SCEC Award Number 13159 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Routine Measurement of Seismic Strains: Analyzing and Improving Optical-Fiber Strainmeter Data
Investigator(s)
Name Organization
Duncan Agnew University of California, San Diego Mark Zumberge University of California, San Diego Frank Wyatt University of California, San Diego
Other Participants
SCEC Priorities 6, 5, 2 SCEC Groups Seismology, GMP
Report Due Date 03/15/2014 Date Report Submitted N/A
Project Abstract
 This SCEC grant supports data analysis of the Trench Optical Fiber Strainmeter, or TOFS, installed at Pinyon Flat Observatory (PFO); this long-base instrument is much less expensive and far easier to install than previous strainmeters, though relatively sensitive to temperature and lacking long-term stability. TOFS offers the prospects for a broad deployment of strainmeters. We have compared data collected on two TOFS, 180 and 90 m long, both installed parallel to the 730 m reference- NWSE laser strainmeter (LSM) at PFO, examining relative noise levels and the relative sensitivity to seismic waves, microseisms, and tides. The instrument calibrations are all known to much better than 1%, but we observe systematic differences in the admittance between the systems that exceed this level, with the admittance from the LSM to the TOFS being, in response to difference signals, 5% or more above the expected value. After considerable efforts to reconcile these observations, we find the most plausible explanation for the source of these departures to be that the strain field is rendered slightly inhomogeneous, even for these long-wavelength signals, by local inhomogeneities in the rock; we will be testing this--the new instrument and the Earth--using a third TOFS installed at a different azimuth.
Intellectual Merit This project, by developing instruments than enable more detailed measurements of the deformations associated with seismic waves, provides additional insight into the fine structure of the wavefield, information that can potentially be used to refine models of earth structure.
Broader Impacts The broader impacts of this project include the training of a graduate student in the methods of fiber-optic strain measurement, the development of new geophysical infrastructure (inexpensive and precise strain measurements) and the potential for improvements of strong ground motion prediction from more accurate modeling of the seismic wavefield.
Exemplary Figure Figure 2
Power spectral densities for four strainmeters at PFO: the NWSE LSM, the PBO borehole sensor (GTSM) and the two TOFS. In each panel the inset shows the spectrum in the tidal bands in mode detail, for both raw and detided data. The spectral levels are roughly equivalent in the microseism band, but at a period of 1000 s the TOFS is about 15 dB noisier than either the borehole or longbase strainmeter, a discrepancy that rises to 20 dB at tidal frequencies and 60 dB at periods of a week or more.