As we all know, the Cascadia Megathrust is an unusual “quiet” subduction zone, which rarely produces earthquakes noticeable to humans. However, following the earthquakes over the last few months, the USGS has reanalysed their data on the zone, and concluded that their probabilities of a major earthquake along the Cascadia Megathrust were higher than initially thought. And during this time, the link between Cascadia and the famed San Andreas was also solved, a mystery going back almost 30 years.
It all began with the magnitude 3 earthquakes that were detected by the newly installed Pacific Northwest Seismic Detection Network, which has enabled new, real time data to be transmitted to the Environment Canada and the USGS, allowing for a better understanding of the Megathrust off the Pacific Northwest, and the link between Cascadia and San Andreas was discovered.
Since then, there have been bigger and bigger earthquakes along both faults. Throwing in the biggest tremors along the San Andreas and Cascadia, and the link can sort of be seen by a casual observer. There was a magnitude 4.3 earthquake along the San Andreas Fault on September 12, followed by the 4.1 shock along Cascadia off Oregon. The 5.0 magnitude earthquake along the coast of Washington/Oregon on September 10 was preceded by a 3.8 along San Andreas, and the 4.6 aftershock the following day was preceded by a 4.1 along San Andreas.
Going back to July’s magnitude 5.6 along San Andreas, and yet another link can be seen. 18 hours after that earthquake, there was a magnitude 3.9 along the Cascadia zone, which shows that there was a build-up in the pressure following the San Andreas earthquake, which was able to be released.
Anyway, back to the point. In early September, the USGS said there was a 50% chance of a magnitude 8+ event off the coast of Oregon by 2066, up from 31% in 2014’s study. Also revised were the magnitude 9+ probabilities, which were raised to 24% at the same time, up from 17-20% back in the 2014 study.
Currently, using the USGS forecasts for Los Angeles, there is a 90% chance of a Great Earthquake along that section of the San Andreas, and a 70% chance for San Francisco in the next 30 years, which, in my opinion, would mean an 80% chance of a megathrust along the Cascadia Megathrust as a compromise between the two. To narrow down these chances, I would say, given the predictions by the USGS, that there is a 26% chance of a megathrust earthquake along Cascadia in the next 10 years, a 23% chance of one striking San Francisco in the next 10 years, and a 30% chance of one occurring near Los Angeles in the next 10 years if this link really can be taken as being reliable.
If I was to make a forecast for the Cascadia megathrust earthquake, id say it will hit in the next 10 years, and be of a likely magnitude 8.5 to 9.0. Based on where the observed foreshocks have taken place, I would say the earthquake is likely to be a full margin rupture event, which, as the USGS has said in its analysis of the new data from the probes, could reach magnitude 9.1+.
Now, imagine this. The earthquake would take 9 minutes to travel from one end to the other of the 955-mile-long megathrust fault, which would result in 6 minutes shaking in most areas, which would put the recently seismically upgraded areas of the Pacific Northwest to the test. Although Oregon and Washington’s buildings have to be able to withstand a magnitude 9.0 earthquake by law, they would probably collapse in the resulting tsunami, which would come ashore between 12-22 minutes after the main event. Likely aftershocks may also trigger secondary tsunamis in the area, which would reach the other side of the Pacific Ocean 17-22 hours later, wreaking havoc wherever they make landfall.
Those who live along the Pacific Northwest need to start making their preparations as the government has been saying since the Zone was discovered in 1991. This kind of nature is not to messed with, as those along the famed San Andreas Fault know from their previous experiences.
Based on my own research form this summer, I could also add some more findings to this ground-breaking research. I have found a link between large earthquakes along the Cascadia Megathrust and the San Andreas Faults as well. Let me prove my point. Every Big One along the San Andreas has been either preceded, or followed, by a megathrust along the Cascadia.
In October 1699, a magnitude 8.2 earthquake struck along the middle section of the San Andreas Fault line, which is thought to have lasted around 3 minutes and killed around 500 people in the poorly built area at that time. On January 7, 1700, a magnitude 8.7-9.2 megathrust occurred along the entire length of the Cascadia Megathrust, which is estimated to have lasted for 9 minutes as it traversed the entire 955-mile fault.
So let me explain how these links cancel out. The San Andreas is a strike slip, which means one plate rubs against another plate without subduction. In this case, the North American Plate rubs up against the Pacific Plate. As the plates move at different angles and speeds, it leads to a build-up of enormous pressure, leading to what is referred to as a Great Earthquake, which exceeds magnitude 8 on the Moment Magnitude Scale.
Once the San Andreas passes through San Francisco Bay, it becomes a subduction zone, where the angles, plates and speeds change again. From the Northern California Coast to the coast of British Colombia, there is the subduction of the Explorer Plate, the Juan de Fuca plate, the Gorda Plates all subduct, along with the Pacific Plate, under the North American Plate off the Pacific Northwest, allowing for the build-up of pressure needed for the megathrust to occur.
When there is the build-up of pressure along one of these seismic zones, an increase of pressure is generated along the other one as well, leading to major earthquakes along both plates within a few months of each other. There are also links between the Great earthquakes in 1190, 780, 730, 690, 420, 350, 440BC, 660BC, 890BC, 980BC, 1340BC and 1440BC, which all show inexplicable links between both margins, which all had Great Earthquakes around the time of the megathrust on Cascadia and San Andreas.
There was the 8.2 earthquake in 1699 a few months along San Andreas before the 8.7-9.2 1700 Cascadia earthquake. The 7.4 earthquake in California that followed an 8.2 along Cascadia in 1190. The 8.5 along Cascadia in 780 was preceded by the 7.3 in California a few months earlier. A magnitude 8.0 along Cascadia in 730 was followed by a 6.7 along San Andreas a few months later. The 8.5 along Cascadia in 690 was preceded by a 7.2 along the northern section of San Andreas by 2 months. Going further back – the 8.7 along Cascadia can be shown to be followed by an 8.0 in the middle zone of San Andreas, and the 8.2 along Cascadia was preceded by a 7.0 in California by 3.5 months. Again, in 440BC, a magnitude 9.0 was thought to have occurred along the Cascadia following an 8.0 magnitude earthquake along San Andreas several months earlier. Another megathrust occurred in 660BC, around the time a magnitude 8.0 can be dated along the middle San Andreas System, and magnitude 8.6 and 9.0 earthquakes can be traced to being occurred a few months after 8.0+ earthquakes along the San Andreas Fault System.
Therefore, I can conclude this blog by saying that if there is a Great Earthquake along the San Andreas Fault, it means there is a build-up of pressure along Cascadia as well, which has been demonstrated throughout this blog. If you don’t believe this, then check up on the latest USGS research on both of the faults, and you might be able to finally make that link for yourself.