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

A central goal of seismology is to understand the physical conditions under which earthquakes occur Key question seismology can address in the near-source region include: Minimum size of earthquakes Magnitude dependence of source parameters Rupture nucleation r D Illustrations by Peter Shearer

A central goal of seismology is to understand the physical conditions under which earthquakes occur Key question seismology can address in the near-source region include: Minimum size of earthquakes Magnitude dependence of source parameters Rupture nucleation . u(t) σd Dc ES

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

May 2006 Multiplet at SAFOD This multiplet occurred at distance of about 600 m (S-P time is 0.1 s). Spectrogram of M1.12 event M1.12 M1.25 0.1 s High signal-to-noise ratio High frequency energy is observed.

Static Stress Drop Mw: 0.38 and 0.11 Multi-Window Spectral Ratio Eshelby (1957) Sato & Hirasawa (1973) Static Stress Drop Mw: 0.38 and 0.11 Multi-Window Spectral Ratio Method (Imanishi & Ellsworth, 2006) 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.

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

Static Stress Drop Measurements at SAFOD Hickman & Zoback (2004) Average value is near the strength of the rock.

Stress Drop Scaling

Stress Drop Scaling

The existence of picoearthquake implies lab-like values for Dc Stress Drop Scaling The existence of picoearthquake implies lab-like values for Dc

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

Kostrov (1964) solution for a growing circular crack: Dynamic Stress Drop Kostrov (1964) solution for a growing circular crack: sD = r vp3 r ü /(k vr2Vs)

Kostrov (1964) solution for a growing circular crack: Dynamic Stress Drop Kostrov (1964) solution for a growing circular crack: sD = r vp3 r ü /(k vr2Vs) Some events begin simply, while others have a nucleation phase. 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 in Initial Millisecond sD = r vp3 r ü /(k vr2Vs) Dynamic Stress Drop = 4 – 7 MPa

Critical Dimension for Instability in Rate & State Theory h Critical Dimension for Instability in Rate & State Theory h* = G Dc /(s-P0)(b-a) Laboratory August 11, 2006 “Hawaii” M 1.8 at 0.5 ms.

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.