Scientific Drilling Into the San Andreas Fault zone

San Andreas Fault Observatory at Depth (SAFOD)

Objectives• Understand the physical and chemical processes that control

deformation and earthquake generation within active fault zones

• Make near-field observations of earthquake nucleation, propagation and arrest to test laboratory-derived concepts of faulting physics.

• Mechanically weak– Low heat flow– High-angle maximum principal stress SHmax

Why Parkfield?

• Transition between the locked segment and aseismic creeping segment of the SAF

• 1857 M 8.2 Fort Tejon event

• 1906 M 7.8 San Francisco event

• Repeating earthquakes

Shaded relief map of California (Hickman et al., 2004)

Repeating earthquakes• M 6 EQs have occurred on the Parkfield section at fairly regular

intervals - in 1857, 1881, 1901, 1922, 1934, 1966 and 2004• 1922, 1934 and 1966 events ruptured the same segment of the

fault in a similar manner

Significant EQs in the Parkfield since 1850 Comparison between 1922 and 1934 events

InSAR measurement

Linear surface displacement rate between 1993 and 2004 (de Michele et al., 2011)

SAFOD

• The bimodal distribution is consistent with right strike-slip motion

• The sharp discontinuity in the InSAR signal is a direct consequence of the surface creep

• The sharpness fades progressively to SE

PKF

Other measurements• The coseismic moment release of the 2004 event is as little as 25% of

the total (Johanson et al., 2006)• The creep rate from InSAR is consistent with short-range EDM,

creepmeter, and alignment array

Along-strike surface slip of the SAF from NW to SE (de Michele et al., 2011)

Geological & Geophysical Cross-sections• Pilot Hole: 2.2-km-deep, drilled through Salinian granite• Main Hole: penetrated two actively deforming zones as

SDZ and CDZ at ~2.7 km vertical depth

Resistivity structure from surface magnetotellurics (Hickman et al., 2004)

Geologic cross-section parrallel to the trajectory of SAFOD (Zoback et al., 2011)

Geophysical logs from Main Hole

Geophysical logs as a function of measured depth (Hickman et al., 2004)

• Damage Zone– ~200-m-wide– Low P and S

velocities, low resistivity

– Result of both physical damage and chemical alteration of the rocks due to faulting and fault-related minerals

Friction experiment

• Stepping tests– slip velocity is suddenly

increased by an order of magnitude

– Friction increases immediately then decay to a new steady-state value

• Slide-hold-slide tests– Steady-state sliding is followed

by a holding for t– followed by a resumption of slip

at the former slip velocity

Experimental data and frictional-healing determination (Carpenter et al., 2012)

Friction behavior: fault gouge• Powdered, clay-rich foliated gouge• Frictional strength is as low as μ=0.21 in the fault zone

SEM image showing shear zones in clay-rich foliated gouge (Carpenter et al., 2011)

Frictional strength and healing behavior(Carpenter et al., 2011)

Friction behavior: intact cores

• Intact fabric, saponite and smectite clay• Fault zone rock is extraordinary weak (μ=0.09-0.25), with the

lowest friction values in the center of the fault

Coefficient of sliding friction(Carpenter et al., 2012)

Mohr-Coulomb failure envelope(Carpenter et al., 2012)

Summary

• The laboratory data offer a coherent explanation for the weakness of the SAF

• The intact samples have different friction behavior from the powdered samples due to the mineral fabric.