principles and applications of microearthquake networks

70 3. Data Processirig Procedures
ignored and the same procedure was repeated for the next first P-arrival
time detected. For a small microearthquake network in which the first
P-arrivals of the desired earthquakes can be expected to reach all the
stations, the time window may be set to be the maximum propagation time
across the network. For example, Crampin and Fyfe (1974) used a 16-sec
time window for the 100-km aperture LOWNET array in Scotland. They
required that a valid earthquake must have at least three stations reporting
first P-arrival times within this time window.
Because more than one earthquake may occur within one minute in a
large microearthquake network, Stewart (1977) carried out additional
tests on the first P-arrival time data to separate earthquakes with similar
origin times. His approach considers first the time relationship and then
the spatial relationship of the first P-arrival time data. If a gap of 8 sec or
more is found between successive arrival times, then the arrival times
before that gap are separated out as one subset of time-coherent data to be
examined further for spatial coherence. The 8-sec gap is chosen for the
USGS Central California Microearthquake Network from considerations
of the station spacing and the P-velocity structure beneath this network.
In order to consider the spatial relationship of the first P-arrival time data,
the network is subdivided into eight distinct geographic zones and each
station is assigned to only one zone. For a given subset of time-coherent
data, the number of first P-arrivals in pairs of adjacent zones is counted. If
this number is greater than 3, then the first P-arrival time data in such
zones are considered to belong to one earthquake event. If this is not the
case, then those data that fail the test are discarded and the remaining data
in this time-coherent subset are examined in a similar manner. The details
of this approach are described in Stewart (1977).
3.5. Computing Hypocenter Parameters
The hypocenter parameters calculated routinely for microearthquakes
include the origin time, epicenter coordinates, focal depth, magnitude,
and estimates of their errors. If enough first-motion data are available, a
plot of the first P-motions on the focal sphere is also made. In this section,
we describe some of the practical aspects of computing these parameters
with examples taken from two large microearthquake networks in California.
The mathematical aspects of hypocenter calculation, fault-plane
solution, and magnitude estimation will be treated in Chapter 6.
Because hypocenter parameters are usually calculated by digital computers,
there are three basic approaches in obtaining the results. Each
approach is iterative in nature since errors in the input data are unavoid-