hypothesis that the trigger time is generally correlatable with the P-wave 
arrival time, but in the case of WKR, the trigger time is clearly late with respect o first 
motion. 
CONCLUSIONS 
The broadest significance of the earthquake accelerograms and preliminary results 
discussed in this paper lies simply in the fact that a large number of accelerograms of 
10 RICHARD J. DIELMAN, THOMAS C. HANKS, AND MIHAILO D. TRIFUNAC 
14.0 15,1 N40E BCN 1 I 
I I 
06227:5 J I DOWN 
I,t | ~ , 
0129:12.2 
I I 
L, 
I I 
S40E 
14.6 16.0 
P NM ! ~,:., ... N50W 
i I' 
I I 
06227:5 ~ J DOWN 
I 
, I 
0129:12.2' 'p N40E 
15.0 17.1 
JaR ,, ,,~ S40W 
I I 
06227:5 i . . . .  i DOWN 
I E 
I I 
I N50W 0129:12 .2  1 
I 
16.2 IZI 
WKR , 
,, ,, 
06227:5 ~ ~, 
I I 
0129'. 12.2 ~=~v~ 
I I 
S40W 
DOWN 
N50W 
15.8 18.5 
UBR .~>m~--~ N40E 
062275 
0t29:12.2 
I 
I I DOWN 
I 
I 
S50E 
m 
FIG. 5a. Accelerograms of 062273 0129 M = 3.9 with T and S times. 
STRONG-MOTION ACCELEROGRAPHS IN BEAR VALLEY, CALIFORNIA 11 
15.8 
GCR N40E 
06227:5 
0129:12.2 
DOWN 
S50E 
15.5 8VF N40E 
I 
06227:5 i~ DOWN 
I 
0129:12.21 S50E 
I 
16.1 
SMR I -~  N40E 
I 
I 
062275 1 DOWN 
- - r  
[ 
O129:12.2 '~ S50E 
MLR 16.6 ~.~ s 4ow 
062275 I , DOWN 
't 
%. -  
I 
I 
I 
0129:12.2 
I 
N50W 
16.6 
HSR N40E 
062275 DOWN 
0129:12.211 ' S50E 
FIG. 5b. More accelcrograms of 062273 0129 M = 3.9 with only T times given. 
12 RICHARD J. DIELMAN, THOMAS C. HANKS, AND MIHAILO D. TRIFUNAC 
small magnitude arthquakes (3 ~< M < 4) have been obtained by a rugged, reliable, and 
very inexpensive network of strong-motion accelerographs. The strong-motion accelero- 
graph, of course, was not designed for this purpose, but the instrumental capability of 
writing WWVB time code directly on the accelerogram together with a dense spacing 
(~ 5 km) of instruments will evidently allow for precise amplitude-frequency studies of 
small magnitude arthquakes in the frequency range of approximately 1 to 20 Hz at a 
number of recording sites. With appropriate modifications to accommodate local radio 
time codes, this strong-motion i strumentation system could be used throughout he 
world and would be particularly valuable in those regions where seismological nd earth- 
quake engineering investigations are yet in an embryonic stage. The accelerograms 
obtained for the 062273 0129 M = 3.9 earthquake bear ample witness to the capability 
of such a system to provide the basic data for source mechanism, wave propagation, and 
local ground-motion studies from earthquakes with magnitude as small as 3.5--4.0 over a 
relatively broad frequency range. Because of the rapid increase of the frequency of earth- 
quake occurrence with decreasing magnitude, the potential scope of  "strong-motion 
seismology" investigations has been considerably broadened. 
ACKNOWLEDGMENTS 
We have appreciated the cooperation and patience of the property owners and local residents who have 
generously provided'space for instrument installations: AGH, the Almaden Winery and Mrs. L. Dewitt; 
BCN, WMV, and MLR, Charlotte Berberick; BVF, Bear Valley Fire Station and Richard T. Gilbert; 
GCR, George H. Callens; HSR, Hannes Schroll; JSR, Mr. P. James; PNM, Pinnacles National Monu- 
ment and Pat Patterson; SCO, Stone Canyon Observatory and Mr. R. Thompson; SMR, Jeff H. Schmidt; 
UBR, Charley Strohn; WBR, Mrs. L. E. Webb; WKR, Mr. F. Wilkinson; WMR, Thomas H. Williams. 
W. E. Ellsworth of NCER, U.S. Geological Survey, kindly provided the earthquake locations and 
determined the station residuals. We also thank the Seismic Engineering Branch of the U.S. Geological 
Survey for allowing us to incorporate three of their existing accelerograph stations into the Bear Valley 
strong-motion array. 
This research was supported in part by the Advanced Research Project Agency of the Department of 
Defense and was monitored by the Air Force Office of Scientific Research under Contract F44620--72-C- 
0097 and by the Earthquake Research Affiliates Program of the California Institute of Technology. 
REFERENCES 
Boore, D. M. and D. P. Hill (1973). Wave propagation characteristics n the vicinity of the San Andreas 
Fault, in Proc. Conf. Tectonic Probl. San Andreas Fault System, R. L. Kovach and A. Nur, Editors, 
pp. 215-223, Stanford Univ. Publ., Univ. Ser., Geol. Sci., 13. 
Kurita, T. (1973). Attenuation of shear waves along the San Andreas Fault in central California, Tech- 
nical Report No. 2, AFOSR-72-2392, 2392A, U.C. Berkeley. 
EARTHQUAKE ENGINEERING RESEARCH LABORATORY 
CALIFORNIA INSTITUTE OF TECHNOLOGY 
PASADENA, CALIFORNIA 91109 
Manuscript received April 10, 1974. 


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An array of strong-motion accelerographs in Bear Valley, California

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