Running more than 800 miles, the San Andreas Fault runs from the Salton Sea in the south to meet the Cascadia Megathrust Fault at the Mendocino Triple Junction in the north.
Southern San Andreas Fault
According to the State of Nevada Joint Information Center, a Southern San Andreas Fault (SSAF) earthquake with an epicenter near the Salton Sea could reach a magnitude 7.8. Its impact area could damage infrastructure 60 miles out from the fault and could result in 50,000 people seeking temporary shelter in Nevada. Of course, the SSAF isn’t the only fault in the area. LA County has a List of Major Active Surface Faults in Southern California. The list itself is two pages long and is followed by 22 pages of fault-specific information.
Northern San Andreas Fault
More relevant to the topic of this website, however, is the Northern San Andreas Fault (NSAF). A Bay Area Threat and Hazard Identification and Risk Assessment (THIRA) lays out a Northern San Andreas Fault rupture scenario, and provides the following details.
| Epicenter: Just outside the entrance to the San Francisco Bay, west of the Golden Gate Bridge | Completely Collapsed Buildings: >1,000 |
| Rupture Length: 300 miles | Rupture: Between the San Juan Bautista area in the south to Cape Mendocino in the north |
| Shaking Duration: 20 – 25 seconds (foreshock) & 45 – 60 seconds (main shock) | People Trapped in Completely Collapsed Buildings: >5,000 |
| Fatalities: 6,600 | Additional People in Need or Search & Rescue: >30,000 |
| Magnitude: 7.9 |
So why is the NSAF relevant to this website? What if a Cascadia rupture could trigger a major earthquake on the Northern San Andreas Fault? It’s possible. And it’s happened before. Several times.
The connection between Cascadia and the NSAF is also discussed in OPB’s article, The Big One, Times 2: Research Shows Cascadia Quakes Sometimes Trigger San Andreas Fault, as well as in the Oregonian, KOIN 6, and by many other news organizations.
Scientists have compared evidence of past earthquakes on both faults. Ages for Cascadia’s quakes are located on page 97 of the USGS publication, Turbidite Event History—Methods and Implications for Holocene Paleoseismicity of the Cascadia Subduction Zone. Check out Surviving Cascadia’s Likelihood of an 8.0 and Exploring the Probabilities pages for more details.
While this image, explained in Step 1: Cascadia’s Impact Area, provides general patterns for event sizes, the slideshow images below detail the estimated size of each Cascadia megathrust earthquake during the last ≈10,000 years. T1 is the most recent, T19 the oldest in the series.
These images are part of a PowerPoint presentation saved to the Oregon Legislative Information System (OLIS) website.
Image credit: Goldfinger, C., et al., The importance of site selection, sediment supply, and hydrodynamics: A case study of submarine
paleoseismology on the northern Ca…, Marine Geology (2016), http://dx.doi.org/10.1016/j.margeo.2016.06.008
Here are those same images provided in a timed sequence by OSU Professor Chris Goldfinger.
Aside from just being really cool to see, the images above tell an important story. To better understand that story, check out the college below. It displays the same images from the slide deck above, with one BIG exception. 4 events have been removed from their chronological spots & are now grouped together in the box at the bottom right of the image collection. They are the smallest of the set.
Those 4 events are assigned to Segment E, enlarged here, and, as stated in Step 1, their inclusion in the official CSZ record has come into question since the release of the Goldfinger et al 2012 publication that first listed all 46. Because of their small size, scientists just aren’t sure if Cascadia was the fault that ruptured in that event. The evidence of a major earthquake could be from a San Andreas rupture or one along another fault in the area.
Setting Segment E events “aside” in the collage below gives us a chance to view the historical patter of just the “CSZ-certain” events.
And now the colors (plural!) in each image below become very important. As before, the red shading shows the CSZ rupture area during each event… Shading in orange represents where the Northern San Andreas Fault (NSAF) ruptured close in time.
Again, these are shown in chronological order with the oldest on the upper left. We can see that the most recent 14 Cascadia megathrust earthquakes match up with a Northern San Andreas Fault (NSAF) rupture.
That’s every event from T1 in 1700 to T6b which is estimated to have occurred 2,896 years ago. In other words, for nearly 3,000 years, Cascadia has been triggering the NSAF… consistently. But does the triggering happen instantly? Sometimes. More details below.
A recent technical report, Paleoseismic Record of Peninsula Segment Earthquakes on the San Andreas Fault near San Francisco, CA and possible NSAF linkage to Cascadia, from the National Earthquake Hazards Reduction Program (NEHRP) concludes:
- Cascadia triggers the Northern San Andreas fault (not the other way around).
- In several cases, there were only hours of separation at most. Time separation for other CSZ/NSAF ruptures is difficult to gauge but likely does not exceed ~ 30 years.
Image Credit: California Cascadia Subduction Zone Earthquake and Tsunami Response Plan
The ages of the past major San Andreas “big ones” can be found in the Uniform California Earthquake Rupture Forecast, Version 3 ( UCERF3 ), Appendix G — Paleoseismic Sites Recurrence Database .
I’ve added the dates from the paper to my San Andreas Story Map. It takes you through ≈3,000 Years of earthquake history along California’s longest fault.
The Story Map starts at the southernmost point of the fault and journeys north toward Cascadia.
If Cascadia, the NSAF and the SSAF were all to rupture at their full lengths, the entire west coast of the US and part of Canada would be affected… and each event would be catastrophic in their own right.
| Cascadia Megathrust Fault | Northern San Andreas Fault | Southern San Andreas Fault | |
| Magnitude | 9.0+ (would include tsunami) | 7.9 | 7.8 |
Thankfully, these faults often break in segments. More, there is a section between the northern and southern San Andreas faults that behaves differently. Part 1: Cascadia’s Impact Area discusses ways that the middle section of Cascadia behaving differently, as well.
Which sections of the San Andreas are most likely to rupture? There are some assumptions, however, seismology is still a fairly young science. Researchers are learning all the time… and sometimes, faults surprise us.
The image and quote below come from a 2017 article, “Where the San Andreas Goes to get away from it all,” by Dr. Chris Rollins. Previously with CalTech at the time of this article, Dr. Rollins is now a researcher with GNS Science in New Zealand.
“It is commonly thought that the fault properties of the creeping section favor stable sliding and essentially kill off earthquakes that propagate into it – as evidenced by the 1906 earthquake’s end at San Juan Bautista. These same fault properties should in theory prevent earthquakes from nucleating (originating) in the creeping section, or at least from reaching magnitudes larger than 5 or so if they do.
However, the shallowest portion of the Japan Trench offshore east Honshu was thought to have the same earthquake-killing fault properties, and to slip gradually and stably, until it slipped 50 meters in a few seconds during the 2011 M=9.0 Tohoku-oki earthquake, directly causing the devastating tsunami that followed…
The question in California, if this hypothesis is true, is whether an earthquake rupturing towards San Juan Bautista from the north, such as the 1906 San Francisco earthquake, could rupture clean through the creeping section and continue past Parkfield into southern California as a result of the same mechanism.”
Portland State University research, Central Cascadia Subduction Zone Creep, states that “The Cascade thrust under central Oregon may be partially creeping for at least 6500 years”. It’s worth noting that the Cascadia Subduction Zone has had full-margin ruptures during that time, so the creep may not prevent a full-margin rupture in the next go-round.
Imagine for a second that:
- The Northern San Andreas Fault with its potential to produce an M7.9 ruptures and doesn’t stop at the “creeping” section
- Instead, it flows through to meet with the Southern San Andreas Fault (or visa versa) with its potential to produce an M7.8.
- Taking into consideration the extra miles between
- While remembering that magnitude is essentially a function of rupture length…
The earthquake could be quite large. A reminder, the full length of San Andreas is ≈800 miles whereas Cascadia is “only” 621 miles. Food for thought.
Examples of earthquakes triggering other faults with short gaps between
| Saddle Mountain (estimated Mw 7.3) and Seattle fault (estimated Mw 7.5): Mainshocks between late fall 923 CE and early spring 924 CE, rupturing within 6 months of each other. (these may have ruptured as a single, larger event (estimated Mw 7.8) | Ansei earthquake pair along Japan’s Nankai subduction zone: Mainshocks in December of 1854, occurring 32 hours apart. |
| New Madrid, USA, sequence: Mainshocks on December 16, 1811, January 23, 1812, and February 7, 1812. | Herat earthquake sequence: 2 earthquakes on October 7, 2023, a third on October 10th and a fourth on October 15th, each measuring Mw 6.3. According to this Temblor article, “It’s unlikely that these four events happened so close together just by chance. It’s far more likely that the earthquakes are triggering each other, like a chain of dominoes being pushed over. When an earthquake occurs, the stress released by the event can be transferred to surrounding faults via coulomb stress transfer.” |
| East Anatolian fault zone (estimated Mw 7.8) and the adjacent Sürgü fault (estimated Mw 7.5): Mainshocks on February 6, 2023, ruptured 9 hours apart |
Examples of multi-fault ruptures occurring as a single event
| Denali earthquake in southern Alaska: 2001 Mw 7.9 | Landers earthquake in southern California: 1992 Mw 7.3 |
| New Zealand Kaikōura earthquake: 2016 Mw 7.8 (involved more than 20 distinct faults) | San Andreas Fault and the San Jacinto Fault rupture together 20% – 23% of the time |
It’s possible. And both events happening together would make response times longer, resources scarcer. Be the hero of your story and spend some time this week working on your family’s 2-weeks-ready plan. You’ve got this!
Surviving Cascadia 