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  

25

Science Reports

~2.5 km from the array. The EM trace appears three times 

because the EM signal was recorded at three different gains. 

Note that the EM signal appears at the same time as the seis-

mic waves. Hence, the EM signal is the result of shaking of 

the coil within the Earth’s magnetic field by the seismic 

waves as they pass the instrument.

selected that isolated all electrical and opti-

cal control lines and all sensors from contact 

with the wellbore fluid. The system was desi-

gned to be positively coupled to the casing 

and fully retrievable for maintenance when 

required. 

The installation of the SAFOD observa-

tory was completed on 28 September 2008. 

The observatory instruments were deployed 

approximately 100 m above the Hawaii tar-

get earthquake zone (Figs. 2, 5). As shown 

schematically in Fig. 7, the observatory 

instrumentation consisted of five pods con-

taining different types of sensors. Pods 1 

and 3 each contained a 3-component seismo-

meter and a 3-component accelerometer, 

Pods 2 and 4 each contained a 2-axis tilt-

meter, and Pod 5 contained a 3-component 

seismometer and accelerometer as well as a 

passive electromagnetic (EM) coil. The goal 

of the EM ex-periment was to determine if 

electromagnetic waves are radiated by the earthquake 

source. All of the instruments were housed in sealed steel 

pods that isolate them from contact with the wellbore fluids. 

The pods were attached to the outside of steel pipe (6-cm 

‘EUE’ tubing) and coupled to the casing by decentralizing 

bow springs. The seismic and tilt systems were completely 

independent of each other, with separate power and data tele-

metry lines encapsulated in 6.4-mm-diameter stainless steel 

tubing with pressure-tight connections in and out of the 

pods. 

The seismic system was based on the Oyo Geospace 

DS150 digital borehole seismometer with a set of 

3-component, 15-Hz Omni-2400 geophones in each sonde. 

MEMS accelerometers replaced the geophones in addi- 

tional DS150 units. The passive EM coil in Pod 5 was also 

digitized by a DS150. Fiber-optic telemetry was used to 

transmit the 4000-sample-per-second data from all seven 

DS150 units to the surface, where they were recorded on a 

USGS Earthworm computer system. The Earthworm system 

archived the data locally on LT3 tapes, downsampled se- 

lected channels to 250 samples per second and transmitted 

them to the Northern California Seismic Network (NCSN) 

where they were integrated into the real-time data system 

and archived at the Northern California Earthquake Data 

Center (NCEDC). Continuous full-sample-rate data are 

archived at the NCEDC and at the IRIS Data Management 

Center. The two borehole tiltmeters were manufactured by 

Pinnacle Technologies. Each tiltmeter produced two chan-

nels of tilt data—recorded at one sample per 3 seconds—

which were transmitted to the NCEDC for processing and 

archiving. 

An example of the data produced by the SAFOD observa-

tory instruments is shown (Fig. 8) for an earthquake located 

Tiltmeter

Tiltmeter

Tiltmeter

EM coil

Magnetometer

EUE

 Tub

ing

Tubing-encapsulated

conductors and optical fibers

Tubing

encapsulated

coaxial cable

Cable Head

Pressure transducer

Seismometer

MEMS accelerometer

Seismometer

MEMS accelerometer

Seismometer

MEMS accelerometer

50 m

50 m

Steel Casing

Figure 7.

 Schematic diagram of the instrumentation deployed in the SAFOD observatory 

above the location of the HI repeating earthquake sequence (see Fig. 5).

Figure 8.

 Seismograms from an M 1.3 microearthquake on 30 

September 2008 recorded on the SAFOD observatory. The origin 

time of the microearthquake is shown by the dashed red line. The 

lower three traces are the output of the passive electromagnetic 

coil.

0.0

0.5

1.0

1.5

2.0

2.5

3.0

September 30, 2008 04:15:51 M 1.3

Seconds

Pod 1 Seis #1

Pod 1 Seis #2

Pod 1 Seis #1

Pod 1 Accel #1

Pod 1 Accel #2

Pod 1 Accel #1

Pod 3 Seis #1

Pod 3 Seis #2

Pod 3 Seis #1

Pod 3 Accel #1

Pod 3 Accel #2

Pod 3 Accel #1

Pod 5 Accel #1

Pod 5 Accel #2

Pod 5 Accel #1

Pod 5 Seis #1

Pod 5 Seis #2

Pod 5 Seis #1

Pod 5 EMI

Pod 5 EMI

Pod 5 EMI

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Scientific Drilling, No. 11, March 2011

Science Reports

then cemented in place. Each loop was anchored at the upper 

end at 9 m depth. One loop was anchored at the lower end at 

864 m, and the other at 782 m, making strainmeters of 855 m 

and 773 m length, respectively. Although the longer loop 

failed in September 2007, vertical strain data continues to be 

produced from the shorter loop. Coseismic strain steps for 

local events have been reported by Blum et al. (2010) that are 

in general agreement with elastic dislocation theory.

Summary

We have already learned much about (i) the structure and 

physical properties of the fault zone at depth, (ii) the compo-

sition of fault zone rocks, (iii) the stress, temperature, and 

fluid pressure conditions under which earthquakes occur, 

and (iv) the absence of deep-seated fluids in fault zone proc-

esses. With the distribution of the Phase 3 core to research-

ers around the world now underway, we can expect new 

insights into the physical and chemical mechanisms control-

ling faulting and fault zone evolution within this major plate 

boundary fault. In addition, the observatory, even in its cur-

rently reduced state, is providing high-quality near-field 

seismograms that may lead to novel observations of rupture 

nucle-ation and other insights into the nature of the earth-

quake source and structure of the fault at seismogenic depth. 

Acknowledgements

Scores of scientists, graduate students, engineers and 

technicians too numerous to name contributed immeasur-

ably to the success of SAFOD. We would particularly like to 

thank Louis Capuano and Jim Hanson of ThermaSource, 

Inc., the prime drilling contractor, and the many individuals 

who served on the SAFOD Advisory Board and Technical 

Panels. We would especially like to thank Roy Hyndman of 

the Pacific Geoscience Center who served as Chair of the 

SAFOD Advisory Board. Funding for the project was provid-

ed by the NSF’s EarthScope Program, with additional sup-

port from the USGS, the ICDP, Stanford University, and 

NASA. Any use of trade, product, or firm names is for 

descriptive purposes only and does not imply endorsement 

by the U.S. government.

References

Almeida, R., Chester, J., Chester, F., Kirschner, D., Waller, T., and 

Moore, D., 2005. Mesoscale structure and lithology of the 

SAFOD Phase I and II core samples. Eos Trans. AGU, 86 

(52), Fall Meeting Suppl., Abstract T21A-0451.

Bakun, W., and McEvilly, T., 1984. Recurrence models and Parkfield, 

California, earthquakes. J. Geophys. Res., 89(B5): 

3051–3058.

Blum, J., Igel, H., and Zumberge, M., 2010. Observation of 

Rayleigh-wave phase velocity and coseismic deformation 

using an optical fiber, interferometric vertical strainmeter 

at the SAFOD Borehole, California. Bull. Seismol. Soc. Am., 

100(5A):1879–1891, doi:10.1785/0120090333.

Unfortunately, the SAFOD observatory instruments 

began to develop electronic problems soon after installation, 

and attempts to keep the instruments running were ulti-

mately unsuccessful. An expert panel convened by NSF is 

currently in the process of examining the failed instrumen-

tation. Leakage of water into pods was the probable cause of 

failure, although the actual failure point will not be known 

until the NSF panel report is completed. Fortunately, the 

SAFOD observatory was designed to permit ongoing access 

to the deepest part of the Main Hole through the inside of the 

EUE tubing (Fig. 7) to which the instrument pods were 

attached. A seismometer with three 15-Hz Omni-2400 geo-

phones was deployed on wireline inside the EUE tubing in 

early December 2008, and this continues to operate as of 

March 2011. Data are digitized at the surface at 1000 sam-

ples per second and transmitted directly into the NCSN and 

are archived at the NCEDC (Table 1). While not a substitute 

for the observatory’s full suite of digital seismometers and 

accelerometers, this interim instrument has allowed contin-

uous observation of the target earthquakes to continue, and 

has produced important data including recordings of the SF 

and LA target earthquakes repeat in December 2008 (Fig. 9). 

The temporary geophone is planned to remain in operation 

until NSF develops a plan for installation of a new observa-

tory.

In addition to the SAFOD observatory, an optical-fiber 

interferometric strainmeter was permanently installed at the 

conclusion of Phase 1 drilling in 2004 (Blum et al., 2010). 

Two optical-fiber loops were placed in the annulus formed by 

the 311-mm inside diameter (ID) initial casing and the 

245-mm OD casing. The fiber sensors were attached to the 

outside of the inner casing as it was lowered into the well and 

Figure 9.

 Seismograms from an SF and LA repeating earthquake 

sequence that occurred in December 2008. This earthquake was 

recorded on a 3-component seismometer deployed within the EUE 

tubing at a position just above the SAFOD observatory.

−0.1

0.0

0.1

0.2

0.3

0.4

December 2008 S.F. − L.A. Sequence

Seconds

L.A. M 1.8

Aftershock M 0

S.F. M 2.1

Aftershock M 1

2.7 X

141 X

1 X

4.7 X

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