principles and applications of microearthquake networks

8.1. Srismic.itJ Ptrtterns 207
of the main shock, and their epicenters were concentrated spatially.
Wyss et al. (1978) examined the seismicity before four large earthquakes
(Kamchatka, USSR; Friuli, Italy: Assam, India; and Kalapana,
Hawaii). On the basis of this, as well as results reported by other investigators,
they concluded that systematic patterns observed in foreshock
seismicity were similar for earthquakes throughout the world. They also
suggested that some "precursory cluster events" appeared to have radiation
properties different from the background earthquakes (see Section
8.3.3).
Utsu ( 1978) attempted to discriminate foreshock sequences from
swarms on the basis of magnitude statistics as follows. Let M1. M2, and
M3 be the magnitudes of the largest, second largest, and third largest
events in a foreshock sequence or a swarm. He selected 245 shallow
swarms or foreshock sequences in Japan using the following criteria: (1)
MI - M2 5 0.6, and M, 2 5.Ofor the period 1926-1964; and (2) M, - M2 5
0.6, and M, 2 4.5 for the period 1965-1977. From this data set, Utsu was
able to identify 77% of the foreshock sequences (10 out of 13) if he stipulated
the following conditions: (I) 0.4 5 M, - M2 5 0.6; (2) M, - M3 2
0.7: and (3) M2 precedes M3 in time. He would have mistaken 7% of the
swarms (17 out of 232) as foreshock sequences.
Jones and Molnar ( 1979) summarized the characteristics of foreshocks
associated with major earthquakes throughout the world. Their primary
data sources were issues of the International Seismological Summary and
bulletins of the lnternational Seismological Center. They observed that
foreshock seismicity was often characterized by a pattern of increases and
decreases, accompanied by changes in its geographic location relative to
the forthcoming main shock epicenter and aftershock zone. The number
of foreshocks per day appeared to increase as the time to the main shock
approached. This relationship seemed to be independent of the main
shock magnitude. The magnitude of the largest foreshock did not appear
to be related to the magnitude of the main shock. Extension of the work
by Jones and Molnar to lower magnitude earthquakes should be possible
as more earthquake catalogs are produced from microearthquake networks.
Kodama and Bufe (1979) studied foreshock occurrence for 29 earthquakes
of magnitude 3.5 or greater located along the San Andreas fault
in central California. Only 10 of the 29 earthquakes appeared to have
foreshocks. The foreshock pattern often was one of an increase of activity,
a period of quiescence, and then more foreshocks within 24 hr of the
main shock. The time interval between the start of the seismicity increase
and the occurrence of the main shock was found to be weakly dependent
on the main shock magnitude.