Hollister - the Creeping Calaveras Fault

Every region has its particular strengths and weaknesses with regards to the type of geology which is easily accessible for student field trips. In the areas surrounding UCT, we have some seriously awesome geology but there are at least two things my students have to accept without seeing any really clear direct evidence in the field:
  1. plates really move
  2. plates really subduct.
Since I have a student from UCT with me in San Francisco this week for the AGU meeting (who gave a badass poster presentation by the way), it's a good opportunity to fill some of these gaps. We took an afternoon drive down to Hollister and San Juan Bautista to see some evidence for recent fault creep offsetting sidewalks and walls.

Hollister is positioned just north of the split where the Calaveras Fault branches from the main strand of the San Andreas Fault. The Calaveras Fault is creeping through Hollister, but rates vary along the fault in space and time from 3-18mm/yr (http://funnel.sfsu.edu/creep/SiteTable.htm).
(Map from http://quake.usgs.gov/recenteqs/)

My student is clearly excited by this right-lateral bulge in a garage wall.

Tension gashes where the fault crosses the street at a high angle and disappears straight under the middle of a house.
wonky sidewalk
another wonky sidewalk
seriously wonky sidewalk, and the steep small hill on the left of the photo is a pressure ridge
wonky sidewalk
more tension gashes in the street
carly will be creeping to your right as you look at this photo.

It's cool to see how different sidewalks and houses of different ages have accumulated different amounts of offset. We also couldn't help noticing new skirting and lots of concrete repairs which presumably addressed the larger offsets. Also - in some places the total offset was accommodated by narrow strands (usually ~ 1m wide) but in others, the deforming zone seemed to be much wider (10m). Seems like this depends on local soil conditions as well as the rigidity of the surface features. Sometimes it is wide under a sidewalk and sometimes all the strain seems to accumulate on one joint between sidewalk panels, as in the last photo.

Today we are off to Ring Mountain with Åke to see some evidence for #2.