principles and applications of microearthquake networks

3.5. Computing Hypocenter Parameters 73
from the noise events by visual examination. In the third stage, the seismic
data are displayed on a graphics terminal, and the analyst identifies and
picks each arriving phase using an adjustable cross-hair cursor. The computer
then calculates the arrival times and stores the phase data. After a
small group of earthquakes is processed, the computer works out preliminary
locations. In the fourth stage, retiming and final analysis are performed.
The phase data can be corrected by reexamining the digital seismic
data, and data from other sources can be added. A final hypocenter
location and magnitude estimation can then be calculated.
In another example, staff members of the USGS Central California
Microearthquake Network have developed an interactive processing system.
The input data for this system are derived from the four analog
magnetic tapes recorded daily from this network. A daily event list is
prepared by examining the monitor and microfilm recordings of the seismic
data. Events in this list are dubbed under computer control from the
four daily analog tapes onto a library tape. Earthquakes on the library tape
are digitized and processed by the Interactive Seismic Display System
(ISDS) written by Stevenson (1978). The algorithm developed by Allen
(1978) is used to automatically pick first P-arrival times. The seismic
traces and the corresponding automatic picks are displayed on a graphics
terminal. The analyst can use a cross-hair cursor to correct the picks if
necessary. An earthquake location program (Lee ef a/., 1981) is executed,
and the output is examined for errors. This procedure can be repeated
until the analyst is satisfied with the results.
Ichikawa (1980) and Ichikawa ef d. ( 1980) described an interactive system
for processing seismic data from a telemetered network operated by
the Japan Meteorological Agency. This system was installed in 1975 and
monitors more than 70 stations. Its operation is similar to that described in
the preceding paragraph.
For the second approach, an interactive data processing system for
earthquakes recorded on 16-mm microfilms has been developed by W. H.
K. Lee and his colleagues. This system consists of three components: (1)
an optical-electromechanical unit that advances and projects four rolls of
16-mm microfilm onto a digitizing table; (2) a digitizing unit with a cursor
that allows the analyst to pick seismic phases; and (3) a microcomputer
that controls the bookkeeping and processing tasks, reduces the digitized
data to phase data, and computes hypocenter location and magnitude
from the phase data. Usually, one earthquake is processed at a time until
the analyst is satisfied with the results. Because all processing steps for
one earthquake are carried out while the seismic traces are displayed,
errors can be easily identified and corrected by the analyst. This method
can process 16-mm microfilms about twice as fast as a noninteractive