principles and applications of microearthquake networks

8 1. Ititrodiictioti
that changes in the travel time ratio tJt, preceded earthquakes of moderate
size (Kondratenko and Nersesov, 1962; Semenov, 1969).
Fedotov and his colleagues at the Institute of Volcanology,
Petropavlovsk-Kamchatsky, USSR, made detailed studies of seismicity
in the Kamchatka-Kuriles region. They developed the concept of seismic
gaps and seismic cycles for the prediction of large earthquakes (Fedotov,
1965, 1968; Fedotov et al., 1969).
Telemetered microearthquake networks were developed only recently
in the Soviet Union, in cooperation with American seismologists (Wallace
and Sadovskiy, 1976).
1 .I .7.
Development in the United States
Systematic studies of local earthquakes in southern California began in
the 1920s through the effort of H. 0. Wood. Anderson and Wood (1925)
developed the Wood-Anderson seismograph, a remarkably simple and
sensitive instrument at that time. From the 1920s to the 1950s Wood-
Anderson seismographs formed the backbone of the Southern California
Seismic Network, which was first operated by the Carnegie Institution
of Washington and later by the California Institute of Technology. Similar
studies of local earthquakes in northern California were carried out by the
Seismographic Stations of the University of California. Even though microearthquakes
occurring near a station could be recorded, these regional
networks were not adequate to investigate microearthquakes, because
station spacing was sparse and instrument sensitivity was not high
enough. The first reported microearthquake study in the United States
was made by Sanford and Holmes (1962) near Socorro, New Mexico, in a
manner similar to that used by Asada (1957).
In the 1950s, J. P. Eaton developed a telemetered seismograph system
(peak magnification of 40,000 at 5 Hz) for the Hawaii Seismic Network.
The stations were deployed densely enough to permit precise location of
microearthquakes on a routine basis (Eaton, 1962). From 1961 to 1963,
Don Tocher upgraded the Seismographic Stations of the University of
California by telemetering seismic signals from individual stations via
telephone lines to a central recording center at Berkeley (Bolt, 1977a).
Thus by the early 1960s the key elements for a telemetered seismic network
were known.
At about the same time, portable seismographs of high sensitivity were
also developed (e.g., Lehner and Press, 1966). Several groups in the
United States began microearthquake surveys. Notable among these were
J. Oliver and his colleagues at the Lamont-Doherty Geological Observatory
(e.g. , Oliver et nl., 1966), and J. N. Brune and his colleagues at the