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



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Science Reports

Scientific Drilling Into the San Andreas Fault Zone 



An Overview of SAFOD’s First Five Years



by Mark Zoback, Stephen Hickman, William Ellsworth, 

and the SAFOD Science Team

doi:10.2204/iodp.sd.11.02.2011

14  

Scientific Drilling, No. 11, March 2011

Science Reports 



Abstract

The San Andreas Fault Observatory at Depth (SAFOD) 

was drilled to study the physical and chemical processes 

controlling faulting and earthquake generation along an 

active, plate-bounding fault at depth. SAFOD is located near 

Parkfield, California and penetrates a section of the fault that 

is moving due to a combination of repeating microearth- 

quakes and fault creep. Geophysical logs define the San 

Andreas Fault Zone to be relatively broad (~200 m), contain-

ing several discrete zones only 2–3 m wide that exhibit very 

low P- and S-wave velocities and low resistivity. Two of these 

zones have progressively deformed the cemented casing at 

measured depths of 3192 m and 3302 m. Cores from both 

deforming zones contain a pervasively sheared, cohesion-

less, foliated fault gouge that coincides with casing deforma-

tion and explains the observed extremely low seismic veloci-

ties and resistivity. These cores are being now extensively 

tested in laboratories around the world, and their composi-

tion, deformation mechanisms, physical properties, and 

rheological behavior are studied. Downhole measurements 

show that within 200 m (maximum) of the active fault trace, 

the direction of maximum horizontal stress remains at a 

high angle to the San Andreas Fault, consistent with other 

measurements. The results from the SAFOD Main Hole, 

together with the stress state determined in the Pilot Hole, 

are consistent with a strong crust/weak fault model of the 

San Andreas. Seismic instrumentation has been deployed  

to study physics of faulting—earthquake nucleation, propa-

gation, and arrest—in order to test how laboratory-derived 

concepts scale up to earthquakes occurring in nature.



Introduction and Goals

SAFOD (the San Andreas Fault Observatory at Depth) is 

a scientific drilling project intended to directly study the 

physical and chemical processes occurring within the San 

Andreas Fault Zone at seismogenic depth. The principal 

goals of SAFOD are as follows: (i) study the structure and 

composition of the San Andreas Fault at depth, (ii) deter-

mine its deformation mechanisms and constitutive proper-

ties, (iii) measure directly the state of stress and pore pres-

sure in and near the fault zone, (iv) determine the origin of 

fault-zone pore fluids, and (v) examine the nature and signif-

icance of time-dependent chemical and physical fault zone 

processes (Zoback et al., 2007). 

Detailed planning of a research experiment focused on 

drilling, sampling, and downhole measurements directly 

within the San Andreas Fault Zone began with an interna-

tional workshop held in Asilomar, California in December 

1992. This workshop highlighted the importance of 

deploying a permanent geophysical observatory within the 

fault zone at seismogenic depth for near-field monitoring of 

earthquake nucleation. Hence, from the outset, the SAFOD 

project has been designed to achieve two parallel suites of 

objectives. The first is to carry out a series of experiments in 

and near the San Andreas Fault that address long-standing 

questions about the physical and chemical processes that 

control deformation and earthquake generation within active 

fault zones. The second is to make near-field observations of 

earthquake nucleation, propagation, and arrest to test how 

laboratory-derived concepts about the physics of faulting 

Figure 1


. Map of the Parkfield segment of the San Andreas Fault 

showing the epicenters of the 1966 and 2004 Parkfield earthquakes 

and the SAFOD drillsite (Rymer et al., 2006). The air photo shows 

the terrain in the area of the SAFOD drill site and the epicenter of 

the 1966 Parkfield earthquake.

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Scientific Drilling, No. 11, March 2011  

15

Science Reports

Science Reports 

scale up to earthquakes occurring in nature. In the years 

following the Asilomar workshop, dozens of planning 

meetings were held to synthesize the research questions of 

highest scientific priority that were deemed to be operation-

ally achievable. Numerous other meetings were also held 

related to site selection and to detailed operational plans for 

drilling, sampling, downhole measurements, and long-term 

monitoring.

When planning of the EarthScope initiative got underway 

at the National Science Foundation (NSF) in the late 1990s, 

the project was named SAFOD and became one of the three 

components of EarthScope along with the Plate Boundary 

Observatory (PBO) and USArray. In 2002, a 2.2-km-deep 

Pilot Hole was funded by the International Continental 

Scientific Drilling Program (ICDP) and was drilled at the 

SAFOD site. The main SAFOD project started when NSF 

funded the EarthScope proposal in 2003, with substantial 

cost sharing and operational support for SAFOD provided by 

the U.S. Geological Survey (USGS), ICDP, and other agen-

cies.

The SAFOD operational plan was designed to address a 



number of first-order scientific questions related to fault 

mechanics in a hostile environment where the mechanically 

and chemically altered rocks in the fault zone are subject to 

high mean stress, potentially high pore pressure, and ele-

vated temperature. Some of these questions are listed below.

• What are the mineralogy, deformation mechanisms, and 



constitutive properties of fault gouge? Why do some 

faults creep? What are the strength and frictional 

properties of recovered fault rocks at in situ conditions 

of stress, fluid pressure, temperature, strain rate, and 

pore fluid chemistry? What determines the depth of 

the shallow seismic-to-aseismic transition? What do 

mineralogical, geochemical, and microstructural 

analyses reveal about the nature and extent of water-

rock interaction?

• What is the fluid pressure and permeability within and 



adjacent to fault zones? Are there super-hydrostatic 

fluid pressures within some fault zones, and through 

what mechanisms are these pressures generated 

and/or maintained? How does fluid pressure vary 

during deformation and episodic fault slip (creep and 

earthquakes)? Do fluid pressure seals exist within or 

adjacent to fault zones, and at what scales?

• What are the composition and origin of fault-zone fluids 



and gases? Are these fluids of meteoric, metamorphic, 

or mantle origin (or combinations of the three)? Is 

fluid chemistry relatively homogeneous, indicating 

pervasive fluid flow and mixing, or heterogeneous, 

indicating channelized flow and/or fluid compart- 

mentalization?

• How do stress orientations and magnitudes vary across 

fault zones? Are principal stress directions and magni-

tudes different within the deforming core of weak fault 

zones compared to the adjacent (stronger) country 

rock, as predicted by some theoretical models? How 

does fault strength measured in the near field com-

pare with depth-averaged strengths inferred from 

heat flow and regional stress directions? What is the 

nature and origin of stress heterogeneity near active 

faults?

• How do earthquakes nucleate? Does seismic slip begin 



suddenly, or do earthquakes begin slowly with accel-

erating fault slip? Do the size and duration of this pre-

cursory slip episode, if it occurs, scale with the magni-

tude of the eventual earthquake? Are there other 

precursors to an impending earthquake, such as 

changes in pore pressure, fluid flow, crustal strain, or 

electromagnetic field?

• How do earthquake ruptures propagate? Do they propa-

gate as a uniformly expanding crack, as a slip pulse, or 

as a sequence of slipping high-strength asperities? 

What is the effective (dynamic) stress during seismic 

faulting? How important are processes such as shear 

heating, transient increases in fluid pressure, and 

fault-normal opening modes in lowering the dynamic 

frictional resistance to rupture propagation?

• How do earthquake source parameters scale with magni-



tude and depth? What is the minimum size earthquake 

that occurs on faults? How is long-term energy release 

rate partitioned between creep dissipation, seismic 

radiation, dynamic frictional resistance, and grain 

size reduction (determined by integrating fault zone 

monitoring with laboratory observations on core)?

• What are the physical properties of fault-zone materials 

and country rock (seismic velocities, electrical resistiv-

ity, density, porosity)? How do physical properties from 

core samples and downhole measurements compare 

with properties inferred from surface geophysical 

observations? What are the dilational, thermoelastic, 

and fluid-transport properties of fault and country 

rocks, and how might they interact to promote either 

slip stabilization or transient over-pressurization dur-

ing faulting?

• What processes control the localization of slip and strain? 

Are fault surfaces defined by background microearth-

quakes and creep the same? Would active slip surfaces 

be recognizable through core analysis and downhole 

measurements in the absence of seismicity and/or 

creep?


In addition, a substantial body of evidence indicates that 

slip along major plate-bounding faults like the San Andreas 

occurs at much lower levels of shear stress than expected, 

based upon laboratory friction measurements on standard 

rock types and assuming hydrostatic pore fluid pressures 

(i.e., it is a weak fault). Yet, the cause of this weakness has 

remained elusive (Hickman, 1991). In the context of the San 

Andreas, two principal lines of evidence indicate that the 




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