principles and applications of microearthquake networks

60 3. Datu Processirig Procedures
fI, is taken to be the onset time of an event for this input signal. If qk
exceeds its threshold level within 1 sec after time fk, then the event is
confirmed to be an earthquake. Otherwise, the onset time fk is discarded,
and no earthquake is confirmed for this input signal. So far, the detection
and confirmation tests are performed on individual input signals, and it is
common for nonseismic events to be confirmed as earthquakes. To minimize
this difficulty, Stewart et ctl. (1971) utilized simple properties of the
arrival time pattern in a microearthquake network. In other words, the
event must be confirmed on some minimum number of seismic stations
within some small interval of time. This time interval must be consistent
with the station spacing and the velocity of seismic waves in the earth’s
crust. The system measured onset times and maximum amplitudes for up
to 32 incoming seismic signals, but hypocentral locations and magnitudes
were not computed.
Massinon and Plantet (1976) summarized an on-line earthquake detection
and location system for a telemetered seismic network in France.
Their method of event detection is similar to that described in Section
3.3.3. The signals from each of 20 short-period seismic stations were
transmitted to a central site where they were monitored by event detectors
as well as recorded directly for subsequent off-line processing. If
more than five stations reported a detection, then the detection times were
taken as the first arrival times and an epicenter was calculated automatically.
Massinon and Plantet (1976) were interested primarily in regional or
teleseismic earthquakes.
Kuroiso and Watanabe (1977) used the short-term average [Eq. (3.1)] to
detect local earthquakes in an 1 1-station telemetered network in Japan.
They computed the short-term average by first bandpass filtering and
rectifying the incoming seismic signal. The detected event was written to a
disk. If subsequent analysis of the data on the disk confirmed that the
event was a local earthquake, then its onset time and other parameters
were determined automatically. The hypocenter and magnitude were then
computed.
3.4. Event Processing
Event processing is the procedure for measurin som basi paramet t-s
from the recorded seismic traces that describe the earthquake ground
motion. These parameters are generally known as phase data and include:
(1) the onset time and the direction of motion of the first P-arrival; (2) the
onset time of later arrivals, such as the S-arrival (if possible); (3) the
maximum trace amplitude and its associated period; and (4) the signal