Stress Orientations Around the San Andreas Plate Boundary System
Heidi Houston
Dept. of Earth and Spaces Sciences, UCLA
595 Young Drive East
Los Angeles, CA 90095-1567
[email protected]
The state of stress around the San Andreas Fault system has provided key constraints on
the mechanical behavior of the strike-slip portion of the Pacific-North America plate
boundary, with considerable potential implications for earthquake generation and hazard.
My current and recent work on stress orientations with graduate student Ann-Sophie Provost
furnishes constraints on fault friction and mechanics, issues that will be addressed by the PBO
and SAFOD components of EarthScope.
We have recently completed a study of the orientations of the principal stresses in the
vicinity of the creeping section on the central SAF based on inversions of thousands of
earthquake focal mechanisms (Provost and Houston, 2001). The key result is that maximum
horizontal compression immediately adjacent to the SAF is oriented at very high angles to its
trend; this implies that the creeping section on the central SAF forms a very narrow
mechanically-weak zone. The cause of the weakness may be either low intrinsic friction or
high pore pressure, but the stress orientations are not consistent with velocity-weakening
behavior and a "normal" level of friction. The stress orientations around the creeping section
contrast somewhat with those in southern California, where the SAF is locked (although it
may still be weaker than expected from laboratory studies of friction).
The extension of this analysis to the Bay Area and farther north along the strike-slip
plate boundary system shows that the extreme weakness of the creeping section also contrasts
with a stronger strike-slip fault system to the north (Figure 1, after Provost and Houston,
submitted). Towards the north, the angles between the orientations of maximum compression
and the trends of the major strike-slip faults decrease in a rather systematic manner. Thus, the
fault system in the north appears mechanically stronger than in the creeping section,
suggesting that the SAF system has evolved from a young, multiple-stranded plate boundary
in the north to a mature strike-slip plate boundary in the central, creeping section where
greater total slip has occurred. The major strike-slip faults in the north dip more shallowly
than in the creeping section; it has been suggested that that they were originally thrust faults
in the forearc of the Cascadia subduction zone that have been reactivated in a strike-slip sense
following the northward passage of the Mendocino triple junction (Castillo and Ellsworth,
1993). We further note anomalous stress orientations in a portion of the Sacramento Valley.
Anomalous east-west orientations of maximum horizontal compression are seen for three
groups just west of Sutter Buttes (groups 48, 49, and 50 in Figure 2, after Provost and
Houston, submitted). Only 10 to 20 km away the maximum horizontal compression lies in
the more typical nearly north-south direction for groups 51, 52, and 53. In both cases (i.e.,
for groups 48 through 53), the seismicity in this region is relatively deep, between 10 and 25
km (Figure 3, after Provost and Houston, submitted). The cause of the anomalous
orientations is currently unknown. They may be related to stresses induced by a putative
"isolated fragment of subducted slab" that has been tomographically imaged below this
region by Benz et al. (1992). Another possible cause could be connected to the cessation of
large-scale subduction-related volcanism as the southern edge of the Juan de Fuca slab
migrates northward. Indeed, the current location of the slab edge is believed to be close to
latitude 39.5°, which raises the further possibility that asthenospheric flow around the edge of
the plate could be affecting the stress orientations.
Our studies of stress orientations in California have relevance to the eventual analysis and
interpretation of PBO and SAFOD data. In general, the measurement of deformation
provides information complementary to the stress measurements; analysis of both stress and
deformation could constrain crustal and fault rheology. Stress orientations near the creeping
section of the SAF may facilitate interpretation of data on intrinsic friction and pore pressure
from SAFOD. Our results are germane to PBO siting issues as well. Good spatial coverage in
the region of the anomalous stress could be instrumental in illuminating the origin of this
feature, as well as its implications for crustal strength. Consideration should also be given, in
PBO design, to clarifying the role of the Eastern California Shear Zone, and thus to
delineating the geometry and mechanical working of the broader plate boundary.
References
Benz, H.M., G. Zandt, and D.H. Oppenheimer, Lithospheric Structure of Northern California
from Teleseismic Images of the Upper Mantle, J. Geophys. Res., 97, 4791-4807, 1992.
Castillo, D.A., and W.L. Ellsworth, Seismotectonics of the San Andreas Fault System between
Point Arena and Cape Mendocino in Northern California: Implication for the
Development and Evolution of a young transform, J. Geophys. Res., 98 (B4), 65436560, 1993.
Provost, A.-S., and H. Houston, Orientation of the stress field surrounding the creeping
section of the San Andreas fault: Evidence for a narrow mechanically-weak fault zone, J.
Geophys. Res., 106 (B6), 11,373-11,386, 2001.
Provost, A.-S., and H. Houston, Constraints from stress orientations on the evolution of
frictional strength along the San Andreas fault, J. Geophys. Res., submitted.
40˚
39.5˚
39˚
38.5˚
38˚
37.5˚
37˚
36.5˚
Pacific
Ocean
36˚
km
0
-124˚
50
-123˚
100
-122˚
-121˚
-120˚
Figure 1. Orientations of maximum horizontal compressive stress from Provost and
Houston (2001) combined with those from Provost and Houston (submitted). Light grey
bars represent stress orientations from groups of seismicity classified as on-fault
seismicity, that is, events occurring on a major regional strand of the strike-slip
plate-boundary fault system; dark grey bars represent stress orientations from groups
of seismicity classified as off-fault events. Note the systematic increase in the
difference between on-fault and off-fault stress orientations as one moves farther south,
primarily due to the increase in the angle between maximum horizontal compression
and the trend of the major faults. After Provost and Houston (submitted).
40˚
24
14
53
25
11
52
19
12
5
39.5˚
13
10
2
21
15
7
9
6
39˚
4
3
38.5˚
17
20
22
16
40
38
39
33
27 35
29
46
43
41
45
58
34
44
37
42
50
55
38˚
-124˚
Sutter
Buttes
48
31
30
26
28
km
0
49
32
Pacific
Ocean
54
50
8
36
1
51
23
18
57
56
-123.5˚
-123˚
-122.5˚
-122˚
-121.5˚
Figure 2. Orientation of maximum horizontal compressive stress in the Bay Area and Northern
California determined from inversions of focal mechanisms. Plus symbols show locations of
seismicity used. There are ~2800 events with ML • 1.5. Numbers identify groups of seismicity.
Note anomalous stress orientations in groups 48, 49, and 50 near Sutter Buttes. Region inside
dashed line is expanded in Figure 3. After Provost and Houston (submitted).
40.0
40˚
11
Latitude (degrees)
53
53
24
14
19
52
52
12
5
39.5
18
54
13
2
51
39.5˚
21 50
10
7
51
23
15
17
9
49
48
8
10
20
4
Depth (km)
36
km
0
6
0
Sutter
Buttes
49
16
48
39.0
54
50
30
40
39˚
46
-122.5˚
43
-122˚
10 20
-121.5˚
1
38
39
41
Figure 3. (right) Focal3 mechanisms of seismicity45
in vicinity of Sutter Buttes, CA (dashed
32 show locations of seismicity. Note that groups 50 and 51
region in Figure 2). Plus symbols
33 (left) North-south cross-section of seismicity
have quite different focal mechanisms.
illustrating anomalously large depths
for groups 48 to 53. Different symbols
differentiate the
27 35
58
34
groups of seismicity.
44
29
31
30
26
28
37
42
55
57
56