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Zumberge, MA, Hatfield W, Wyatt FK.  2018.  Measuring seafloor strain with an optical fiber interferometer. Earth and Space Science. 5:371-379.   10.1029/2018ea000418   AbstractWebsite

We monitored the length of an optical fiber cable stretched between two seafloor anchors separated by 200m at a depth of 1900m, 90km west of Newport, OR, near the toe of the accretionary prism of the Cascadia subduction zone. We continuously recorded length changes using an equal arm Michelson interferometer formed by the sensing cable fiber and a mandrel-wound reference fiber. A second, nearly identical fiber interferometer (sharing the same cable and housing), differing only in its fiber's temperature coefficient, was recorded simultaneously, allowing the separation of optical path length change due to temperature from that due to strain. Data were collected for 100days following deployment on 18 October 2015, and showed an overall strain (length change) of -10.7 epsilon (shorter by 2.14mm). At seismic periods, the sensitivity was a few n epsilon; at tidal periods the noise level was a few tens of n epsilon. The RMS variation after removal of a -79n epsilon/day drift over the final 30days was 36n epsilon. No strain transients were observed. An unexpected response to the varying hydrostatic load from ocean tides was observed with a coefficient of -101n epsilon per meter of ocean tide height.

DeWolf, S, Wyatt FK, Zumberge MA, Hatfield W.  2015.  Improved vertical optical fiber borehole strainmeter design for measuring Earth strain. Review of Scientific Instruments. 86   10.1063/1.4935923   AbstractWebsite

Fiber-based interferometers provide the means to sense very small displacements over long baselines, and have the advantage of being nearly completely passive in their operation, making them particularly well suited for geophysical applications. A new 250 m, interferometric vertical borehole strainmeter has been developed based completely on passive optical components. Details of the design and deployment at the Pinon Flat Observatory are presented. Power spectra show an intertidal noise level of -130 dB (re. 1 epsilon(2)/Hz), consistent within 1-3 dB between redundant components. Examination of its response to Earth tides and earthquakes relative to the areal strain recorded by an orthogonal pair of collocated, 730 m horizontal laser strainmeters yield a Poisson's ratio for local near surface material of 0.25 that is consistent with previous results. (C) 2015 AIP Publishing LLC.

Pettit, EC, Waddington ED, Harrison WD, Thorsteinsson T, Elsberg D, Morack J, Zumberge MA.  2011.  The crossover stress, anisotropy and the ice flow law at Siple Dome, West Antarctica. Journal of Glaciology. 57:39-52.   10.3189/002214311795306619   AbstractWebsite

We used observations and modeling of Sip le Dome, West Antarctica, a ridge ice divide, to infer the importance of linear deformation mechanisms in ice-sheet flow. We determined the crossover stress (a threshold value of the effective deviatoric stress below which linear flow mechanisms dominate over nonlinear flow mechanisms) by combining measurements of ice properties with in situ deformation rate measurements and a finite-element ice flow model that accounts for the effects of viscous anisotropy induced by preferred crystal-orientation fabric. We found that a crossover stress of 0.18 bar produces the best match between predicted and observed deformation rates. For Sip le Dome, this means that including a linear term in the flow law is necessary, but generally the flow is still dominated by the nonlinear (Glen; n = 3) term. The pattern of flow near the divide at Sip le Dome is also strongly affected by crystal fabric. Measurements of sonic velocity, which is a proxy for vertically oriented crystal fabric, suggest that a bed-parallel shear band exists several hundred meters above the bed within the Ice Age ice.

Zumberge, MA.  1997.  Precise optical path length measurement through an optical fiber: Application to seafloor strain monitoring. Ocean Engineering. 24:531-542.   10.1016/s0029-8018(96)00029-7   AbstractWebsite

An optical tiber strainmeter intended for measuring tectonic strains on the seafloor is under development. In this instrument, an optical fiber is stretched between two points fixed to the ocean bottom; relative displacement of these points causes a change in the elongation of the fiber. This associated change in optical path length is monitored by an electronic distance meter. The dominant sources of noise in determining the optical path length of the fiber stem from the dependence of the fiber's index of refraction on both wavelength and temperature. In a 50 day long experiment performed in the shallow ocean, a test fiber was installed along a 210 m long baseline on the bottom. The RMS Variation in length was 5 mm except for two displacements of order 10 cm caused by known effects. (C) 1997 Elsevier Science Ltd.

Sasagawa, G, Zumberge MA.  1991.  Absolute Gravity Measurements in California, 1984-1989. Journal of Geophysical Research-Solid Earth and Planets. 96:2501-2513.   10.1029/90jb02283   AbstractWebsite

Repeated absolute gravity measurements have been made at 12 sites in California between 1984.3 and 1989.7. As determined in laboratory tests, the instrument used has an estimated accuracy of 10-mu-Gal (approximately 10(-8) g). The repeatability of the measurements is consistent with this accuracy assessment. No gravity changes above the limits set by instrumental uncertainty and environmental noise are observed in California during this period; the field observations provide upper limits on the rates of secular gravity changes which could be attributed to crustal deformation with a resolution corresponding to vertical displacement rates of 1-2 cm/yr.