THE PLATE BOUNDARY OBSERVATORY BOREHOLE STRAINMETER NETWORK: OBSERVATIONS FIVE YEARS INTO FULL NETWORK DATA COLLECTION

The Plate Boundary Observatory (PBO) borehole strainmeter network became fully operational in late 2008, with the completed 75-station network now reaching five years into data collection. The earliest PBO Gladwin borehole strainmeter sites were installed in 2005 and have been collecting data for eight full years. Each BSM site contains a Gladwin tensor strainmeter, a Malin three component borehole seismometer, and in some cases GPS, pore pressure, and/or tilt meter. The strainmeters and geophones are at depths between 400 to 800 feet, which is free of most surface noise. Over the years the Gladwin strainmeters have observed numerous geophysical phenomena, in some locations repeatedly.

Site selection focused on specific areas of geologic interest. The Olympic peninsula in Washington State was targeted due to the peninsula’s westward above sea-level extension over the subducting Juan de Fuca plate. These borehole strainmeter sites observe the almost annual episodic tremor and slip events that range from Vancouver Island, Canada to Olympia, WA. The strainmeters in Parkfield, CA span the transition zone between the locked segment of the San Andreas fault to the south and the creeping section to the north. The northernmost sites observe creep events. The network also includes two volcanic arrays: Mt St Helens and the Yellowstone caldera. The Yellowstone strainmeters have observed seiches in Yellowstone lake. Over time, repetitive observations like these will aid in the understanding of the regional fault mechanics and volcanic processes.