Scientific Drilling Into the San Andreas Fault zone

1. Scientific Drilling Into the San Andreas Fault zone San Andreas Fault Observatory at Depth (SAFOD)

2. 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

3. Why Parkfield? • Transition between the locked segment and aseismic creeping segment of the SAF • 1857 M 8.2 Fort Tejonevent • 1906 M 7.8 San Francisco event • Repeating earthquakes Shaded relief map of California (Hickman et al., 2004)

4. Repeating earthquakes • M 6 EQs have occurred on the Parkfieldsection 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

5. InSAR measurement • 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 SAFOD PKF Linear surface displacement rate between 1993 and 2004 (de Michele et al., 2011)

6. 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)

7. 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)

8. Geophysical logs from Main Hole • 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 Geophysical logs as a function of measured depth (Hickman et al., 2004)

9. 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)

10. Friction behavior: fault gouge • Powdered, clay-rich foliated gouge • Frictional strength is as low as μ=0.21 in the fault zone Frictional strength and healing behavior (Carpenter et al., 2011) SEM image showing shear zones in clay-rich foliated gouge (Carpenter et al., 2011)

11. Friction behavior: intact cores • Intact fabric, saponiteand 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)

12. 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.