Hydrological Signals in Plate Boundary Observatory Borehole Strainmeter DataHodgkinson, Kathleen; Mencin, Dave; Borsa, Adrain; Jackson, Mike The Plate Boundary Observatory includes 75 borehole strainmeters installed in targeted arrays between southern California and Vancouver Island, British Columbia. The purpose of the strainmeters is to capture short-term strain transients with periods of hours to weeks that fall between the detection levels of GPS and seismology. One of the main sources of non-tectonic noise in the borehole strainmeter data is that of irrigation and hydrological pumping. This noise must be extracted from the time-series in order to isolate any tectonic transients. Two strainmeter sites in particular, one on Vancouver Island and another close to the San Jacinto fault in southern California provide an opportunity to study the effect of irrigation and hydrological pumping on strain measurements. A seasonal signal is visible in the measurements from three strainmeters, all located within a few hundred meters of each other, on southern Vancouver Island. The source is believed to be the pumping of water from wells and the irrigation of fields about 1 km from the instruments. The signal is large enough that it can complicate the identification of any strain transient generated by Episodic Tremor and Slip events along the Cascadia subduction zone. In southern California, two strainmeters are located within 50 to 200 meters of a man made lake into which water is pumped. Isolation and removal of the hydrological signal would improve the ability to detect changes in strain rate along the San Jacinto fault in the strain data. The Vancouver Island and southern California instruments both provide an opportunity to study the effect of hydrological changes in strain measurements. Also, since the origin of the signal is known and the sites consist of two or more strainmeters the data allows the study of how the hydrological signal in the strain data can vary with distance from the noise source. |