principles and applications of microearthquake networks

6 1. Introduction
the initial P-onsets was devised. The early foresight of this research group
in specifying the requirements of instrumental networks to study microearthquakes
and their ingenuity in the design, construction, and operation
of this particular network deserve to be recognized.
1.1.5. Development in Japan
Because of frequent damaging earthquakes occurring in and near Japan,
Japanese seismologists pioneered in many aspects of earthquake research.
In order to observe earthquakes in epicentral areas, Japanese
seismologists (notably A. Imamura and M. Ishimoto) developed portable
seismographs to study aftershocks and to monitor seismicity. The works
by Imamura (1929), Imamura et a!. (1932), Nasu (1929a,b, 1935a,b, 1936),
Iida (1939, 1940), Omote (1944, 1950a,b, 1955), and many others greatly
contributed to our knowledge of aftershocks. The paper that first introduced
the word “microearthquake” was by Asada and Suzuki (1949).
They chose this word to be equivalent to the Japanese word bisho zisin,
which means very small or smaller than small earthquake (Z. Suzuki,
written communication, 1979).
Asada (1957) reported the first detailed study of microearthquakes. He
demonstrated that many microearthquakes could be detected in and near
Kanto district by ultrasensitive seismographs having a magnification of
10‘ at 20 Hz. Asada understood the technical requirements of recording
microearthquakes and the advantages of studying microearthquakes because
of their higher rate of occurrence. He investigated the frequencymagnitude
relationship for microearthquakes, and tried to determine
whether microearthquakes also occur in seismic regions where larger
earthquakes occur. He foresaw the potential of using microearthquakes
to predict larger earthquakes.
Detailed studies of microearthquakes in the Kii Peninsula, central
Japan, using temporary arrays of short-period instruments and radiotelemetry
seismograph systems were carried out by Miyamura and his
colleagues in the 1950s and early 1960s (1966, with references therein
to earlier works). Timing accuracy was stressed in order to obtain precise
locations for the microearthquakes. Miyamura (1960) achieved an accuracy
of k0.05 sec by using a recording paper speed of 4 mdsec. Miyamura
et al. (1964) also used triggered magnetic tape recorders with a small
four-station array in Gifu Prefecture, central Japan. An endless tape loop
with a 20-sec delay time allowed recording of the initial P-onsets. An
automatic gain control system was used to allow better readings of the
S-phases.
The well-known Matsushiro earthquake swarm was first observed at