Observing the Cascadia Subduction Zone in action
Residents of southern British Columbia, Washington, Oregon, and northern California know that great (magnitude 8 or 9) earthquakes have occurred every few hundred years on the west coast. These earthquakes take place offshore on the shallow part of a major fault where the ocean floor slides under, or subducts beneath, western North America.
A schematic cross-section for the Cascadia Subduction zone derived from observations of crustal deformation. The convergent motion of the subducting Juan de Fuca plate is not continuous but determined by the frictional strength on the plate interface. On the shallowest portion of the interface, from 0 to 15 km depths, friction is strong and the plates are locked together causing the overlying crust to deform for hundreds of years. At intermediate depths of the plate interface, 15 to 25 km, temperatures increase and, along with increases in released fluids, the frictional strength becomes gradually weaker over this depth range called the transition zone. The plate interface from 25 to 45 km depths which underlies Vancouver Island is characterized by frictional strength that appears to change with time. For about 15 months, plate motion is resisted by frictional strength which then disappears over a period of several weeks allowing the plates to slip a few centimetres. This region of the plate interface with this short-term stick-slip behavior is called the slip zone.
A team of scientists at the Geological Survey of Canada of the federal Department of Natural Resources have observed unexpected behavior of the deeper parts of the same fault underlying southern Vancouver Island and the Olympic Peninsula. Their study showed that instead of slipping steadily, as was generally assumed, this deeper fault interface exhibits episodes of slip of several centimetres over periods of one to two weeks. The discovery of this behaviour by GSC scientists at the Pacific Geoscience Centre, Sidney, B.C., was published in the journal Science in 2001.
In this publication, the authors suggest that an episode of slip adds stress to the shallow, locked part of the fault bringing it slightly closer to failing, or giving way, resulting in the next great earthquake. There's no need for alarm, however. These episodes of slip occur every 13 to 16 months and therefore represent "business as usual" for the fault. Instead of steadily building up stress on the locked zone, as was formerly thought, it now looks as though stress is added episodically over a period of a few weeks.
The orginal slip event was detected with a network of sensitive Global Positioning System (GPS) sites established to monitor the crustal stretching and squeezing of this earthquake-prone subduction zone. This is the same satellite technology that is used for navigation and general positioning but much greater precision is achieved by using specialized GPS receivers, stable antenna mounts and careful data processing. It is thus possible to measure horizontal positions to about a millimetre over a few hundred kilometres.
The location of continuous GPS stations operating in 1999 in the coastal region of southwest British Columbia and northwest Washington State. At that time, there were 9 regional Canadian GPS stations operated by the Geological Survey of Canada and 5 US regional GPS stations operated by various agencies in the United States. The spatial coverage provided by these 14 sites, the generation of more precise satellite orbits from the International GPS Service (IGS), and improved GPS data analysis techniques facilitated the first detection of slow slip events in the Cascadia Subduction Zone. The map also shows the epicentres of three large (M~7) historical crustal earthquakes: two in central Vancouver Island in 1918 and 1946; one south of Chilliwack in 1872, and the epicentre of the M=6.8 Nisqually earthquake which occurred in the subducting ocean plate at a depth of 55 km beneath Olympia, WA.
 |
The red stars in the picture represent the earthquakes greater than magnitude 7 on North American Plate |
 |
The blue star represents the earthquake greater than magnitude 7 on Juan de Fuca Plate |
 |
The red square is the reference GPS station used in our data processing. |
 |
The green squares are the GPS sites in Canada |
 |
The yellow square yellow squares are the GPS sites in the USA |
| The green and red dotted lines show the nominal downdip limits of the locked and transition zones, respectively, from the model of Flueck et al. (1997). | |
GPS and other geodetic observations over the past two decades have established that the western margin of North America, from southern British Columbia to northern California, is being slowly compressed eastward (black arrows) because the subducting oceanic plate drags the North America plate margin with it.
GPS-derived long-term motions and position offsets due to the August 1999 slip event. Based on 5 years of continuous GPS data, a pattern of long-term deformation (shown by black arrows) was resolved for regional GPS stations, with stations located on the outer coast moving the most, at rates of up to 1.5 cm/yr in a northeast direction. This long-term deformation is due to the fact that the plate interface underlying the offshore continental slope is locked. The red arrows show the observed displacements over a 3-week period in August 1999. Key features for this transient event are: 1) the directions of the offsets are predominantly southwest, opposite to the long-term deformation motion; 2) the offsets are seen at only 7 contiguous GPS stations in southern Vancouver Island and northwest Washington; and 3) the maximum offsets are not at outer-coast stations but at inner-coast locations such as Victoria (GPS station ALBH) and Sidney (GPS station PGC5). These features imply that the offsets are caused by slow plate slip on the deep (~40 km) plate interface underlying southern Vancouver Island.
Time series of changes in the east-west position of the Victoria GPS station (ALBH) over a 16 month period spanning the August 1999 slip event. Blue triangles plot the day-to-day changes in the east-west component with respect to the NA plate. The linear portions of the plot before and after the transient westward offset exhibit the long-term eastward deformation motion at a rate of about 4 mm/yr. Note that the reversed motion is not instantaneous but occurs over a period of about two weeks. Furthermore, an examination of analogous east-west positional time series at the other GPS sites shows that the deep slip started in the Seattle region and propagated along the strike of the subduction zone at a rate of about 8 km/day, reaching the Nanaimo region after about 35 days
The 1999 slip episode was detected from GPS observations that showed that in the summer of 1999, sites located in Puget Sound and southern Vancouver Island briefly reversed their direction of motion (red arrows). These motions of up to 6 mm occurred over about 10 days at any given GPS site, and took about 35 days to travel from Puget Sound to central Vancouver Island. These motions can be modelled by slip of ~ 2 cm occurring on the fault over an area of about 50 km by 300 km (about 30 mi by 190 mi) at depths of about 25 km to 45 km (about 15 mi to 28 mi). If this slip had occurred suddenly, it would have been a magnitude 6.7 earthquake, similar in magnitude to the 2001 Nisqually earthquake near Seattle. But because it took several weeks to complete, no shock waves were generated, and it was only detected with the sensitive GPS measurements.
The Nisqually earthquake is an example of an "intraslab" earthquake, one of the other kinds of earthquakes that can occur in the area. Intraslab earthquakes occur in the subducting oceanic slab and shallow crustal earthquakes occur in the North America plate above the subduction zone. These earthquakes are smaller than megathrust earthquakes, but are also hazardous because they can occur near populated areas, in contrast to the offshore megathrust earthquakes. GPS observations from Washington State have documented the crustal displacement from the Nisqually earthquake and illustrate the numerous uses of the technique.
Understanding the relationship between the different kinds of earthquakes and the silent slip is a new area of study. More GPS stations are needed to better understand how frequently, and over what area, these slip events occur. Future study of this new aspect of fault behavior will lead to a better understanding of earthquake hazard and may guide the first steps towards monitoring pressure build-up on the subduction fault as it is occurring.