the san andreas fault observatory at depth (safod) an integrated study of a major plate-bounding...

The San Andreas Fault Observatory at Depth (SAFOD)

An integrated study of a major plate-bounding fault at seismogenic depths

Amy Day-LewisMark Zoback

Department of Geophysics, Stanford University

Stephen HickmanU.S. Geological Survey

SAFOD A borehole observatory across the San Andreas Fault to directly measure the physical conditions under which earthquakes occur

Plate Boundary Observatory A fixed array of GPS receivers and borehole strainmeters to measure real-time deformation on a plate-boundary scale

USArray A continental-scale seismic array to provide a coherent 3-D image of the lithosphere and deeper Earth

EarthScope – “A New View into the Earth”

A. Day-Lewis, CLSI Workshop, Tokyo, October 3-4 2005

The site is small,…

SAFOD


To directly measure the physical and

chemical processes that control

deformation and earthquake

generation within an active, plate-

bounding fault zone.

…but the goal is big.


our target

Microseismicity 1984 - 1999, up to M 5, F. Waldhauser & B. Ellsworth).

Slip rate inferred from geodetic measurements 1966-1991 (Murray et al. 2001).

1966 2004


GROUP 1 (10/20/03)

GROUP 2 (10/21/03)

GROUP 3 (6/27/01?)

primarySAFODtarget

in plane of SAF perpendicular to SAF

mainSAF

repeating micro-earthquakes

[Waldhauser, 2004]U.C. Berkeley (HRSN) stations JCN, MMN and VCA

S.F. L.A.


1) 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.

2) 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 – are earthquakes predictable?

specific objectives

1) 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.


multi-phase approach

Comprehensive Site Characterization

C. Thurber, S. Roecker



Pilot Hole• drilled in 2002 to 2.2

km MD/TVD• laid the scientific and

technical groundwork for SAFOD

• constrained local geology

• improved locations of target earthquakes M 2.1

Target Earthquake

San

An

dre

as F

ault

Zo

ne

San

An

dre

as F

ault

Zo

ne

Resistivities: Unsworth & Bedrosian 2004

Earthquake locations: Roecker & Thurber 2004




Pilot Hole Phase I Main Hole

• drilled in 2004 vertically to 1.5 km TVD, deviated 55° to 2.5 km TVD

• intense physical sample collection

• geophysical logging• hydrofracture tests• coring

M 2.1 Target Earthquake

San

An

dre

as F

ault

Zo

ne

San

An

dre

as F

ault

Zo

ne




Pilot Hole Phase I Main Hole Phase II Main Hole

• drilled in 2005 through the San Andreas Fault Zone to a final depth of 3.1 km TVD

• On-site mineralogical analysis

• MWD, LWD, and pipe-conveyed logging

• spot and sidewall coring

San

An

dre

as F

ault

Zo

ne

San

An

dre

as F

ault

Zo

ne




Pilot Hole Phase I Main Hole Phase II Main Hole Phase III Main Hole

• dedicated coring phase in 2007

• 4 multi-lateral cores drilled 250 m from the main hole

• on site core processing

San

An

dre

as F

ault

Zo

ne

San

An

dre

as F

ault

Zo

ne




Pilot Hole Phase I Main Hole Phase II Main Hole Phase III Main Hole Multi-stage

Observatory• 2007-2027• monitor strain, tilt,

pore pressure, temperature

• observe earthquakes in the near-field

San

An

dre

as F

ault

Zo

ne

San

An

dre

as F

ault

Zo

ne

Retrievable geophone, accelerometer, tilt meter, fluid pressure and temperature monitoring array inside casing

Fiber optic strain meter cemented behind casing

Retrievable geophone, accelerometer and tilt meter inside casing



benefits of this approach

Paulsson Geophysical ArrayPASO Array

earthquake locations by Zhang and Thurber


In the Field

continuous cuttings analysis

The mudloggers were the first to recognize SAFOD’s entry into sedimentary rock!

graniteseds

Fra

nci

scan

X


real-time mud gas logging

• Recognition of shear zones during drilling

• C isotope, 3He/4He → fluid origin (e.g., biogenic, mantle-derived)


geophysical logging

• hole orientation

• hole diameter (caliper)

• temperature

• gamma

• density

• seismic velocities

• electrical resistivity

• electrical and acoustic wellbore images


(Boness andZoback, 2004)

shear zones

stress relief

zones

log analysis


stress and wellbore stability analysis


coring


hornblende-biotite granodiorite

Phase I core: 1.5 km


Phase I core: 2.5 km (top)pebble conglomerate and arkosic sandstones• consists almost entirely

of granitic debris, very little weathering

• grains poorly sorted and very poorly rounded

• grains tightly packed, with abundant grain-to-grain cracking

• pressure-solution features common

• matrix filled with crushed and recrystallized phylllosilicates, plus zeolites and carbonates


Phase I core: 2.5 km (bottom)Core catcher contents from final run (Run #5, deepest rock cored): Heavily fractured and recemented granite or granite cobble conglomerate

Shear Zone

fine to very fine siltstone


• structural, petrologic and geochemical study of deformation and diagenesis

• mineral transformations and fabrics

• thermochronolgy (zircons)• brittle fracture, deformability,

permeability and seismic anisotropy

• thermal conductivity and radiogenic heat production

• fluid inclusion volatiles analysis

core “Sample Party,” February 2005


Phase II core

Inoceramus fossils &

bioturbation

very fine sandstone, siltstone, and shale

photography (flat and 360° scans) &

continuous physical property scans


• one-foot-thick clay-rich zone, with abundant internal shearing (polished surfaces and slickensides)

• separates siltstone from heavily fractured and recemented granite cobble conglomerate.

shear zone at 3,067 m MD


on-site mineralogypetrographic examination of grain-mount thin sections

XRD and XRF analysis for mineralogy

magnetic susceptibility and remanant magnetization

heavy mineral separations for high-pressure Franciscan metamorphic minerals

All helped identify shear zones & lithologic changes (and the east side of the fault!).


real-time decision-makingft MD

firstmudstone

firstserpentine

changein beddingin image logs

major drillingbreak, gas kick


side-wall coring

52 1” cores recovered over open-hole interval (3,066–3,953 m MD)

depths selected to best sample lithologic, structural and physical property variations identified in real-time cuttings analysis (optical and XRD) and geophysical well logs, while avoiding highly washed-out zones


shales and mudstones

Core 23: 3,722 m

Core 38: 3,387 m Core 67: 3,126 m

Core 58: 3,213 m

fine- to coarse-grained sandstones

conglomerates (granite cobble?)

sidewall core examples


2005 to 2007

• repeat logging for casing shear

• collection of new regional and borehole seismic data to refine the location of the target events

• data integration to determine the optimum locations for Phase III coring

Hypocentroid locations determined by Felix Waldhauser using cross correlation measurements of NCSN waveforms and hypoDD.

Red: “S.F.” targetBlue: “L.A.” targetGreen: “S.W.” targetYellow: July 16, 2005 S.F. and

August 2, 2005 S.W.


3-Comp. Seismometer

Laser Strainmeter

Borehole Tiltmeter

early monitoring success

M 2.8 at 4 km distance


Geophysical Research Letters Volume 31 2004

Special Sections on the San Andreas Fault Observatory at Depth

• Part 1: Earthquakes and Crustal Structure (no. 12, 10 papers)

• Part 2: Thermomechanical Setting, Physical Properties and Mineralogy (no. 15, 10 papers)

Pilot Hole Results


AGU Fall Meeting, Session T11

San Francisco, CaliforniaDecember 5–9, 2005

Sponsored by the Tectonophysics and Seismology Sections

Conveners: Naomi Boness, Stanford andJohn Solum, USGS

Phase I and II Results

• physical properties• state of stress• geochemical analysis of

gases and fluids• borehole seismic studies• mechanical, petrological,

structural and microbiological analyses of cuttings and core


SAFOD is funded by:

National Science Foundation

and the EarthScope Project

With co-principal investigators:

William Ellsworth and Stephen Hickman, U.S. Geological Survey, and

Mark Zoback, Stanford University

And assistance by:

International Continental Scientific Drilling Program


the san andreas fault observatory at depth (safod) an integrated study of a major plate-bounding...

Documents

clsi workshop

san andreas fault observatory

earthquake locations

geological survey slide

chemical processes

earthquake generation

earthquake nucleation

physical conditions