NOTE: Simply visiting the site is not enough to claim a find (per earthcaching guidelines) - you must message/email answers to the cache owner in a timely fashion. After one week if I haven't received answers I will send you a reminder, if no response after that, the log will unfortunately be deleted.
Situated on the southwestern edge of the Long Valley Caldera and the southern tip of the Mono-Inyo volcanic chain, it is no small exaggeration to say that the town of Mammoth Lakes very literally sits on a hotbed of geologic activity. So maybe it's not too surprising to know that earthquakes occur quite frequently in the area. The last major quake to hit Mammoth occurred on May 25, 1980 and measured magnitude 6.2. More recently, a magnitude 4.2 quake struck on August 24, 2011. Interestingly however, an earthquake has never been recorded at the very spot of this earthcache, so then why is it called the Mammoth Earthquake Fault?
fault /fôlt/
n. Geology
A fracture in the crust of the earth accompanied by a displacement of one side of the fracture with respect to the other, usually in a direction parallel to the fracture.
Faults occur at the boundaries of tectonic plates and movement of earth along a fault generally results in earthquakes. There are several kinds of faults (see diagrams above), and in all cases both plates shift relative to each other - either horizontally or vertically.
Examine the crack in the rhyolite lava flow at the posted coordinates (one of many such flows on Mammoth Mountain). Walk around the fenced-off area of the fault, noting especially the relationship between how the edges of both sides used to fit together.
To log this earthcache, please email me the answers to these questions:
1. Estimate the width and depth of the fault.
2. Using your GPS, measure the length of the fenced-off portion of the fault (in feet) and determine its bearing.
3. Are the features of this fissure consistent with an earthquake fault as defined in this cache description? If so, which kind of fault depicted above does it resemble? If not, propose a possible method for its formation.
The kiosk nearby may also be a useful resource (but as of June 2021 it is unavailable and hopefully being reconstructed - use internet resources as needed), but you will still need to visit the fault to take the measurements. Photos at the site are encouraged.
Enjoy your visit, and many thanks to Jon at the Inyo National Forest, with whose permission this earthcache was placed.
References:
1. U. S. Geological Survey website, specifically the section on earthquakes
2. Inyo National Forest