principles and applications of microearthquake networks

8.3. Temporiil Vuriiitiotis qf Otlier Seismic Parutmeters 215
read reliably. During the anomalous period, some of the earthquakes used
also had shallow focal depths relative to the majority of the earthquakes in
the source region.
Along more theoretical lines, a number of authors have studied the
problems associated with investigating velocity changes. Three examples
are given below.
The effect on the Wadati diagram of a layered medium in which the ratio
V,/V, differs by a small but reasonable amount from one layer to another
was studied by Kisslinger and Engdahl (1973). They found that at distances
large compared to the focal depth, the slope obtained from a
Wadati diagram was most representative of the V,,/V, ratio in the deepest
layer encountered by the seismic wave. In this case the origin time was no
longer given by the intercept on the P-arrival time axis.
Picking S-wave onsets is usually difficult, especially if only verticalcomponent
seismograms are available. Kanasewich et a/. ( 1973) computed
synthetic seismograms to illustrate the effect that a S- to P-wave conversion
near the receiver had in producing an earlier apparent S-arrival. They
demonstrated that large errors in picking S-arrivals might occur if only
vertical-component seismograms were used. For stations underlaid by
sedimentary layers only a few kilometers thick, errors of a few seconds
might occur even if horizontal-component seismograms were read. They
suggested that misidentification of S-arrivals might be fairly common in
practice. In regions with lateral velocity changes, such as across a fault
zone, it might be difficult to identify the same arrival phase from station to
station because of phase conversions along the travel path.
Crampin (1978) studied the propagation of seismic waves through a
medium characterized by systems of parallel cracks. He demonstrated
that crack orientation and geometry could play a primary role in producing
velocity anomalies and in distinguishing between the various dilatancy
models. He suggested that the orientation of the principal stress axes
could produce an anisotropic crack system, making some velocity
anomalies very difficult to observe. This might explain why velocity
anomalies were most often reported in regions with thrust-type focal
mechanisms and rarely in regions with strike-slip mechanisms. On the
other hand, polarization anomalies for the shear phases should always be
observable, and thus might be a reliable means to recognize dilatant episodes
(see, also, Crampin and McGonigle, 1981).
8.3. Temporal Variations of Other Seismic Parameters
Many other kinds of earthquake precursors have been reported. These
include temporal changes in h-slope, amplitude or amplitude ratio of cer-