My first Earthcache, I've always wanted to place one so here goes. There is no parking directly at GZ so please park at the associated parking waypoint on a street right off Foothill Rd.
The Calaveras Fault branches off from the San Andreas around Hollister and continues into the Diablo Range, splitting off around Milpitas to form the Hayward Fault to the northwest but continuing on its own throught the San Ramon Valley to Danville. The more southern reaches of the fault are responsible for the 1984 Morgan Hill Earthquake of magnitude 6.2 and the recent and memorable 2007 Alum Rock Earthquake of magnitude 5.6. Unlike the Hayward Fault, the Calaveras Fault (especially this northern section following I-680) is remarkably difficult to find any evidence of. This is ostensibly due to the sharp escarpment that leads up from the Amador Valley to the mountains directly west, which over millions of years has deposited landslides around the area. The steepness of these mountains can be seen from many miles east and is most likely responsible for unstable land causing landslides to fall down into the valley. These landslides have covered any notable surface evidence of the fault, but the evidence of the landslides themselves is here showing the dynamic differences in landforms between the valley, the gentle foothills transition area and the upper reaches of the mountains.
Refer to the additional image in the gallery for a small map showing the landslide area. Please walk from the parking waypoint to GZ and look southwest uphill to DM me the answers to these quick questions:
1.) How high do you think the small grassy hill to the southwest rises up from the land around it from this vantage point? Do you think it was caused by a landslide millions of years ago due to its appearance and relationship to the land below and above it?
2.) Based on elevation differences and other observations here, where do you think GZ is relatively speaking, between the mountains, landslide impacted area or flat valley? Where do you think the Calaveras Fault lies in relation to GZ?
3.) Post a photo of yourself or your geocaching name written down/some personal object to your log in front of the Longview/Foothill Rd street signs near the parking waypoint to further confirm you visited the site. Please take care not to show the hill in question to the west.
I have always been interested in learning about the whole Bay Area fault system and noticed there were a lot of Hayward Fault caches so I thought I would try to showcase a little-known fault here in this one.
Sources: Wikipedia, MST Geology Dr. J. David Rogers
Congrats to Azuljewel and JMobious for the FTF!
**UPDATE**: On October 25, 2022 at 11:42 AM PDT a quite palpable magnitude 5.1 earthquake occured on the Calaveras Fault in the mountains to the east of SE San Jose. A 3.1 magnitude aftershock followed minutes later. It just goes to show how active and dynamic this fault still is.