By Mark Zoback, Stephen Hickman, William Ellsworth, and the safod science Team doi: 10. 2204/iodp sd. 11. 02. 2011

Scientific Drilling, No. 11, March 2011  

21

Science Reports

that the San Andreas Fault has very low permeability and 

hydrologically separates the Pacific and North American 

plates (Wiersberg and Erzinger, 2008). 

Downhole Measurements.

  A wide range of downhole 

measurements was carried out as part of SAFOD Phases 1 

and 2 (Table 3). As the structure and properties of the San 

Andreas Fault Zone are of most importance, we show in 

Fig. 4A a summary of the geophysical logs from Phase 2 

along with some of the main lithologic units encountered. 

An approximately 200-m-wide damage zone of anoma-

lously low P-  and S-wave velocities and low resistivity 

(Fig. 4A) is interpreted to be the result of both physical 

damage and chemical alteration of the rocks due to faulting 

as well as the unusual, fault-related minerals (discussed 

above) that were noted during drilling. There are also a num-

ber of localized zones where the physical properties are even 

more anomalous. Repeated measurements of the shape of 

the steel casing deployed in the borehole revealed that the 

steel casing was being deformed by fault movement in at 

least two places. Figure 4C shows the casing radius (as 

measured using a 40-finger caliper) as a function of position 

around the hole. While the amount of deformation associated 

with the 3302-m shear zone is more pronounced than the 

Andreas Fault Zone (Solum et al., 2006). Moore and Rymer 

(2007) demonstrated that some of the serpentinite in the 

fault zone has been altered to talc, an unusual mineral in that 

it has exceptionally low frictional strength and is thermo-

dynamically stable over the range of depths and pressures 

characteristic of the upper crust in this region. They spec-

ulated that if talc is widespread in the fault zone, it could 

explain both the strength of the fault and its creeping behav-

ior.

Gases coming into the well as the borehole was being 

drilled yielded a great deal of useful data. This technology, in 

which gas is separated from the drilling mud as it comes to 

the surface, was also used in the Pilot Hole where gas anoma-

lies correlated with shear zones in the granite/granodiorite 

(Erzinger et al., 2004). During Phases 1 and 2, implementa-

tion of this technology showed a number of important corre-

lations with major faults and geologic boundaries. One 

finding of particular interest reported by Wiersberg and 

Erzinger (2007) is that there is a marked difference in the 

concentration of 

3

He/

4

He across the San Andreas Fault. On 

the southwest side of the fault this ratio is ~0.4, whereas on 

the northeast side of the fault it is ~0.9. This data and differ-

ences in the relative concentrations of hydrogen, carbon 

dioxide, and methane on the two sides of the fault indicate 

Table 2.

 Summary of Physical Samples Obtained from SAFOD.

Types of samples

Phase 1

Phase 2

Phase 3

Washed cuttings, small sample bags

3 sets, every 3 m

3 sets, every 3 m

intermittent depths

Washed cuttings, large (15 cm x 25 cm) sample bags

every 30 m

every 30 m

Washed cuttings, large (25 cm x 43 cm) sampole bags

every 91 m

every 91 m

Unwashed cuttings

every 3m

every 3 m

Drilling mud

every 30 m

every 30 m

Core

8.5 m at 1.5 km MD,  

10 cm diameter

3.7 of 6.6 cm diameter 

core at 4 km MD

Core 1.1 run, 11.08 m 

3141.1–3153.6 m MD

10 cm diameter

11 m at 3.0 km MD,  

10 cm diameter

Core 2 runs 1–3, 12.03 m, 

3186.7–3200.4 m MD

10 cm diameter

Core 3 runs 4–5, 16.15 m,

3294.9–3313.5 m MD

10 cm diameter

Sidewall cores

52 small (2 cm dia. 

x 2.5 cm) side wall 

cores between 3.1 

and 4.0 km MD

Miscellaneous rock samples

3 samples

40 samples

Table 3.

 SAFOD Geophysical Logging Data.

Run

Depth Range 

(Measured Depth)

Logging Technique

Parameters Measured

Run 1

602.5–1443.5 m

Open Hole, Wireline

Density, porosity, gamma, caliper, resistivity, cross-dipole sonic 

velocity, FMI

Run 2a

1368–2030 m

Open Hole, Wireline

Density, porosity, gamma, caliper, resistivity, sonic velocity, FMI, 

UBI, ECS

Run 2b

1890–3043 m

Open Hole, Pipe Conveyed

Density, porosity, gamma, caliper, resistivity, sonic velocity, FMI

Run 3

1356–3033 m

Cased Hole, Wireline

Sonic velociy, elemental chemistry, cement bond

Run 4

3045–3712 m

Open Hole, Logging While Drilling

Density, porosity, gamma, caliper, resistivity, FMI

Run 5

3045–3965 m

Open Hole, Pipe Conveyed

Density, porosity, gamma, caliper, resistivity, sonic velocity, FMI

Runs 6–11*

2953–3815 m

Cased Hole, Wireline

Caliper, direction, temperature

* Runs 6–11 include caliper logs run 6 different times between September 2005 and June 2007

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22  

Scientific Drilling, No. 11, March 2011

Science Reports

(Fig. 1). Recent relocations of the SAFOD target earth- 

quakes indicate that the SF/LA cluster correlates with the 

fault at 3413 m, as shown in Fig. 2D (Thurber et al., 2010). 

This fault defines the northeastern edge of the damage zone 

and has geophysical characteristics very similar to the SDZ 

and CDZ (Fig. 2A); hence, it has been designated as the 

Northeast Boundary Fault (NBF). However, unlike the SDZ 

and CDZ, no casing deformation was detected on the NBF in 

any of the caliper logs run in 2005 through 2007 (Runs 6–11, 

Table 3).

A number of other important downhole measurements 

were made during Phases 1 and 2. Boness 

and Zoback (2006) reported that to within 

200 m of the active trace of the fault, the 

direction of maximum horizontal stress re-

mains at a high angle to the San Andreas 

Fault, consistent with measurements 

 

made in SAFOD at greater distances and 

with regional data that imply that fault  

slip occurs in response to low resolved 

shear stress.  Zoback and Hickman (2007) 

reported that stress magnitudes are 

consistent with the prediction of high 

 

mean stress within the fault zone (Rice, 

1992; Chery et al., 2004) and a classical 

Anderson/Coulomb reverse/strike-slip 

stress state outside it. Together with the 

stress state determined in the Pilot Hole 

(Hickman and Zoback, 2004), the results 

from the SAFOD Main Hole are consistent 

with a strong crust/weak fault model of the 

San Andreas. Almeida et al. (2005) carried 

out a paleostress analysis using slip direc-

tions on the faults encountered in the core 

obtained at the end of Phase 1 and also 

found a direction of maximum horizontal 

compression at a very high angle to the 

San Andreas Fault.

Further support for the low frictional 

strength of the San Andreas comes from 

temperature measurements in the SAFOD 

Main Hole. Heat flow data from the Pilot 

Hole were consistent with measurements 

made at relatively shallow depth and imply 

no frictionally generated heat by the San 

Andreas Fault (Williams et al., 2004). Heat 

flow measurements made in the Main Hole 

indicate no systematic change in tempera-

ture as a function of distance from fault. 

Hence, these data are also consistent with 

an absence of frictionally generated heat 

(Williams et al., 2005).

The possibility of extremely high pore 

pressure within the San Andreas Fault 

3192-m shear zone, both of these zones represent portions of 

the overall San Andreas Fault Zone in which active creep 

deformation is occurring. We refer to the actively deforming 

zones at 3192 m as the Southwest Deforming Zone (SDZ) 

and 3302 m as the Central Deforming Zone (CDZ). Note the 

remarkable similarity of the anomalously low compressional 

(Vp) and shear (Vs) wave velocities and resistivity within 

these two deformation zones (Fig. 4B). These two shear 

zones were primary targets for coring during Phase 3.

The HI earthquake cluster occurs on the SDZ about 100 m 

below the point where the borehole passed through this fault 

Figure 4.

 [A] Selected geophysical logs and generalized geology as a function of measured 

depth along the Phase 2 SAFOD borehole. The dashed red lines indicate some of the many 

faults encountered. The thick red lines indicate where fault creep deformed the Phase 2 

cased borehole at the SDZ and CDZ. Depth in this figure represents the measured depth 

along the length of the wellbore. [B] The SDZ and CDZ correlate with localized zones (shown 

in red) where the geophysical log properties from Phase 2 are even more anomalous than in 

the surrounding damage zone. The same is true of the fault at the northeast boundary of the 

damage zone, the NBF. [C] After the borehole was cased and cemented, a 40-finger caliper 

(see photo) was used to measure the casing radius at various times (the depth scales 

are the same as in [B]). The caliper data obtained on 6 October 2005 showed significant 

casing deformation within the CDZ. When the casing was resurveyed on 5 June 2007, more 

deformation was observed at the depth of the CDZ, and slight deformation was observed 

at the SDZ. Although the NBF is geophysically quite similar to the SDZ and CDZ (see [A]) 

and is associated with the SF and LA earthquake sequences (Figs. 2 and 3), no casing 

deformation was identified at that depth.

3000

3100

3200

3300

3400

3500

3600

3700

3800

3900

4000

Measured Depth (m)

Arkosic

Sandstone

Shale

Siltstone

Claystone

10

0

Ω−m

10

2

10

1

Vp

3

4

5

6

km s

-1

2

3

km s

-1

Vs

Damage Zone 

1

2

3

4

5

6

7

 

 

1

2

3

4

5

6

7

3185

3190

3195

3200

Measured Depth (m)

Resistivity

Resistivity

Resistivity

Vp

Vp

Vp

Vp

Vp/Vs

Resistivity

Vs

Vs

Vs

Vs

Vp/Vs

Vp/Vs

Vp/Vs

   Vp, Vs (km sec    )

-1

 

Resistivity (

Ω

-m)

         Vp/Vs

Geologic Plate Boundary Casing Deformation

Siltstone

Siltstone

Shale

Shale

Shale

3302 m

Central 

Deforming Zone

3192 m

Southwest 

Deforming Zone

Resistivity

3295

3300

3305

3310

6 Oct. ‘05

5 June ‘07

5 June ‘07

Casing Deformation

SDZ

CDZ

NBF

SDZ

CDZ

NBF

A

B

C

Measured Depth (m)

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