International Federation of Digital Seismograph Networks

XA (1990-1990): Piñon Flats Small Aperture Array, High Frequency Array

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FDSN code XA (1990-1990) Network name Piñon Flats Small Aperture Array, High Frequency Array (Piñon I or SSA1)
Start year 1990 Operated by
End year 1990 Deployment region

The Piñon Flat Passive Source High Frequency Array Experiment of the IRIS Eurasian Seismic Studies Program was conducted in April and May, 1990, at Piñon Flat Observatory, California. Goals of the field portion of the experiment were to obtain a detailed, unaliased record of the seismic wavefields from local and regional events, as well as to test and prepare equipment and personnel for similar ESSP deployments in the Soviet Union during 1991 and 1992.

Piñon Flat Observatory (PFO) was chosen as the experiment site for its high levels of local seismic activity, the potential for recording regional earthquakes from California and Baja, and its proximity to the Nevada Test Site. Existing seismic coverage at PFO and the surrounding area provided guidance concerning event location as well as expected site response. A total of 325 events were recorded between April 18, 1990, and May 27, 1990.

The array operated in master/slave mode, with elements of the array triggered by two borehole sensors located at depths of approximately 150 and 275 meters. The borehole instruments are part of a permanent
USGS installation at PFO and have been in operation for several years. Use of the borehole sensors as masters in trigger decisions significantly reduced false triggers. Over the duration of the experiment, approximately 87% of the slaves triggered in response to a master trigger command. A large percentage of the "failure to trigger" data loss was due to problems with the central power distribution system early in the project. Very few data were lost due to instrument failure.

The array contained 58, 3-component, L-22 (2 Hz) sensors and 29 PASSCAL data recorders, each record­ ing on 6 channels. Signals from the two borehole receivers were recorded using one additional PASSCAL recorder, bringing the total number of seismometers to 60 and the total number of PASSCAL recorders to 30. The externally triggered data streams were recorded at a sample rate of 250 samples/sec. The triggers were STA (short-term average) to LTA (long-term average) ratios. The STA and LTA windows were 0.1 and 30 seconds respectively, and the STAJLTA trigger threshold was set at 10. The data streams used pre-event buffers of 15 seconds and an overall record length of 90 seconds. Data recording was done in 16 bit mode, so an event triggering all receivers generated just over 8 Mb of data.

The array was configured as a grid with two orthogonal arms. Sensor spacings of 7 meters within the grid and 21 meters on the arms were chosen to aid analysis of body to surface wave conversions at high wavenumbers. All elements of the array were linked to a common time signal from a single OMEGA time clock to ensure precise timing over short distances. The sensors were oriented, leveled, and set in a plaster pad at the bottom of pits dug 1 to 4 meters through an unconsolidated sandy surface layer and into the underlying weathered granitic material.

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