Seismology in the Source: The San Andreas Fault Observatory at Depth Bill Ellsworth Steve Hickman (USGS Menlo Park) Mark Zoback (Stanford University) San Andreas Fault Zone Target Earthquakes
San Andreas Fault Observatory at Depth (SAFOD) North American Plate The central scientific objective of SAFOD is to directly measure the physical and chemical processes that control deformation and earthquake generation within an active plate-bounding fault zone.
SAFOD goals: • Exhume rock and fluid samples for extensive laboratory studies • Conduct a comprehensive suite of downhole measurements to determine the physical and chemical conditions under which earthquakes occur • Monitor changes in deformation, fluid pressure and seismicity directly within the fault zone during multiple earthquake recurrence cycles San Andreas Fault Zone Target Earthquakes
In Plane of SAF Perpendicular to SAF Primary SAFOD Target SAFOD Drilling Target: Repeating Earthquakes Key objective of SAFOD is to observe the nucleation and propagation of repeating earthquakes in the near field region SAFOD Depth Depth ~140 m Main SAF Strand Fault Creeping Avg. ~ 2.5 cm/yr ~200 m Distance Perp. to Strike Distance Along Strike Nadeau et al. 2004, Waldhauser and Ellsworth 2004 .
Sep. 28, 2004 Parkfield Earthquake Experiment North American Plate High-resolution seismological and geodetic data are available at Parkfield for more than 20 years. We recently captured the long-anticipated M 6.0 Parkfield earthquake (Bakun, et al., Nature, 2005).
Comparing the 1966 and 2004 Aftershocks Both Earthquakes Ruptured the Same Segment
Parkfield MainshockObserved on the Pilot Hole Array P S 2 seconds
New seismological methods for locating earthquakes make it possible to routinely determine position (x, y, z) with an accuracy that is comparable or smaller than the source dimension over large distances. South Moat of Long Valley Caldera
1997 Long Valley Swarm (Prejean, et al, JGR, 2002)
Double difference earthquake locations after Waldhauser, Ellsworth, Schaff and Cole (GRL, 2004). Displayed with LiveGraphics3D
NW SE Seismicity Patterns • Seismicity is organized in both space and time. • Repeating earthquakes at precisely the same location and of the same size dominate and are broadly organized into horizontal lineations or “streaks”. • “Holes” in the seismicity correspond to regions where the fault is locked or retarded (Waldhauser, et al., GRL, 2004). • Earthquakes during 30+ years before the 2004 earthquake and the 2004 aftershocks broadly follow the same spatial distribution. • Many aftershocks are in-fact repeats of earlier earthquakes.
Persistence of Seismicity Patterns NW SE M 4.2 Aftershock on November 29, 2004 Repeat of 1934 foreshock, and 1935, 1939 and 1945 events
Persistence of Seismicity Patterns NW SE M 5.0 Aftershock, Sept. 29, 2004 Repeat of 1934 foreshock, and 1939, 1956 and 1993 events Ellsworth; Waldhauser et al. S54B-02
1966 fore and main shocks 2004 hypocenter 2004 aftershocks background seismicity 2004 Slip distributionfrom Jessica Murray(BSSA Parkfield Special Issue) • Estimated using data from strainmeters, continuous and campaign GPS • Peak slip: 34 cm; Mw 6.1
Relative Locations of SAFOD Target Earthquakes (Repeaters) GROUP 1 In Plane of SAF Perpendicular to SAF Primary SAFOD Target GROUP 2 GROUP 3 SAFOD Depth Depth ~140 m Main SAF Strand Fault Creeping Avg. ~ 2.5 cm/yr ~200 m Distance Perp. to Strike Distance Along Strike Nadeau et al. 2004, Waldhauser and Ellsworth 2005 Repeat rate of SAFOD target earthquakes increased in response to M 6 Parkfield Earthquake of Sept. 28, 2004 (surface creep rate also up, now ~ 5 cm/yr) .
Monitoring Earthquakes in the Near Field in Stage 2 • Stage 2: Monitoring • 3-component geophone • 3C MEMS accelerometer January 23, 2005 Recurrence of SAFOD Target Earthquake (M 1.8 “L.A.” Repeater observed by Stage 2 Sonde in Main Hole at a distance of 420 m)
January 23, 2005 “L.A.” Recurrence Near field recording on Stage 2 sonde at 420 m range and whole-space synthetics (40 msec triangular source time function)
What’s to come? SAFOD Observatory • Stage 1 – Pilot Hole • Seismometers • Tilt • Stage 2 – Main Hole • Laser Strainmeter • Seismometers • Accelerometers • Stage 3 - Long-Term Monitoring • 2006 prototype • 2007 long-term deployment of • Seismometers • Accelerometers • Tilt • Pore pressure • Temperature Fiber optic strain meter cemented behind casing San Andreas Fault Zone Retrievable geophone, accelerometer, tilt meter, fluid pressure and temperature monitoring array inside casing Target Earthquakes Retrievable geophone, accelerometer and tilt meter inside casing
Conclusions • Rewarding – Engagement of Scientists and Students from a Wide Range of Disciplines and Countries • Gratifying – To Have the Project Come Together After 13 Years • Successful – We Have Established Access to the San Andreas Fault at Seismogenic Depth • Exciting – To Consider the Many New Aspects of Earthquake Research That Can Now Take Place • Earthquake Physics • Fault Rock Geology • Rock Mechanics • Role of Fluids and Gases