HFS #10: Hayward Fault Rift Zone – ADA Accessible EarthCache
HFS #10: Hayward Fault Rift Zone – ADA Accessible
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This relatively simple earth cache. In addition to the requires to log and learn, two of the three way points are at a couple of informational boards. You will need to visit GZ and the two waypoints to complete this cache. GZ and the north waypoint are easily accessible by folks of limited mobility. For the southern way point limited mobility folks will be able to approach by driving by in a car. The feature you need to observe is easily viewable from a car. None of the logging requirements are provided by the informational boards; however, the provide some very interesting information on this park, the Hayward Fault, and the Hayward Fault Rift Valley.
This earthcache is located in Temescal EBRP, which is located in a rift valley of the Hayward fault. Although most of the movement along the Hayward fault is a right-lateral strike-slip fault, there also is a component of compressive forces. Right-lateral strike-slip movement means that if you stand facing the fault plane, the movement on the opposite site of the fault will be to the right regardless of which side of the fault you stand on. The compressive forces along this portion of this portion of the fault causes hills to rise on either side of the fault. Highway 13 (from Hwy 24 to Hwy 580) and Temescal Park lie right along the axis of the Hayward Rift Valley created by these forces. At some distance to a fault plane is a zone of highly fractured rock. Directly adjacent to the fault plane is a zone of “fault gouge”. Fault gouge develops along long active faults, and this material is a weak, powdery, clay like material generated when the host rock has been totally pulverized by movement along the fault. This gouge is much weaker than rock and is highly erodible. This rift valley was made deeper by the ease with which Temescal Creek eroded the weak gouge along fault zone. On the info board you can see how Temescal Creek is redirected by the fault.
GZ for this earthcache is located at the first of two information board located in front of the park office. Where you stand to read the info board puts you standing directly on top of the Hayward fault. As you face the info board, to your right you’ll see a series of “en echelon” cracks in the asphalt. These fractures are at an oblique angle to the movement along the fault. In your mind, connect the edges of the fractures along the fracture set and along the other edge of the fractures connect the edges of the fractures. These two imaginary lines will represent a fracture zone that is subparallel to a particular fracture associated with the fault. Subparallel a term used in geology to mean almost be not exactly parallel to some linear feature. Try to estimate and record the compass bearing of the fracture causing this set of en echolon fracture set. In geology compass bearings are always recorded as the deviation from north, that is N45W or 315° as opposed to S45E or 135° (all of which represent the bearing of the same line measured and reported in different methods).
Go to the first waypoint for the info board by the kiosk at the entrance to the swim area. This board explains a lot about the lateral and compresses forces (transpression) that generated the hills either side of this rift valley. From here (or on top of the dam is even better), look straight down the rift valley to the south and try to estimate the bearing of the whole rift valley.
The final waypoint is at the south side of the car tunnel. If on foot, beware the sidewalk is narrow and a car may approach. The butteress wall and sidewalk have been offset by the movement of the fault. Estimate the amount of displacement in the sidewalk; how many inches.
To log this cache provide the following information:
1) The name and GC code for this cache in the subject line
2) What is the bearing of the fracture zone you found at the GZ?
3) What is bearing you estimated for the whole rift valley when at waypoint 1, by the swim area?
4) Speculate on the difference between the bearing of the fracture zone at GZ and of the whole rift valley.
5) What is the distance in inches for the offset sideway just south of the tunnel at the of the southern park entrance?
Because of the number of stops I added to the difficulty, but left the terrain at 1T because it is accessible.
Additional Hints
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Treasures
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