principles and applications of microearthquake networks

1.2. Oljen*ieit* crrtd Scope 9
Caltech Seismological Laboratory (e.g., Brune and Allen, 1967). A detailed
study of the aftershocks of the 1966 Parkfield earthquake by Eaton
et a/. (1970b) demonstrated that microearthquakes were useful in delineating
the slip surface of the Parkfield earthquake in three dimensions. This
experience led Eaton and his colleagues of the U.S. Geological Survey
(USGS) to develop the Central California Microearthquake Network on a
large scale (Eaton et al., 1970a). Beginning with three local clusters of
stations in 1967, this network has grown to include 250 stations covering
central California in 1980.
Figures I and 2 illustrate one of the capabilities of a dense microearthquake
network (such as the USGS Central California Microearthquake
Network). Figure la shows numerous faults mapped in the network region.
Figure Ib shows earthquake epicenters determined by a good regional
network operated by the Seismographic Stations of the University
of California at Berkeley for a nine-year period. With a dense network of
stations, as shown in Fig. 2a, the spatial distribution of earthquake epicenters
delineates their relationship to active faults in only one year's observation,
as shown in Fig. 2b.
In addition to several microearthquake networks developed and operated
by the U.S. Geological Survey, others have been implemented by
various universities, for example, University of Washington (Crosson,
1972), St. Louis University (Stauder et ctl., 1976), and the Lamont-
Doherty Geological Observatory of Columbia University (Sykes, 1977).
In the 1970s, various agencies of the United States government became
concerned about earthquake hazards and supported the development and
operation of many microearthquake networks. Today, about 50 permanent
networks are operating in the United States (see Section 7. I).
Aggarwal et ul. (1973) were the first to establish a network of portable
seismographs in the United States specifically to search for precursory
velocity anomalies similar to those reported from the Garm region, USSR.
They studied a swarm of microearthquakes in the Blue Mountain Lake
area of New York State. Detailed analysis of V,/V, ratios showed the
same type of behavior as in the Garm region, and changes preceding
earthquakes as small as magnitude 2.5 and 3.3 were observed (Aggarwal
er al.. 1973, 1975). Using stations from the Southern California Seismic
Network, Whitcomb et cil. ( 1973) reported similar precursory behavior
preceding the destructive San Fernando earthquake of February 9, 1971.
1.2. Overview and Scope
This work reviews some principles and applications of microearthquake
networks. In summarizing the present status, we emphasize methods and