Origins of earthquakes on Salt Spring Island
We live in one of the world's crumple zones. It's not a technical term, but it seems to describe my home. The crumpling started about 200 million years ago when the Atlantic Ocean started to open and our newly formed continent started its journey west. At the time, the edge of North America was a lot further east, somewhere near the Rocky Mountains. In the Atlantic, new ocean crust was being created, pushing North America up and over the ocean crust of the Pacific. For the next 200 million years the continent would glide across the ocean collecting debris along the way. The bits of debris consisted of islands and archipelagos or basically anything sticking up above the ocean floor. Scientists have a name for these bits, they're called terranes.
The terranes weren't just peacefully nudged up on to the side of North America, they were plowed, folded and crumpled. The last major crumpling events happened 55 and 42 million years ago when the Pacific Rim and Crescent terranes were accreted to southern Vancouver Island. These are the impacts that created the Gulf Islands. A crack from the last event is still visible in form of the St Mary Fault on Salt Spring Island. The notch in the ridge that connects Mt Belcher to Mt Erskine, along with the drop off on the north shore of St Mary lake are the result of this fault.
So what's going on today. Thankfully, there are no terranes currently being accreted to North America but we still a have earthquakes. If you live in the Salt Spring area, try this. Stand up and point to the nearest ocean plate. A few years ago I tried this when I was a substitute teacher at the local high school. Arms went up and pointed in a variety of directions, most pointed somewhat west. A good guess but not entirely accurate. From my house, if you travel roughly 235km west, you will find yourself floating above the nearest surface boundary between the Juan de Fuca plate and the continent of North America. When all eyes turned to me and I pointed straight down. It's hard to be sure but the Juan de Fuca plate is estimated to be 50 to 60km below us. The good news is that the epicenter of the next big earthquake will probably be out at sea. While that distance makes us safer than places like to Tofino, it still puts us in a high danger area compared to most of Canada. According to the BC government there is 10% chance a major quake in the next 50 years.
I can already hear people saying what about tsunamis? Well, short of living a very long way from the ocean, Salt Spring is as safe as you can get. According to the CRD, about the a biggest a tsunami can get around here is 2m. Even in low lying areas most people wouldn't even get their feet wet.
So what could be worse than living on a subduction zone? Well, I can think of few. We could live in Mexico City. Like us, the plate boundary is hundreds of kilometers away. However, the city is built on an ancient lake bed. The sediments of the lake bed amplify the shaking, making the destruction far worse. Or how about our neighbors to the north, Delta and Richmond; these municipalities are built on the Fraser River delta, so they have same amplification problem as Mexico City. To make matters worse, in many places the ground is saturated with water. The combination of water and shaking can cause the ground to liquefy; buildings can literally sink into the ground. Then there is California. In California the plate boundary runs through land putting them a lot closer to the epicenter. Fortunately for California, the plates, instead of moving towards each other, are sliding past each other. Sort of the difference between a head on collision and glancing blow.
The worst places, however, are where one continental plate is pushing against another. In Nepal, the Indian plate is pushing north against the Eurasian plate making the tallest crumple zone in the world, more properly called the Himalayan Mountains. In the Middle East, the Arabian plate is also pushing against Eurasian plate causing major earthquakes.
So, although the thought of an earthquake scares me, things could be worse. In my view, B.C.'s south coast is the most beautiful place on earth, it's my home, and earthquakes or not I wouldn't live anywhere else.
For more information you can click on the diagrams above or the links below.