The largest earthquake to hit California in more than 15 years occurred at

02:28:58.9 GCT on 9 April 1968 near Borrego Mountain, on the western edge of

the Imperial Valley. The Seismological Laboratory at Pasadena has tentatively

assigned the shock a magnitude of 6.5, an epicentral location of 33 ° 08.8' N, 116 °

07.5' W, and a focal depth of 20 km. The earthquake was felt throughout most

of southern California and adjacent areas, but the absence of severe damage and

casualties was in large part due to the relatively undeveloped nature of the epicentral

region. Indeed, it would have been difficult to pick a location in the southernmost

part of the State more remote from centers of population

Allen, C. R.

Brune, J. N.

Clark, M. M.

Grantz, A.

Sharp, R. V.

Theodore, T. G.

Wolfe, E. W.

Wyss, M.

Caltech Authors - Main

Bulletin of the Seismological Society of America Vol. 58, No. 3r pp. 1183-1186. June, 1968 
THE BORREGO MOUNTAIN, CALIFORNIA, EARTHQUAKE OF 
9 APRIL 1968: A PRELIMINARY REPORT 1 
[BY C. R. AJ.I~, A. GRANTZ, J. N. ]3RUNE, M. M. CLARK, R. V. SHARP, T. G. 
THEODORE, E. W. WOLFE, AND M. WYss 
The largest earthquake to hit California in more than 15 years occurred at 
02:28:58.9 GCT on 9 April 1968 near Borrego Mountain, on the western edge of 
the Imperial Valley. The Seismological Laboratory at Pasadena has tentatively 
assigned the shock a magnitude of 6.5, an epieentral location of 33 ° 08.8' N, 116 ° 
07.5' W, and a focal depth of 20 km. The earthquake was felt throughout most 
of southern California and adjacent areas, but the absence of severe damage and 
casualties was in large part due to the relatively undeveloped nature of the epi- 
central region. Indeed, it would have been dittieult o pick a location in the south- 
ernmost part of the State more remote from centers of population. 
Although the earthquake occurred in the evening (6:29 p.m., local time), rapid 
computer location of the epicenter using the California Institute of Technology 
telemetered seismograph network allowed placement of the first portable seismo- 
graph unit within 3 km of the epicenter soon after midnight, and the fault trace 
was identified and the first displacement measurements made by 2:00 a.m. De- 
tailed mapping of the fault trace by U. S. Geological Survey and Calteeh personnel 
commenced the next day, which was fortunate in view of the fact that within four 
days much of the fault trace had already been obscured by blowing sand and 
dune buggy tracks. By April 12th, 14 portable seismograph units were in operation 
throughout the aRershock region. 
The earthquake was accompanied by right-lateral displacement along a 33-kin 
segment of the Coyote Creek fault (Figure 1)--a major branch of the San Jacinto 
fault zone, which in turn is the most active member of the San Andreas fault system 
in southern California. Earthquakes of comparable magnitude had occurred earlier 
along the San Jaeinto fault zone just to the north of this area in 1954 and just to the 
south in 1942, but no known major historic activity had occurred along the exact 
segment broken on April 9th. The Coyote Creek fault in this region has been 
mapped and its regional setting described by Dibblee (1954) and Sharp (1967). 
The fault break consists of two northwest-trending en echelon segments that 
overlap and are separated by a 2 km wide zone of highly fractured terrane just 
southeast of the settlement ofOcotillo Wells. The maximum displacement observed 
was along the northern segment, 5 km northwest of Ocotillo Wells, where 38 em 
of right-lateral slip was indicated by offset channels and vehicle tracks and by 
displaced slabs of dried mud crust. The maximum horizontal slip observed along 
the southern segment, 20 em, was noted 10 km southeast ofOeotillo Wells. Vertical 
displacements were nil in most areas where the fault cut level ground, but wherever 
the break lay along a pre-existing fault scarp or bordered hills, vertical displace- 
1 A joint contribution of the California Institute of Technology, Division of Geological 
Sciences (Contribution No. 1532) and the National Center for Earthquake Research, U.S. 
Geological Survey. 
1183 
1184 BULLET IN  OF  THE SE ISMOLOGICAL  SOCIETY  OF  AMERICA 
ments of up to 20 cm occurred--almost invariably with the same sense of displace- 
ment as that of the earlier movements that produced the fault scarps and hills. 
Neither side was consistently uplifted with respect o the other. 
/ 
f- 
..~? 0 \1 
~ ~,  %. ",~. ... 
OCOTILLO 
WELL~ 
12 
.L 
FAULT TRACE,  "~X 
SHOWING RELAT IVE  
DIRECTION AND AMOUNT 
OF HORIZONTAL D ISPLACE-  
MENT IN CENTIMETERS 
8 
? 
FAULT TRACE, SHOWING 
VERTICAL DISPLACEMENT IN 
CENTIMETERS; BALL ON RELA-  
T IVELY  DOWN-THROWN SIDE 
O 4, KM 
l I I I I 
116°07~30 H Los Ange SI  e s ~  
OCEAN 
San B~ego 
s S 
: CC s~z~o. Sea 
c~LI  FOBNI A - "--'~'- "  "7""  
" ' -T ix I~;  . . . .  ~ " 
y -;.\ 
-~TENTATIVE, EPICENTER 
~-~OF MAIN SHOCK 
~...~7~'1G2!29 GCT 9 APRIL 1968) 
>0 
dl " 0 ~ 
-.~'-,j ~ o~ i .A ,, / 
. . . . .  ~J  - / : / 
Cree 
r ~X>J- . - '~  
/ L 
SA = SAN ANDREAS F, 
S,J = SAN JACINTO E 
CC = COYOTE CREEK E 
SH= SUPERSTITION 
HILLS E 
I=  IMPERIAL  F, 
EV = EARTHQUAKE 
VALLEY F. 
E= ELS INORE E 
B = BANNING R 
" '~" ,x~ 116 o O0 i 
.. 
oz 
% 
'~ ,  y- J  
Fig. 1. The Borrego Mountain area, southern California, showing the tentative picenter 
earthquake of9 April 1968, and the displacements that accompanied the earthquake along 33 
km of the Coyote Creek fault trace. 
In most areas, the fault trace comprised a series of overlapping en echelon 
cracks. The total width of the fractured zone along the main fault trace varied from 
less than i meter to several hundred meters, but in the wider parts of this zone most 
of the displacement took place in a narrower zone of fractures that ranged in width 
from less than 1 to 15 or 20 meters. Had the zone of surface faulting traversed a
built-up area, most of the damage from fault displacement would have occurred 
within this narrower zone--a zone which varied in position from the center to either 
side of the entire fracture zone. The trace was entirely within Quaternary alluvium 
SEISMOLOGICAL NOTES 1185 
and lake beds, except at the northernmost extremity where branches cut steeply 
dipping beds of the Pliocene and Pleistocene Palm Spring Formation, and in the 
Ocotillo badlands where the Borrego Formation and Oeotillo Conglomerate (both 
Quaternary) were displaced (Downs and Woodard, 1962; Theodore Downs and 
J. A. White, oral communication, 1968). In every area where a pre-existing fault was 
indicated by a scarp or ground-water barrier, the rupture took place almost exactly 
along the line of earlier breaks. Theodolite surveys across the fault at Highway 78 
near Ocotillo Wells, repeated nine times between April 9th and April 27th, have as 
yet given no indication of post-earthquake cr ep, such as was so obvious following 
the Parkfield-Cholame earthquakes of 1966 (Brown and others, 1967; Smith and 
Wyss, in press). 
A single foreshock of magnitude 3.7 preceded the main shock by about a minute, 
but a Caltech micro-earthquake trailer operating at Obsidian Butte, 50 km east 
of the epicenter, ecorded no other precursory activity; indeed, the seismograph 
trace was remarkably quiet in the hours preceding the shock, and the general level 
of activity at this station had been very low during the preceding 4 months. Pre- 
liminary aftershock locations, based on four permanent s ations within 80 km and 
utilizing some data from temporary stations within the epicentral area, indicate a 
zone of activity extending along the projected fault trace from about 33 ° 18 ~ N, 
116 ° 19 ~ W to 32 ° 59 t N, 115 ° 57' W--a total length of about 50 km. Unlike many 
other large earthquakes, the epicenter of the main shock lay midway along the fault 
trace and the zone of aftershock activity. 
In addition to the primary displacement of the Coyote Creek fault, minor 
horizontal displacements or creep episodes must have taken place at about he same 
time on the Superstition Hills fault, Imperial fault, and Banning-Mission Creek 
(San Andreas) fault--45 kin, 70 kin, and 50 km, respectively, from the epicenter. 
Although these fresh breaks were not noticed until a few days after April 9th, no 
other significant earthquakes have been identified in these areas, and the remark- 
able freshness of the surficial cracks makes it highly likely that the displacements or 
creep episodes were triggered by the Borrego Mountain earthquake. In any ease, 
the fresh cracks must postdate the last major rainfall in this region on March 8th. 
Theodolite resurveys of several small nets that had been established across these 
faults 3-11 months prior to the earthquake indicate the following cumulative right- 
lateral displacements a  of April 28th: 1.2 cm at the Imperial fault where it crosses 
Highway 80, 1.1 cm where the fault crosses Worthington Road, 6 km northwest, 
1.3 cm at Harris Road, 11 km northwest; 2.2 cm where the Superstition Hills fault 
crosses Imler Road; and 0.9 cm on the Banning-Mission Creek fault at a point 
east of Thermal. Fresh breaks along the Imperial fault extend for more than 20 
km, distinctly farther both to the north and south than the 10-km segment broken 
during the magnitude 3.6 shock of 1966 (Brune and Allen, 1967). Fresh breaks along 
the Superstition Hills fault extend at least 8 km north from Imler Road, and new 
cracks along the Banning-Mission Creek fault have been observed at several points 
along the 30-kin segment from Salton Creek northwest into the Mecca Hills. In 
all three areas the fresh cracks clearly indicate right-lateral displacement of 1-2 cm. 
Although most of the displacements took place prior to the first remeasurements 
following the April 9th event, surveys that have been repeated an average of 4 
1186 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 
times at each locality between April 9th and April 28th suggest hat minor hori- 
zontal creep may be continuing at some places. These observations of displace- 
ments or creep episodes on nearby faults that  probably were triggered by the 
Borrego Mountain earthquake are the most surprising, and perhaps the most 
significant, result of the investigation thus far. 
ACKNOWLEDGMENT 
Participation in this study by the California Institute of Technology was supported by 
National Science Foundation Grant GA-1087. 
I~EFERENCES 
Brown, 1%. D., Jr., J. G. Vedder, 1%. E. Wallace, E. F. 1%oth, R. F. Yerkes, R. O. Castle, A. O. 
Waananen, 1%. W. Page, and J. P. Eaton, 1967, The Parkfield-Cholame, California, earth- 
quakes of June-August 1966--Surface geologic effects, water-resources a pects, and pre- 
liminary seismic data: U.S. Geol. Survey Prof. Paper 579, pp. 66. 
Brune, J. N. and C. it. Allen (1967). A low-stress-drop, low-magnitude earthquake with surface 
faulting: The Imperial, California, earthquake of March 4, 1966: Bull. Seism. Soe. Am. 
57, 501-514. 
Dibblee, T. W., Jr. (1954). Geology of the Imperial Valley region, California, [Pt.] 2 in Chap. 2 
of Jahns, R. H., ed., Geology of southern California: California Div. Mines Bull. 170, 21- 
28. 
Downs, Theodore and G. D. Woodard (1962). Middle Pleistocene xtension of the Gulf of 
California into the Imperial Valley [abs.]: Geol. Soc. Am. Spec. Paper 68, 21. 
Sharp, 1%. V. (1967). San Jacinto fault zone in the Peninsular 1%anges of southern California: 
Bull. Geol. Soe. Am., 78, 705-730. 
Smith, S. W. and M. Wyss (1968). Displacement on the San Andreas fault initiated by the 196g 
Parkfield earthquake: Bull. Seism. Soc. Am. in press. 
SEISMOLOGICAL LABORATORY 
C&LIFORNIA INSTITUTE OF TECHNOLOGY 
PASADENA, CALIFORNIA 
(C.1%.A., J.N.B., AND M.W.) 
U.S. GEOLOGICAL SURVEY 
MENLO PARK, CALIFORNIA 
(A.G., M.M.C., 1%.V.S., T.G.T., AND E.W.W.) 
Manuscript received April 29, 1968 


The Borrego Mountain, California, earthquake of 9 April 1968: A preliminary report

Bulletin of the Seismological Society of America Vol. 58, No. 3r pp. 1183-1186. June, 1968 
THE BORREGO MOUNTAIN, CALIFORNIA, EARTHQUAKE OF 
9 APRIL 1968: A PRELIMINARY REPORT 1 
[BY C. R. AJ.I~,  A. GRANTZ, J. N. ]3RUNE, M. M. CLARK, R. V. SHARP, T. G. 
THEODORE, E. W. WOLFE, AND M. WYss 
The largest earthquake to hit California in more than 15 years occurred at 
02:28:58.9 GCT on 9 April 1968 near Borrego Mountain, on the western edge of 
the Imperial Valley. The Seismological Laboratory at Pasadena has tentatively 
assigned the shock a magnitude of 6.5, an epieentral location of 33 ° 08.8' N, 116 ° 
07.5' W, and a focal depth of 20 km. The earthquake was felt throughout most 
of southern California and adjacent areas, but the absence of severe damage and 
casualties was in large part due to the relatively undeveloped nature of the epi- 
central region. Indeed, it would have been dittieult to pick a location in the south- 
ernmost part of the State more remote from centers of population. 
Although the earthquake occurred in the evening (6:29 p.m., local time), rapid 
computer location of the epicenter using the California Institute of Technology 
telemetered seismograph network allowed placement of the first portable seismo- 
graph unit within 3 km of the epicenter soon after midnight, and the fault trace 
was identified and the first displacement measurements made by 2:00 a.m. De- 
tailed mapping of the fault trace by U. S. Geological Survey and Calteeh personnel 
commenced the next day, which was fortunate in view of the fact that within four 
days much of the fault trace had already been obscured by blowing sand and 
dune buggy tracks. By April 12th, 14 portable seismograph units were in operation 
throughout the aRershock region. 
The earthquake was accompanied by right-lateral displacement along a 33-kin 
segment of the Coyote Creek fault (Figure 1)--a major branch of the San Jacinto 
fault zone, which in turn is the most active member of the San Andreas fault system 
in southern California. Earthquakes of comparable magnitude had occurred earlier 
along the San Jaeinto fault zone just to the north of this area in 1954 and just to the 
south in 1942, but no known major historic activity had occurred along the exact 
segment broken on April 9th. The Coyote Creek fault in this region has been 
mapped and its regional setting described by Dibblee (1954) and Sharp (1967). 
The fault break consists of two northwest-trending en echelon segments that 
overlap and are separated by a 2 km wide zone of highly fractured terrane just 
southeast of the settlement of Ocotillo Wells. The maximum displacement observed 
was along the northern segment, 5 km northwest of Ocotillo Wells, where 38 em 
of right-lateral slip was indicated by offset channels and vehicle tracks and by 
displaced slabs of dried mud crust. The maximum horizontal slip observed along 
the southern segment, 20 em, was noted 10 km southeast of Oeotillo Wells. Vertical 
displacements were nil in most areas where the fault cut level ground, but wherever 
the break lay along a pre-existing fault scarp or bordered hills, vertical displace- 
1 A joint contribution of the California Institute of Technology, Division of Geological 
Sciences (Contribution No. 1532) and the National Center for Earthquake Research, U.S. 
Geological Survey. 
1183 
1184 B U L L E T I N  O F  T H E  S E I S M O L O G I C A L  S O C I E T Y  O F  A M E R I C A  
ments of up to 20 cm occurred--almost invariably with the same sense of displace- 
ment as that  of the earlier movements that  produced the fault scarps and hills. 
Neither side was consistently uplifted with respect to the other. 
/ 
f- 
..~? 0 \1 
~ ~ ,  %. ",~. ... 
OCOTILLO 
WELL~ 
12 
.L 
FAULT TRACE,  " ~ X  
SHOWING R E L A T I V E  
DIRECTION AND AMOUNT 
OF HORIZONTAL D I S P L A C E -  
M E N T  IN C E N T I M E T E R S  
8 
? 
FAULT TRACE, SHOWING 
VERTICAL DISPLACEMENT IN 
CENTIMETERS; BALL ON RELA-  
T IVELY  DOWN-THROWN SIDE 
O 4, KM 
l I I I I 
116°07~30 H Los Ange S I  e s ~  
O C E A N  
San B~ego 
s S 
: CC s~z~o. Sea 
c ~ L I  FOBNI A - "--'~'- - "  " 7 " "  
" ' - T i x I ~ ;  . . . .  ~ " 
y -;.\ 
-~TENTATIVE, EPICENTER 
~ - ~ O F  MAIN SHOCK 
~...~7~'1G2!29 GCT 9 APRIL 1968) 
>0 
dl " 0 ~ 
-.~'-,j ~ o~ i .A ,, / 
. . . . .  ~ J  - / : / 
Cree 
r ~X>J- . - ' ~  
/ L 
SA = SAN ANDREAS F, 
S,J = SAN JACINTO E 
CC = COYOTE CREEK E 
SH= SUPERSTITION 
HILLS E 
I =  I M P E R I A L  F, 
EV = EARTHQUAKE 
VALLEY F. 
E= E L S I N O R E  E 
B = BANNING R 
" ' ~ " , x ~  116 o O0 i 
.. 
oz 
% 
' ~ ,  y - J  
Fig. 1. The Borrego Mountain area, southern California, showing the tentative epicenter 
earthquake of 9 April 1968, and the displacements that accompanied the earthquake along 33 
km of the Coyote Creek fault trace. 
In  most areas, the fault trace comprised a series of overlapping en echelon 
cracks. The total width of the fractured zone along the main fault trace varied from 
less than i meter to several hundred meters, but in the wider parts of this zone most 
of the displacement took place in a narrower zone of fractures that  ranged in width 
from less than 1 to 15 or 20 meters. Had the zone of surface faulting traversed a 
built-up area, most of the damage from fault displacement would have occurred 
within this narrower zone--a  zone which varied in position from the center to either 
side of the entire fracture zone. The trace was entirely within Quaternary alluvium 
S EIS M OLOGICAL NOTES 1185 
and lake beds, except at the northernmost extremity where branches cut steeply 
dipping beds of the Pliocene and Pleistocene Palm Spring Formation, and in the 
Ocotillo badlands where the Borrego Formation and Oeotillo Conglomerate (both 
Quaternary) were displaced (Downs and Woodard, 1962; Theodore Downs and 
J. A. White, oral communication, 1968). In every area where a pre-existing fault was 
indicated by a scarp or ground-water barrier, the rupture took place almost exactly 
along the line of earlier breaks. Theodolite surveys across the fault at Highway 78 
near Ocotillo Wells, repeated nine times between April 9th and April 27th, have as 
yet given no indication of post-earthquake creep, such as was so obvious following 
the Parkfield-Cholame earthquakes of 1966 (Brown and others, 1967; Smith and 
Wyss, in press). 
A single foreshock of magnitude 3.7 preceded the main shock by about a minute, 
but a Caltech micro-earthquake trailer operating at Obsidian Butte, 50 km east 
of the epicenter, recorded no other precursory activity; indeed, the seismograph 
trace was remarkably quiet in the hours preceding the shock, and the general level 
of activity at this station had been very low during the preceding 4 months. Pre- 
liminary aftershock locations, based on four permanent stations within 80 km and 
utilizing some data from temporary stations within the epicentral area, indicate a 
zone of activity extending along the projected fault trace from about 33 ° 18 ~ N, 
116 ° 19 ~ W to 32 ° 59 t N, 115 ° 57' W--a total length of about 50 km. Unlike many 
other large earthquakes, the epicenter of the main shock lay midway along the fault 
trace and the zone of aftershock activity. 
In addition to the primary displacement of the Coyote Creek fault, minor 
horizontal displacements or creep episodes must have taken place at about the same 
time on the Superstition Hills fault, Imperial fault, and Banning-Mission Creek 
(San Andreas) fault--45 kin, 70 kin, and 50 km, respectively, from the epicenter. 
Although these fresh breaks were not noticed until a few days after April 9th, no 
other significant earthquakes have been identified in these areas, and the remark- 
able freshness of the surficial cracks makes it highly likely that the displacements or 
creep episodes were triggered by the Borrego Mountain earthquake. In any ease, 
the fresh cracks must postdate the last major rainfall in this region on March 8th. 
Theodolite resurveys of several small nets that had been established across these 
faults 3-11 months prior to the earthquake indicate the following cumulative right- 
lateral displacements as of April 28th: 1.2 cm at the Imperial fault where it crosses 
Highway 80, 1.1 cm where the fault crosses Worthington Road, 6 km northwest, 
1.3 cm at Harris Road, 11 km northwest; 2.2 cm where the Superstition Hills fault 
crosses Imler Road; and 0.9 cm on the Banning-Mission Creek fault at a point 
east of Thermal. Fresh breaks along the Imperial fault extend for more than 20 
km, distinctly farther both to the north and south than the 10-km segment broken 
during the magnitude 3.6 shock of 1966 (Brune and Allen, 1967). Fresh breaks along 
the Superstition Hills fault extend at least 8 km north from Imler Road, and new 
cracks along the Banning-Mission Creek fault have been observed at several points 
along the 30-kin segment from Salton Creek northwest into the Mecca Hills. In 
all three areas the fresh cracks clearly indicate right-lateral displacement of 1-2 cm. 
Although most of the displacements took place prior to the first remeasurements 
following the April 9th event, surveys that have been repeated an average of 4 
1186 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 
t imes at each locality between April 9th and April  28th suggest tha t  minor hori-  
zontal  creep ma y  be continuing at some places. These observations of displace- 
ments  or creep episodes on nearby  faults t ha t  p robab ly  were tr iggered by  the  
Borrego Moun ta in  ear thquake  are the mos t  surprising, and perhaps the mos t  
significant, result of the invest igat ion thus far. 
ACKNOWLEDGMENT 
Participation in this study by the California Institute of Technology was supported by 
National Science Foundation Grant GA-1087. 
I~EFERENCES 
Brown, 1%. D., Jr., J. G. Vedder, 1%. E. Wallace, E. F. 1%oth, R. F. Yerkes, R. O. Castle, A. O. 
Waananen, 1%. W. Page, and J. P. Eaton, 1967, The Parkfield-Cholame, California, earth- 
quakes of June-August 1966--Surface geologic effects, water-resources aspects, and pre- 
liminary seismic data: U.S. Geol. Survey Prof. Paper 579, pp. 66. 
Brune, J. N. and C. it. Allen (1967). A low-stress-drop, low-magnitude earthquake with surface 
faulting: The Imperial, California, earthquake of March 4, 1966: Bull. Seism. Soe. Am. 
57, 501-514. 
Dibblee, T. W., Jr. (1954). Geology of the Imperial Valley region, California, [Pt.] 2 in Chap. 2 
of Jahns, R. H., ed., Geology of southern California: California Div. Mines Bull. 170, 21- 
28. 
Downs, Theodore and G. D. Woodard (1962). Middle Pleistocene extension of the Gulf of 
California into the Imperial Valley [abs.]: Geol. Soc. Am. Spec. Paper 68, 21. 
Sharp, 1%. V. (1967). San Jacinto fault zone in the Peninsular 1%anges of southern California: 
Bull. Geol. Soe. Am., 78, 705-730. 
Smith, S. W. and M. Wyss (1968). Displacement on the San Andreas fault initiated by the 196g 
Parkfield earthquake: Bull. Seism. Soc. Am. in press. 
SEISMOLOGICAL LABORATORY 
C&LIFORNIA INSTITUTE OF TECHNOLOGY 
PASADENA, CALIFORNIA 
(C.1%.A., J.N.B., AND M.W.) 
U.S. GEOLOGICAL SURVEY 
MENLO PARK, CALIFORNIA 
(A.G., M.M.C., 1%.V.S., T.G.T., AND E.W.W.) 
Manuscript received April 29, 1968 


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The Borrego Mountain, California, earthquake of 9 April 1968: A preliminary report

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