My Faults: A Journey Along the Pacific and North American Plates, P...

Looking at a large map, most earthquake faults make sense. They are the boundaries between continent blocks, nature’s version of borders. They are cracks between the big plates beneath our feet, tectonic plates that move endlessly over the molten core of the earth. Where these plates collide, slip, and slide beneath one another, we get seismic activity like earthquakes and volcanoes. Even in the middle of plates, where there are thin spots in the earth’s crust, events can occur. But most big seismic action happens along the periphery where plates come together.

Over the last 10 years or so, I’ve become fascinated by this geology. If I’m honest, then “worried” might be a better word, since I live in earthquake country. But while I think about earthquakes, that concern doesn’t keep me up at night. My pre-occupation centers more on how to be prepared as well as possible, not if but WHEN the next big one comes. Also, I really enjoy the geography of it all.

Since I’m based on the west coast of the United States, my main interest is the northeastern boundary of the Pacific Plate, where it scrapes up against the North American Plate. From Alaska and the Pacific Northwest through California and into Mexico, I have a great passion for the west coast of North America, the region that I call home.

In the world of disclaimers, I’m not a geologist or seismologist, so my understanding of these disciplines is amateur at best. And I certainly do not understand regions beyond this one very well, so I’ll let others write about those. This is my attempt to trace that a long set of fault lines down the map, discussing what I understand about its various parts. This line also forms part of the Pacific Ring of Fire, the plate perimeter where seismic activity often occurs through earthquakes and volcanoes.

At the Top of the Ring of Fire

The line between the Pacific and North American Plates begins at the top of the eastern Pacific Ocean, traced by Alaska’s Aleutian Islands. From the Bering Strait that separates Asia from North America, we could turn west. That would take us towards Siberia and down the Kamchatka peninsula, which looks very much like Alaska, down the Kuriles and into Japan. But instead, we’ll go east from the Bering Strait, tracing the Aleutian island chain towards Alaska proper and following the coastline as it curves to the southeast, beginning to mark the west coast of North America.

The image above (at the top of this post) shows some of the Aleutian islands; they include 41 volcanoes, more than half of them active.

The first, top section of this boundary is called the Aleutian subduction zone (fault). It’s scary as hell because it’s a subduction zone, the same kind that has produced mega-earthquakes and tsunamis off the coasts of Japan and Sumatra. In 1964, this fault segment was jolted by an earthquake centered in the area of Prince William Sound. The Good Friday quake, as it was called later, was one of the most powerful earth movements ever recorded (measuring 9.2 on the Richter scale). It sent tsunamis all over the Pacific: to Japan, Papua New Guinea, New Zealand, various islands, Mexico, Peru, and even as far down as Antarctica.

Here is a U.S. government model of tsunami arrival times around the North Pacific, following a hypothetical major earthquake on the Aleutian fault:

In the last 75 years, this fault has produced four earthquakes of 8.0 or greater intensity, which is quite severe. Because it’s a good distance from most population centers around the Pacific, there should be ample warning for people on distant shores to avoid the beaches and run for high ground after a tsunami-producing quake in the Aleutian zone. However, Alaska’s largest city of Anchorage, just to the north of the fault line, has several hundred thousand residents who are at risk from a large quake on the eastern side of the fault.

Tracing that line further east, it begins to curve downwards along the coast of Alaska. There is a transition zone as it turns southeast, which is a source of some strong seismic activity in its own right as these plates push into and beneath one another. Then the Fairweather-Queen Charlotte Fault takes the plate boundary down past the coasts of southeastern Alaska and British Columbia, Canada. The Alaskan section is known as the Fairweather fault and it runs through Glacier Bay, where giant cliffs of ice calve and crash into the water even without any help from tectonics.

Queen Charlotte and Haida Gwaii

On the Canadian side, this fault takes on the name Queen Charlotte. The islands along this coast once had that name also. While some residents still call them “the Charlottes”, they are now known more officially as Haida Gwaii, their native name. The Queen Charlotte fault is oriented much like California’s San Andreas fault, which has strike-slip movement as the plates move there horizontally. However, in the 2012 Haida Gwaii earthquake (7.9 magnitude), a seafloor map showed that the quake included a thrust component and was more like what scientists would expect along the Cascadia fault to the south. Some of nature’s mysteries are still being revealed to science.

If you are travelling south by boat through Queen Charlotte Sound, there is a big gap between the Haida Gwaii archipelago and Vancouver Island to the south. Deep ocean swells rock the boat through this strait until you get into the lee of Vancouver Island. Smaller boats can head in to the main coastline and seek the shelter of smaller islands that line it; thread those needles and you can travel up or down much of the coast that way.

This gap in the ocean is also the approximate home of the Queen Charlotte triple junction, a place where three faults come together. The Fairweather-Queen Charlotte Fault ends, joined by the Explorer Ridge and the Cascadia Subduction zone. It is a junction that has produced some strong seismic activity in the past and promises more in the future.

This was Part 1 of 3. Please continue reading when the next segment is published!

References for all three parts. All images are public domain or produced by a U.S. government agency and not subject to copyright:

My Faults: A Journey Along the Pacific and North American Plates, Part 1 of 3 (Alaska to Haida Gwaii)

At the Top of the Ring of Fire

Queen Charlotte and Haida Gwaii

This was Part 1 of 3. Please continue reading when the next segment is published!