principles and applications of microearthquake networks

8.1. Seismicit! Plitterrzs 203
between earthquake magnitude M and precursor time T generally agreed
with those found by other investigators using other precursory phenomena.
Yamashina and Inoue (1979) studied the seismicity patterns preceding a
shallow earthquake of magnitude 6.1 that occurred on June 3, 1978, in
southwest Japan. They traced the development of a circular gap, about 5
km in diameter, starting 6 months before the main shock. However, they
did not observe an increase in seismicity within the gap before the occurrence
of the earthquake.
These results show that for some tectonic environments great shallow
earthquakes are repetitive in time, and that the seismicity preceding them
has some identifiable patterns. More importantly, it appears that similar
seismicity patterns can be identified for earthquakes of all magnitudes.
This raises the possibility that the long-term seismicity of a region could
be estimated from the short-term seismicity which is easier to obtain. In
other words, how is the short-term seismicity related to the long-term?
Although many short-term microearthquake surveys have been made
throughout the world (see Section 7.2 for a summary), this question does
not seem to have a simple answer. For instance, a microearthquake survey
by Brune and Allen (1967) suggested that there might be an inverse
correlation between short-term microseismicity and long-term seismicity
in some places along the San Andreas fault in southern California. They
found that the microseismicity at the time of the survey was at a low level
along the portion of the San Andreas fault that broke in the 1857
earthquake. Results from the USGS Central California Microearthquake
Network were similar: microseismicity along the portion of the San Andreas
fault that broke in 1906 was also low. On the other hand, Ben-
Menahem et d. (1977) studied the seismicity of the Dead Sea region by
combining data from a 2000-year historical record, from a few decades of
macroseismic data and instrumental recordings, and from a 5-month microearthquake
survey. They reported that all these sources of data were
consistent with a 6-slope value of 0.86.
Although the microseismicity recorded over a few years may be similar
to the seismicity recorded over a longer period of time (e.g., Asada, 1957;
Rikitake, 1976), one would not rely too heavily on short-term microseismicity
data for earthquake prediction purposes. One would always like
to work with the longest possible earthquake history in order to avoid
seismicity fluctuations that may not be significant.
8.1.3. Pattern Recognition Techniques
In recent years pattern recognition techniques have been developed and
applied to data from earthquake catalogs. The goal has been to identify