MAGNETIC-FIELD OBSERVATIONS IN THE NEAR-FIELD THE 28 JUNE 1992 M(W) 7.3 LANDERS, CALIFORNIA, EARTHQUAKE

Recent reports suggest that large magnetic field changes occur prior t o, and during, large earthquakes. Two continuously operating proton ma gnetometers, LSBM and OCHM, at distances of 17.3 and 24.2 km, respecti vely, from the epicenter of the 28 June 1992 M(w) 7.3 Landers earthqua ke, recorded data through the earthquake and its aftershocks. These tw o stations are part of a differentially connected array of proton magn etometers that has been operated along the San Andreas fault since 197 6. The instruments have a sensitivity of 0.25 nT or better and transmi t data every 10 min through the GOES satellite to the USGS headquarter s in Menlo Park, California. Seismomagnetic offsets of -1.2 +/- 0.6 an d -0.7 +/- 0.7 nT were observed at these sites. In comparison, offsets of -0.3 +/- 0.2 and -1.3 +/- 0.2 nT were observed during the 8 July 1 986 M(L) 5.9 North Palm Springs earthquake, which occurred directly be neath the OCHM magnetometer site. The observations are generally consi stent with seismomagnetic models of the earthquake, in which fault geo metry and slip have the same form as that determined by either inversi on of the seismic data or inversion of geodetically determined ground displacements produced by the earthquake. In these models, right-later al rupture occurs on connected fault segments in a homogeneous medium with average magnetization of 2 A/m. The fault-slip distribution has r oughly the same form as the observed surface rupture, and the total mo ment release is 1.1 x 10(20) Nm. There is no indication of diffusion-l ike character to the magnetic field offsets that might indicate these effects result from fluid flow phenomena. It thus seems unlikely that these earthquake-generated offsets and those produced by the North Pal m Springs earthquake were generated by electrokinetic effects. Also, t here are no indications of enhanced low-frequency magnetic noise befor e the earthquake at frequencies below 0.001 Hz.