principles and applications of microearthquake networks

220 8. Applications fw Earthquake Prediction
of the fault plane, of the order of 5-20', at the time of the main shock. As
an alternative explanation, they suggested changes in the attenuation
properties of the material surrounding the focal regions.
Jones and Molnar (1979) measured amplitude ratios (P/S) for foreshocks
from three earthquake sequences-ne in the Philippines and two in eastern
Europe. They used seismograms recorded at WWSSN stations that
were sufficiently close to these earthquakes so that the foreshocks were
well recorded. The amplitude ratios were found to be stable with time for
each of the foreshock sequences, suggesting that the focal mechanisms for
the foreshocks did not change.
Theoretical expressions for the amplitude ratio P/SV derived by
Kisslinger (1980) show that this ratio cannot distinguish between conjugate
mechanisms or determine the sense of slip on the fault plane. He
suggested that routine observations of the P/SV ratio at a well-placed
station could detect changes in focal mechanisms without the need for
more detailed fault-plane solutions. Such solutions usually are based on
the directions of the first P-motions from vertical-component seismograms
(see Section 6.2) and may be supplemented by the polarization directions
for the S-arrivals from three-component seismograms. The orientation of
the principal stress axes may be inferred from fault-plane solutions.
Changes in the orientation of the compressional stress axes have been
investigated as a possible earthquake precursor, and we now summarize a
few examples.
The orientation of the compressional stress axes using small earthquakes
has been studied in the Garm region, USSR. Nersesov et NI.
(1973) identified three stages of temporal behavior: (1) a quiet period,
lasting several years, was characterized by random distribution of the
azimuths of the compressional stress axes; (2) a shorter period, of the
order of I year, was distinguished by an alignment of the compressional
stress axes along one of the two preferred directions, in this case 110-170";
and (3) a period beginning 3 to 4 months before the main shock was
characterized by a process of realignment, in which the axes of compressional
stress rotated to the second preferred direction, in this case 15-70'.
After the main shock, the stress system returned to its initial state as
described by (1).
Bolt et al. ( 1977) studied the Briones Hills earthquake swarm of January
1977 in central California, in which the largest event (ML = 4.3) was
considered to be the main shock. The fault-plane solution for the main
shock was found to be consistent with the regional right-lateral, strike-slip
motion along a northwest trending fault. The largest foreshock (ML = 4.0)
had a fault-plane solution that was rotated approximately 90" clockwise
from that of the main shock. The waveforms recorded at a station about 6