Back to the Future on the San Andreas Fault (2023)

Geoscientists have collected important datapaleoseismic sitesalong sections of the San Andreas fault to discover the past chronology of earthquakes at each location. The data shows that in many places along the San Andreas fault we exceed the mean time between major earthquakes. Because we're above average, a lot of people use the term "late", but it's more complicated than that. First, let's zoom out and see the big picture.

San Andreas Fault Zone: The Big Picture

Scientists have a good overall picture of the San Andreas Fault Zone (SAFZ). The ZAF started moving about 28-30 million years ago and has been sliding horizontally in general (transformation motion).300–350 km (186–220 miles)since it started moving. The SAFZ is the main part of the boundary between the Pacific tectonic plate on the west side and the North American plate on the east side. The "zone" part of the name means that it is a system with the main fault and many subparallel faults, all of which accommodate movement between the two plates. In northern California, the zone includes Hayward, Calaveras and northern San Andreas and other falls, and in southern California, the zone is even broader and includes southern San Andreas, San Jacinto and other falls in the area from Angels.

Diethe relative motion between these two tectonic plates is 50 mm/year (about 2 inches/year), but this rate is distributed among all the faults that are part of the ZSAF. Faults are boundaries between blocks, and each block is in constant motion, which we can see by analyzing GPS (Global Positioning System) data. However, the edges of the blocks, the faults themselves, are stuck and only move during a big earthquake (some faults drag a bit, but most are locked). An earthquake occurs when the stress from the moving plate force overcomes the friction, causing the edges of the plate boundary, the fault, to stick together. The stuck section slides and the edge of each block reaches the remainder of the slab. The board moves slowly all the time, but the edges move erratically.

Many of the sites that paleoismologists have studied are along important stretches of the SAFZ where there is a large population or infrastructure that would be affected by a major earthquake in the future. Let's start in Southern California and work our way north.

southern california

There are only two important historical earthquakes known in the San Andrés Falls in southern California, the most recent in 1857 and the previous one in 1812. With approximately 45 years between historical earthquakes but approximately 160 years since the last one, it is clear that the error does not exist. don't behave like a clock with a regular chime. Historical information does not provide enough data to determine whether or not there is a pattern in the timing of earthquakes, but paleoseismology has provided a wealth of data.

Along the southern tip of San Andreas fromPalm Springs al mar de Salton, earthquakes occur infrequently, approximately every 200-300 years. The last earthquake occurred about 300 years ago, during the Spanish exploration period, but there are no historical records of the event. Instead, radiocarbon dating gives the age of the most recent earthquake and six others that have occurred since around 800 AD.

A paleoseismological site inWrightwood, Califórniahas been studied by several scientists, and recently (in 2010) detailed data from several studies were merged to create a single timeline. The resulting 3,000-year record includes 29 surface earthquakes. Careful analysis of the ages of earthquakes, including uncertainties in radiocarbon dating (see Determining the Age of a Paleearthquake inIntroduction to paleosismology), showed that the average time between earthquakes is about 100 years.

Repeated intervals (times between earthquakes) in Wrightwood are regular rather than frequent (determined by mathematical analysis), and historically the interval between two major earthquakes has been only four times as long as the current interval (since 1857). The results of this study indicate that this section of the San Andreas fault is likely to experience a major earthquake in the not-too-distant future.

Another site is located about 100 km northwest along the fault.Frazier Mountainhas been examined. At this location, the record spans approximately 1000 years and during this period approximately 9 major earthquakes were recorded in the sediments, including the 1857 rupture.

By comparing data from places like Wrightwood and Frazier Mountain, earthquake scientists are working to understand the pattern of large earthquakes, asking questions like: How typical was the great earthquake (M7.9) of 1857? Or is the magnitude of the 1812 earthquake (~M7.1) more common? Note that since the magnitude scale is a logarithmic scale, the energy released by these different earthquakes is about 25 times different.

northern california

Diehayward errorin the San Francisco Bay Area, it cuts across a densely populated area, so it has been fairly well studied. The last major earthquake on this fault was around M6.9 and occurred in 1868. The fault has moved at about 4.6 mm/year (0.2 in/year) over the last few decades, but that's only half its size. hatch in the long run. rate, so the stress is based on this error. A paleoseismic study from 2007Laguna Tysons(now a BART station) found evidence of 12 earthquakes (including the historic 1868 earthquake) with an average time between earthquakes of about 160 years. The average time interval between the 5 most recent earthquakes is slightly shorter, around 140 years. The study concluded that there is a 33% chance of a surface earthquake occurring in the next 30 years. (To watchEarthquake Prediction for the San Francisco Bay Area 2014–2043).

Diebedbug macamait is the northern continuation of the Hayward-Rodgers Creek fault system in northern California. In 2014 a paleosismology website was launched inhael creekon the Maacama fault echoed the results found on the southern Hayward fault, insidious with rare large earthquakes and a large one expected in the not too distant future.

Diehasel dellThe site near Corralitos, CA, was excavated in 2013 to characterize the portion of the San Andreas Fault in the Santa Cruz Mountains. The Santa Cruz section stretches 39 miles (62 km) from Los Gatos (near San Jose) to San Juan Bautista, California, and was last destroyed in the famous 1906 San Francisco earthquake. Radiocarbon dating of coal, wood fragments and small plant remains, combined with a reassessment of three previously surveyed nearby paleoseismic sites, revealed past variations in seismic activity. Three earthquakes occurred in 70 years between 1838 and 1906, but there had been no earthquakes in the 500 years before that, and there had been no earthquakes in the 110 years since 1906.

This shows that the average time between earthquakes contains some short and long intervals. New studies further northwest along thePeninsular section of the San Andreas faultthey also show a long gap between the 1906 earthquake and the previous earthquake, which occurred around 1300. Before 1300, the gaps are shorter, around 200 years. The northern coastal part of the San Andreas Fault is north of San Francisco. Studies of this section of the fault indicate an average repetition interval of 200-300 years.

Now that?

Paleoseismic data in various parts of the San Andreas fault zone tell us that some sections appear to be above average or "overdue" for a significant earthquake. But the data can't be used to make predictions: We don't understand earthquakes well enough to know exactly where the next earthquake will strike, how strong it will be, or exactly when it will strike.

Let's imagine for a minute that we knew where, how big and when an earthquake would happen. You might think it would be good because then you could leave the area earlier and come back after the earthquake. But just focusing on avoiding an earthquake doesn't fix most tremor effects. Returning home, you are likely to see damaged and collapsed buildings and bridges, broken pipes and power lines, and burnt debris from the fires. Entering her home, she finds fallen bookshelves, broken glass from mirrors that no longer hang on the walls, and the contents of kitchen cupboards piled on the floor.

What we do know is that California is "earthquake country" and we need to be prepared for that.In particular, we need to design buildings and infrastructure that can withstand the impacts of earthquakes or that can be easily repaired. Scientists are working to improve predictions that estimate how often future earthquakes will occur and how much the ground will shake, so that engineers and planners know where to focus their efforts to mitigate the effects of damaging earthquakes. Using forecasts, we can build structures properly, plan for earthquake response, and be prepared at home to make a big difference in the effects of a major earthquake.

-written by Lisa Wald, Kate Scharer and Carol Prentice, US Geological Survey

...and many thanks to the Facebook poster who gave us the idea for the title!

additional resources

• Map of active faults and historic fractures in California- paleoseismic data compiled from Table H3 ofUCERF3(under)
• Introduction to paleosismology
• Paleosismology and the Hayward Fault
• New information about the San Andreas Fault
• Centennial earthquake information
• Science of the Seismic Zone of New Madrid - Paleosismology
• UCERF3: A new seismic forecast for California's complex fault system
• earthquake preparedness