Earthquakes have certainly been in the news over the past few years and rightly so. Although the earth and its inhabitants suffer from a variety of natural disasters every year from hurricanes to floods, most of them can be predicted in advance. Hurricanes can generally be predicted at least a few days in advance and even tornadoes can usually be predicted at least hours in advance. But earthquakes are an entirely different matter. Despite the best efforts of geologists over the past decades, earthquakes still come with zero warning.
We can, with reasonable certainty, predict where earthquakes will occur. but unfortunately not when. We know for instance that the entire rim of the Pacific Ocean is a very active area for earthquakes and thus we hear that coastal B.C. is someday going to get hit by “the big one”, meaning a quake of magnitude 8 or larger. We also know when the last big one was, and by examining the relatively recent geologic record we can see when large earthquakes happened over the past 10,000 years or so. That unfortunately still only gives us an average time between earthquakes, not an exact time.
All of this discussion about “the big one” at the Coast naturally raises the question, “could we have a big one here in the Okanagan?” The short answer is “highly unlikely”. In the 11 years that I have lived in the Okanagan I’ve felt about four earthquakes, all of them very small and many more have occurred that few people actually felt — most recorded earthquakes in the Okanagan Valley are less than magnitude 2 and only a few reach as high as magnitude 4. Most people know that earthquakes are associated with faults in the earth’s crust — when movement takes place along the fault, the result is an earthquake.
The Okanagan Valley is situated along a large fault — likely the reason the Valley is here — erosion by water and glaciers could more easily remove the broken-up rock along the fault and thus carve out a valley. There is no geologic evidence that I’m aware of to indicate significant activity along this fault in the past 10,000 years and there is nothing to suggest that is going to change.
None of this is to say that big earthquakes don’t occur away from the coast. Perhaps the largest earthquake in our general vicinity in the last 200 years occurred in Washington state in the Cascade Mountains west of Lake Chelan in 1872. There were no seismographs to get an accurate reading of that quake’s magnitude, but from various evidence geologists estimate it was about 7.3 — a very significant earthquake.
However, there were so few people living in the area at the time and no large buildings, so damage was minimal. But the shaking was so severe that reports indicate that people in New Westminster ran from the buildings fearful that they might collapse and there are also reports that the shaking was felt as far away as Quesnel. That quake did cause large landslides in the Cascade Mountains and in the silt cliffs along the Columbia River in Washington state.
My opinion is that the possible collapse of the silt cliffs along Okanagan and Skaha Lakes would be the most significant potential danger in any future large quake in our vicinity. Notice that I said “possible” not “probable”. My own house sits on the silt cliffs above Skaha Lake. The problems associated with collapse of the silt cliffs in Penticton area in the last hundred years have been caused by overwatering and poor irrigation practices, not by earthquakes.
The South Okanagan Naturalists’ Club will meet at 7:30 p.m. on April 28 in the basement hall of the Penticton United Church on Main Street. The speaker for April is Lisa Scott, professional biologist, who will report on endangered cavity-nesting birds in the Okanagan and the importance of wildlife trees for these birds. All are welcome.
Robert Handfield is a retired geologist as well as past president of the South Okanagan Naturalists’ Club.