San Andres Faults
San Andres Faults
San Andres Faults
a r t i c l e
i n f o
Article history:
Received 1 April 2009
Received in revised form 12 August 2009
Accepted 13 August 2009
Available online 18 September 2009
Edited by R.D. van der Hilst
Keywords:
fault strength
friction
lithosphere
nite element model
a b s t r a c t
The strength of secondary faults within plate-boundary zones and of master faults like the San Andreas has
been controversial for decades. We use a global nite-element code with a variable-resolution grid and
global plate-driving forces to determine whether the effective friction on the San Andreas fault is high
(* 0.61), intermediate (* 0.30.5) or low (* 0.2), whether a single value of * can be used for all
mapped faults within California, and whether weakening of the ductile lower crust associated with faulting is
important. We compare our model results with existing data on fault slip-rates, GPS velocities, stress eld,
and earthquake depth distribution. The comparison indicates that all faults are weak (* 0.2), and that
additional weakening of major faults is important. All viable solutions also indicate that weakening of the
lower crust below major faults is necessary. The strongest faults in the region have * in the range 0.20.05.
The San Andreas fault is a very weak fault among weak faults, with * < 0.05. Our results also show that a
global code with appropriate grid-renement and driven by global plate motions can reasonably reproduce
regional tectonics.
2009 Elsevier B.V. All rights reserved.
1. Introduction
There is little agreement concerning the brittle strength of large
strike-slip faults like the San Andreas, or even the strength of faults in
general. Determining the strength of faults has signicant implications for structural geology, tectonics, and seismology. According to
classic fault mechanics theory (Anderson, 1942; Byerlee, 1978), the
San Andreas fault should not be able to slip in its current orientation.
Other faults, like low-angle normal ones, should not even exist, and
those that do exist exhibit anomalous seismicity (Wernicke, 1995;
Axen, 2007). Several authors have shown that at least some large
faults in different tectonic settings appear to be much weaker than
predicted by Byerlee's Law (e.g. Mount and Suppe, 1987; Zoback et al.,
1987; Bird and Kong, 1994; Carena et al., 2002; Townend and Zoback,
2004; Bilotti and Shaw, 2005; Suppe, 2007).
Concerning the San Andreas fault, there are arguments both in
favor of it being weak (Lachenbruch and Sass, 1992; Bird and Kong,
1994; Zoback, 2000; Hardebeck and Hauksson, 2001; Townend and
Zoback, 2004) and of it being strong (Scholz, 2000a,b). Even though
the hypothesis of a weak San Andreas fault currently encounters more
favor, the denition of weak San Andreas itself varies considerably
and the range of proposed effective friction coefcients (*) is rather
wide, from 0.05 (Zoback et al., 1987; Bird and Kong, 1994; Townend
and Zoback, 2004), to 0.1 (Humphreys and Coblentz, 2007) to 0.3
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Fig. 1. Global grid showing temperature at 100 km depth as deviation from the average
calculated at that depth, and MCM velocity vectors (blue arrows) at 100 km depth.
Faults and plate boundaries are in yellow. PA = Pacic, NA = North America,
CO = Cocos, CA = Caribbean, NZ = Nazca, SA = South America. (For interpretation of
the references to color in this gure legend, the reader is referred to the web version of
this article.)
Table 1
Parameters used in SHELLS calculations.
Parameter
Value
0.85
2816 kg m 3
3332 kg m 3
3125 kg m 3
1032 kg m 3
1.00
9.8 m s 2
273 K
1223 K
1673 K
2.7 J m 1 s1 K 1
3.2 J m 1 s 1 K 1
7.27 10 7 J m 3 s 1
1412 K
6.1 10 4 K m 1
2.4 10 5 K 1
3.94 10 5 K 1
0.333333
4000 K
18,314 K
2.3 109 Pa s1/n
9.5 104 Pa s1/n
North America
4761
6996
1363
Most of these parameters are dened and described in detail in Bird (1989).
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Fig. 2. Variable-resolution grid, global view. The largest elements at global level have
sides of 2 103 km away from plate boundaries, and 1 103 km at plate boundaries.
NA = North American plate, PA = Pacic plate, CO = Cocos plate, NZ = Nazca plate,
CA = Caribbean plate. Plate boundaries from Bird (1999).
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Fig. 3. Local, high-resolution grid, with topography from ETOPO2 (National Geophysical Data Center, 2006) and fault elements (thick black lines). M = Mendocino triple junction,
WH = WasatchHurricane fault system, SAF = San Andreas fault.
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Fig. 4. Perspective view of the SCEC Community Fault Model, simplied version that
represents fault segments as rectangular patches (Plesch et al., 2007). SAF = San
Andreas fault, SJFZ = San Jacinto fault zone.
Fig. 5. Overview of results of simulations performed with different driving mechanisms (NUVEL-1A, or MCMs from Schuberth et al. (2009)). For each value of * we also test various
combinations of slip-dependent weakening (as dened by Bird and Kong (1994)) both in the upper and lower crust. Red box shows the range in which all acceptable models (as
dened in section 3) exist. No acceptable models can be produced outside this range of fault strengths. Key for reading the x-axis: fri0.01_weak0.9_bwk0.5 = effective friction of 0.01
with 90% slip-dependent weakening in the upper crust and 50% slip-dependent weakening in the lower crust. Slip-rates and horizontal velocities are sensitive to changes in all input
parameters, while the sensitivity of SHmax is mostly limited to weakening. The characteristic saw-tooth pattern reveals the prominent effect of weakening the faults in the upper and
lower crust, while the broad overall trend of the curves reects the changes in initial m*. It is apparent that the effects of initial * and weakening are of rst-order importance, while
the effects of changing the plate-driving mechanism are second-order. (For interpretation of the references to color in this gure legend, the reader is referred to the web version of
this article.)
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Fig. 6. Effect of introducing linear slip-dependent weakening in the upper (ucw) and lower crust on slip-rate RMS errors. Signicant upper and lower crustal weakening are needed
to achieve good results. The star represents our best model, which corresponds to model 2 of Figs. 8 and 10.
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Fig. 7. Examples of calculated depth to the brittleductile transition (BDT). Maximum depth of seismicity, which we use as a proxy for BDT depth, is shown in the top left corner. We
created this map by taking the deepest event in each 5 5 km cell from the combined Southern California Earthquake Center (SCECDC) and Northern California Earthquake Center
(NCEDC) earthquake catalogs, without any averaging or interpolation: blank cells have no earthquakes. (a) represents one of our best models with upper-crust linear slip-dependent
weakening, showing a reasonable BDT depth along faults. (b) is the strong-faults case (model 1, Fig. 10) with no weakening, which shows too shallow BDT depth. (c) is a model with
high initial friction and signicant weakening, which scores at acceptable levels on slip-rates, GPS velocity and SHmax mean error, but for which the resulting BDT depth is
unacceptably low. ucw = upper crust slip-dependent weakening. lcw = lower crust slip-dependent weakening, which is set as follows for the three fault maps: 30% below the SAF
alone in (a), none in (b), and 60% uniform (non slip-dependent) below all faults in (c).
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Fig. 8. Diagrams showing the results of several different combinations of parameters for models driven by NUVEL-1A. Models are grouped according to presence or absence of slipdependent weakening of faults in the upper and lower crust. ucw = upper crust slip-dependent weakening, lcw = lower crust slip-dependent weakening. Acceptable models must
have all four vertices of the corresponding polygon fall within the gray-shaded acceptable range (as dened in section 3 of the main text) on each axis of the diagram. Top left
diagram represents the no-weakening case. The results are improved signicantly by the introduction of slip-dependent weakening at faults in at least one crustal layer, as shown in
the two bottom diagrams. The best results are obtained when weakening is introduced in both upper and lower crust (top-right diagram). Our best model is represented by the solid
red line. Both extremely low initial friction (* = 0.01) and the absence of weakening coupled with high initial friction (* = 0.6) consistently produce poor results. (For
interpretation of the references to color in this gure legend, the reader is referred to the web version of this article.)
Andreas fault is one of the very few known examples worldwide, need
to have very low (<<0.1) intrinsic *. In all other cases there is no
need to invoke exceedingly weak materials or anomalously high pore
uid pressures on large swaths of these faults in order to explain the
apparent low frictional strength.
A second important outcome of our simulations is that the
integrated strength of faults must include contributions from both
the brittle and the ductile parts of the crust, at least for major faults.
The signicance of this is that major faults like the San Andreas, the
Altyn Tagh, the Red RiverAilao Shan, the North Anatolian Fault and
many others likely exist at lithospheric level as postulated by several
authors (Tapponnier et al., 1986, 2001; Thatcher, 1995; Jackson,
2002), while secondary faults in the network may well be conned to
the brittle crust. Lack of signicant weakening in the lower crust is the
probable cause of failure to match observed slip-rates in the case of
several faults in our models that are effectively branches of the San
Andreas system (e.g. San Jacinto, Hayward and Calaveras), but which
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Fig. 9. Fault slip-rates plotted as slip patches for (a) our best model (model 2 of Figs. 8 and 10) and (b) for the high-friction, strong-faults case (model 1 of Fig. 10). The color of sliprate patches represents the dominant slip component. Numbers next to the patches are slip-rates in mm/yr. Both slip-rates and type of faulting match observations much better
when the strength of faults is low and slip-dependent weakening is introduced. In addition to a marked slip-rate increase in (a) when compared to (b), we also observe that the
compression along the Big Bend of the SAF visible as thrust faulting (blue) in (b) disappears in (a). The SAF becomes a dominantly right-lateral strike-slip fault and compression is
now conned to faults on either side of it. SAF = San Andreas fault, SJFZ = San Jacinto fault zone. (For interpretation of the references to color in this gure legend, the reader is
referred to the web version of this article.)
Acknowledgments
This project is funded by Deutsche Forschungsgemeinschaft (DFG)
grants CA691/1-1 and CA691/1-2. The authors would like to thank
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Fig. 10. Comparison between observed slip-rates and slip-rates calculated for several models. Model 1 is the strong-faults case: all fault slip-rates are reduced to as little as 5%10% of their observed values. Model 2 is our best model (star in
Fig. 6), model 3 is another model that shows a slightly better t for slip-rates on several important faults and that still produces acceptable results for the three other parameters (BDT depth, SHmax, GPS velocities, see Fig. 8).
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