Hi All. It has been said by a few that the West Coast of the U.S. is living on borrowed time. At present time I have no idea if this is true or not, but from the article below it would appear that we are. Just how much borrowed time is anyone’s guess.

With the thrust fault locked, the 40 mm/yr convergence of the Juan de Fuca plate is taken up as elastic shortening across the continental margin. GPS measurements show Victoria to be moving landward at a rate of 7 mm/yr with respect to the stable North American continent (Penticton site) (after Dragert and Hyndman, 1995). The remainder of the shortening is across the continental shelf. Sooner of later it will give and like a giant recoil spring it will try to return to it’s original position, or allow it self to collapse. Take Care…Don in creepy town.

Geological Survey of Canada
R.D. Hyndman, G.C. Rogers, H. Dragert, K. Wang, D. Oleskevich, J. Henton,
J.J. Clague, J. Adams, P.T. Bobrowsky

The failure of the magnitude-frequency of occurrence approach is especially serious in the coastal region of western North America between Cape Mendocino in northern California and the Queen Charlotte Islands in British Columbia. In this region, the Juan de Fuca oceanic plate is underthrusting the North American continent at the Cascadia subduction zone. Although the regional earthquake activity is quite high in some areas (magnitude 7 events occurred in 1918, 1946, 1949 and 1965), no earthquakes of any size have been detected on the subduction thrust fault itself. This is so even with the extensive seismograph networks established in southwestern British Columbia and western Washington over the past 20 years.

In a global perspective, the lack of thrust earthquakes, large and small, is surprising. Most of the world's great earthquakes (defined as magnitude 8 and greater) have occurred on subduction zone thrust faults, and most subduction zones have experienced historical great earthquakes. Such events are especially concentrated around the Pacific Ocean where the majority of subduction zones are located. The Cascadia subduction zone appears to be an anomaly. But, we need to remember that the written historical record in this area is short. It is only a little more than 200 years since the first visits to the region by Captains Juan Perez in 1774 and James Cook in 1778.

This limited written history is in marked contrast to the detailed Japanese record of great subduction zone earthquakes and tsunami waves that extends back to the 7th century (e.g., Ando, 1975).

There are three possible explanations for the lack of historical great earthquakes along the Cascadia coast: (1) The Juan de Fuca plate is no longer converging and underthrusting North America. (2) Underthrusting is continuing, but it is accommodated by smooth stable sliding, not punctuated by the stick-slip behavior of earthquakes. (3) The thrust fault is completely locked with not enough motion to generate even small earthquakes. The first two options imply that the earthquake hazard estimates for the region based on historical seismicity are appropriate. The third option implies that there is a potential for very large and damaging earthquakes that has not been included in most past hazard estimates.

Twenty years ago the first option was much discussed, and Riddihough and Hyndman (1976) presented a variety of evidence that there is present convergence and underthrusting. Since that time, extensive studies have been carried out across the Cascadia continental margin. They allow us to now say with considerable assurance that convergence is indeed continuing. One type of evidence is the folding and faulting seen in seismic reflection images of young sediments at the base of the continental slope. These sediments were laid down on the deep sea floor as flat layers but even the Quaternary sequences less than a 1 m.y. old are strongly folded and faulted (e.g., Davis and Hyndman, 1989). They continue to be scraped off the underthrusting oceanic crust by the bulldozer blade of the continental crust. Perhaps the most dramatic evidence for active subduction was the volcanic eruption of Mt. St. Helens in 1980. Such eruptions have their origin in melting that occurs when the oceanic crust reaches a depth of about 100 km beneath the continent. A chain of "arc" volcanoes inland of the coast extends from northern California to southern British Columbia. They are geologically active, although many are historically dormant. The north-south geographic limit of the volcanoes corresponds to the extent of the subducting Juan de Fuca plate. The principle of plate tectonics, that arc volcanoes only occur where there is active subduction, indeed applies here. The debate over the second possibility, smooth aseismic underthrusting, has continued until quite recently. Again, the contrary evidence is now strong, especially from paleoseismicity (the traces of past great earthquakes preserved in the geological record), and from measurements of present elastic strain building up in the continent near the coast. The observed deformation corresponds to that expected for a locked thrust fault. These two types of evidence are discussed below.

We are left with the third alternative. Great earthquakes do occur, but the last one was more than 200 years ago, prior to the historical written record. Thus, we must face the serious consequence that the earthquake hazard is substantially greater than previously thought.

• Cascadia Thrust Fault