Bill Ellsworth U.S. Geological Survey Near-Source Observations of Earthquakes: Implications for Earthquake Rupture and Fault Mechanics JAMSTEC International Technical Committee on Long Term Monitoring System Development for Ocean Boreholes Yokohama, Japan February 16-17, 2009

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 Stress evolution during and following rupture 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 Stress evolution during and following rupture u (t). σdσd DcDc ESES

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 Stress evolution during and following rupture. σdσd DcDc ESES

Shaking Forecast Model

World-wide Strong Motion Data Set (as of 2004)

2002 M 7.9 Denali Earthquake Near-Field Ground Motion at Pump Station 10

Accelerations were modest (0.36 g) Fault Normal Fault Parallel Vertical

Velocities were pulse-like (180 cm/s) Fault Normal Fault Parallel Vertical

Displacements were geodetic (2.8 m) Fault Normal Fault Parallel Vertical

Where Does the Energy Come From? Isochrones and Fault Parallel Displacement Intensity for Pump Station 10 Note change in scale

How high are the stresses that cause big earthquakes?

San Andreas Fault Observatory at Depth (SAFOD)

Increase in Least Principal Stress Observed in the San Andreas Fault Zone SAFOD Stress Magnitudes: Pilot Hole, SAFOD SVSV Lithostat

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

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

Parkfield Mainshock Observed on the Pilot Hole Array  2 seconds  P S

2004 Parkfield Main Shock and Early Aftershocks

4 Minutes of Aftershocks of 2004 Parkfield Earthquake

World-wide Strong Motion Data Set (as of 2004)

World-wide Strong Motion Data Set (as of 2004) NanTroSEIZE?