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Borehole strainmeter measurements spanning the 2014 M w 6.0 South Napa Earthquake, California: The effect from instrument calibration
Author(s) -
Langbein John
Publication year - 2015
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2015jb012278
Subject(s) - borehole , geology , seismology , calibration , vertical seismic profile , geodesy , earthquake prediction , geotechnical engineering , physics , quantum mechanics
The 24 August 2014 M w 6.0 South Napa, California earthquake produced significant offsets on 12 borehole strainmeters in the San Francisco Bay area. These strainmeters are located between 24 and 80 km from the source, and the observed offsets ranged up to 400 parts per billion (ppb), which exceeds their nominal precision by a factor of 100. However, the observed offsets of tidally calibrated strains differ by up to 130 ppb from predictions based on a moment tensor derived from seismic data. The large misfit can be attributed to a combination of poor instrument calibration and better modeling of the strain field from the earthquake. Borehole strainmeters require in situ calibration, which historically has been accomplished by comparing their measurements of Earth tides with the strain tides predicted by a model. Although the borehole strainmeter accurately measures the deformation within the borehole, the long‐wavelength strain signals from tides or other tectonic processes recorded in the borehole are modified by the presence of the borehole and the elastic properties of the grout and the instrument. Previous analyses of surface‐mounted, strainmeter data and their relationship with the predicted tides suggest that tidal models could be in error by 30%. The poor fit of the borehole strainmeter data from this earthquake can be improved by simultaneously varying the components of the model tides up to 30% and making small adjustments to the point source model of the earthquake, which reduces the RMS misfit from 130 ppb to 18 ppb. This suggests that relying on tidal models to calibrate borehole strainmeters significantly reduces their accuracy.