Hi Michael. The magnetic field around rocks containing magnetic material will change when stress is applied to the rock. This is known as the piezomagnetic effect, a couple of good references are Stacey, 1964 (The Piezomagnetic Effect) and Nagata, 1969 (Basic Magnetic Properties of Rocks Under the Effects of Mechanical Stresses).

The idea in earthquake and volcanic studies is to monitor the magnetic field at the Earth's surface to detect time dependent changes related to stress changes in the crust. They have measured changes in magnetic field related to stress drops at the time of major earthquakes (Loma Prieta, 1989, Landers, 1992; North Palm Springs, 1986;
The changes for these events are on the order of 1 or 2 nT(gammas) and detection of changes prior to events is unclear at this time.

The creep meters you refer to are XMM1which is Middle Mountain. XMD1 is Middle Ridge located 3Km south of Middle Mountain.

In the Parkfield area there is almost always continuous creep. It’s when it stops that strain is starting to build. At present most of the strain build up seems to be in the area of Middle Mountain. At present there is no strain in that area. However I have seen that change in less then 24 hours. The biggest problem with creep meters is that they are located close to the surface and as such rain, or water can affect them. The ground that they are buried in gets soft which can give a false reading.

Creep meter XHSW shows acceleration in creep, which is a good indication of no strain in that area. Take Care…Don in creepy town.