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Wang, TH, Cochran ES, Agnew D, Oglesby DD.  2013.  Infrequent triggering of tremor along the San Jacinto Fault near Anza, California. Bulletin of the Seismological Society of America. 103:2482-2497.   10.1785/0120120284   AbstractWebsite

We examine the conditions necessary to trigger tremor along the San Jacinto fault (SJF) near Anza, California, where previous studies suggest triggered tremor occurs, but observations are sparse. We investigate the stress required to trigger tremor using continuous broadband seismograms from 11 stations located near Anza, California. We examine 44 M-w >= 7.4 teleseismic events between 2001 and 2011; these events occur at a wide range of back azimuths and hypocentral distances. In addition, we included one smaller-magnitude, regional event, the 2009 M-w 6.5 Gulf of California earthquake, because it induced extremely high strains at Anza. We find the only episode of triggered tremor occurred during the 3 November 2002 M-w 7.8 Denali earthquake. The tremor episode lasted 300 s, was composed of 12 tremor bursts, and was located along SJF at the northwestern edge of the Anza gap at approximately 13 km depth. The tremor episode started at the Love-wave arrival, when surface-wave particle motions are primarily in the transverse direction. We find that the Denali earthquake induced the second highest stress (similar to 35 kPa) among the 44 teleseismic events and 1 regional event. The dominant period of the Denali surface wave was 22.8 s, at the lower end of the range observed for all events (20-40 s), similar to periods shown to trigger tremor in other locations. The surface waves from the 2009 M-w 6.5 Gulf of California earthquake had the highest observed strain, yet a much shorter dominant period of 10 s and did not trigger tremor. This result suggests that not only the amplitude of the induced strain, but also the period of the incoming surface wave, may control triggering of tremors near Anza. In addition, we find that the transient-shear stress (17-35 kPa) required to trigger tremor along the SJF at Anza is distinctly higher than what has been reported for the well-studied San Andreas fault.

Wyatt, FK, Morrissey ST, Agnew DC.  1988.  Shallow Borehole Tilt - A Reprise. Journal of Geophysical Research-Solid Earth and Planets. 93:9197-9201.   10.1029/JB093iB08p09197   AbstractWebsite

We describe results from nearly a decade of tilt measurements produced by two arrays of shallow borehole tiltmeters: one in a semiarid environment at Piñon Flat Observatory (PFO), California (depth of burial 4.5 m) and the other in a maritime-Arctic environment at Adak, Alaska (depth of burial 2 m). Although renovation and reinstallation of the instruments at the two sites reduced thermal noise, it did not change the secular records significantly. This implies that the large tilts observed reflect instability of the ground rather than the sensor, so that deeper installations should give better results. The PFO data show large rainfall-related tilts (caused by near-surface weathering) and also periodic temperature-related tilts (from several thermoelastic effects). The Adak data are dominated by tilts from the annual temperature cycle (though by a smaller amount than at PFO), but at other frequencies they show essentially the same power levels as at PFO. Both data sets confirm earlier results that burial at such shallow depths, even in apparently stable material, is inadequate for the measurement of tectonic tilts.

Wyatt, F, Agnew D, Linde A, Sacks IS.  1983.  Borehole Stranimeter studies in Pinon flat observatory. Carnegie Institute of Washington, Yearbook 82, Washington DC. :533-538. Abstract
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Wyatt, FK, Agnew DC, Gladwin M.  1994.  Continuous Measurements of Crustal Deformation for the 1992 Landers Earthquake Sequence. Bulletin of the Seismological Society of America. 84:768-779. AbstractWebsite

We describe, and attempt to interpret, continuous measurements of strains and tilts made at Pinon Flat Observatory (PFO) before, during, and after the Landers and Joshua Tree earthquake sequences. These data show substantial transient deformation following the Landers mainshock, with a total amplitude of several percent of the co-seismic deformation, and a decay time of at least several days. Comparing data from the many types of instruments at PFO allows us to infer possible sources for this deformation. The immediate postseismic transient was nearly the same size on three long-base strainmeters, suggesting either broad-scale deformation or local motion near one part of the observatory. The latter can largely be ruled out by the similarity of many other measurements in the area covered by these strainmeters and the observations by others of significant postseismic displacements nearer the source. Possible mechanisms for broad-scale deformation include postseismic fault slip, time-dependent creep in near-surface rocks, and elastic or thermal responses to water-table changes. The first two agree best with the observations from PFO, but if postseismic fault slip is the source, it must have been distributed differently than the co-seismic slip, and may have included faults other than those that ruptured seismically. If one of the other mechanisms is the main source, the PFO data imply that the postseismic slip must have been very much smaller than the seismic slip, perhaps 2% or less. No significant preseismic deformation was observed, at a level of 2 X 10(-3) of the co-seismic deformation, for the days to minutes before the earthquake.

Wyatt, F, Bilham R, Beavan J, Sylvester AG, Owen T, Harvey A, Macdonald C, Jackson DD, Agnew DC.  1984.  Comparing Tiltmeters for Crustal Deformation Measurement - A Preliminary Report. Geophysical Research Letters. 11:963-966.   10.1029/GL011i010p00963   AbstractWebsite

A collection of high-precision tiltmeters is being operated at Piñon Flat Observatory, southern California, both to compare instruments and to measure tectonic deformation. We report on 1.2 years of data from four of these: two Michelson-Gale long fluid tiltmeters, one long center-pressure tiltmeter, and a shallow borehole tiltmeter. The three long-base instruments are all located on the same baseline, with a precise leveling line running between their end-monuments. At nontidal frequencies, only the two Michelson-Gale instruments show some coherence (γ² = .3 for periods of 2 to 4 days), while the center-pressure instrument is correlated with air temperature at periods from a few days to a few weeks. The most stable tilt record shows a secular rate of 0.28 µrad/a, which may be real. Over much longer times, leveling to specially stabilized benchmarks should confirm this. Comparing instruments has identified more and less successful measurement techniques; it appears that low-noise data will most probably be produced only by relatively complex and expensive instruments, though even for these, the operating costs over any reasonable lifetime will exceed the capital cost. Even the best existing sensors must be improved to measure continuous tectonic motions.

Wyatt, F, Cabaniss G, Agnew DC.  1982.  A Comparison of Tiltmeters at Tidal Frequencies. Geophysical Research Letters. 9:743-746.   10.1029/GL009i007p00743   AbstractWebsite

Different techniques for measuring crustal deformation are now being tested at Piñon Flat Geophysical Observatory in southern California. As a part of this comparison, we have analyzed five months of data from four different tiltmeters: three borehole instruments, two 4.5 m and one 26 m deep; and a long-base (535 m) instrument at the surface. For periods from 1 hour to 10 days, the signals from the deep borehole and long-base instruments show comparable noise levels; the shallow instruments are much noisier. Tidal analyses give results for the different sensors which agree to within the errors, which are several percent.