The Cascadia and San Andreas faults may be linked

Last week, there were reports that three earthquakes were recorded off the West Coast near the border of California and Oregon. The strongest one measured a 5.7 and was located about 100 miles to the west of Pistol River, according to the U.S. Geological Survey. These quakes were also near the Cascadia Subduction Zone, which served as a reminder of the potentially damaging earthquakes this region can generate.

The western coastline of North America is one of the most active seismic regions in the world. Powerful geological forces constantly shape the landscape as tectonic plates collide, slide past one another, and sink beneath neighboring plates. These movements have produced some of the continent’s largest earthquakes throughout history, including the big San Francisco earthquake in 1906. Research performed by Professor Chris Goldfinger at Oregon State University, published in Scientific American late last year, now suggests that two of the region’s most important fault systems, the San Andreas Fault and the Cascadia Subduction Zone, may be more closely connected to one another than previously thought. Scientists have discovered evidence indicating that major earthquakes along these faults may have occurred in close succession several times during the past 3,000 years.

As I’ve mentioned in previous articles, the Cascadia Subduction Zone extends from northern California to Vancouver Island in Canada. It forms where the Juan de Fuca and Gorda tectonic plates are forced beneath the North American Plate. The San Andreas Fault stretches approximately 800 miles through California, marking the boundary where the Pacific Plate and North American Plate slide horizontally past each other.

Based on historical evidence, there was a huge megathrust earthquake Jan. 26, 1700, that was known as the Cascadia earthquake. It happened across a 620-mile area along the Cascadia subduction zone from the middle of Vancouver Island in British Columbia, Canada, southward along the Pacific Northwest coast down into northern California where it meets the San Andreas Fault. The magnitude of the large earthquake in 1700 was estimated between 8.7 and 9.2. Researchers recently studied sediment samples collected from the ocean floor near the Mendocino Triple Junction, an area off the coast of northern California where the two fault systems meet. Geological evidence suggests that large earthquakes on the San Andreas Fault and the Cascadia Subduction Zone have often happened within relatively short periods of one another.

To investigate the relationship between the two major fault systems, the article states that scientists analyzed more than 130 sediment cores collected from the seafloor that provided a record of geological events stretching back approximately 3,000 years. Within the samples, researchers found layers of deposits created by underwater landslides that would often be generated by major earthquakes.

During their investigation, scientists noticed an unusual pattern at the Mendocino Triple Junction. Under normal conditions, the underwater layers from earthquake activity would contain coarse sand at the bottom and finer sediments near the top. However, many samples from this region appeared reversed, with sand-rich layers sitting above finer materials. After further investigation, it was concluded that these formations likely resulted from two separate earthquake-triggered landslides occurring in rapid succession. The first earthquake deposited a layer of sediment, and before it could fully settle. Then, a second earthquake caused another underwater landslide that covered it with additional material.

The timing of these events was found to be particularly significant. Some of the paired sediment layers indicated that the earthquakes may have occurred within minutes, years, or decades apart. Scientists analyzed the ages of shells found within the sediments and discovered at least eight major earthquakes on the San Andreas Fault that occurred within decades of large Cascadia earthquakes during the past 3,000 years.

The concept of earthquake synchronization is not a new concept. Scientists have suspected that nearby fault systems can influence one another. However, this study provides some of the strongest evidence that such interactions may occur between massive fault systems over very long periods of time. Despite this apparent relationship, the faults do not always react in perfect unison. The devastating 1906 San Francisco earthquake was likely caused solely by movement along the northern San Andreas Fault and had no effect on the Cascadia Subduction Zone. This demonstrates that while synchronization of the major fault systems may occur at times, each one can still generate earthquakes on its own.

According to a recent study published in Earth, Atmospheric and Planetary Sciences, there is an approximate 15 percent chance that the Cascadia Subduction Zone will trigger a major earthquake within the next 50 years and about a 29 percent chance of one by 2100.

In terms of our local weather, the drier than normal weather pattern continues across the Inland Northwest. Through the first week of June, only 0.03 inches of moisture has fallen at Cliff Harris’s station in Coeur d’Alene. Showers and a few thunderstorms are possible early this week before the big high pressure ridge rebuilds over the region. Unless there are a few thunderstorms that drop a lot of rain, the long-range computer models indicate that June may be another drier than normal month. Looking farther down the meteorological roadway, our upcoming summer season is still expected to start out drier and warmer than normal, but as sea-surface temperatures warm, we could start seeing moisture from the south later in the summer and fall. Stay tuned. 

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Contact Randy Mann at [email protected].