Science Spotlight

Station ALBH

Researcher: Herb Dragert
Natural Resources Canada, Emeritus Research Scientist

The most interesting aspects of my job have been the constant changes brought about by technological developments and the fact that answering one research question usually gives rise to ten new research questions.

ALBH is located on Vancouver Island in British Columbia.

Country: Canada
Elevation: 31.7 m
Lat/Long:  48.3898 / -123.4875

Episodic Tremor and Slip

ALBH was one the very first continuously operating GPS sites to be installed in the world. It is located in the Cascadia Subduction Zone. Here a small tectonic plate called the Juan de Fuca plate subducts beneath the North American plate (Figure 1). Instead of moving freely past each other, the two stick together in what is called a "locked" zone. When this locked zone breaks, every 500 to 600 years or so, a massive earthquake rocks the region, causing the land along the coast to sink and move seaward. The last great earthquake in this region was in 1700.

The GPS data from ALBH were vital to the discovery of what is known as episodic tremor and slip. ALBH continues to be key in on-going investigations of this phenomenon. In a transformative paper published in 2001, scientists at the Pacific Geoscience Centre pointed out that instead of moving steadily in one direction, as we would expect, the GPS sites moved in the opposite direction for several weeks (Figure 2) This observation was subsequently discovered to happen every 14-15 months (Figure 3). A follow-on paper showed that these slip episodes were accompanied by seismic tremor.

Figure 1. Top: the Cascadia Subduction Zone. Below: Side view of the Juan de Fuca subducting plate. The downgoing Juan de Fuca plate moves eastward, but is stuck to the North American plate along the locked zone.

Figure 4. Position changes for ALBH in a North American fixed reference frame. (For help interpreting the graphs, see the GPS Data page.)


Figure 2. Black vectors are long-term velocities relative to stable North America and red vectors are anomalous displacements during the 1999 slow slip event. The inset shows relative timing of the events.

Figure 3. Top: day-by-day changes in the location of ALBH in an east-west direction. The long-term upward trend (green line) shows that, on average, this site moves east at a rate of five millimeters per year. The red saw-tooth line shows that for about 15 months, the GPS site moves more rapidly east than the long-term average. However, at the end of each 15-month period, the site moves west about four millimeters over a period of two weeks. These temporary reversals or breaks mark episodic tremor and slip events. Below: Tremor activity.

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Last modified: 2019-12-26  16:24:58  America/Denver  


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