Investigation of Earthquake Sources and Fracture Origin of Crustal Seismic Coda Using Seismograms Recorded at Depth in the Cajon Pass Drillhole

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This research is to continue the recording and analysis of a triaxial set of 10 Hz geophones placed at a depth of 2.5 km in the Cajon Pass deep drill hole near the San Andreas fault in southern California. To date, the instrumentation has recorded several hundred earthquakes at frequencies up to 400 Hz. Most larger events were recorded simultaneously by a 2 Hz triaxial surface seismometer. Results suggest that surface seismograms are severely depleted in high frequency wave amplitudes and downhole seismograms have high corner frequencies and high frequency power law amplitude decays. Additional wide-band seismic sensors will be installed at 2.0 and 3.5 km depths in order to give an improved measure of natural seismicity and record microearthquake scaling in a volume of seismically active crust. This research is a component of the National Earthquake Hazard Reduction Program.