principles and applications of microearthquake networks

3.5. Coniputing Hypoceriter Parameters 75
system detected 86% of these events, but missed 4%. The remaining 10%
of the earthquakes occurred in a sparsely monitored portion of the network
and would not be expected to be detected. About half of the detected
earthquakes had computed hypocenter parameters that agreed
favorably with those obtained routinely.
Anderson (1978) compared 44 first P-arrival times obtained by his algorithm
with those determined by an analyst. The data were taken from a
magnitude 3.5 earthquake with epicentral distances to stations in the
range of 4.5- 1 15 km. The comparison showed that 77% of Anderson’s first
P-arrival times were within kO.01 sec, and 89% were within 20.05 sec.
Similarly, Allen (1978) reported that his algorithm timed about 70% of the
first P-arrivals within t0.05 sec of the times determined by an analyst in
several test runs.
So far, the analyst does a betterjob in identifying poor first P-onsets and
in deciding which stations to ignore in calculating hypocenter parameters.
Nevertheless, automated methods are useful in obtaining rapid earthquake
locations and in processing large quantities of data. The quality of
the results is often good enough for a preliminary reporting of a main
shock and its aftershocks (e.g., see Lester et af., 1975). We expect that
improved automated methods will soon take over most of the routine data
processing tasks now performed by analysts.