The San Andreas Fault Observatory at Depth: Challenges and Opportunities for Computational Science a

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The San Andreas Fault Observatory at
DepthChallenges and Opportunities for
Computational Science and Information
TechnologyFred Pollitz, Bill Ellsworth and
Steve HickmanU.S. Geological Survey, Menlo Park,
CAandMark ZobackDepartment of Geophysics,
Stanford University EarthScope Computational
Science/Information Technology WorkshopMarch
24-27, 2002Snowbird, Utah
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SAFOD
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• PARKFIELD EARTHQUAKE EXPERIMENT HIGHLIGHTS TO
  DATE
• Creation of the most complete active fault
  observatory in the world.
• Continuous operation of real-time warning system
  for over 15 years, and expansion of its rapid
  earthquake reporting capability to cover the
  entire state of California.
• Open and unrestricted access to monitoring data
  through the Internet to permit the entire
  scientific community to build and test models of
  the earthquake cycle.
• Direct measurement of stress build-up on the San
  Andreas Fault, and recognition that stress
  build-up is not uniform with time.
• Discovery that many small-magnitude earthquakes
  at Parkfield are virtually identical and
  repeatedly rupture the same area on the fault.

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SAFOD
Source Jessica Murray Felix Waldhauser Bill
Ellsworth
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San Andreas Fault Observatory at DepthProject
Overview and Science Goals

• Test fundamental theories of earthquake
  mechanics
• Determine structure and composition of the fault
  zone.
• Measure stress, permeability and pore pressure
  conditions in situ.
• Determine frictional behavior, physical
  properties and chemical processes controlling
  faulting through laboratory analyses of fault
  rocks and fluids.
• Establish a long-term observatory in the fault
  zone
• Characterize 3-D volume of crust containing the
  fault.
• Monitor strain, pore pressure and temperature
  during the cycle of repeating microearthquakes.
• Observe earthquake nucleation and rupture
  processes in the near field.
• Determine the nature and strength of the
  asperities that generate repeating
  microearthquakes.

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Chemical Effects of Fluids in Faulting
Grain-Scale Effects
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A key opportunity for SAFOD will be creation of
system-level models making testable predictions
about the future behavior of this segment of the
San Andreas Fault

• Why do some earthquakes have remarkably regular
  recurrence intervals while others are highly
  irregular?
• What controls the magnitude of these repeating
  earthquakes?
• How do these sources evolve through multiple
  seismic cycles?

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Pre-SAFOD Activities

• Seismic array deployments (temporary permanent)
• Microearthquake relocations seismic tomography
• High-resolution seismic reflection refraction
• Magnetotelluric profiling
• Fault zone guided waves
• Gravity surveys
• Fluid geochemistry
• Aeromagnetic ground magnetic surveys
• Environmental assessment permitting
• Geophysical Field Camp Oct. 2000
• 2 km pilot hole will be drilled in Summer 2002

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PASO 3-D Tomography(Courtesy of Cliff Thurber
and Steve Roecker)

• 70 PASSCAL stations in a 15 x 15 km array
• There are marked variations in seismic velocity
  and Vp/Vs ratio associated with the fault zone
  (and with local sedimentary basins)
• Microearthquakes align both with the surface
  trace of SAF and subsurface velocity
  discontinuities
• The crust at Parkfield is highly heterogeneous

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• SAFOD Integrated Science Team
• Downhole Measurements
• stress magnitudes and directions
• permeability pore pressure
• temperature
• geochemical logging lithologic reconstruction
• seismic velocities attenuation
• fracture fault geometry
• porosity, resistivity density
• Lab Measurements on Core, Cuttings Fluids
  
• mineralogy isotope geochemistry
• deformation microstructures
• water gas geochemistry (bulk and isotopic)
• frictional strength properties (gouge and
  country rock)
• matrix permeability resistivity
• seismic poroelastic properties

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All components of EarthScope must work together
in the planning, execution and interpretation of
experiments aimed at understanding the SAFOD
crustal volume.
SAFOD
Integrated physics-based models for faulting and
earthquake generation along the San Andreas Fault
that combine measurements made in the drill hole
with crustal-scale measurements of the
displacement field, seismic velocity, density,
conductivity and other properties.
PBO
USArray
InSAR
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• THEORETICAL MODELING
• Small Scale
• poroelastic stresses
• thermal pressurization vs. pore dilatation
• dynamic wave propagation
• frictional constitutive relations (including
  time-dependant chemical effects)
• Large Scale
• tectonic loading
• viscoelastic relaxation
• MODEL INPUT
• fault geometry and crustal structure
• constitutive laws for process of breakdown
  leading to rupture
• fault fluid pressure and permeability structure
• poroelastic and viscoelastic structure
• tectonic loading boundary conditions

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• SAFOD and YOUSAFOD is an open
  experiment.
• Monitoring data from the observatory will be
  freely available to all without restriction (and
  we hope in near real-time).
• To learn more visit www.earthscope.org and follow
  links to SAFOD web page.

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• SAFOD Pilot Hole Objectives
• Facilitate precise location of earthquakes for
  SAFOD
• Record surface seismic sources during
  long-baseline seismic reflection/refraction
  experiment in Sept. 2002
• Develop and test broad-band seismic, pore
  pressure, strain and temperature monitoring
  instrumentation for SAFOD
• Measure stress, fluid pressure and heat flow
  adjacent to the fault zone (thermomechanical
  setting)
• Compare physical properties from geophysical
  surveys (seismic velocities, density,
  resistivity) with downhole and core measurements
• Reveal nature and extent of fluid/rock
  interaction adjacent to fault zone (lab studies
  on core and formation fluid samples)
• Provide technical information about drilling
  conditions prior to SAFOD drilling
• Attempt high-resolution, real-time imaging of
  fault zone during SAFOD drilling using drill bit
  as seismic source

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• KEY QUESTIONS TO BE ADDRESSED
• Why are major plate-boundary faults like the San
  Andreas weak?
• What is the fluid pressure within and adjacent to
  the fault zone and how does it vary during a
  seismic cycle?
• What is the origin and composition of fault zone
  fluids?
• How do stress orientations and magnitudes vary
  across the fault zone?
• What factors control the nucleation, propagation,
  arrest and recurrence of earthquake rupture
• How are stress and strain transferred along or
  between faults over different time scales?
• What are the width and structure (geologic and
  thermal) of the active fault zone at depth?
• What are the mineralogy, deformation mechanisms
  and constitutive properties of the fault rocks?
• What processes lead to the localization of slip
  and strain rate?
• What are the permeabilities of fault-zone
  materials and country rock?
• What is the extent of chemical water-rock
  interaction and how does this effect fault-zone
  rheology?
• What is the origin of low-velocity/high-electrical
  -conductivity zones associated with the fault
  zone?
• How is energy partitioned within the fault zone
  between seismic radiation, frictional heating,
  grain size reduction and chemical reactions?

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Full-Cycle Earthquake Models
L creepmeter S borehole strain W water level
CREEPING
COLOR CODE FAULT log (Vfault/Vplate), from
dark blue (Dilational strain

• INPUT
• Plate tectonic loading
• Regional distribution of locked vs
• creeping sections
• Fault friction laws (rate- state-variable)
• Deformation data (creep, EDM, GPS, etc)

• OUTPUT
• Map of fault slip and stress vs time
• 3-D distribution of strain accumulation
• Likely location of earthquake nucleation
• Estimate of strong ground motion for
• scenario earthquake (eventually)

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• SAFOD SCIENCE AND EARTHQUAKE HAZARDS
• Determine processes responsible for earthquake
  initiation, leading to
• Better earthquake forecasts, based on improved
  understanding of the seismic cycle and earthquake
  recurrence.
• An assessment of whether or not short-term
  earthquake prediction is even possible and, if
  so, how it might be accomplished.
• Test and improve models for earthquake rupture
  dynamics, leading to
• Improved predictions of strong ground motion.
• Better models for dynamic stress transfer and
  rupture propagation between fault segments.
• Evaluate the roles of fluid pressure, rock
  friction, chemical reactions and other factors in
  controlling fault strength, leading to
• More accurate earthquake simulations in the lab
  and on the computer using representative
  fault-zone properties and physical conditions.
• Improved models for earthquake triggering.
• Better understanding of factors controlling
  creeping vs locked behavior.

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Pilot Hole
Source Bob Jachens Rufus Catchings
Source Mike Rymer Rufus Catchings
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SAFOD Project History

• 1992 Asilomar Workshop on San Andreas Fault Zone
  Drilling
• 1994 Marconi Workshop on Site Characterization
• 1994 Menlo Park Workshop on Site Selection
• 1994 Geophysical Surveys of Middle Mountain
  Begin
• 1996 NSF Project Proposal
• 1998 NSF Proposal Package (with 68 PIs from U.S.
  Universities, U.S. Geological Survey, DOE Labs
  and Foreign Institutions)
• 1999 EarthScope Initiative
• 2000 Near-Surface Geophysical Field Camp
• 2001 International Workshop on Borehole
  Instrumentation and Near-Source Seismology
  (March)
• 2001 Parkfield Seismic Imaging Workshop (June)
• 2001 Pilot Hole Funded by ICDP Allied Science
  Proposals Submitted to NSF, USGS and
  International Agencies

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MECHANICAL IMPLICATIONS Heat-flow
constraint and fault-normal compression (SHmax
at 75or more to SAF) require either
1) Low friction (m 0.1) along the fault and
high friction elsewhere or
2) Super-lithostatic pore pressure confined to
the fault zone and/or 3)
Dynamic weakening mechanisms
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