Completion of the PBO Borehole Strainmeter Network: Network Results and Review of Processing Techniques. Abstract

abstract

  • By October 2008 the NSF funded geodetic component of Earthscope, the Plate Boundary Observatory will be complete. As of September 2008 the strainmeter network within PBO consists of 70 co-located borehole strainmeters and seismometers, with borehole tiltmeters included at volcanic sites. The instruments are installed in arrays from Vancouver Island, Canada, to Anza in southern California. The network provides an improved ability to record the evolution of strain transients such as those associated with Episodic Tremor and Slip (ETS) events and slow fault slip. Already, the strainmeters in the Pacific North West have recorded three Cascadia ETS events, providing unique temporal and spatial resolution of the signals. Raw data are available to the community from the IRIS DMC and the NCEDC in SEED format within minutes of being downloaded from the loggers. In addition to the raw data, UNAVCO's PBO-Borehole Strainmeter Analysis Center provides a processed (Level 2) data set with a barometric pressure, tide and long-term trend correction that is updated every 14 days. The main purpose of the strainmeter network is to record strain transients that fall between the detection levels of seismology and GPS. To identify such transients, strain changes induced by non-tectonic signals must be removed from the data. Two main sources of noise are barometric pressure and borehole relaxation. The current Level 2 barometric pressure corrections are produced assuming pressure and strain changes are linearly related. However, this simple model does not completely remove the pressure signal and, in some cases, can even introduce noise. We shall compare the current linear correction with a frequency- dependant correction to seek an improved pressure correction technique. We shall also compare strain residuals from trend models generated using a combination of linear and exponential terms with residuals calculated using high pass filtering to remove the long-term borehole trends. If the improved pressure and trend corrections lead to a cleaner processed data set then these new methods should be included in routine Level 2 data strainmeter data products.

publication date

  • 2008

presented at event