High-resolution seismic data from the Frijoles Fault of the San Gregorio Fault zone, Año Nuevo, California

The U.S. Geological Survey acquired high-resolution P- and S-wave seismic data across the Frijoles Fault strand of the San Gregorio Fault Zone (SGFZ) at Año Nuevo, California in 2012. The SGFZ is a Holocene-active, dominantly right-lateral fault system that trends more than 200 km along the California coastline. The Frijoles Fault is one of several onshore strands of the SGF system, and together the strands represent a 3- to 4-km wide fault zone at Año Nuevo. Prior paleoseismology studies indicate highly variable slip-rate estimates, indicating considerable uncertainty about the slip history of the SGFZ and the seismic hazard it poses. Amongst the onshore strands of the SGFZ at Año Nuevo, the Frijoles Fault presented the fewest logistical challenges for our seismic survey; thus, tthe goal of this data acquisition was to better understand the structure, geometry, and precise location of the Frijoles Fault strand of the SGFZ at Año Nuevo.

We deployed 118 geophones (channels) at 5-m spacing along a linear profile centered on the mapped surface trace of the Frijoles Fault and co-located active P- and S-wave sources at ~1-m offset from the geophones. Channel numbers increase from southwest to northeast along the profile. We generated P-waves using a seisgun and S-waves by horizontally striking an aluminum block on both sides with a sledgehammer. We first deployed vertical-component geophones (40-Hz, SercelTM L40A, sensitivity of 22.34 volts/meter/second) to record P-wave sources, after which we replaced the vertical-component geophones with horizontal-component geophones (4.5-Hz, SercelTM L28-LBH, sensitivity of 31.3 volts/meter/second) to record S-wave sources. We connected geophones to takeout connectors on the refraction cables, which connected to two 60-channel Geometrics Stratavisor NX-60TM seismographs with 24-bit analog-to-digital converters. Two geophones from each refraction cable overlapped to provide accurate timing and quality control. Each shot was recorded at a 0.5-ms sampling rate for two seconds, with data recording for 100 ms before the actual time of the shot. This data release provides the metadata needed to utilize the seismic data.

Data Format and Files
We combined seismic traces for a given shot time into shot gathers (Figure 3), and the traces in each shot gather are ordered by channel numbers (1-118), which are based on the position of the geophones along the profile. Furthermore, we assigned a unique field file identification number (FFID) to each shot, and we combined the shot gathers recorded from both seismographs into two SEG-Y files (Barry et al., 1975), AN12.78023.segy and AN12.marine.segy.  The SEG-Y files are stored in big-Endian, 4-byte, IBM-floating-point format (format code 1). Data samples are in millivolts and can be converted to velocity using the geophone sensitivity values. SEG-Y header values are incorrect due to seismograph limitations; therefore, one should refer to the metadata for correct geophone locations. 
Metadata for all profiles are contained in two text files, AN12.setup.csv, AN12.location.csv, and one xml file,AN12Metadata.xml. The setup file identifies the shot (FFID) recorded by the two seismographs, channel number, recording stations (geophones), and the source type for the SEG-Y files. The location file includes the channel number, latitude, and longitude of all geophone locations. 

Reference
Barry, K.M., Cavers, D.A., and Kneale, C.W., 1975, Recommended standards for digital tape formats: Geophysics, vol. 40, no. 2, p. 344-352, doi: 10.1190/1.1440530.