NYLAND RANCH NAIL ARRAY
Latitude: N 36°51'18"
Longitude: W 121°32'43"
Length: 43 m
Number of Nails: 28
Number of Surveys to 2009: 46
Date of Initial Survey: October 1967
Date of Latest Survey: 3 November 2008
Trespass Permission: Permission to trespass is required.
The nail line is located about one kilometer northeast of San Juan Bautista
in a ranch driveway on the southwest side of San Juan Bautista Highway.
It begins at the entrance to Nyland Ranch and continues southwestward
up the driveway and across the San Andreas fault.
The Nyland Ranch nail line was established by Robert Nason in October
1967. UCSB began measurements in February, 1976. Right slip occurred at
a rate of 7 mm/yr (Sylvester, 1986) until six weeks after the M6.9 Loma
Prieta earthquake of October 1989, when it accelerated to 19 mm/yr (Sylvester
et al., 1989). The line was paved over in January 1995 and reestablished
in June 1995 as shown in the graph below. The surveys to 9
July 2006 have established an average creep rate of 12 mm/yr since 1995.
(see Photos of nail line and
bent fence). University of Colorado maintains a creepmeter
at this site since 2004. The driveway and nails were paved over since the 3 November 2008 survey, so this site is no longer being monitored.
It is noteworthy that the creep rate slowed perceptibly before local
earthquakes (M= or >5), as is shown clearly in the graph below
of the residuals to a least squares line constructed through the data
plotted above. The decreasing creep rate is especially pronounced
in the years before the M 6.9 Loma Prieta earthquake. Since the August
2002 earthquake, a strong retardation signal has last six years and thus
ought to presage a M>5 earthquake within 20 km of San Juan Bautista,
although just how large and when depend on the magnitude and duration
of the retardation signal.
Behr, J., R. Bilham, P. Bodin, K. Breckenridge, and A. Sylvester, 1997.
Accelerated afterslip on the San Andreas fault following the Loma Prieta
earthquake. U.S. Geological Survey Professional Paper 1550.
Bilham, R., N. Suszek, and S. Pinkney, 2004. California Creepmeters.
Seismological Research Letters 75 (4), 481-492.
Sylvester, A. G., 1986. Near-Field Tectonic Geodesy, Chap. 11, in Wallace,
R. E., Active Tectonics. National Academy Press, Washington, D.C., 266
Sylvester, A. G., R. O. Burford, and S. S. Schulz, 1989. Almost no surface
displacement occurred at San Juan Bautista as a result of the Loma Prieta
earthquake. EOS 71 (8), 290. (But it sure
did six weeks afterward! - see Behr et al reference)
Sylvester, A. G., 1995. Nearfield vertical displacement in the
creeping segment of the San Andreas fault, central California, 1975
to 1994. Tectonophysics 247, 25-47. (see esp. pp. 29-30).
Updated 12 November 2008
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