Cecil and Ida Green Piñon Flat Observatory (PFO)

Introduction

      PFO is operated by the Institute of Geophysics and Planetary Physics of the University of California, San Diego (IGPP-UCSD), and is intended...

• To measure, as accurately as possible, crustal deformation in a tectonically active area, so as to improve our understanding of the earthquake cycle, and so improve estimates of the earthquake hazard throughout southern California.
• To serve as a testing ground for new geophysical instrumentation. Investigators from throughout the world operate instruments at this site.

Tectonic Setting

      The observatory is located between the San Andreas and San Jacinto faults the two most active faults on the Southern California segment of the Pacific/North-American plate boundary. The distances to these faults (25 km to the San Andreas and 12 km to the San Jacinto) are about the same as the depth of seismicity; the observatory is thus close enough to monitor the deformations that drive the earthquake process. Large earthquakes are expected on both faults in the near future: conservatively, 30% odds in the next 30 years or so. The 30 kilometer section of the San Jacinto fault closest to PFO has not broken in this century, although sections north and south of it have; indeed, since 1900 this fault has been the most frequent source of earthquakes larger than magnitude 6 in southern California, as shown by a map of historic earthquakes. The nearby San Andreas has not broken in historic time, and most estimates of plate motion suggest that as much as 10 m (33 feet) of slip may be stored up. Should all this slip be released in one event, it would likely result in a great (magnitude 8+) earthquake. The 1992 Landers, California, earthquake increased the stress on a large portion of the nearby San Andreas fault.

Development of the Observatory

      The specific location of PFO was chosen because it was near these faults and because the size of the instruments required a large flat area not covered by alluvium. The flat area (Pinyon Flat) gave the name to the observatory (though we spell the title somewhat differently). The surface material is decomposed granite, grading with depth into unweathered Mesozoic granodiorite. Work at PFO began in 1970, at which time the land was owned by the U.S. Forest Service; it was purchased by the University in 1980 with generous support from Cecil and Ida Green. Operation of the observatory is currently supported by the U.S. Geological Survey, the Incorporated Research Institutions for Seismology (IRIS), and the Southern California Earthquake Center (SCEC).

Data Available

      Files of raw data are available; these should not be interpreted without detailed knowledge of how the strainmeter operates. These data begin on 20 May 2005 (2005:140) when our newest datalogger was installed; for older data contact Frank Wyatt (see below).

Log of Some Events at PFO

• Information about the 2004/5 construction of the (four-vault) Seismic Test Facility (STF) at Pinon Flat Observatory.
• Information about the 2002 upgrade of the EW strainmeter adding anchoring and other new features to this sensor.
• Information about the 1995 upgrade of the NW-SE laser strainmeter to reduce maintenance costs and improve strain recordings.
• Information on the July 1994 brushfire at PFO (now completely recovered from).

Additional Information

• A summary of the different long-term measurements made at PFO.
• A bibliography of research papers based on measurements at PFO
• A collection of background information about PFO (facilities and instruments present) for investigators interested in using the facility, along with a map of the site.
• Various photographs of the site and some of the people who have worked there.

For more information about PFO write to:
Frank Wyatt
IGPP 0225
University of California, San Diego
La Jolla, CA 92093-0225

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