Hi Scorpio. You have asked some questions about Hilton Creek Fault before. The answer I gave you then was not the best, but since then I have learned a little more about the area. In order to get a clear picture of what is going on at Long Valley Caldera you have to look at the whole area. I figure a good place to start is with McGee Creek Fault.

McGee Creek flows down from the heights of the Sierra Nevada, whose steep eastern facade is tilting upwards even today. The creek flows into the region of downshifting, which accompanies the Sierra's rise. In doing so, it crosses the Hilton Creek Fault: a dividing line between the rise and fall. But what makes this specific place so unique? McGee Creek and the Hilton Creek Fault happen to lie just outside the Long Valley Caldera, The McGee Creek fault scarp is most visible in the afternoon, when the sun is falling behind the Sierra Nevadas and a shadow is cast along the 1100 meter length of the 15 meter high scarp.

The scarp cuts across the tip of the two terminal moraine "fingers" which are separated by McGee Creek. The scarp on the northern moraine is approximately twice the height of that on the southern moraine, and a question has arisen as to whether the scarp splits into two smaller segments that run parallel up the southern moraine.

The Long Valley Caldera formed 708 +/- 15 thousand years ago with the ejection of the Bishop Tuff. A subsurface magma chamber rose to the surface around the rim of what is now to long valley caldera. The caldera itself formed as the magma chamber was evacuated and the surface collapsed to fill the void. Even today, the caldera is not stable, having molten lava in the subsurface. This lava has a damping effect on the faulting. At the northern end of the Hilton Creek Fault, it enters the caldera. It splinters into several smaller faults, and dies out quickly as it moves northward. The slip rate today at McGee Creek is 1.4 to 3 millimeters per year, 5 kilometers to the north, it is 1.1 to 2. 15 kilometers later, the fault ends. This is evidence of the damping effect of the caldera's molten subsurface.

The moraine appears to present a paradox, for it is clearly of a dip-slip nature, but when standing at the site, there appears to be a strike-slip component as well. The strike-slip component of the scarp is an illusion, which results from the fact that the fault and the moraine do not intersect at a perpendicular angle. When you examine an aerial photo, or take bearings with a compass, the explanation behind this illusion becomes obvious.

In 1980, the Mammoth Lakes area underwent a series of four earthquakes above 6 in magnitude. Surprisingly, there was very little activity along the Hilton Creek Fault. Rinehart and Smith wrote, Movement on the Hilton Creek Fault was a prime suspect for the May 25 1980 earthquakes, but the data proved it innocent. Surface ruptures probably do not reflect movement along the fault. More likely, they reflect slumping of the loose overburden (sand and gravel) along the old scarp Rinehart, 1982.

There was, however a rockfall off McGee mountain. In Harp and Keefer's paper, they report, many boulders were also dislodged from high, steep glacial moraines; several narrowly missed striking residences in McGee Creek canyon and along the Sierra Nevada range front. Numerous other boulders in glacial moraines in McGee Creek Canyon were shifted as much as .5 meters during the main shocks; these remain balanced precariously and could be dislodged by future earthquakes.

However as with anything you must always keep in mind that the answer is no more then an educated guess as to what you see. Nature is very secretive. She won’t tell you anymore then what she has to. And only then when you least expect it.
Hope this helps. Take Care…Don in creepy town.