Bulletinofthe SeismologicalSocietyofAmerica,Vol.72,No. 1,pp. 181-196,February1982
SEISMICITY IN THE EPICENTRAL REGION OF T H E 1887
N O R T H E A S T E R N SONORAN EARTHQUAKE, MEXICO
BY STEVEN G. NATALI* AND MARC L. SBAR
ABSTRACT
The 1887 Sonoran earthquake, in northeastern Sonora, Mexico, has an 80-km
rupture length, with an average 3-m displacement, and an assumed 16-km depth
of rupture. This corresponds to a seismic moment of 1,27 x 1027 dyne-cm and
an Ms of 7.4. Seven to ten portable seismographs with an average spacing of 10
km were operated in a 2800-km 2 region surrounding the 1887 fault scarp. Thirtythree earthquakes were detected during 30 days of usable recording. For all
events, - 1 . 3 ~ M < 2. The most accurate locations (horizontal error ~ 5 km)
define a west-dipping, normal, range-bounding fault. All events occurred within
15 km of the surface. The name Pitaycachi fault (after the most prominent peak
of the horst block) is proposed for this zone of active faulting. Seismicity along
the zone terminates abruptly 13 km north of the fault scarp and extends to the
south for at least 32 km beyond the southern. A composite focal mechanism
from events at the southern end of the 1887 scarp indicates a normal fault
striking N35°E and dipping 72 ° to the northwest.
INTRODUCTION
On 3 May 1887, a rupture took place in northeastern Sonora, Mexico (Figure 1),
that is one of the largest historic normal faulting earthquakes in North America.
The calculated seismic moment (1.27 × 1027 dyne-cm) corresponds to M s = 7.4
(Hanks and Kanamori, 1979). An average 3-m displacement, 80-km rupture length,
and 16-km rupture depth were used in the moment calculation. Tabulations by
Slemmons (1977) also indicate M s = 7.5 based on maximum ground displacement
(4.5 m) and rupture length. The 700-km average radius of felt reports (DuBois and
Sbar, 1979) is consistent with the magnitude assigned this earthquake. Fifty-one
persons were killed in communities near the epicenter. All deaths were from collapse
of adobe structures. For a detailed account of the earthquake's effects, see Aguilera
(1920) and Dubois and Smith (1980).
Events of up to MMI VI are still reported in the remote town of Colonia Morelos,
Sonora, located near the southern end of the 1887 fault scarp (Figure 1). As recently
as December 1977, persons were awakened to the sounds of "train-like rumblings
from the mountains to the south." Furniture moved about, adobe walls and foundations cracked, and unreinforced brick chimneys sheared at rooflines. Landslides
in Cajon de Alamo 10 km to the south blocked an irrigation ditch.
Because the 1700~m Pitaycachi Peak dominates the landscape of San Bernardino
Valley, the name Pitaycachi fault is proposed for the 1887 zone of faulting. The
faulting extends along the eastern edge of the San Bernardino Valley from N31 ° 16'
to N30°33 ' latitude (Herd, personal communication, 1981). Most of this zone is
shown in Figure 1.
A detailed study of Pitaycachi fault and the effects of the 1887 Sonoran earthquake
is essential to earthquake hazard evaluations in Arizona, New Mexico, and Sonora.
While calling attention to the potential for large ruptures in the southern Basin and
* Present address: AmocoProductionCompany,AmocoBuilding,Denver Colorado80202.
181
182
STEVEN G. NATALI AND MARC L. SBAR
EXPLANATION
[]
/~
....
,==="=
........
~, "',
North Net,July 5 - 1 4 , 1 9 7 9
Improved Dirt Roods
1887 Fault Scarp
0
Intermittent Streams
CP
L
\[~I
]
b ;' "'-...
', I,,
l
,' ~F~
"'-.
L
/ i
L,.'~
- AR,ZO,A"'.~.NEW
ME.X,CO
/I
' ~'~.
', r---'~. ]
Recon Net, June 6 - 1 6 , 1978
South Net,dune 2 0 - 2 9 , 1 9 7 9
i
'1 I OF "~ SONORA
, "-",
AREA
I
FIG. 1. Geology of the San Bernardino Valley, northeastern Sonora, Mexico, showing the 1887 fault
scarp (solid line) and (?) older scarps (dashed line). A 20-km southern extension of the 1887 scarp
proposed by Herd (personal communication, 1981} is not shown. U n s h a d e d areas labeled TQs are
alluvial basins of Tertiary and Quaternary sediments. TQb, Tertiary and Quaternary basalts; MTv,
undifferentiated Mesozoic or Tertiary volcanics; MTr, Mesozoic or Tertiary rhyolites; Ms, Mesozoic
sediments; Ti, undifferentiated intrusives. Microearthquake station locations are shown for the Recon,
South, and North nets. In the inset, CP, BR, RGR, SMO, and SAF are the Colorado Plateau, southern
Basin and Range, Rio Graade rift and Sierra Madre Occidental structural provinces, and the San Andreas
fault system, respectively.
R a n g e p r o v i n c e , t h e S o n o r a n e a r t h q u a k e also m a y p r o v i d e a l o w e r b o u n d for t h e
m a x i m u m m a g n i t u d e for t h i s r e g i o n .
I n o r d e r to q u a n t i f y t h e l e n g t h , s t r i k e , dip, s e i s m i c i t y level, e a r t h q u a k e focal
d e p t h s a n d slip d i r e c t i o n for t h e P i t a y c a c h i fault, t h e U n i v e r s i t y of A r i z o n a , in
SEISMICITY IN THE REGION OF THE SONORAN EARTHQUAKE
183
cooperation with the University of Sonora, University of Texas at E1 Paso, and New
Mexico State University, conducted a 10-station reconnaissance survey of microearthquake activity along the 1887 fault scarp during the summer of 1978 (Recon
Net: 6 to 16 June 1978). The seven-station south net and eight-station north net
were maintained 20 to 29 June 1979, and 5 to 14 July 1979, respectively. Coverage
by all nets overlapped at approximately Pitaycachi Peak (Figure 1). Average station
spacing was 10 km and the entire area of the three networks is approximately 70 km
north-south by 40 km east-west.
R E G I O N A L SETTING
The 1887 fault scarp is located along the east side of the San Bernardino Valley
of northeastern Sonora, Mexico (Figure 1). This valley is part of a continuous faultcontrolled valley that extends through 4 ° of latitude, from N27 ° to N31 °, along the
west side of the Sierra Madre Occidental structural province (de Cserna et al., 1961;
King, 1969).
The main features associated with the 1887 scarp are a north-south-trending
alluvial basin filled with Tertiary and Quaternary sediments that is flanked on either
side by horsts of mid-Tertiary undifferentiated volcanics, Mesozoic sediments
(chiefly carbonates), and Quaternary basalts (0.2 to 4 m.y.; Lynch, 1972). The
Tertiary volcanics probably correspond to what was first termed the "mid-Tertiary
orogeny" by Damon (1964). During late Oligocene and mid-Miocene times (30 to 13
m.y.; Shafiqullah et al., 1980) massive amounts of calc-alkalic volcanic rocks were
erupted in southern Arizona and northern Sonora in voluminous ignimbrite sheets
and andesite flows. Subsequently termed the "ignembrite flare-up" by Coney (1976),
this volcanic episode is viewed by some geologists as the surface expression of
shallow subduction under southwestern North America during a greatly accelerated
surge of North American-Pacific plate convergence. Subsequent onset of Basin and
Range high-angle faulting (13 to 10 m.y.; Eberly and Stanley, 1978) left the San
Bernardino Valley with internal drainage. Sediments filling this low-elevation basin
produced gypsum-rich late Miocene (?) lake deposits following evaporation of
mineral-laden waters (Scarborough, 1979). In several places the Pitaycachi fault is
parallel to the mountain front, but displaced valleyward by several kilometers.
Dissected bedrock sediments that locally reach 1 km in width attest to the amount
of mountain front retreat since onset of Basin and Range faulting.
The 1887 scarp (Figure 1) was mapped by the authors from aerial photographs at
a scale of 1:50,000 and ground-checked along most of its length. In more extensive
mapping, Darrel Herd (personal communication, 1981) has extended 1887 ground
breakage an additional 20 km to the south (Figure 5) for a total ground rupture
length of 80 km. Offset along the scarp is in a normal sense with the westward basin
having dropped an average of 3 m during the 1887 earthquake. Subsequent lateral
stream erosion along the scarp has enhanced its apparent height in several places.
(W. B. Bull, Geosciences Department, University of Arizona, written communication, 1980). Field measurements do not confirm the 8-m maximum displacement
reported by Aguilera (1920). Measurement of the offsets between pre-1887 surfaces
yields a maximum displacement of 4.5 m. Slickensides near the JAV station site
(Figure 1) indicate pure dip-slip displacement on a normal fault with 70 ° dip to the
west and approximately north-south strike.
Caliche cementation, high-soil carbonate content, and arid climate have left the
scarp free-face well preserved along the central 35 km of its length. At its southern
end, the scarp enters the Sierra E1 Tigre. While fresh rhyolites provide excellent
184
STEVEN
G. N A T A L I A N D M A R C L . S B A R
o u t c r o p , difficult access h a s p r e v e n t e d t h e f a u l t b e i n g m a p p e d t h r o u g h t h e s e
m o u n t a i n s . M i c r o e a r t h q u a k e s i n v a l l e y s s o u t h e a s t a n d s o u t h w e s t of t h e 1887 s c a r p ' s
s o u t h e r n t e r m i n a t i o n s u g g e s t a b r a n c h i n g of t h e P i t a y c a c h i f a u l t n e a r t h e t o w n of
C o l o n i a M o r e l o s a n d a s o u t h w a r d c o n t i n u a t i o n i n t o o n e or b o t h of t h e s e valleys.
METHODS
Instrumentation. T h e field s e i s m o g r a p h s u s e d were T e l e d y n e G e o t e c h P o r t a c o r ders w i t h v e r t i c a l c o m p o n e n t K i n e m e t r i c s SS-1 R a n g e r g e o p h o n e s ( n a t u r a l freq u e n c y 1 Hz) set a t 70 p e r c e n t of critical d a m p i n g a n d S p r e n g n e t h e r M E Q 800
TABLE 1
STATIONDATA
Station
Code
N. Lat.
(Deg, Min)
W. Long.
(Deg, Min)
Elevation
(m)
Gain
10 Hz@
(× 10~)
VMOD
Thickhess
Dates
Installed
5.44
5.44
2.72
5.44
5.44
5.44
0.68
5.44
5.44
5.44
1.0
1.0
2.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
6-16 June
6-16 June
15-16June
6-16 June
8-16 June
6-16 June
6-16 June
6-16 June
6-16 June
14-16June
1.59
0.79
1.59
0.79
1.59
1.59
1.0
2.0
1.0
1.0
1.0
1.0
20-29 June
20-29 June
20-29 June
20-29 June
21-29 June
20-29 June
0.79
0.79
1.59
0.40
1.59
1.59
0.79
0.79
1.0
3.0
1.0
3.0
1.0
1.0
3.0
0.0
6-14 July
5-13 July
5-14 July
6-13 July'
4-14 July
5-14 July
4-14 July
5-14 July
Recon Net (6-16 June 1978)
Buena Vista
Dam
Oscar
Pitaycachi
Prickly Pear
Rancho Capadero
Rancho Cherriones
Rancho Javalina
Rancho Juracal
Tip
BVS
DAM
OSC
PIT
PKP
CAP
CHE
JAV
JUR
TIP
30
30
31
31
30
31
31
30
31
30
53.50
48.83
01.00
03.66
52.50
05.87
05.76
59.00
11.44
57.20
109 09.70
109 10.52
109 18.47
109 08,16
109 16.50
109 05.79
109 14.00
109 09.00
109 06.45
109 02.11
1100
1000
900
1300
900
1275
1050
1300
1250
1900
South Net (20-29 June 1979)
Buena Vista
Colonia Oaxaca
Dam
E1 Tigre
Prickly Pear
Rancho Javalina
BVS
COA
DAM
ELT
PKP
JAV
30
30
30
30
30
30
53.50
45.00
48.83
45.25
52.50
59.00
109
109
109
109
109
109
09.70
02.81
10.52
15.62
16.50
09.00
1100
1000
1000
900
900
1300
North Net (4-14 June 1979)
Pitaycachi
Rancho Bolsa
Rancho Capadero
Rancho Cherriones
Rancho Juracal
Rancho San Fernando
Rancho Virgen
Ruins
PIT
BOL
CAP
CHE
JUR
RSF
VIR
RUI
31
31
31
31
31
31
31
31
03.66
13.79
05.87
05.76
11.44
01.27
13.21
15.92
109
109
109
109
109
109
108
109
08.16
09.69
05.79
14.00
06.45
05.65
59.82
14.31
1300
1100
1275
1050
1250
1400
1500
1100
r e c o r d e r s c o n n e c t e d to e i t h e r v e r t i c a l c o m p o n e n t S p r e n g n e t h e r 500 ( n a t u r a l freq u e n c y = 1 Hz) or K i n e m e t r i c s SS-1 R a n g e r g e o p h o n e s . W h e r e possible, g e o p h o n e s
were p l a c e d o n b e d r o c k . A t a l l u v i a l sites, t h e y were b u r i e d a m i n i m u m of 30 cm.
M o s t s y s t e m s t y p i c a l l y m a g n i f i e d g r o u n d d i s p l a c e m e n t b y 1 to 6 m i l l i o n a t 10 Hz
(see T a b l e 1).
C o m p a r i s o n of e a c h u n i t ' s i n t e r n a l c r y s t a l clock w i t h a W W V t i m e s t a n d a r d a t
t h e b e g i n n i n g a n d e n d of e a c h r e c o r d a l l o w e d d e t e r m i n a t i o n of a b s o l u t e t i m e to
w i t h i n +_0.01 sec. A m i c r o s c o p e was u s e d to r e a d i m p u l s i v e P a r r i v a l s to +_0.05 sec,
SEISMICITY IN THE REGION OF THE SONORAN EARTHQUAKE
185
while the poorest P arrivals were read to ___0.2sec or better. S arrivals could usually
be read to _+0.3 sec, but were sometimes emergent or lost in the P-wave coda,
resulting in errors of 1 sec or more.
Location procedure and errors. All events were located using the U.S. Geological
Survey Hypoellipse program (Lahr, 1979). T he program uses the standard deviation
of the travel-time residuals to calculate a standard error ellipsoid in which the
horizontal projection of the major and minor axes (ERH) and vertical axis (ERZ)
are listed, thus providing a quantitative estimate of horizontal and depth control for
each solution. Qualitatively, one finds a broad region of good horizontal control in
the centers of the arrays, with lesser control toward the peripheries. At distances of
more than one station spacing outside the edge of the nets, radial distances to the
event remain well constrained, but azimuthal control is poor. Depths are characteristically well controlled only in the vicinity of a station. Depths calculated for events
within the periphery of the nets were determined to be stable by varying the crustal
models for a number of events. T he different velocity models described below caused
depth changes of no more than 3 km.
Structure determination. The accuracy of event locations is a function of the
accuracy of the velocity model specified. T h e velocity model was derived from a
synthesis of refraction studies in the southern Basin and Range province (Warren,
1969; Gish et al., 1981; Sinno et al., 1981). These workers consistently identified
three velocities: 6.1 km/sec in the upper crust (1- to 13-km depth) 6.4 to 6.7 km/sec
for the lower crust (13 to 25 km), and an upper mantle velocity of 7.7 to 7.8 km/sec
below 24 to 27-km depth.
Starting with this simple model, gravity information was used to further manipulate the crustal structure in such a way as to minimize travel-time residuals. J. R.
Sumner (1977), in performing an east-west gravity transect across the center of the
San Bernardino Valley, found a very close fit to observed data with the interpretation
shown in Figure 2a. Three layers of increasingly compacted sediments fill an alluvial
basin whose eastern margin is downdropped more than 3 km. The steepest gravitational gradients coincide with the 1887 fault scarp. Depth to bed~gck gradually
decreases to the west.
~
In modeling seismic velocities, a dramatic decrease in travel-time residuals was
found when using a variable thickness layer that closely mimicked the basement
structure deduced by Sumner (Figure 2b). Stations to the east of the 1887 scarp rest
on bedrock, while those just west of the scarp rest on a wedge of slower velocity
sediments that thins as one approaches the western valley margin. Other velocity
structures yielded significantly larger travel-time residuals.
Station delay. Station delays were determined independently of hypocenter
calculations to avoid biasing true residual anomalies caused by velocity model
errors. Crosson (1972) argues that such independence is crucial because these same
residual anomalies are then used to make corrections in velocity structure. Four
quarry blasts from widely varying azimuths at distances of approximately 120 km
were recorded with impulsive P arrivals. Using a plane wave fit over the entire
array, it was found that an elevation delay of 4 km/sec in conjunction with the
velocity model shown in Figure 2b was sufficient for all stations.
RESULTS
Forty events were detected over a period of 30 days during two summer field
seasons. Seven of these were suspected of being quarry blasts and were eliminated.
A summary of the events believed to be microearthquakes is provided in Table 2. A
186
STEVEN G. NATALI AND MARC L. SBAR
histogram displaying the number of earthquakes/day shows the events to be
approximately evenly distributed in time (Figure 3a). There appears to be an
approximately uniform level of activity, both from summer to summer and at the
northern and southern ends of the 1887 scarp. Events greater than the minimum
detection capability (ML = --1.3) occur at an average rate of 1.1 events/day.
E a r t h q u a k e magnitudes. Local Richter magnitudes were calculated using the
formula (Lahr, 1979)
FA M W A (f)
ML = log L 2~1o ~
I
-B~ + b21og D 2 + G
(1)
(a)
RESIDUAL GRAWT¥ MODEL
~
Elev.(km)
.p:-~.5~/¢m3
fi
0
2
-1: . .'-".] '
: : :: fto
------
east
west
(b)
P-WAVE VELOCITY MODEL
sc~
z
i
0
Elevotion delay = 4.0 km/sec,
-I
Vp = 4.0 km/sec.
-2
-2
--1
-3
-4
0
-I
-3
Vp = 4 , 8 km/sec.
-4
-5
-5
-5
-5
l
-io
Vp= 6.1 km/sec.
-15
-15f
-I0
-20
-2:5
-30
-20
Vp= 6.8 km/sec.
-25
-30
Vp=7.9 km/sec.
I
J
5 km
Fro. 2. (a) Gravity model from an E-W transect across the San Bernardino Valley at N31°05 ' latitude.
density contrast. No vertical exaggeration (after J. R. Sumner, 1977). (b) P-wave crustal velocity
model, showing 4.0 km/sec variable thickness layer. See Table 1 for layer thicknesses for each station
(VMOD thickness).
hp =
where
A
= maximum peak-to-peak amplitude of largest phase (ram)
C10
= scaling factor that varies with instrument gain setting
M W A (f) = magnification of ground displacement by Wood-Anderson seismometer as function of frequency
M(f)
= Portacorder-Ranger system magnification of ground displacement
for the frequency of the observed phase
187
S E I S M I C I T Y I N T H E R E G I O N OF T H E S O N O R A N E A R T H Q U A K E
TABLE 2
A SUMMARY OF THE EVENTS BELIEVED TO BE MICROEARTHQUAKES
Event
OriginTime (GMT)
(Yr Mo Day Hr Min Sec)
N. Lat.
(Deg, Min)
W. Long.
(Deg, Min)
Depth
(kin)
ML
ML(s.d.) Res.rms No. of
(sec) Stations
R e c o n N e t {6-16 J u n e 1978)
R5
R6
R7
R8
R9
Ell
R13
El4
R15
El6
78
78
78
78
78
78
78
78
78
78
0609
0610
0610
0610
0610
0614
0615
0615
0615
0615
1313
0843
1114
1158
1753
1143
0607
0833
1305
1333
22.86
03.02
55.22
25.43
38.85
45.16
26.64
19.85
15.30
53.12
30
30
30
31
31
30
30
30
30
30
40.23
38.88
53.78
08.02
21.32
51.89
51.85
50.12
48.66
50.08
109
109
109
109
108
109
109
109
109
109
02.91
03.81
09.38
03.52
54.83
13.60
08.85
11.38
09.21
11.33
9
13
11
8
25*
5
11
2
3
10
-0.3
-1.3
-0.7
-1.2
-0.4
-1.0
-0.3
-1.2
-1.3
-0.1
0.2
0.3
0.1
--0.8
0.4
0.5
0.5
0.4
0.09
0.15
0.13
0.27
0.18
1.42
1.17
0.69
0.99
1.58
3
3
7
5
3
5
10
8
8
8
-1.1
-1.2
>1.2
-0.8
-0.5
-0.7
-1.3
0.5
-0.8
-0.1
-1.1
0.4
--0.3
0.4
-0.3
0.2
0.3
0.2
0.5
0.84
-0.30
1.15
3.04
-0.37
2.16
0.65
0.27
0.51
5
1
6
3
6
1
6
5
7
5
4
0.5
>1.3
>1.5
0.2
-0.4
-0.7
-0.3
-0.9
-0.3
0.41
0.8
-0.9
---0.1
0.1
0.2
--0.2
-0.2
--
-0.58
1.12
0.90
0.42
0.57
-0.18
0.51
-1.75
--
1
6
5
6
6
6
1
3
7
1
6
1
S o u t h N e t {20-29 J u n e 1979}
$1
S2
$3
$4
$5
$6
$7
$8
$9
S10
$11
79
79
79
79
79
79
79
79
79
79
79
0622
0622
0622
0623
0623
0623
0625
9625
0627
0628
0628
1624
1657
1923
0832
1041
1602
1400
2208
0740
1628
1632
56.81
39.7
55.93
24.93
54.89
39.7
11.39
41.11
25.58
11.80
12.61
31
30
30
31
30
30
30
30
30
31
30
N1
N2
N3
N4
N5
N7
N8
N10
N12
N13
N14
N15
79
79
79
79
79
79
79
79
79
79
79
79
0707
0707
0707
0708
0709
0709
0709
0710
0711
0712
0713
0713
1157
2002
2214
0021
1722
1816
1817
1118
1217
0000
0232
1758
02.6
55.85
38.16
10.27
21.59
39.13
21.3
33.03
25.20
34.9
28.28
05.5
31
30
30
30
31
31
31
31
31
31
31
31
03.63
52.50
19.06
06.57
39.62
52.50
48.16
34.58
53.98
09.44
54.89
109
109
109
109
108
109
109
109
109
108
109
24.42
16.50~
19.82
05.41
47.79
16.50~
13.77
11.44
14.16
58.83
15.33
3
-29*
1
15"
-4
1
7
3
2
N o r t h N e t (4-14 J u l y 1979)
05.76
38.38
35.80
39.53
13.19
14.21
15.92
04.50
10.84
15.92
05.57
11.44
109
109
109
109
109
109
109
109
109
109
108
109
14.00~
12.14
29.21
23.24
08.93
10.98
14.31~
09.54
12.91
14.31~
22.68
06.45~
-2
29*
1
4
3
-3
7
-18"
--
* T h e s e d e p t h s a re poorly constrained.
t L o c a t i o n s sh own are t h e c o o r d i n a t e s of t h e single s t a t i o n on w h i c h t h e s e e ve nt s were recorded. All
e v e n t s h a d S - P i n t e r v a l s of less t h a n 1 sec.
D
B1
B2
G
= hypocentral distance
= 0.15
for 1km_-_D_-<200km
= 0.80
= magnitude correction applied to each station (not used in this
study).
The attenuation constants B1 and B2 are approximately equal to Richter's - l o g A0
correction, and are those employed by the U.S. Geological Survey central California
network. In the absence of published attenuation data for this paper's study area, it
is assumed that these same attenuation constants apply to northeastern Sonora.
188
STEVEN G. NATALI AND MARC L. SBAR
A s s e e n i n F i g u r e 3b, m o s t of t h e e v e n t s are small. ML v a l u e s v a r y b e t w e e n - 1 . 3
a n d a p p r o x i m a t e l y +2, w i t h m o s t e v e n t s falling b e t w e e n t h e v a l u e s of - 1 a n d +1.
All ML v a l u e s h a v e s t a n d a r d d e v i a t i o n s of h a l f a m a g n i t u d e u n i t or less. V a r i a t i o n s
w i t h i n t h e n e t are p r o b a b l y d u e to d i f f e r e n c e s i n r e s p o n s e b e t w e e n b e d r o c k a n d
a l l u v i a l sites.
EVENTS
N6E
6
8
I0
12
14
n
16
20
PER DAY
22
+5
24
26
28
6
8
I0
12
14
M L VALUES
+2
ML +l
I'
0
t+
,,,
o,
÷
•
+
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.
-I
-2
I
6 p
I
8
I
I
I0
1121
I
I
I
14
I
t
16
20
I
I
I
22
I
I
24
I
26
I
I
I
I
r
I
28
I
6
I
I
8
I
I I 21
I
I0
I
14
SOUTH NET
NORTH NET
RECON NET
JUNE 2 0 - 2 9 , 1979
JULY 5-14, 1979
JUNE 6-16, 1978
FIG. 3. (a) Events per day as a function of time. (b) Local Richter magnitude (ML) of events as a
function of time. Error bars show standard deviation. Events recorded by only one station show no error
bars. Events clipping the entire net are shown with arrows.
8
7
,
V p / V S = 1.68 ; Z,/= 0.25
/
6
5
S-P
(sec.)
/
,
_
•
4 _
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M
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• HIGH QUALITY ARRIVALS
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2
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5
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6
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8
i
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9
,
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,
P - T o (sec.)
Fro. 4. Plot of S-P versus P arrival times projected to a common origin for events with three or more
reliable S-P readings.
Vp/Vs a n d Poisson ratio. A p l o t of S-P i n t e r v a l s v e r s u s a r r i v a l t i m e y i e l d e d a
c o n s i s t e n t Vp/Vs r a t i o of 1.68 ( F i g u r e 4) a n d a c o r r e s p o n d i n g P o i s s o n r a t i o of 0.23.
A Vp/Vs r a t i o of 1.68 was t h e r e f o r e u s e d i n t h e l o c a t i o n p r o g r a m .
Epicenter locations. M i c r o e a r t h q u a k e l o c a t i o n s f r o m all t h r e e n e t w o r k s are
189
SEISMICITY IN THE REGION OF THE SONORAN EARTHQUAKE
plotted together in Figure 5. The 1887 fault scarp is shown schematically for
reference, as is the U.S.-Mexico border. Locations with less than 5 km of horizontal
error are plotted as solid circles, while those exceeding this limit are shown as open
circles. The northernmost event was located with a fourth 12-station net operating
in the northern San Bernardino Valley (22 May to 4 June 1978; Sbar, 1980) and is
included here for completeness. This was the only event recorded by that network,
although its coverage extended to N31°50 '.
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FIc. 5. Composite map of microearthquake locations for all nets. The Pitaycachi fault (solid line) and
the U.S.-Mexico border are drawn schematically for reference. Shown south of Colonia Morelos is an
interpreted extension of the Pitaycachi fault into the E1 Tigre Valley based on microearthquake locations
and ground fault scarps mapped by Darrel Herd (personal communication, 1981). Locations with
epicentral errors less than 5 km are plotted as solid circles, while those having larger errors are shown as
open circles. Shown also are the events used in composite focal mechanism solutions A and B (Figure 6).
190
STEVEN G. NATALI AND MARC L. SBAR
All well-constrained locations fall to the west of the 1887 scarp, and are consistent
with a steeply dipping, west-side-down normal fault. The single easternmost event
approximately midway along the scarp is probably associated with another fault.
All other open-circle locations north of N30°50 ' latitude could be explained by a
single fault within the location errors. This is not to imply that the Pitaycachi fault
is a single, simple, normal fault along the eastern boundary of the San Bernardino
Valley. Possible splays of the fault with lengths of several kilometers have been
observed to the west of the main range-bounding fault (see Figure 1).
Activity continues south of observed 1887 ground breakage into the Bavispe and
E1 Tigre valleys southeast and southwest of Colonia Morelos, respectively. With the
present station configuration, azimuthal control of these distant events is poor, and
the accurate mapping of seismicity to the south cannot be performed with these
locations.
Depths. Depth control exists only for those events within the periphery of the
networks. All well-located events occur above 15-km depth.
Composite fault plane solutions. It is assumed that focal mechanisms for a
number of events closely associated in space and time are similar. Several highquality locations were chosen to create two composite fault plane solutions. First
motions were plotted on equal-area, lower hemisphere projections. All trace polarities were checked by noting that nearby quarry blasts yielded compressional first
motions.
Hamilton (1971) noted that layered ray-path models cause large discontinuities in
take-off angle at the focal sphere when the focus lies near a layer boundary. These
discontinuities in turn cause erroneous interpretations produced solely by inadequacies in the velocity model. To avoid this problem, a second crustal model was
constructed whose velocity increased linearly with depth. This continuous velocity
gradient causes all rays to be projected back to the focal sphere as arcs instead of
discrete, discontinuous line segments. Focal mechanism solutions computed using
the layered and linear-gradient velocity models were essentially identical.
The composite solution A (Figure 6a) is composed of three well-constrained
events located at the southern end of the 1887 scarp, where the fault strikes N25°E
to N5°E and dips 70 ° to 80 ° to the northwest. The focal mechanism best fitting all
points is a normal fault striking N35°E. The fault plane is assumed, based on
geological information, to correspond to the nodal plane dipping 72 ° to the northwest
with the west side down. Although it is difficult to escape the interpretation of a
steeply dipping, west-side-down normal-faulting mechanism, the strike of the nodal
planes is thought to be constrained to no better than +_ 20 °. Nodal planes striking
anywhere from N-S to NE-SE would violate no more than two points in Figure 6a.
Composite solution A implies a component of crustal extension in the WNW-ESE
direction (_+20°), and is consistent with the stress field proposed for the southern
Basin and Range province by Zoback and Zoback (1980) in which
Svert ~ SNNE >> SWNW.
A WNW-ESE orientation of least-principal stress direction is also indicted by dike
and cinder cone alignments (<3 m.y. old) in the northern San Bernardino Valley.
Luedke and Smith (1978) report a direction of N62°W for the least-principal stress
in that locality. To the northeast, in southwestern New Mexico, Quaternary dikes
and cinder cones in the Tres Hermanas Mountains (Balk, 1962) and 100,000-yearold cinder cones in the Potrillo volcanic field (Hoffer, 1976; Luedka and Smith, 1978)
SEISMICITY IN THE REGION OF T H E SONORAN E A R T H Q U A K E
191
indicate least-principal stress directions of E-W and NS0°W, respectively. Zoback
and Zoback (1980) caution that dike and cinder cone alignments probably have
errors of at least _+20°. The pure dip-slip slickensides of the Pitaycachi fault where
it strikes N5°E indicate approximately E-W extension. All of these geological
indicators of least-principal stress direction lend credence to the WNW-ESE (+20 °)
extension inferred from composite solution A.
Composite solution B (Figure 6b) contains events from the northern end of the
1887 scarp where it bends through almost 50 ° of strike. The assumption that a single
focal mechanism controls events clustered in space and time appears to be inadequate at this locality. The kinematics associated with the tight bend in the Pitaycachi
fault require oblique slip. Thus, some events may exhibit components of right-slip
while nearby events either have little or no strike-slip component. Shown in Figure
N
N
,/.'/
(o)
•
COMPRESSIONAL
O
DILATATrONAL
x
NODAL
(b)
Fro. 6. Composite focal mechanism solutions. In these equal-area lower hemisphere projections,
compressional arrivals are solid dots and dilatations are open. E m e r g e n t P arrivals with impulsive S
phases are infested nodal arrivals and are plotted as X's. Compressional quadrants are accented with
slashes. (a) Southern end of 1887 scarp. Events $7, $9, and S l l . P axis: 63 ° plunge, 125 ° trend; T axis:
27 ° plunge, 305 ° trend; inferred fault plane: N35°E strike, 72°NW dip. (b) N o r t h e r n end of 1887 scarp.
Events N5, N7, N10, and N12. Two interpretations are shown, with the solid line interpretation being
preferred as more consistent with local geology. For preferred solution, P axis: 30 ° plunge, 10 ° trend; T
axis: 0 ° plunge, 100 ° trend; inferred fault plane: N32°W strike, 70°SW dip.
6b are two possible interpretations for the focal mechanism: oblique thrusting
(dashed lines) and oblique normal faulting (solid lines). The latter interpretation is
thought to be more consistent with local geology. The northwest-striking nodal
plane is assumed to cmTespond to the fault and exhibits a right-slip.
The preferred interpretation for composite solution B (oblique normal faulting)
is consistent with the E-W to WNW-ESE extension inferred from solution A and
slickensides along the Pitaycachi fault. Thompson and Burke (1973) demonstrated
that relative components strike versus dip-slip on a single fault system are dictated
by the orientation of that fault relative to regional extension. At its northern
termination, the Pitaycachi fault bends sharply to become subparallel to inferred
regional extension. One would therefore expect events in this locality to have oblique
right-slip focal mechanisms.
192
STEVEN G. NATALI AND MARC L. SBAR
DISCUSSION
Distribution of seismicity. Projecting surface displacement and event locations
onto a north-south profile indicates that the zone of seismicity is at least 115 km
long, significantly greater than the 80-km length of ground rupture associated with
the Sonoran earthquake (Figure 7). Also, as one proceeds from north to south along
the profile, the northernmost onset of microearthquake activity and ground rupture
is approximately coincident with an abrupt 500 m lowering in elevation of the San
Bernardino Valley floor. A 12-station, 12-day microearthquake network located in
the northern San Bernardino Valley detected no events north of this point. Coverage
extended as far north as N31°50 ' (Sbar, 1980).
SAN
1400
A
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50'
40 ~
30'
20'
10'
31°00 '
50'
40'
30'
20'
I0'
30°00 '
N e LATITUDE
Fro. 7. Projections onto a north-south profile. (a) Elevation of the floors of the San Bernardino,
Bavispe, and E1 Tigre Valleys. (b) Ground displacement due to the 1887 earthquake (from Aguilera,
1920). Checks indicate measurements confirmed on the ground by the authors. Also indicated as
h a t c h u r e d a r e a in (b) is recently mapped extension (Darrel Herd, personal communication, 1981). (c)
Number of epicenters in each 5' latitude segment. Microearthquakes are from nets with detection
capabilities extending from N30 ° 15' to N32°00 ' {this paper and Sbar, 1980).
To the south, seismic activity continues to the limit of this study's detection
capability (N30°19'). One well-located event is in the Bavispe Valley, while the
mapped 1887 rupture follows the E1 Tigre Valley. Thus, most of the activity appears
to be associated with the Pitaycachi fault and its extensions to the north and south.
A few events cannot be on this fault and indicate the existence of other active faults
to the east and southeast of the Pitaycachi fault. In general, this activity has the
appearance of an aftershock sequence, which may explain the realtively high activity
of this region as compared with southeastern Arizona.
Relation to regional tectonics. The relationship between the Sonoran earthquake
and regional seismicity to the north is poorly understood. The Intermountain seismic
belt (Smith and Sbar, 1974) extends southward into northwestern Arizona along
SEISMICITY IN T H E R E G I O N OF T H E S O N O R A N E A R T H Q U A K E
193
north-south-trending faults in the Grand Canyon area. Fault plane solutions along
this transition zone between the Great Basin and Colorado Plateau indicate vertical
motion with a component of east-westerly regional extension. The seismicity and
Quarternary faults in northern Arizona are concentrated along the southern Basin
and Range-Colorado Plateau boundary from the northwest corner of the state to
about Phoenix (Mokhtar, 1979). The southern Basin and Range Province to the
southwest and the Colorado Plateau to the Northeast are virtually inactive. South
of Phoenix historic earthquakes (Sumner, 1976; DuBois et al., 1981) as well as
Quaternary faults are scattered over the southeastern quarter of Arizonain a diffuse
region that may extend into New Mexico. Based on geomorphic recurrence rates
and the short seismic history (Sbar et al., 1980), the rate of activity in this area
appears lower than to the northwest.
Because of the scattered Quaternary fault scarps and diffuse historical seismicity
%¢~/
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Fro. 8. 1887 fault scarp and related features plotted on a transverse-mercator projection about the
pole of rotation (N53 ° latitude, W53 ° longitude) between the North American and Pacific plates (after
Atwatar, 1970 and Smith, 1978). Shown also is the WNW-ESE direction of extension inferred from
composite focal mechanism solution A (Figure 6a).
in southeastern Arizona, Sumner (1976) and Sbar et al. (1980) propose a belt of
seismicity connecting the northwest corner of Arizona with the Sonoran earthquake
epicentral area as one possibility. The existence of such a seismic belt remains
unresolved. The relationship between the Sonoran earthquake and seismicity in the
Rio Grande Rift area (Sanford et al., 1979) or to the south or west is also unknown.
Thus, the connection of the large, seemingly isolated, Sonoran earthquake to other
tectonic features remains problematic.
Two hypotheses to explain the faulting in the San Bernardino Valley will be
considered below. Atwater {1970) suggests that extensional deformation of continental lithosphere due to transform motion between the North American and Pacific
plates may extend as much as 1000 km inland from the plate boundaries. Coney
{1976) refines this hypothesis in arguing that Basin and Range extension is best
194
S T E V E N G. N A T A L I A N D M A R C L. S B A R
developed in regions previously affected by widespread thermal disturbances, possible thinning and melting of continental lithosphere, and the vast "ignimbrite flareups" of Oligocene-Miocene age. Thus, as relative motion between the North American and Pacific plates changed from convergence to transform, the thermally
weakened ignimbrite belts may have failed as a wide transform and spreading
boundary. Distributed right-lateral shear related to the San Andreas transform
superimposed upon fundamental extension within the southern Basin and Range
province (probably initiated as a form of back-arc extension) is a slight variant of
the above (Scholz et al., 1971).
Following an exercise by Atwater (1970) and Smith (1978), the Pitaycachi fault
was plotted on a transverse-mercator projection about the pole of rotation of (N53 °
latitude, W53 ° longitude) between the North American and Pacific plates (Figure
8). Shown also is the direction of extension inferred from composite fault plane
solution A. The inferred extension direction of the Pitaycachi fault is consistent with
the passive extension first hypothesized by Atwater. So also are the Quaternary
north-striking normal and northwest-striking strike-slip faults found to the west of
the Pitaycachi fault in Caborca (Merrian and Eells, 1978) and Pitiquito-La Primavera quadrangles (Longaria and Perez, 1978) of northwestern Sonora.
As a second hypothesis, there are reasons to believe that the San Bernardino
Valley may be different from surrounding Basin and Range crust. Seager and
Morgan (1979) suggest that there may be several discrete, currently active rift
systems similar to (but separate from) the Rio Grande rift within the Basin and
Range province. They believe that the San Bernardino Valley and parts of the
adjacent Animas Valley to the northeast exhibit many of the traits of the Rio
Grande rift. Anomalously low elevations for the southern San Bernardino Valley,
young volcanism (0.2 to 4 m.y.) and presumably high-heat flow suggest an active
intra-Basin and Range rift system with its own anomalous strip of mantle and crust.
Zoback and Zoback (1980) have documented abrupt transitions in orientation in
lithospheric stress in areas of extensional deformation that closely correspond to
anomalous heat flow features (e.g., Rio Grande rift and Snake River Plain). They
conclude that the streses responsible for rifting in areas of anomalously high-heat
flow are intimately linked to thermal processes. It is therefore reasonable to
postulate that the localized high-heat flow in the San Bernardino Valley could result
in localized extensional stresses and rifting.
CONCLUSIONS
A magnitude 7.4 for the 1887 Sonoran earthquake determined from seismic
moment is consistent with calculations based on maximum fault displacement,
average fault displacement, length of rupture, and felt area.* Length of the microearthquake activity is about 1.5 times the north-south extent of surface rupture,
indicating that the Pitaycachi fault is currently active over a much longer length
than previously known.
Most microearthquakes between N31 ° 15' and N30°50 ' latitude appear to define
a single westward-dipping normal fault for which the name Pitaycachi fault is
proposed. Focal depths are confined to within 15 km of the surface. The Pitaycachi
fault extends into the El Tigre Valley and may continue southward along the
western flafik to the Sierra Madre Occidental through as much as 4 ° of latitude.
The southern extent of active faulting is unknown. But probably extends beyond
this study area.
The WNW-ESE (_+20°) extension inferred from composite solution A is consistent
S E I S M I C I T Y IN T H E R E G I O N OF T H E S O N O R A N E A R T H Q U A K E
195
with regional extension predicted by transcurrent motion between the Pacific and
North American plates, and is also consistent with an hypothesis in which Seager
and Morgan (1979) suggest an active intra-Basin and Range rift zone centered on
the San Bernardino Valley. In the latter, crustal blocks, instead of responding
passively to regional extension, are controlled by a localized stress field associated
with anomalous thermal processes in this locality. The data contained within this
study do not permit differentiation between these two hypotheses.
ACKNOWLEDGMENTS
The authors wish to thank Fred Homuth of LASL and Professors Salas, Keller, and Morgan of the
University of Sonora, University of Texas at E1 Paso, and New Mexico State University, respectively,
and their students for their field support during this project.
Professors J. S. Sumner and R. F. Butler gave valuable comments and guidance during the writing of
the manuscript.
Professor W. B. Bull kindly allowed us to accompany him on two geomorphology field trips to the
1887 fault scarp and to use unpublished data in this manuscript. Darrel Herd of the U.S. Geological
Survey provided aerial photographs of the entire study area. J. R. Sumner, through personal comments,
provided valuable insight into the area's geology.
• Most importantly, we are sincerely grateful to Dan Gish for accompanying us as a field assistant in
Mexico during the entire project. Without his untiring aid, this project could never have been undertaken.
This project was funded by NSF Grant EAR-7863648 and DOE Contract EG-77-S-02-4362.
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S T E V E N G. N A T A L I A N D MARC L. S B A R
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DEPARTMENT OF GEOSCIENCES
UNIVERSITY OF ARIZONA
TUCSON, ARIZONA87521
Manuscript received June 16, 1980