Intensive observations of the geomagnetic field have been carried out since 1976 in the Izu Peninsula, Japan. An array of observation sites covers the area of anomalously high microearthquake activity persisting since 1975. The array also extends over the region of crustal uplift amounting to 15cm or thereabouts which was revealed at the beginning of 1976. At present an observation system consisting of four types of stations is in operation. The total intensity of the geomagnetic field has been measured continuously by proton precession magnetometers at four stations. At two other temporary stations the total intensity is to be measured whenever earthquake activity becomes high in their vicinity. Three components of geomagnetic field variations have also been measured by a flux-gate magnetometer at another station. Finally, 45 observation sites have been established for repeated surveys of the total intensity. In addition to the repeated surveys at these sites, a small-scale experiment based on a synchronized measurement of difference in the total intensity between station pairs has been under way at an array of 14 sites located near the center of the crustal uplift.
During the period from 1976 to 1978, four major earthquakes took place within a distance of 30km from the array: a M5.4 event of August 1976, a M7.0 one of January 1978, a M5.0 one of November 1978, and a M5.4 one of December 1978. In association with these earthquakes, some changes were detected in the total intensity and also in short-period geomagnetic variations. In the case of the largest event of magnitude 7.0, the difference in the total intensity between the two stations, where continuous measurements had been made, underwent a change about two months prior to the shock. The change was very similar in time sequence to that in the electric self-potential observed near one of the above two stations. Amplitudes of short-period geomagnetic variations also changed about two months before the shock. The result of repeated surveys disclosed a striking pattern of spatial distribution of presumably coseismic changes which is quite opposite to that obtained during a post-earthquake period. About two months prior to the M5.0 earthquake which occurred several kilometers east of another station for continuous measurements, the total intensity remarkably decreased by 5nT or so at the station. The field abruptly recovered after its occurrence; a coseismic change amounted to about 5nT. In the case of the M5.4 event of 1978, no coseismic change was detected at a temporary station located a few kilometers west of the aftershock area. Changes in amplitude of short-period geomagnetic variations also appeared before this M5.4 earthquake.

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Before magnetic field variations in tectonically active regions can be used to infer changes in the crustal stress distribution, it must be demonstrated that piezomagnetism is a stress dominated rock property. Changes in many rock properties of geophysical interest such as electrical conductivity and seismic wave velocity during compressional loading are dominated by changes in microcrack porosity. If piezomagnetism were to fall into such a category its applicability as a method for determining in situ stress fluctuations would be limited. In order to assess the influence of microcrack dilatancy on magnetic susceptibility and remanent magnetization a series of cyclic loading experiments were conducted at confining pressures up to 200MPa. The results of these tests indicate that piezomagnetism is a stress related property, that is, it is controlled by differential stress and only marginally, if at all, affected by changes in microcrack porosity. Magnetic susceptibility decreased with increasing differential stress. The change in susceptibility was uniquely related to differential stress and totally independent of confining pressure, the number of stress cycles or the magnitude of the inelastic volumetric strain. Remanent magnetization, on the other hand, showed a pronounced demagnetization on the first cycle which rapidly decreased on subsequent cycles as long as the peak cycle stress remained constant. An increase in peak stress caused a pronounced decrease in magnetization at the termination of the cycle. There is a striking similarity between this observation and the change exhibited in volumetric strain; that is, an increase in crack porosity at the termination of the first cycle that became progressively smaller with increasing cycle number. Both remanent magnetization and volumetric strain became reproducible on the same cycle. In spite of this apparent correlation other test suggest that such results are merely fortuitous and changes remanent magnetization during cyclic loading are dominated by differential stress.

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Since early 1975, we have monitored magnetotelluric apparent resistivities at three sites close to the Los Angeles metropolitan area. We estimate the repeatability of our calculated apparent resistivities to be 10-20%. A major problem has been the removal of bias in the resistivity estimates caused by cultural noise, mainly on the magnetic field components. We have attempted to remove this bias using a variety of techniques including singular value decomposition of the signal matrix. None of these procedures was entirely successful when the signal predictabilities fell below 80%.
The only major earthquake to occur in our study area was the Santa Barbara shock on August 13, 1978 (M_L=5.2). No significant changes were observed in apparent resistivities measured at a site situated 80km to the east of the epicenter both 25 days before and 7 days after this shock. Significant changes in low frequency apparent resistivities were observed at a site in West Antelope Valley, 50km northwest of Palmdale. Between April 1976 and July 1978 the rotated minimum apparent resistivities in the frequency range 0.001-0.1Hz increases 10-15%, while the maximum resistivities remained almost constant. The direction of the maximum rotated apparent resistivity also changed by as much as 20°. These changes cannot be correlated with any obvious change in seismicity. However, they may be related to the uplift and subsidence which is known to have occurred in this area.
We have also observed changes in the high frequency (>1Hz) apparent resistivities following a heavy rainfall which occurred in the spring of 1978. At one site, the apparent resistivities at 10Hz dropped two orders of magnitude. This effect was modelled by adding a surface layer with a thickness of 10m and a resistivity of 10Ωm.

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