Pallet Creek is the site of a world famous paleoseismic trench site, where geologist can look for past earthquakes along the San Andreas Fault. Here you can visit the world's most famous trench site, which is a natural hillside exposure that cuts perpendicular across the San Andreas Fault trace. What do scientists look for when they are wanting to know the past rupture history of the San Andreas? And how do we know when these earthquakes occurred in the past? Based on these findings, what is the recurrence interval of large earthquakes on this section of the San Andreas? These questions will be answered, in detail, below. But first, let's discuss the importance of the San Andreas Fault as a whole.
The geology of the San Andreas Fault
The San Andreas Fault is a transform plate boundary entirely within the state of California. Its southern end is about a mile southeast of Bombay Beach in the Salton Sea while its northern end is about 50 miles south of the city of Eureka in northern California, along the Pacific coast. The fault connects the Salton Trough and the spreading center (more on this later) in the Gulf of California to the Mendocino Fracture Zone and the Cascadia Subduction Zone.
There are two types of plates on Earth: Continental and Oceanic. Continental plates are much thicker (30-70km) and are much lighter. They are mostly made of felsic materials such as granites, which contain feldspars and quartz minerals. Unlike continental crust, oceanic crust is much denser and thinner (5-25km thick). A common misconception is that the mantel is liquid when in fact it is solid. It has different physical properties than the crust in that the crust is rigid and tends to fracture (brittle behavior) while the mantle can "flow" and deform (strain) under stress (ductile behavior). The mantel has a constancy similar to silly putty, which is solid but has some properties of a liquid. It can flow away from an input of stress, while the crust cannot. While the temperature of the mantel is high enough to melt most minerals (and rocks), the high pressure from the kilometers of rocks above allow the material to stay a solid rather than melt into a liquid. Changing a phase (solid, liquid, gas, plasma) is dependent only on the temperature and pressure exerted on the material. The ductile properties of the solid mantle allow the plates to effectively float on top of it.
The mantle, being the largest structure of earth's interior, making up over 50% of Earth's volume, has currents (called convection currents) similar to ocean currents. Hotter material from the lower part of the mantel is less dense and will begin to rise towards the upper mantle. As is rises, its temperature drops and begins to cool and shrink, becoming less dense. Eventually it will fall back down to the lower where it is heated up again and the process starts over. These cycles, known as convection currents, are what drives plate tectonics. Earth is the only place in the universe where this occurs, although some have argued Jupiter's icy moon Europa may also have plate tectonics to an extent. Some argue that plate tectonics could only happen with just the right amount of water in the mantle (3-5 wt%), which allows oceanic plates to bend and not fracture. Note: wt% is equivalent to weight percent.
There are three main types of plate boundaries. However, in this earthcache I will only be discussing the most relevant one to the San Andreas Fault system which is a Transform Plate Boundary. Below is a diagram showing the evolution of the San Andreas fault from 30 million years ago to today.
Pallet Creek Paleoseismic Site
The most recent earthquake to rupture this segment of the San Andreas was the largest earthquake in California's history (magnitude 7.9) which occurred on 9 January 1857, the aptly named Fort Tejon Earthquake. This specific earthquake is preserved here, if you know where to look. What scientists look for are offset sedimentary layers. It is assumed these layers must have been deposited some time prior to them becoming offset. Therefore, the offset of these layers must have occured at the time of the earthquake rupture. Below is a map of the 1857 Fort Tejon earthquake rupture (shown in red) along the San Andreas Fault. The red circle is the epicenter. Pallet Creek about due north of the "L" in Angeles.
Some of these sedimentary layers are contain organics and carbon rich molecules, which were preserved when ancient wildfires occured in the area. These charcol layers then became offset when an earthquake occured. And because charcol is carbon rich, it is possible to use radiocarbon dating to determine an age for when the plant that burned died. Note that radiocarbon dating does not tell us the age of the earthquake, rather it tells us the age at which the plant stopped taking in carbon dioxide from the atmosphere. It is assumed that the earthquake that caused these offset layers occurred some time immediately after the plant's death which in reality could be decades.
Carbon has several isotopes. An isotope of an element means it has the same number of protons and electrons, but a different number of neutrons. One isotope of carbon, in particular, is being created in the atmosphere at a constant rate called Carbon-14 and it has a known decay rate as it is radioactive. It's half-life is 5,730 years meaning it takes that long for half of a sample of Carbon-14 to decay into smaller isotopes of other elements. This makes it possible to date anything with Carbon accurately to about 100,00 years. Anything over this age it would be quite difficult to discern any detectable Carbon-14 in a sample.
Samples of these charcoal layers are then collected and sent to labs where the Carbon-14 is measured against Carbon-12 to determine when the sample stopped taking in carbon, which occurred at the time of death. That calculated age is then checked with other samples from the same site (and across other paleoseismic sites) to refine the age as to when the earthquake may have occurred. Here, about 8 different earthquake ruptures along the San Andreas have been identified in the record spanning about 1500 years, giving an average recurrence interval of about 190 years. The last earthquake on this section occurred 167 years ago (as of 2024) so it is possible another large rupture here can happen any day.
Works Cited:
https://sites.google.com/site/geoearthquakesadelaide/fault-systems
https://youtu.be/1vWvHaaMJnM?t=863
https://www.nps.gov/subjects/geology/plate-tectonics-transform-plate-boundaries.htm
https://oceanexplorer.noaa.gov/facts/plate-boundaries.html
https://www.skepticblog.org/2013/04/24/pilgrimage-to-pallett-creek/
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JB008099
https://www.snexplores.org/article/fossil-fuels-confusing-carbon-isotope-dating-measurements
https://science.howstuffworks.com/environmental/earth/geology/carbon-14.htm
https://scvhistory.com/scvhistory/meltzner1857.htm
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL THE QUESTIONS BELOW. YOU CAN CONTACT ME THROUGH MY EMAIL OR THE GEOCACHING MESSAGE CENTER TO SEND YOUR ANSWERS. ANY INCORRECT ANSWERS MAY RESULT IN A CLARIFICATION RESPONSE FROM ME.
1. "San Andreas Fault - Pallet Creek Paleoseismic Site" on the first line of your email AND list all geocaching names of your party so I can match your answers to them. If you all want to learn something, I would prefer each cacher send me individual emails in the spirt of earthcaching.
2. Take a photo of you (or your signature item if you don't want to show your face) at the paleoseismic site with the hillside trench in the background.
3. As you face the trench (hillside), do you see any evidence of fault offsets? If so, how many do you count? If not, why do you think you don't see any?
4. Based on your answers to number 3, what does this tell you about the frequency of earthquakes at this site. Are they occurring regularly or do they rarely occur?
5. If an earthquake were to occur today, what might you expect to see preserved here in 100 years?