principles and applications of microearthquake networks

8.2. Temporal V'iricrtions of Seismic. Velocity 209
occurrence in the Garm region were published. The paper by Kondratenko
and Nersesov (1962) often is cited as being the first to report a
positive correlation between temporal changes in the velocity ratio V,/V,
and the later occurrence of a significant earthquake. From the early 1960s
until the late 1970s, reports on changes in velocity or velocity ratio increased
dramatically. In addition to those in the USSR, premonitory velocity
anomalies for some earthquakes in China, Japan, the United States,
and other places were also reported. Readers may refer to Rikitake (1976)
for a general review.
In this section, we summarize both the positive and negative results in
the search for precursory velocity anomalies, and discuss a few of the
problems that should be considered before relating a temporal velocity
anomaly to the future occurrence of an earthquake.
8.2.1. Investigations of Velocity Anomalies
Table IX summarizes some examples of investigations in which V,/V,
or P-residual anomalies have been reported. It is apparent that the inferred
velocity changes are quite small, regardless of the magnitude of the
targeted earthquakes. In this table, many of the figures given for precursor
times and sizes of velocity changes are estimated from illustrations in the
original papers, as some authors do not specifically list these data. Others
may obtain slightly different results.
Table X summarizes some examples of investigations reporting negative
results in the search for precursory velocity anomalies. Most negative
results reported are from California along the San Andreas fault system.
The reason may be the predominantly strike-slip focal mechanisms and
relatively shallow focal depths.
Temporal variations of velocity have also been investigated by techniques
other than the Wadati method or P-residual method. For example,
McNally and McEvilly (1977) studied the first P-motions for a set of 400
earthquakes along the San Andreas fault in central California. They found
that the inconsistencies in first P-motions could be explained by lateral
refraction of the P-waves along the higher velocity material southwest of
the fault zone from earthquakes located to the northeast. Velocity contrast
across the fault could be determined by geometrical interpretation of the
laterally refracted wave path. McNally and McEvilly suggested that a
closely spaced array of seismometers normal to the fault should be capable
of monitoring stress-related velocity changes within the earthquake
source region.
As another example, Fitch and Rynn (1976) described an inversion
method in which localized volumes of anomalously low seismic velocity