Measurements of regional magnetic field during gravity, strain and leveling surveys near the San Andreas fault at Cajon, Palmdale and Tejon are strongly correlated with changes in gravity, areal strain, and uplift in these regions during the period 1977-1984. This correlation principally depends on data taken during 1978-79 and 1981-82 when episodes of the ‘Palmdale Uplift’ occurred in this general region. Because the inferred relationships between these parameters are in approximate agreement with those obtained from simple deformation models, the preferred explanation appeals to short-term strain episodes independently detected in each data set. Transfer functions from magnetic to strain, gravity, and uplift perturbations, obtained by least-square linear fits to the data, are -0.98nT/ppm, -0.03nT/μGal, and 9.1nT/m respectively. Tectonomagnetic model calculations underestimate the observed changes and those reported previously for dam loading and volcano-magnetic observations. A less likely alternative explanation of the observed data appeals to a common source of meteorologically generated crustal or instrumental noise in the strain, gravity, magnetic, and uplift data.

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A variety of tectonic models can be constructed by superposing the displacement field solution of a single force operative at a point in an elastic half-space. Piezomagnetic changes associated with the same models are given as well by the linear combination of fundamental piezomagnetic potentials, which arise from stress-induced magnetization produced by a unit single force at a point. The earth is assumed a homogeneous and isotropic elastic semi-infinite medium having a uniformly magnetized top layer. The method is adaptable to (a) surface load, (b) strain nuclei, (c) dislocation and (d) volume source problems. The prescribed potentials are obtained with the aid of a surface integral representation of the tectonomagnetic field (SASAI, 1983). As an application example, piezomagnetic field is calculated for a uniform circular load.

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Precursory changes both in th electrical earth resistivity and in the geomagnetic total force gradient were clearly observed in relation to the earthquake of M 5.6 which occurred near the Yamasaki fault on May 30, 1984. The observation site was at a distance of several kilometers from the epiceter. The precursors were recognized about 60-70 days before the event. First the resistivity gradually decreased during 10 days by 30% of its ordinary value. After keeping a low level for about 50 days, the resistivity began to increase, exceeding the ordinary value 2 days before the shock, and attained to a maximum of 400% after the shock. The extremely large value returned to its ordinary value within two weeks after the event. The geomagnetic total force gradient also showed about 10% increase synchronously with the resistivity change. Almost 2 days after the shock, the gradient recovered to its ordinary value. No coseismic change was recognized. Other precursor like changes of the resistivity possibly associated with several earthquakes in southwest Japan were also detected at the same observation site within the last year. We consider that the resistivity change may be caused by a small amount of change in the water content within the rocks or fissures around the fault and that the fault region is a place particularly sensitive to tectonic stress changes. A resistivity structure model to account for the observed precursors will be presented.

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The main factors which are responsible for an annual variation of apparent resistivity are variations of ground water level and ground temperature. In order to estimate quantitatively the influence of these factors, apparent resistivity changes were calculated by using the finite element method. The calculation results yield the following conclusions. (1) The influence of variation of ground water level on the annual variation of apparent resistivity is larger than that of ground temperature. (2) The depth of bedrock strongly controls the amplitude of annual variation of apparent resistivity; that is, shallower bedrock enhances the amplitude more greatly. (3) In order to suppress the annual variation of apparent resistivity, it is necessary to select an appropriate electrode span. For example, if the depth of bedrock is 10m, the electrode span AB/2 must be larger than 1000m to reduce the amplitude of annual variation to about 1%. If the depth is larger than 200m, the electrode span of 50m will reduce the amplitude of annual variation to less than 1%. Accordingly, in order to monitor changes in the resistivity of sediments associated with earthquakes, it is better to establish a geoelectric station at a location where bedrock is deeper.

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Electromagnetic response is examined for a local three-dimensional resistivity anomaly embedded in the earth's crust of uniform resistivity, in order to interpret time-dependence of horizontal amplitudes as well as transfer functions for short-period geomagnetic variations at an observation site in the Izu region, Japan. The results of numerical estimation indicate that the detected enhancement of about ten percent in horizontal amplitude can be well explained by a local decrease, amounting to one order of magnitude, in crustal resistivity, on the condition that the observation site is located over the area of resistivity decrease. In this situation no marked change is expected for transfer functions; this is consistent with the observation. The results of numerical estimation also suggest that electric as well as magnetic field measurements are useful for detecting crustal resistivity changes; for instance, changes are expected in the amplitude of induced electric field, as would be reflected in apparent resistivity changes in magnetotellurics, and also in polarization angle of the electric field. It should be noted, however, that no detectable precursory changes are expected unless observation sites are located close to the area of resistivity change presumably corresponding to an earthquake source area and its vicinity.

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Anomalous variation of the electric self-potential difference between carbon rod electrodes fixed in an underground geophysical observation tunnel took place preceding a considerably strong earthquake (M=5.6) which occurred at an epicentral distance of 3.2km. The precursor-like changes ranging 40mV at maximum began about 40 days before and also 55 hours before the earthquake occurrence and the recovery began co-seismically coming back to the initial level after 13 hours. The geophysical observation tunnel is located in the fractured and electrically conductive zone of the Yamasaki active fault, a target field for the earthquake prediction study.
It was elucidated from the results of observation of the electric self-potential differences with the components of various directions and electrodes spacing and from the measurements of the self-potential across the fault and in the tunnel that the observed anomalous variation does not imply the variation of the earth-current observed due to the voltage drop through the ohmic resistance between the electrodes, but implies the decrease of the self-potential of the electrode, fixed in the fractured zone of the fault, itself, to that of another which served as a reference.
As for the cause of the decrease of the self-potential, there may be two possibilities: (1) it was due to the time variation of the contact electric potential difference between the electrode and the surroundings at each electrode, (2) it was caused by the time variation of the self-potential of the fractured rock or of the water in the cracks of the fault where the electrode is fixed.
As the electrodes used are carbon rod electrodes, non-equilibrium balanced type electrodes, the first possibility may be plausible. However, as all electrodes had already aged and become stable enough at that time and also considering the other observational results, the second possibility is likely to be more probable.

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In order to observe anomalous electromagnetic radiations associated with occurrences of earthquakes, a simple system was adopted for a continuous observation in the LF range using a radio receiver as a sensor. The frequency of 163kHz was chosen as a monitoring frequency based on the result of a careful survey of spectra of artificial radio signals. The accumulated number of pulse type noises was continuously recorded by this system.
Anomalous LF noises have been observed before and after shallow earthquakes which occurred in the inland area or in the shallow sea region. They appeared for a few hours to several days before and after the main shocks. The total number of them depends on the magnitude of the main shock. No significant increments of noises have been observed in the case of earthquakes whose epicenters were located in the deep sea region.
These results suggest that electromagnetic noises are emitted from the focal region of shallow earthquakes and this phenomenon is important for the study of focal processes and the earthquake prediction.

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