Bill Ellsworth U.S. Geological Survey and Kaz Imanishi Geological Survey of Japan A.I.S.T. Near-Source Observations of Earthquakes: Implications for Earthquake Rupture and Fault Mechanics

Deep Geophysical Observatories in California San Andreas Fault Observatory at Depth (SAFOD) 2.7 km deep Long Valley Exploratory Well (LVEW) 2.7 km deep

Earthquakes at 300 m distance to seismometer in LVEW with magnitudes as small as Mw -2.5 Seconds Analog 4.5 Hz seismometer digitized at surface (500 samples/s)

Digital 15 Hz seismometer with optical fiber transmission to surface at 4000 samples/s Installed at 2.7 km depth, 120 C M1.12 M1.25 May 2006 multiplet occurred at distance of about 600 m (S-P time is 0.1 s). Spectrogram of M1.12 event High signal-to-noise ratio High frequency energy is observed. 0.1 s 4000 samples/s

Static Stress Drop Stress drops range from approximately 1 to 100 MPa. For any given cluster, the stress drops are nearly constant within a factor of 2-3. Eshelby (1957) Sato & Hirasawa (1973) Multi-Window Spectral Ratio Method (Imanishi & Ellsworth, 2006) M w : 0.38 and 0.11

Aftershocks of M1.8 “Hawaii” Target (August 11, 2006) M w –2.1 M w –2.7 M w –2.5 M w –2.6 Spectral ratios relative to EV1 Spectral ratios are almost constant. Corner frequencies of these events are beyond the frequency band Or all the events have the same corner frequency

Stress Drop Scaling

The existence of picoearthquake implies lab-like values for D c

Ide and Beroza (2001) SAFOD Pilot Hole Apparent Stress Measurements (Imanishi and Ellsworth, 2006) Apparent Stress Scaling (Radiated Energy / Seismic Moment)

Kostrov (1964) solution for a growing circular crack:  D =  v p 3 r ü /( k v r 2 V s  Dynamic Stress Drop

Some events begin simply, while others have a nucleation phase. Kostrov (1964) solution for a growing circular crack:  D =  v p 3 r ü /( k v r 2 V s  Dynamic Stress Drop 1.1 MPa 3.2 MPa 5.0 MPa 2.2 MPa

August 11, 2006 M 1.8 Hawaii Repeat Seismometer at 2.65 km depth at a distance of 120 m

Dynamic Stress Drop = 4 – 7 MPa Dynamic Stress Drop in Initial Millisecond  D =  v p 3 r ü /( k v r 2 V s 

Critical Dimension for Instability in Rate & State Theory h* = G D c /(-P 0 )(b-a) August 11, 2006 “Hawaii” M 1.8 at 0.5 ms. Laboratory

Permanent Monitoring Array Instrumentation Design goals Record weak motion at the maximum gain consistent with high signal-to-noise in the 10 – 2000 Hz band. Record on scale motion of M 2 earthquakes in their near field over a broad band (0.5 – 1500 Hz). Maintain linearity of ground motion recording in the sensor, electronics and mechanical coupling to the Earth. Record aseismic transient deformation at periods from 1 hour (or longer) to 1 s. Record pore pressure fluctuations in the fault zone at periods of days to 1 s.

SAFOD Observatory Pipe deployed system Electrical conductors and optical fibers in stainless steel microtubes. No O-rings (laser welded sondes) Stiff bow spring decentralizers on instrument pods 3 levels of multi-component sondes GERI DS150 3C 15 Hz seismometer Modified GERI DS150 with 3C Colibrys MEMS accelerometer Pinnacle borehole tiltmeter Optical fiber telemetry (4K sps) GERI Geores control computer USGS Earthworm data distribution and archiving system On-site event detection and integration of SAFOD, HRSN and NCSN waveforms using Norsar MIMO system Pinnacle Technologies borehole tiltmeter Optical fiber strainmeter deployed behind casing in vertical section of main hole (M. Zumberge, UCSD) Pore pressure and packer not installed due to hole conditions

SAFOD Seismic Sensors Commercial high-frequency borehole seismometers (Oyo Geospace DS150 – 150 C rating) MEMS accelerometers replace geophones for broad band response

2 3/8” EUE tubing EUE/CS crossover sub 1 ¼” CS tubing crossover sub EM tool carrier Seismic tool carrier Tiltmeter tool carrier 11 mm polypropylene control line containing ¼” SS tube with fiber and electrical conductors 11 mm polypropylene control line containing ¼” SS tube with coax 3/8” SS tube containing electrical conductors Cable Head Seismometer MEMS accelerometer Tiltmeter Seismometer MEMS accelerometer Seismometer MEMS accelerometer EM coil

Tool Carrier Assembly Metal-metal seals on control lines Gas-tight threads on endcaps (NPT-type) Tool carrier filled with ceramic proppant and synthetic oil Low melting point metal (138 C) used to rigidly attach instruments to tool carrier Pressure testing tool carrier

Installation of the Observatory Instrument Pod and Control Line Small Drill Rig

Analog Seismometer with high temperature (200 C) 15 Hz geophones (Institue of Earth Science and Engineering, University of Auckland, New Zealand) Steve and Bill filling the cable head with high-temperature epoxy 1.6” OD 2x 15 Hz geophones/component drives 3 km cable

Conclusions Stress Drop and Apparent Stress are scale-invariant for Mw>0. We do not as yet have measurements for smaller earthquakes. The dynamic stress drops in the first 1-2 ms (rupture dimension ~2-4 m) are typically in the range from 1 – 10 MPa and are comparable to the event stress drops. Earthquakes as small as Mw -3.5 (picoearthquakes) occur along the San Andreas Fault at SAFOD and in Long Valley Caldera. If there is a minimum earthquake magnitude, we have not yet seen it. Lab values of Dc are consistent with the occurrence of picoearthquakes at SAFOD. The August 11, 2006 Mw 1.8 “Hawaii” earthquake began without a Slow Initial Phase and has (b-a) ~ 0.01.