principles and applications of microearthquake networks

48 3. Datu Processirig Procedures
crofilms (1 3 rolldday). The analog magnetic tapes are used because each
can record roughly 300 times more data than a standard 800 BPI digital
magnetic tape. But even four analog magnetic tapes per day are too many
to keep on a permanent basis, so we save only the earthquake waveform
data by dubbing to another analog tape. The dubbed waveform data for this
network (level 2) consists of approximately 5 x l@' bitdyear in analog
form (about 75 analog tapes per year). These analog waveform data can be
further condensed if we digitize only the relevant portions of the analog
waveforms from the stations that recorded the earthquakes adequately.
That amount of digital data is about 5 x 10'O bits/year and requires some
500 reels of 800 BPI tapes. Earthquake phase data (level 3) are prepared
from the dubbed waveform data and/or from the 16-mm microfilms, and
consist of about 10' bitsiyear. Finally at level 4, the event lists consist of
about 5 x lo6 bits/year and are derived from the phase data using an
earthquake location program.
Since data processing is not an end in itself, any effective procedures
must be considered with respect to the users. Because of the volume of
microearthquake data, a considerable amount of processing and interpretation
are required before the data become useful for research. In the
following discussion, we consider the human and technological factors
that limit our ability to process and comprehend large amounts of data.
Although some readers may not agree with our order-of-magnitude approach,
we believe that practical limits do exist and that in some instances
we are close to reaching or exceeding them.
The basic unit of information is 1 bit (either 0 or 1). A letter in the
English alphabet is commonly represented by 8 bits or 1 byte. A number
usually takes from 10 to 60 bits depending on the precision required. A
page of a scientific paper contains about 2 x lo4 bits of information,
whereas a color picture can require up to los bitdframe to display. Thus a
picture is generally much higher in information density than a page of
words or numbers. For a human being or a data processing device, the
limiting factors are data capacity, data rate or execution time, and access
delay time. Although human beings may have a large data capacity and
quick recall time, they are limited to a reading speed of less than 1000
worddmin or lo3 bitsisec, and a computing speed of less than one arithmetic
operation per second. On the other hand, a large computer may have a
data capacity of 1@* bits, a data rate of lo7 bits/sec, and an execution
speed of lo7 instructions per second.
Data processing requires at least several computer instructions for each
data sample, and there are about 3 x lo7 seconds in a year. Hence, it is
clear that a large computer cannot process more than I(Y3 bitsiyear of data
at present, nor can a human being read more than le0 bitdyear of infor-