B. Stage 1 — USArray and San Andreas Fault Observatory at
Depth
San Andreas Fault Observatory at Depth
The
San Andreas Fault Observatory at Depth is the second part of Stage I. The objective
is to install instrumentation to provide the first at-depth observatory within
a major active fault, the San Andreas Fault, at Parkfield, California. A variety
of instrumentation types and techniques will be used. The monitoring instrumentation
is to be emplaced through a 4.0-km deep drill hole that intersects the San Andreas
Fault at the location of the 1966 magnitude 6 Parkfield earthquake shown in Figure
3. At this location, fault slip takes place through a combination of small-to-moderate
magnitude earthquakes and aseismic creep. Emplacement of the observatory will
be done in two stages. The main hole will be rotary drilled vertically to a depth
of ~2.2 km and then deviated through the fault zone at a ~50 degree inclination
from the vertical to a final depth of 4.0 km. After two years of monitoring and
analysis, four depth intervals will be picked for continuous coring within the
fault zone through "windows" cut in the drill casing. A long-term monitoring
package will then be re-inserted within the fault zone. The scientific goals are
to provide direct observational data on the composition, physical state and mechanical
behavior of a major active fault zone at depth as a function of time. Such data
will test and constrain a number of hypotheses pertaining to faulting and earthquake
generation.
The Need for an Observatory at Depth
An enormous amount
of field, laboratory and theoretical work has been directed toward understanding
the mechanical and hydrological behavior of faults. But, it is still difficult
to differentiate or constrain the broad range of conceptual models for the mechanical
behavior of faults at depth. One of the primary causes for this dilemma is the
difficulty of either directly observing or accurately inferring physical properties
and deformation mechanisms along faults at depth with the relevant rock types,
volumes, fluid content, and stresses. We have no direct knowledge of the stress
conditions under which earthquakes initiate nor do we know whether, as is often
proposed, high pore fluid pressure exists within the San Andreas fault zone at
depth or if variations in fluid pressure with time affect fault behavior.
Intensive
downhole geophysical measurements and long-term monitoring are planned within
and adjacent to the active fault zone. Monitoring experiments will include nearfield,
wide-dynamic-range seismological observations of earthquake nucleation and rupture
and continuous monitoring of variations in pore pressure, temperature and crustal
deformation during the earthquake cycle. We hope to directly evaluate the roles
of fluid pressure, intrinsic rock friction, chemical reactions, in situ stress
and other parameters in the earthquake generation process. In this way, we can
provide earthquake researchers with the opportunity to simulate earthquakes in
the laboratory and on the computer using realistic fault zone properties and physical
conditions.
 | Figure
3. A) Earthquake locations at Parkfield from 1984-1990, together with the location
of the proposed drill hole. The 1966 M=6.0 earthquake initiated directly beneath
the proposed drillsite and ruptured toward the south, stopping near the town of
Cholame. B) Southwest-northeast cross section through the proposed drillsite,
showing hypocenters of background microearthquakes (circles) and the 1966 M=6.0
mainshock (star) together with P-wave velocities from a tomographic inversion
by Michael and Eberhart-Phillips (1991). (Figure from M.D. Zoback, S. Hickman
and W. Ellsworth, personal communication, 1999.)
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Planning and the Diverse Science Team
While the idea of instrumenting
the San Andreas Fault has arisen many times over the past several decades, this
project had its origin in December of 1992 when a workshop was convened with support
from NSF. 113 scientists and engineers from seven countries attended the workshop.
Its purpose was to initiate a broad-based scientific discussion of the issues
that could be addressed by direct experimentation and monitoring within the fault
at depth, to identify potential sites, and to identify technological developments
required to make the construction and drilling possible. The fundamental scientific
issue addressed in this effort, obtaining an improved understanding of the physical
and chemical processes responsible for earthquakes along major fault zones, is
clearly of global scientific interest. Throughout the planning process leading
to the development of this proposal, the US scientific community has invited participation
by scientists from around the world, especially through the International Continental
Drilling Consortium.
A proposal was submitted in January 1999 to NSF to
accomplish the goals of the San Andreas Fault Observatory at Depth. At the present,
the science team includes 33 principal investigators from 19 US universities,
about 15 scientists from the US Geological Survey, several scientists from Lawrence
Berkeley and Lawrence Livermore National Laboratories of the US Department of
Energy and scientists from 12 institutions in 4 foreign countries.
Choice
of the Parkfield Site
After consideration of all the candidate sites for
this first at-depth fault observatory, the scientific community chose the Parkfield
location through a consensus process involving preliminary site-characteristic
studies and numerous workshops. The site is intensively studied geologically and
geophysically, has shallow seismicity within reach of the projected observatory
depth, and is now under permit for at least 20 years of scientific monitoring.
Construction-stage Work Plans
The plan of work during construction
is divided into four categories: 1) downhole measurements, 2) measurements on
core, cuttings and fluids, 3) geophysical and geological site characterization,
and 4) fault zone monitoring.
Downhole measurements include logging while
drilling, open hole geophysical logging, cased-hole logging, and stress, pore
pressure and permeability measurements coupled with fluid sampling.
Measurements
on core, cuttings and fluids include studies on the mineralogy, deformation microstructures,
physical properties, and constitutive behavior of rock samples recovered from
the San Andreas zone and country rock at depth. Other studies are aimed at studying
the chemical and isotopic composition of fault zone fluids and gases.
Extensive
geophysical and geological site characterization studies have already been done
during the site selection process. Further studies, with the use of the downhole
seismometers and the USArray concentrated network at the surface, will be aimed
at providing a high-resolution image of the earthquakes and physical environment
of the fault zone and upper crust beneath the drill site. These projects will
address two main objectives. The first is to provide technical information that
is critical to the siting and drilling of the borehole, including the precise
location of earthquake foci that will be intersected and the presence of geological
complications to avoid. The second is to create a comprehensive structural and
physical model of the fault zone. Detailed knowledge of the physical environment
of the fault will be essential to the interpretation of structures and properties
observed in the borehole for the ultimate decision of positioning four offshoot
core holes and their related monitoring instruments.
The goal of the fault
zone monitoring work is twofold: 1) to make in-situ measurements of deformation,
pore pressure, seismic wave radiation and other relevant parameters in the nearfield
of earthquakes, and 2) to select the optimal intervals for instrumentation and
sampling, such as continuous coring through the fault zone. It is expected that
the instruments will catch multiple earthquake cycles for repeating earthquakes
(M~2 or larger) in the target zone at distances of less than a few hundred meters
to about 1.5 km over a 20-year lifetime of the experiment. With luck, they may
catch the rupture process of the fault in a large-magnitude event over this same
time period.
After the first stage of the construction program when the
main observatory is completed, a 2-year-long period of intensive monitoring will
be done using a re-deployable monitoring array shown schematically in Figure 4.
The array will consist of seismometers, accelerometers and a fluid pressure monitor.
The removable monitoring array will be pulled out of the borehole after about
1 year to conduct a high precision-borehole directional survey and ultrasonic
cement imaging (USI) log. The USI log identifies any changes in casing shape or
the cement bond integrity behind the casing. This will determine if the faults
crossed by the hole are actively creeping, or if broad-scale deformation is occurring.
Repeat measurements of casing ovality and trajectory over time using casing shape
logs and gyroscopic directional surveys similar to those proposed here have identified
casing offsets as small as 1 cm over a 5-m-wide shear zone. Following the completion
of coring, the borehole will be re-instrumented with a modified borehole array
that will be augmented with additional pressure transducers, a borehole strainmeter
and other sensors.
 | Figure
4. Schematic illustration of the fault zone monitoring string in place in the
proposed 4-km-deep San Andreas Fault Observatory at Depth. Although not shown
here, additional sensors in the instrument package will monitor borehole deformation
and/or tilt. The monitoring string is designed so that it can be removed for instrument
repair and maintenance and so that the cased hole can be repeatedly logged for
deformation induced by ongoing fault creep and/or earthquakes. (Figure from M.D.
Zoback, S. Hickman and W. Ellsworth, personal communication, 1999.)
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Technology at High Pressures and Temperatures
The San Andreas Fault
Observatory at Depth is intended to stand on its own scientific merits. However,
it is also an important step toward conducting a more ambitious program of deeper
(~10 km) sampling and monitoring that reaches further into the active seismic
zone. The seismogenic zone of the San Andreas Fault extends to 20 km in some regions.
The technological challenges amplify greatly with depth because of the effects
from the high pressures and temperatures encountered. The 4-km monitoring effort
will contribute to the development of technology in drilling and instrumentation
to extend our capability to deeper depths.
Coordination with USArray
One of the goals of the San Andreas Fault Observatory at Depth is to use the strong
geometry of the deep seismometer string in coordination with the surface USArray
instruments to construct a very-high-resolution tomographic picture of the San
Andreas Fault at Parkfield and the surrounding areas. Even though the San Andreas
Fault at Parkfield is one of the most intensively studied fault sites in the world,
the site's earthquake locations still have large uncertainties due to the
strong lateral velocity contrasts as a result of materials on either side of the
fault. The combination of the dense surface spacing of the USArray instruments
and the deep emplacement of seismometers in the fault will resolve this question.
The array combination will simultaneously be locating small earthquakes with unprecedented
accuracy along the fault. The resulting detailed 3-D rheology from the tomographic
inversion and earthquake locations will be used in dynamic computer models of
faulting and strong motion calculations.
Timetable and Hole Design
Pre-project Site Characterization. A variety of site characterization
studies near the proposed observatory have been going on for the past several
years under support of NSF and the USGS. For example, Figure 5 shows a resistivity
cross-section across the fault along with a schematic of the proposed observatory.
The next significant set of studies at the site began in 1998. First, a 7-km long
high-resolution seismic reflection profile was shot across the observatory site
and fault zone with support of the USGS. Twelve portable reflection seismographs
were used with 700 channels of seismometers along a dense, fault crossing profile
with 10-m spacing. The main goals of this experiment were to refine the picture
of near-surface geology in the observatory area, and to assure that the site is
not located directly above small-scale secondary faults which might needlessly
complicate drilling at shallow depth. The second site characterization study will
be to deploy a number of additional seismographs in the region surrounding the
site for earthquake monitoring.
2001/Year 1.
Construction, downhole measurements and casing of the main hole are scheduled
to commence in late 2001 and end in early 2002. The recently developed capability
of the petroleum industry to drill "multi-laterals" — satellite
wells which are emplaced from a single "parent" well — has enabled
a simplification of the project strategy not previously available. Earlier strategies
included the necessity to continuously core an inclined hole across an entire
fault zone. As the fault zone is likely to be severely crushed and altered to
gouge, as well as potentially over-pressured; this was a formidable challenge.
This is especially true with the requirement of maintaining a sufficient hole
diameter to conduct the necessary downhole measurements and deploy fault zone
monitoring instrumentation after casing the hole. With the new "multi-lateral"
strategy, a rotary drilled "main hole" can penetrate the entire fault
zone. Geophysical logging, casing and cementing of such deviated holes are now
routine procedures in the petroleum industry, even in poorly consolidated and
overpressured formations. After appreciable study of the results from the main
hole, we will emplace four continuously cored "multi-laterals" off of
the main hole at carefully selected locations, as outlined below.
2002/Year
2. Fault zone monitoring begins in 2002 and goes on for 2 years. Measurements
on core and cuttings will begin as well as analyses of borehole geophysical data.
2003/Year 3. Fault zone monitoring and measurements on core and
cuttings continue. A comprehensive suite of site characterization studies is begun
in coordination with the USArray instrumentation.
2004/Year 4. Data
from site characterization and fault zone monitoring are analyzed. A comprehensive
analysis of all available data will be used to pick intervals for continuous coring.
Continuous coring within the fault zone will be carried out at four depth intervals
through "windows" cut in the casing. The seismicity rate in the area
is such that after 2 years of fault zone monitoring, there should be a sufficient
number of shallow earthquakes near the observatory hole to accurately locate the
active fault trace(s) suing the clamped seismometer array in the borehole. This
information, when combined with the geologic data from spot cores and cuttings,
geophysical logs, downhole measurements, fluid and gas chemistry data, pore pressure
and in situ stress measurements and the results of site characterization studies
will enable the science team to determine the optimal intervals for continuous
coring. As a result of this strategy, each core will be interpretable in terms
of its proximity to active fault traces, the composition and physical properties
of the fault zone, pore pressure, and stress, etc. A comprehensive scientific
measurement program is planned for the exhumed core. Following core retrieval,
the core holes will be lined with uncemented perforated casing and used for monitoring
fluid pressure at depth.
2005/Year 5. When coring activities have
been completed at the site, a permanent monitoring string will be deployed in
the hole so that the hole can be utilized as a continuous fault zone observatory
well into the future. Investigations of core samples are underway.
2006/Year
6. Data analysis and measurements on core are completed. Fault zone monitoring
continues.
 | Figure
5. Resistivity structure of the primary drill site based on inversion of continuous
magnetotelluric data. (Martyn Unsworth, pers. comm. 1988; see also Unsworth et
al., 1997). The proposed drill hole is also shown along with earthquake hypocenters
(crosses from Michellini and McEvilly, 1991; circles from Eberhardt-Phillips and
Michael, 1991). (Figure from M.D. Zoback, S. Hickman and W. Ellsworth, personal
communication, 1999.)
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References
Eberhart-Phillips,
D., and A.J. Michael, Three-dimensional velocity structure, seismicity and fault
structure in the Parkfield region, central California, J. Geophys. Res.,
98, 15737-15758, 1993.
Michael, A.J., and D. Eberhart-Phillips, Relations
among fault behavior, subsurface geology and three-dimensional velocity models,
Science, 253, 651-653, 1991
Michelini, A. and T.V. McEvilly, Seismological
studies at Parkfield. I. Simultaneous inversion for velocity structure and hypocenters
using cubic B-splines parameterization, Bull.Seis. Soc. America, 81, 524-552,
1991.
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