August 22, 2011
A few months earlier than anticipated, the 2011 Cascadia Episodic Tremor and Slip event may be underway. Based on the start time of last year’s event, this year’s tremor was expected to begin late October 2011. An increase in tremor south of Puget Sound around August 7th, however, and its propagation northeastwards under the Olympic Peninsula over the past 10 days has caused speculation that this is the main 2011 ETS event. (http://www.pnsn.org/WEBICORDER/DEEPTREM/summer2011.html).
UNAVCO maintains a network of borehole strainmeters on the Olympic Peninsula and Vancouver Island, Canada, as part of the Plate Boundary Observatory (Figure 1). The strainmeters are distributed in such a way as to capture the spatial and temporal evolution of Cascadia ETS events. Several of the instruments have recorded the main ETS events of 2005, 2007, 2008, 2009, 2010 and now 2011 (Figure 2). Generally, shear-strains increase in the northeast direction as the tremor moves towards the strainmeters and then decrease as the tremor continues northwest away from the instruments and towards Vancouver Island. Peak to peak amplitudes tend to be on the order of 50 to 100 nanostrain with the signals lasting 2 to 3 weeks.
The animation shows the 2011 ETS strain signal as of mid-August 2011 compared to the propagation of tremor. Strainmeters B005 and B007 are collocated and have the best recording of this year’s events so far. The signal at B003 develops after B005 and B007 as the tremor moves closer to that site. In this animation long term trends have been removed by simply removing s straight line from the the data plotted. The background tidal and barometric pressure signals have been removed to isolate the ETS deformation. The tremor location and times were retrieved from the Pacific Northwest Seismic Network interactive tremor web site. This animation will be updated daily throughout the 2011 ETS. Links to daily updates of ETS signal at these sites can be found at Borehole Strainmeter Data. The processed data used to generate these plots are also updated daily and are available in ASCII format from the same data products page.
Last modified: Monday, 13-Jul-2015 15:22:31 UTC