principles and applications of microearthquake networks

50 3. Data Processitig Procedures
correctness of the station location on the map is to let another person find
the station using the map alone. The station coordinates should be carefully
read from the map, usually to kO.01 minute in latitude and longitude,
and to a few meters in elevation. These coordinates should be checked by
at least one other person. A chronological record should be kept on instrument
characteristics and maintenance services performed. At the central
recording site, it is necessary to have a record keeping procedure for
the raw seismic data and a place to store the data safely. If absolute time
such as WWVB radio signal is not recorded, then clock correction information
must be saved. The one-to-one correspondence between the station
and its seismic signal should be directly identifiable from the recorded
media, such as analog magnetic tapes and 16-mm microfilms. If it is not,
then assignment of recording position for each station must be kept. For
example, Houck et al. (1976) compiled a handbook of station coordinates,
polarity, and recording position on the analog magnetic tapes and 16-mm
microfilms for the USGS Central California Microearthquake Network.
Many steps are involved in processing the raw seismic data into event
lists of hypocenter parameters, magnitude, etc. It is important to keep
records of processing procedures, selection criteria, intermediate data,
and final results. It is also important to publish regularly the basic information
and results of a microearthquake network. Data from a microearthquake
network can quickly fill up a room. Therefore, the data must be
organized, cataloged, and filed properly. The data should not be taken
from the storage room unless it is absolutely necessary; otherwise, they
have a tendency to be lost.
In summary, the goals of record keeping are to make all information
from a microearthquake network easily accessible and to maintain the
long-term continuity of the data.
3.3. Event Detection
Event detection is the procedure for recording the approximate time of
occurrence of an earthquake within a microearthquake network. A scan
list is a tabulation, in chronological order, of the times of the detected
earthquakes. If regional or teleseismic earthquakes are to be detected and
processed, then they are also included in the scan list. Scan lists generated
by visual or semiautomatic methods may be annotated, for instance, with
a remark as to the type of event (local, regional or teleseismic earthquake,
or explosion) and with an estimate of its signal duration. Scan lists are
used to guide the processing of a set of events and to ensure that all events