Characterization of NOTA Borehole Strainmeter Performance for Earthquake Early Warning Applications

Abstract:

Recent work by Barbour, Farghal, and Langbein (2020) at the USGS has demonstrated the potential benefits of using borehole strainmeter (BSM) measurements in earthquake early warning (EEW). These instruments record seismic strain waves with high signal-to-noise ratios, and unlike broadband seismometers do not saturate during large, M>7, earthquakes. UNAVCO currently operates 75 BSMs in the western US as part of the Network of the Americas (NOTA) and the NSF-funded Geodetic Facility for the Advancement of GEoscience (GAGE). We are in the process of upgrading these instruments from hourly downloads to support near real-time dataflow.

ShakeAlert, the western US EEW system operated by the USGS, is currently working on expanding data sources to include real-time GNSS displacements into the rapid magnitude estimation algorithms. Low-latency strain data, once available, could become another high-value, independent data source for ShakeAlert. The distribution of NOTA BSMs in the Pacific Northwest, in particular, could be extremely valuable in helping ShakeAlert reach its four-station alerting threshold faster in the event of a large Cascadia subduction zone event.

To further this goal, UNAVCO has undertaken efforts to characterize the performance of individual stations at higher frequencies. We present here an initial framework for examining ambient noise levels at NOTA BSM stations using power spectral analysis, based on techniques presented in McNamara and Buland 2004. We also will show examples of co-seismic dynamic strains and power spectra and how those compare to ambient noise levels. Finally, we will use Barbour et al., (2020) to present the magnitude and timing of strain-based estimates for several recent earthquakes. The goal of this work is to develop metrics that quantify the sensitivity of individual strainmeters to co-seismic ground motion.