principles and applications of microearthquake networks

3.4. Everrt Processitig 61
duration of the earthquake. Precise measurement of the phase data is
essential because they are used to compute the origin time. hypocenter,
and magnitude of the earthquake, and to deduce its focal mechanism. In
the following subsection, we give some general methods of measuring the
phase data.
The goals of event processing are to measure the phase data as uniformly
and precisely as possible and to prepare the so-called phase list
containing these data for each earthquake. In this section. we discuss
processing the individual seismic traces, while in Section 3.5 we discuss
processing the phase lists.
3.4.1. Visual Methods
The traditional way of measuring phase data is to read visually the
phase time, amplitudes and periods, etc., from the seismograms using
rulers or scales (see, e.g., Willmore, 1979). The phase data are usually
recorded on a printed form to facilitate punching cards or otherwise entering
the data into a computer. The visual method has the advantage that it
is straightforward. It is also a simple matter to read and merge data from a
variety of visual recording media. For a small microearthquake network
operating in an area of moderate seismicity, this may be the most costeffective
method to use. However, the visual method has the following
disadvantages: (1) different analysts may read the seismograms differently:
(2) errors may be introduced in measuring, writing, or keypunching
the data; and (3) it may be difficult to carry out such a tedious task over a
long period of time. Also, visual recording media have a limited dynamic
range and a limited recording speed, so that some of the phase data (such
as first P-motion, S-arrival, and maximum trace amplitude) cannot be read
precisely, if at all.
For a large microearthquake network where the data volume is large,
the visual method of measuring phase data requires a considerable amount
of checking to minimize errors in data handling. This includes systematic
checking of measured phase data by another analyst before keypunching,
and systematic verifying of the phase data after keypunching. Our experience
shows that analysts or keypunch operators occasionally enter the
data in the wrong place or transpose digits in the data. For example, a
P-arrival time of 13.55 sec may be written or keypunched as 3 1.55 sec.
3.4.2. Semiautomated Methods
Semiautomated methods usually are designed to eliminate the tasks of
recording and keypunching data that are required in the visual methods.