Results From a Decade of Optical Fiber Strainmeters at Piñon Flat Observatory

Abstract

Optical fibers, stretched between anchors attached to the Earth and buried in boreholes and shallow trenches, offer an inexpensive and robust means to observe Earth strain. We have deployed several optical fiber strainmeters (OFSs) in boreholes, where thermal stability helps mitigate the noise from temperature changes. We have also constructed horizontal OFSs in 1‐m‐deep trenches of lengths 221 and 174 m. In these, a second optical fiber having a contrasting temperature coefficient provides a means to separate true strain from thermal noise. Signals in the seismic band from the horizontal strainmeters agree well with records from collocated seismometers, and the predominant ambient noise recorded by both techniques are consistent. Solid Earth strain tides with amplitude of 30 nε (nanostrain) are observed in these horizontal OFSs that agree well with those observed by collocated vacuum laser strainmeters. Noise at periods of a few days amounts to 50 nε. The two horizontal strainmeters show drift rates of up to 1 µε per month, and occasional false signals (evident from comparisons with the vacuum laser strainmeters) can accumulate occasionally to the order of 50 nε over times from hours to weeks. Results to date indicate that this technology on land could detect slow‐slip events of amplitude 100 nε and higher.

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“…Strainmeters are designed to measure the strain field (Gladwin & Hart, 1985;Agnew, 1986). However, they often only measure certain components of the strain tensor, such as single axial components (Zumberge et al, 1988;DeWolf et al, 2015;Hatfield et al, 2022), volumetric strain (Linde and Sacks, 1995;Bonnacorso et al, 2016;Canitano et al, 2021) or 3-components of the strain tensor (Maccioni et al, 2019). The volumetric strain (𝜀 𝑥 + 𝜀 𝑦 + 𝜀 𝑧 ) can be accurately measured by dilatometers in boreholes (Sacks et al, 1971).…”