While Southern California, and really all of California, is considered a hotbed of earthquake activity, it is not often we see amazing examples of the direction and magnitude of plate movement right in our own backyard. Walnut Creek Park, found in the northern part of the San Jose Hills, is home to some interesting geological features that are a direct result of seismic activity.
Historically, Walnut Creek had been a seasonal stream, flowing only after a recent rain. Currently, water flows year round as a result of drainage, seep and outflow from nearby Raging Waters water park and Puddingstone Reservoir. Walnut Creek and Puddingstone Reservoir are found in the north and northeast portion of the San Jose Hills. The San Jose Hills trend east-northeast and are uplifted along a west-southwest-plunging anticline underlain by the La Vida member of the Puente Formation, the Glendora Volcanics, and Cretaceous granitic rocks. The San Jose Hills were formed while the area was under water in the Los Angeles Basin. The plate tectonics that created the San Gabriel Mountains to the north continued to shift and fold, creating fissures through seismic and volcanic activity. The Indian Hill fault lies to the north of San Jose Hills and the San Jose fault (north dipping reverse) lies to the south. Walnut creek flows along the general path of what is known as the Walnut Creek fault. Erosion from Walnut Creek has exposed some interesting bedrock formations.
The Walnut Creek fault (blue line in image above) is not well defined nor has it been extensively studied. The Walnut Creek fault is considered a strike/slip (northeast striking, left slip) fault from the quaternary period, forming sometime in the last 2.6 million years. A lateral slip fault is determined to be right or left by determining the direction which the ground has moved while viewing it from across the fault line. This fault separates strongly-folded strata of the San Jose and Puente Hills from nearly flat-lying strata of the San Gabriel Valley. Very little history is known about this fault line; It is unknown when the most recent surface ruptures occurred, intervals between seismic activity or magnitudes. The Upland earthquakes of 1988 and 1990 commonly attributed to the San Jose fault could have been seismic activity along a continuation of the Walnut Creek or Indian Hill faults. While it the potential seismic activity for the Walnut Creek fault is unknown, the San Jose fault has a seismic potential of 6.0 to 6.9 for future earthquakes.
So how are the fault line, direction of movement and the dip or tilt interrelated? The fault line is actually the surface trace of a fault region that is three dimensional and not just a line in the surface of the earth. A fault line or region is an area where tectonic plates are moving against each other and earthquakes are felt as the faults provide pressure relief in movement along the fault line. The direction of movement is the direction that the plates are moving against each other. They may slide against each other laterally as a strike slip; one may move below or over another plate as in a dip-slip type fault; faults may be a combination of both strike-slip and dip-slip as an oblique-slip fault. For a fault to be categorized as an oblique, both components must be clearly measurable. The strike and dip refer to the orientation of a geologic feature, such as a fault. The dip gives the steepest angle of descent of a tilted bed or feature relative to a horizontal plane. This number is between 0°-90° with the dip being described in the direction which the bed is moving (N, S, E, W). Any planar geologic feature can be described by strike and dip. An apparent dip is a measurement of the dip not taken perpendicular to the direction of movement.
Despite being hidden between several housing tracts, this area is also home to many different critters, some of which might not like visitors. If you're not careful, you could find yourself with a bite and a trip to the ER, trying to wash off a stink from a furry striped critter or suffering from a nasty itch. Watch where you walk and put your hands while visiting and enjoying the area.
When logging a find on this EarthCache:
You will need a means of determining the angle of tilt of exposed bedrock. You may use a compass or other device that can measure angles relatively accurately , or a smartphone app designed to measure angles. I used the "inclinometer" app on my android smartphone when creating this earthcache. This was a free download and does not require data or internet connection to use. Similar apps are available for iPhone users.
Visit the two additional wapoints listed. You will need to measure the greatest angle of slope or dip of the fault line along the rocky outcroppings and determine the direction of movement. You should get a number between 0 and 90 degrees.
WP1: N 34° 05.445 W 117° 50.002
WP2: N 34° 05.365 W 117° 50.173
(also listed below)
You may post the log of your find on the day of your visit to this EarthCache.
Send the following information via email within one week of your "found it" log:
- Walnut Creek Fault EarthCache GC5AN4N
- The nicknames of other geocachers with you the day you completed this EarthCache.
- At waypoint 1; What is the slope (in degrees) of the dip and what direction is the dip movement at the large outcropping of rock at which it lies? Is there visible plate movement under, over or side slip where you are standing? What is the fault strike direction? Explain.
- At waypoint 2: What is the slope (in degrees) of the dip and what direction is the dip movement at the large outcropping of rock at which it lies? Is the fault strike direction the same as wp1? Are the rocks moving under, over or alongside the fault line? Explain why you think this.
- Compare the similarities and differences you find at the two locations. If you found differences between the two locations, explain why this may be and what may have happened here.
Post in your online log any additional coordinates which may have fault/seismic/plate tectonic significance for this fault line that you may find while hiking the trails in either direction. Photos describing these locations are encouraged. All photos taken while visiting this EarthCache are always welcome as long as they do not give away any answers.
Walnut Creek Community Regional Park in the city of San Dimas and administered by Bonelli Park/L.A. County Parks Dept., is a minimally developed community park and host to a 3 mile long multi-use trail. Activities include hiking, equestrian, and mountain biking with several creek crossings. (If you're lucky, you might be able to catch a crawdad or two in the creek!) There are parking areas at the west, middle and east entrances to the park, and an equestrian staging area in the middle of the park. There is no potable water available within the park, so make sure you bring enough for the duration of your visit. This area of the park is frequented by various muggle types, so be aware that you may not have the area to yourself. The city of San Dimas has been officially examining the potential for additional development of Walnut Creek Park since 2011. The city recently received an $800,000 grant for an environmental impact study.
Geocaching is permitted within Walnut Creek Park and Bonelli Park, per the Bonelli Park Supervisor. Use good judgement when placing and searching for geocaches and earthcaches so we all may continue to enjoy access.
References:
- http://earthquake.usgs.gov/research/external/reports/00HQGR0070.pdf
- http://www.westcovina.org/cityhall/fire/prepare/nhmp/6.asp
- http://earthquake.usgs.gov/research/external/reports/00HQGR0070.pdf
- http://www.scec.org/research/special/SCEC001activefaultsLA.pdf#page=26&zoom=auto,-265,476
- http://www.data.scec.org/significant/index.html
- http://en.wikipedia.org/wiki/San_Jose_Hills
- http://earthquake.usgs.gov/learn/glossary/?term=strike-slip
- http://en.wikipedia.org/wiki/Strike_and_dip
- San Dimas City Hall in person interview