You are at site F on the above map.
This is my 100th earthcache, and also my first earthcache published since receiving a B.S. in Geology. I wanted to do something special for my 100th earthcache, so I revisited one of the best examples of fault creep in the world. This earthcache will bring you on a 2-site tour of Hollister. If you want to do more and explore the different creep offsets, feel free! I've listed several sites that I think are notable. You are only required to visit one of those additional sites plus the site at the above coordinates for this earthcache. Before I get into too much detail, let me first discuss the Calaveras Fault. It is one of a handfull of offshoot (branch) faults from the much larger San Andreas Fault. Further north, the Calaveras Fault branches into the Hayward Fault. Further south, the Calaveras Fault joins up with the San Andreas Fault. The Calaveras Fault is one of the only 3 faults in California that is activley creeping (the others being the central San Andreas Fault and Hayward Fault). The Calveras creeps at a rate of 15 mm/yr while the San Andreas Fault creeps at roughly double that at 28-32 mm/yr.
Asiesmic Fault creep is the near continuous movement, or displacement, along a fault without the need of earthquakes. It is due to a combination of mineralogy and stress. In the case of the Calaveras, it is due to a high concentration of the mineral talc, which is the softest known mineral. Talc has a defined Mohs hardness of 1. For reference, diamond (the hardest mineral) has a defined Mohs hardness of 10. The talc is compressed and broken into a powder like substance where it lubricates the fault plane, allowing for creep to actively occur. This can cause noticable surface offsets especially in Hollister where streets and curbs cross the fault at right angles. Hollister has one of the world's best examples of aseismic fault creep and is frequently visited by geologists from around the world.
Calaveras Fault trace at GZ indicated by the red line. Image is courtsey of me.
The Calveras Fault is classified as a strike skip fault. However, there are different types of slip slip fault like this one. They vary depending on what direction they are moving relative to the other. You can tell by a simply trick. Stand one one side of the fault and look to the other. If the direction of motion on the other side is moving to your right, then it's a right-lateral strike strip fault. If it's moving to your left, then it's a left-lateral stick strip fault. The Calveras Fault falls into one of these categories.
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. "Calveras Fault - Holister (2.0)" 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.
Questions to be completed at the above coordinates
N36° 51.035 W121° 24.399
2. Take a close up picture of the offset you see (either on the curb, sidewalk, or retaining wall) with some sort of reference scale. Geologist sometimes use coins, books, or even humans! Be creative in what you choose for your scale. Note: this is not a selfie, but rather a documentation of the geology using a scale.
3. How many inches is the offset of the curb here?
4. Using your answer in question 3 to help you, and knowing the average creep rate of the Calveras Fault, (a) estimate the time when the curb was straight. In other words, how long ago was this curb built based on your calulations. (b) Based on your calulations, how many more years will it be until the street curb is no longer touching? Hint: offset distance x time = creep rate. Note: Your units must match. For example, you need to convert everything to milimeters or inches before you do your calulation!
Questions to be completed one of the additional sites of your choosing
See map at the top or GPS coordinates in the waypoint sections
5. Using the same scale you used in question 2, take another close up photo of the offset. Be sure to note in your image title which site your photo was taken from.
6. How many inches is the offset of the curb here?
Questions to be completed after visiting both sites
7. Compare and contrast the extent of noticeable surface features (offsets) at both sites. Which site is more altered? Why?
8. How do your answers from 3 and 6 compare? Are they similar? Different? Explain.
9. Based on your images, what type of strike slip fault would you classify this as?
Works Cited
http://geologycafe.com/fieldtrips/hollister.html
http://sepwww.stanford.edu/oldsep/joe/fault_images/hollmapA.html
https://seismo.berkeley.edu/blog/2016/11/18/the-missing-link.html