PBO Strainmeters: Extending the Spectrum of Fault Deformation Observations

Abstract:

A fundamental goal of the Plate Boundary Observatory (PBO) is to enable investigation of the role that small, short-term strain transients play in the release of accumulated stress along fault zones. The detection of slow earthquakes along the San Andreas in the 1990’s and the discovery of Episodic Tremor and Slip (ETS) events in Japan and Cascadia in the following years suggested that fault slip modes could span the temporal range from seismic to slow-deformation processes. Strainmeters provide the sensitivity required to detect short-term, nanostrain-level transients and so for this reason clusters of co-located strainmeters and seismometers were included in the PBO design. Arrays of strainmeters, each forming a mini fault observatory, were installed along the San Jacinto Fault, the creeping section of the central San Andreas, in the Eastern California Shear Zone, in two volcanic zones, Mt St Helens and Yellowstone and a regional scale network was built along the Cascadia Subduction Zone megathrust fault. Currently the network consists of 75 borehole and 6 long baseline strainmeters. Since the installation of the first strainmeter in 2005 the instruments have provided unprecedented temporal resolution of multiple creep events along the central section of the San Andreas, post-seismic transients in Anza and ETS events in Cascadia,

UNAVCO generates Earthscope Level 2 data products for the strainmeters. These include models for the earth tides, barometric responses, long-term borehole trends plus areal and shear strain time-series. Processed time-series are updated automatically every 24 hours and a follow-up data set reviewed by a data engineer is released every 7 to 10 days. Site information, data quality assessment, strain plots and time-series data for all PBO strain instruments can be obtained from the UNAVCO strain and seismic web page (http://www.unavco.org/data). In this presentation we will highlight some of the strain transients these instruments have measured, outline the processing steps required to extract these subtle transient signals from the data, and describe the strainmeter data products available UNAVCO including newly implemented web services.