The latest with the San Andreas Fault

Last week, there were reports that a series of small earthquakes were recorded in California that generated a few news headlines. The state is notorious for earthquake activity. It’s estimated that California experiences approximately 50 earthquakes per day, but most are so small, they are usually not felt.

Despite the state’s reputation for earthquake activity, scientists are keeping a closer eye on the infamous “San Andreas Fault.” The San Andreas Fault is one of the most significant and studied geological features on Earth. This continental “transform fault” marks the boundary between two of Earth’s massive tectonic plates that include the Pacific Plate and the North American Plate.

The Pacific Plate is moving to the northwest relative to the North American Plate at a rate of about 1.5 inches per year. This movement doesn’t happen smoothly as friction between the large plates causes them to lock together, which results in the building up of stress over time. When the stress exceeds the strength of the rocks, the rock essentially snaps and the vibration of the event results in an earthquake.

These sudden releases of energy can be minor or massive, depending on how much stress has accumulated and how large the rupture along the fault is. Some of the strongest earthquakes in California’s history have occurred when long segments of the fault have slipped at the same time, releasing enormous amounts of energy. The most famous was the big earthquake in San Francisco in 1906.

The San Andreas fault stretches for over 800 miles through California’s coastal regions. In the southern part of California, the fault ends near Bombay Beach, in the Salton Sea. In the northwestern part of the state, the fault terminates offshore near Eureka. This is also the region where three tectonic plates come together as the San Andreas fault intersects with the northwest large plate boundaries, the Cascadia subduction zone.

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. The devastating Alaskan earthquake in 1964, the second largest recorded in history, was 9.2 in magnitude. Scientists are very concerned that a major earthquake in the Northwest could lead to a rupture along the San Andreas Fault.

The big quake in 1700 was believed to have generated a large tsunami that hit the coast of Japan. It may also have been linked to the Bonneville Slide. This was a major landslide that dammed the Columbia River near Cascade Locks in Oregon. The Native Americans referred to this as the “Bridge of the Gods.” However, other investigations state that the landslide occurred around 1450 based on radiocarbon dating and dendrochronology (tree rings).

The San Andreas Fault was officially discovered and named in 1895 by Professor Andrew Lawson of UC Berkeley. He was a Canadian-born geologist working in California to help determine the origin of the big San Francisco earthquake in 1906. While conducting geological surveys in this area, Lawson noticed a prominent linear valley near San Andreas Lake. He identified this valley as a fault line, naming it the San Andreas Fault after the surrounding San Andreas Valley.

The full geological context of the San Andreas Fault became more clear many years later with the development of plate tectonic theory. Scientists realized that the San Andreas Fault is a transform boundary where the Pacific Plate slides past the North American Plate. This explained the earthquake activity as well as the fault’s size and ongoing movement.

By the way, scientists say there is at least a 50% chance of another large earthquake in the San Francisco Bay area by the early 2030s. Scientists also state there is an approximate 37% chance of a major earthquake in the Pacific Northwest by the 2050s.

In terms of our local weather, over the last week, we’ve finally received some much-needed rainfall across the Spokane and Coeur d’Alene region after a dry first 10 days of the month. Despite the moisture, we’re likely to end up well below the monthly normal for May of 2.37 inches in Coeur d’Alene.

As I mentioned earlier, officials are concerned that if the drier-than-normal weather pattern continues across the region, the fire season for 2025 could be another tough one for the northwestern U.S. The latest information from the U.S. Drought Monitor still has moderate to severe drought expanding, which includes Coeur d’Alene. The mountain regions did receive some moisture in recent weeks, but severe drought conditions persist.

The long-range forecast models indicate that after the period of showers this week. As we get close to the normally wet new moon lunar phase at the end of the month, it does look like we’ll have another chance of rainfall across the Inland Northwest into early June.

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Contact Randy Mann at [email protected].