Southern California is notorious for faults and the earthquakes caused by them. Elysian Park is no exception. The aptly named Elysian Park Fault runs directly under the park. The fault is considered a blind thrust fault, meaning it has no surface trace. In essence, the fault does not make it to the surface. A thrust (or reverse) fault is a type of strike-slip fault in which the hanging wall moves up relative to the footwall.
To know what wall is the hanging wall and which one is the footwall imagine you are a miner inside a mine shaft. This mine shaft cuts across a fault which dips at some angle. You hang a lantern on the ceiling. The part of the wall you hung the lantern to is known as the hanging wall. The part of the wall where your feet are standing is known as the footwall. Depending on how the lantern moves relative to your feet will differ depending on the motion and stress of the fault.
If the angle of dip is 90 degrees, then the fault would be classified as a strike-slip fault, as in the case of the San Andreas (among other notable examples). Notice how in the images above, the fault trace makes it to the surface. This means if you were standing at the surface, you would be able to directly see the fault. Take note of the image below, which depicts a blind thrust fault at depth, where the fault does not reach the surface.
Because the fault does not make it to the surface, it does not mean that its effects can't be seen at the surface. Note that there is a hill in the landscape as a result of the motion association with the blind thrust fault in the image above. This is due to tensional stresses from the left and the right of the image which "bunch up" the land and creates a ridge. Over time, these ridges are eroded by wind and water. Erosion is in a constant battle with uplift. If the rate of erosion and the rate of uplift are equal, the ridge will not be noticeable at the surface. This is the case with the fault associated with the 1994 Northridge earthquake. Before this earthquake, the fault was unknown to science. Blind thrust faults are extremely dangerous as it is incredibly difficult to detect and many are capable of causing damaging earthquakes, as seen in 1994 (image below). Elysian Park, for reference, is indicated by the purple star.
Faults typically are discontinuous and are composed of many strands which make up the entire fault system. For example, the San Andreas Fault system is composed of many, smaller segments. When there is a break in this continuity, a step is formed (whether that be convergent or divergent). Depending on the fault mechanics, orientation, and distance between segments, the land can either sink (sag) or bunch up (uplift). When the land sags, it creates a natural low point for water to collect. This is how Lake Palmdale formed along the San Andreas Fault (see GC21VQA for more info). When the land bunches up, hill(s) can develop as the land begins to uplift. This is how Signal Hill formed along the Newport-Inglewood Fault (see GC65F1A for more info).
The "Elysian Park Fault" is called this because its most noticeable surface feature is the hill(s) that make up Elysian Park. If a step occurs in a blind fault, the land can create a series of hills and valleys (as seen below).
Because the fault lies underground, the rocks beyond the fault zones become deformed and start to form anticlines. An anticline is when horizontally deposited layers bend upwards. The opposite of this is known as a syncline, when the layers bend downwards. Below is a diagram of an anticline which is what lies beneath Elysian Park.
Note, anticlines and synclines should not be thought of as hills or valleys. Instead, think of an anticline as the rock layers bending upward and synclines are the rock layers bending downward. Synclines can occur at the tops of hills, as with Siding Hill in Maryland (one of the best examples of a syncline in the world. The image below was taken by bluesnote while he was on a 2019 geology field trip in Maryland (see GC3G49D and GCM8ND for more info). Note how the lowest point of the syncline (the axial fold) is aligned with the top of the hill.
Regarding the axial fold (or top) of the anticline for Elysian Park, it roughly aligned with I-5 on the northeast side of the park. The image below shows where the top of the anticline would be underground. Note that the line indicated in the image below does not indicate where the Elysian Park Fault is as the fault is buried at depth. The purple star is the approximate location of SoCal Spring Fling (GC9P0KE), for which this EarthCache was created.
Works Cited
https://earthquake.usgs.gov/cfusion/qfault/show_report_AB_archive.cfm?fault_id=218§ion_id=
https://usgs.maps.arcgis.com/apps/webappviewer/index.html?id=5a6038b3a1684561a9b0aadf88412fcf
https://planning.lacity.org/eir/2k_Ave_of_Stars/text/geo.pdf
http://ceo.scec.org/wallacecreek/misc/defs.html
https://www3.nd.edu/~cneal/PlanetEarth/Lab-Structural/Faults.html
http://www.geosci.usyd.edu.au/users/prey/Teaching/Geol-1002/HTML.Lect3/sld007.htm
https://scedc.caltech.edu/earthquake/northridge1994.html
https://landslides.usgs.gov/static/lfs/nshm/qfaults/Reports/134.pdf
https://www.pinterest.dk/pin/726486983610466655/
View of outcropping from public road parking area.
TO LOG A FIND ON THIS CACHE YOU MUST ANSWER ALL OF THE QUESTIONS BELOW. You can contact me through the geocaching message center or email to submit your answers. Any incorrect answers may result in a clarification response.
1. "Elysian Park" on the first line of your message 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 spirit of earthcaching.
2. Take close up photo of the rock outcrop using some sort of scale. Geologists often use coins, GPS', pens, shoes, notebooks, etc. You might want to be creative in what you use for your scale, however, the scale that you choose is entirely up to you. This photo must be uploaded to your log.
3. (a) Take an elevation reading at GZ. Most sources have noted that the minimum rate of uplift for the Elysian Park Fault is 2.5 mm/yr (roughly 0.1 inches per year). (b) If the rate is assumed to be constant, what would the elevation be on your GPS in 1,000 years?
4. Take a look at the outcrop in front of you. Would this be a syncline or an anticline? How can you tell? Hint: think about how the rocks are in relation to the ridge caused by a blind thrust fault (which is the top of what you are standing on).
5. Take a look at the outcrop in front of you. What evidence would indicate a fault is nearby?
6. Do you see any hills or valleys which might indicate the presence of another blind thrust fault in the area from this vantage point? If so, note their relative direction and distance from here.
View from lower outcropping reference point.
View from upper outcropping reference point.
Super special thank you to bluesnote for creating this earthcache
for the SoCal Spring Fling Mega-event!