from next week's AGU meeting:

Seismomagnetic Effects from the Long-awaited September 28, 2004, M6 Parkfield Earthquake

Johnston, M J USGS, Menlo Park and others

Precise measurements of local magnetic fields have been obtained with a differentially connected array of seven synchronized proton magnetometers located along 60 km of the locked-to-creeping transition region of the San Andreas Fault at Parkfield, CA. since 1984. The M6 Parkfield earthquake on September 28, 2004, occurred within this array and generated coseismic magnetic field changes of between 0.2 and 0.5 nT at five sites in the network. No preseismic magnetic field changes exceeding background noise levels are apparent in the magnetic data during the month, week and days before the earthquake (or expected in light of the absence of measurable precursive deformation, seismicity or pore pressure changes). Observations of electric and magnetic fields from 0.01 to 20 Hz are also made at one site near the end of the earthquake rupture and corrected for common-mode signals from the ionosphere/magnetosphere using a second site some 115 km to the northwest along the fault. These magnetic data show no indications of unusual noise before the earthquake in the ULF band (0.01 Hz to 20 Hz) as suggested may have preceded the 1989 Loma Prieta earthquake. Nor do we see electric field changes similar to those believed to occur before earthquakes of this magnitude from data in Greece.

[omitted some details].

The absence of electric and magnetic field precursors for this, and other earthquakes with M5-7.3 elsewhere on the San Andreas fault, indicates useful prediction of damaging earthquakes seems unlikely using these electromagnetic data.