principles and applications of microearthquake networks

8.3. TemporuI Variations of Other Seismic Parameters 2 17
1964, near the Matsushiro Seismological Observatory. The data set consisted
of 25 foreshocks and 173 aftershocks, with magnitudes as small as
-2. Foreshock activity started about 4 hr before the main shock, and
aftershock activity lasted about 14 hr. Computed b-slopes were 0.35 for
the foreshocks and 0.76 for the aftershocks. Background seismicity of the
Matsushiro region had a b-slope of 0.8.
Temporal changes in b-slope associated with an earthquake swarm in
mid-1970 near Danville, California, were studied by Bufe (1970). The
swarm activity could not be easily partitioned into a foreshock-aftershock
series. However, Bufe found that the b-slope decreased from a regional
value of about 1.04 to 0.6-0.8 before episodes of relatively large magnitude
earthquakes, and returned to normal soon after these episodes.
About 2400 events were used in this study.
Pfluke and Steppe ( 1973) investigated spatial variations of b-slope along
the San Andreas fault system in central California. The region between
Mount Diablo and Parkfield was divided into 10 geographic zones, based
upon tectonic and seismic considerations. The data set consisted of 1452
earthquakes with magnitude 21.8 from 1969 to 1972 as listed in the
catalogs of the USGS Central California Microearthquake Network.
Using the method developed by Utsu (1965, 1966), they found significant
differences in the b-slopes computed for the relatively small geographic
zones. For example, in two areas characterized by fault creep, the higher
b-slope occurred in the area with low seismicity, and the lower b-slope
occurred in the area with high seismicity.
Using data from the same earthquake catalogs, Wyss and Lee (1973)
studied b-slope variation as a function of time prior to the larger events
listed. Four events ranging in magnitude from 3.9 to 5.0 were selected for
study. A constant event window was used to compute temporal variation
of fi, or equivalently, the b-slope. The number of earthquakes kept
within the event window was either 25, 36, or SO. At times of low seismicity,
the event window might cover a time period as long as 1 year, thereby
reducing the chances of detecting significant temporal changes in b-slope.
Each of the four events studied showed precursory decreases in b-slope
prior to their occurrence. However, not all temporal decreases in b-slope
were followed by a significant earthquake. A precursory decrease in
b-slope was also observed by Stephens et ul. (1980) to have occurred
before the 1979 St. Elias, Alaska, earthquake.
Udias (1977) computed 6-slopes for two aftershock sequences and a
swarm near Bear Valley in central California. He used data from the
short-period, high-gain seismic system operated at the San Andreas Observatory
by the University of California at Berkeley. For the two aftershock
sequences, the b-slope values were 1.12 and 0.96; whereas for the